path: root/prototypes/connections/module.c

#include "module.h"
#include "module-save.h"
#include "module-text.h"
#include <math.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

enum { STATE_BYTES = 16, COLLECTION_BYTES = 12, INPUT_BYTES = 16, FIELD_BYTES = 8,
       CONSTANT_BYTES = 8, RECORD_BYTES = 8, RECORD_FIELD_BYTES = 16, VIEW_BYTES = 16,
       HANDLER_BYTES = 12, OP_BYTES = 16 };
/* T_FIELD and T_ITEM stand for the type of the message or record field named. */
enum { T_NUM = 1, T_BYTES = 2, T_TEXT = 3, T_CURSOR = 4, T_FIELD = 5, T_INDEX = 6, T_ITEM = 7 };
enum { FROM_PAYLOAD, FROM_DOCUMENT, FROM_COPY, FROM_CONSTANT, FROM_TEXT, FROM_ARENA };
/* Where an opcode may appear. */
enum { IN_VIEW = 1, IN_HANDLER = 2, IN_MESSAGE = 4, IN_KEYED = 8, IN_TEXT = 16, IN_ROW = 32 };
enum { READ = IN_VIEW | IN_HANDLER };
/* What an argument names: nothing (it is 0), a state of one type (A_STATE +
 * the type), a list and one of its record's fields, a message field, the
 * collection, a constant, a unit, the column's row, a branch target or a
 * stack depth. */
enum { A_NONE, A_STATE, A_ITEM = A_STATE + PURE_STATE_INDEX + 1, A_FIELD, A_COLLECTION, A_CONSTANT, A_UNIT,
       A_ROW, A_TARGET, A_DEPTH, A_FOLD };
/* A fold's block is verified as a column that leaves one number (count) or
 * two (a value and whether to include it), a table's filter as a column that
 * leaves one: these stand for a view type. */
enum { FOLD_ONE = 100, FOLD_TWO, FILTER };
/* The text or list an opcode works on: its cursor and index operands belong
 * to it, and so does a cursor or index it pushes. O_ANY: no rule. O_OPERAND:
 * it pushes a position in its operand's text or list. O_ARG: the argument.
 * O_REF: the text or list of the argument's cursor or index state. O_LIST:
 * the list of an item argument. */
enum { O_ANY, O_OPERAND, O_ARG, O_REF, O_LIST };
static const unsigned char magic[8] = {'P','U','R','E','M','O','D','5'};

/* Operand types popped (bottom to top), the type pushed, where allowed, what
 * the argument names and the text or list worked on. */
#define NUMS(pops) {pops, {T_NUM, T_NUM, T_NUM}, T_NUM, READ, A_NONE, O_ANY}
#define STATE(type) A_STATE + PURE_STATE_##type
static const struct { unsigned char pops, in[3], out, where, arg, of; } signature[PURE_OP_COUNT] = {
    [PURE_OP_CONST] = NUMS(0), [PURE_OP_ADD] = NUMS(2), [PURE_OP_SUB] = NUMS(2), [PURE_OP_MUL] = NUMS(2),
    [PURE_OP_DIV] = NUMS(2), [PURE_OP_NEG] = NUMS(1), [PURE_OP_LT] = NUMS(2), [PURE_OP_LE] = NUMS(2),
    [PURE_OP_GT] = NUMS(2), [PURE_OP_GE] = NUMS(2), [PURE_OP_EQ] = NUMS(2), [PURE_OP_NE] = NUMS(2),
    [PURE_OP_AND] = NUMS(2), [PURE_OP_OR] = NUMS(2), [PURE_OP_NOT] = NUMS(1), [PURE_OP_MIN] = NUMS(2),
    [PURE_OP_MAX] = NUMS(2), [PURE_OP_CLAMP] = NUMS(3),
    [PURE_OP_LOAD] = {0, {0}, T_NUM, READ, STATE(NUM), O_ANY},
    [PURE_OP_DT] = {0, {0}, T_NUM, IN_HANDLER, A_NONE, O_ANY},
    [PURE_OP_STORE] = {1, {T_NUM}, 0, IN_HANDLER, STATE(NUM), O_ANY},
    [PURE_OP_JZ] = {1, {T_NUM}, 0, READ, A_TARGET, O_ANY},
    [PURE_OP_JMP] = {0, {0}, 0, READ, A_TARGET, O_ANY},
    [PURE_OP_FIELD] = {0, {0}, T_FIELD, IN_MESSAGE, A_FIELD, O_ANY},
    [PURE_OP_DOC] = {0, {0}, T_BYTES, IN_KEYED, A_COLLECTION, O_ANY},
    [PURE_OP_DOC_SIZE] = {0, {0}, T_NUM, IN_KEYED, A_COLLECTION, O_ANY},
    [PURE_OP_LEN] = {1, {T_BYTES}, T_NUM, IN_HANDLER, A_NONE, O_ANY},
    [PURE_OP_REPLACE] = {3, {T_NUM, T_NUM, T_BYTES}, 0, IN_KEYED, A_COLLECTION, O_ANY},
    [PURE_OP_REPLY] = {1, {T_BYTES}, 0, IN_MESSAGE, A_NONE, O_ANY},
    [PURE_OP_REQUIRE] = {1, {T_NUM}, 0, IN_HANDLER, A_NONE, O_ANY},
    [PURE_OP_TEXT_CONST] = {0, {0}, T_TEXT, READ, A_CONSTANT, O_ANY},
    [PURE_OP_TEXT_LOAD] = {0, {0}, T_TEXT, READ, STATE(TEXT), O_ANY},
    [PURE_OP_TEXT_INPUT] = {0, {0}, T_TEXT, IN_TEXT, A_NONE, O_ANY},
    [PURE_OP_NUM_TEXT] = {1, {T_NUM}, T_TEXT, READ, A_NONE, O_ANY},
    [PURE_OP_CONCAT] = {2, {T_TEXT, T_TEXT}, T_TEXT, READ, A_NONE, O_ANY},
    [PURE_OP_CHARS] = {1, {T_TEXT}, T_NUM, READ, A_NONE, O_ANY},
    [PURE_OP_LINES] = {1, {T_TEXT}, T_NUM, READ, A_NONE, O_ANY},
    [PURE_OP_TEXT_BYTES] = {1, {T_TEXT}, T_NUM, READ, A_NONE, O_ANY},
    [PURE_OP_CUR_LOAD] = {0, {0}, T_CURSOR, READ, STATE(CURSOR), O_REF},
    [PURE_OP_CUR_START] = {0, {0}, T_CURSOR, READ, STATE(TEXT), O_ARG},
    [PURE_OP_CUR_END] = {0, {0}, T_CURSOR, READ, STATE(TEXT), O_ARG},
    [PURE_OP_CUR_MOVE] = {2, {T_CURSOR, T_NUM}, T_CURSOR, READ, A_UNIT, O_OPERAND},
    [PURE_OP_CUR_STORE] = {1, {T_CURSOR}, 0, IN_HANDLER, STATE(CURSOR), O_REF},
    [PURE_OP_TEXT_INSERT] = {2, {T_CURSOR, T_TEXT}, 0, IN_HANDLER, STATE(TEXT), O_ARG},
    [PURE_OP_TEXT_REMOVE] = {2, {T_CURSOR, T_CURSOR}, 0, IN_HANDLER, STATE(TEXT), O_ARG},
    [PURE_OP_TEXT_LINE] = {1, {T_CURSOR}, T_TEXT, READ, STATE(TEXT), O_ARG},
    [PURE_OP_TEXT_SET] = {1, {T_TEXT}, 0, IN_HANDLER, STATE(TEXT), O_ANY},
    [PURE_OP_TEXT_FROM] = {1, {T_BYTES}, T_TEXT, READ, A_NONE, O_ANY},
    [PURE_OP_BYTES_FROM] = {1, {T_TEXT}, T_BYTES, READ, A_NONE, O_ANY},
    [PURE_OP_CUR_COUNT] = {1, {T_CURSOR}, T_NUM, READ, A_UNIT, O_ANY},
    /* Record operations also take as many values as the list's record has fields. */
    [PURE_OP_LIST_COUNT] = {0, {0}, T_NUM, READ, STATE(LIST), O_ANY},
    [PURE_OP_LIST_APPEND] = {0, {0}, 0, IN_HANDLER, STATE(LIST), O_ANY},
    [PURE_OP_LIST_INSERT] = {1, {T_INDEX}, 0, IN_HANDLER, STATE(LIST), O_ARG},
    [PURE_OP_LIST_REMOVE] = {1, {T_INDEX}, 0, IN_HANDLER, STATE(LIST), O_ARG},
    [PURE_OP_ITEM_GET] = {1, {T_INDEX}, T_ITEM, READ, A_ITEM, O_LIST},
    [PURE_OP_ITEM_SET] = {1, {T_INDEX}, 0, IN_HANDLER, A_ITEM, O_LIST},
    [PURE_OP_INDEX_LOAD] = {0, {0}, T_INDEX, READ, STATE(INDEX), O_REF},
    [PURE_OP_INDEX_STORE] = {1, {T_INDEX}, 0, IN_HANDLER, STATE(INDEX), O_REF},
    [PURE_OP_INDEX_MOVE] = {2, {T_INDEX, T_NUM}, T_INDEX, READ, A_NONE, O_OPERAND},
    [PURE_OP_INDEX_FIRST] = {0, {0}, T_INDEX, READ, STATE(LIST), O_ARG},
    [PURE_OP_INDEX_LAST] = {0, {0}, T_INDEX, READ, STATE(LIST), O_ARG},
    [PURE_OP_INDEX_NUM] = {1, {T_INDEX}, T_NUM, READ, A_NONE, O_ANY},
    [PURE_OP_ROW] = {0, {0}, T_INDEX, IN_ROW, A_ROW, O_ARG},
    /* Stack copies: their types come from the values they move. */
    [PURE_OP_PICK] = {0, {0}, 0, READ, A_DEPTH, O_ANY},
    [PURE_OP_SLIDE] = {0, {0}, 0, READ, A_DEPTH, O_ANY},
    [PURE_OP_FOLD] = {0, {0}, T_NUM, READ, A_FOLD, O_ANY},
};
#undef NUMS
#undef STATE

static uint32_t read16(const unsigned char *p) { return (uint32_t)p[0] | (uint32_t)p[1] << 8; }

static uint32_t read32(const unsigned char *p) {
    return (uint32_t)p[0] | (uint32_t)p[1] << 8 | (uint32_t)p[2] << 16 | (uint32_t)p[3] << 24;
}

static double read_double(const unsigned char *p) {
    uint64_t bits = read32(p) | (uint64_t)read32(p + 4) << 32;
    double value;
    memcpy(&value, &bits, sizeof(value));
    return value;
}

PURE_COLD static bool key_name(const char *key) {
    static const char *const named[] = {"Space", "Enter", "ArrowLeft", "ArrowRight", "ArrowUp", "ArrowDown",
                                        "Backspace", "Delete", "Home", "End", "Tab", "Escape", "PageUp", "PageDown"};
    for (size_t i = 0; i < sizeof(named) / sizeof(*named); ++i)
        if (!strcmp(key, named[i])) return true;
    if (!strncmp(key, "Key", 3)) return key[3] >= 'A' && key[3] <= 'Z' && !key[4];
    if (!strncmp(key, "Digit", 5)) return key[5] >= '0' && key[5] <= '9' && !key[6];
    return false;
}

/* A document key: 1..64 bytes of [A-Za-z0-9_-], as every Pure store accepts. */
static bool document_key(const char *key) {
    size_t length = key ? strlen(key) : 0;
    if (!length || length > 64) return false;
    for (size_t i = 0; i < length; ++i) {
        char c = key[i];
        if (!((c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z') || (c >= '0' && c <= '9') ||
              c == '-' || c == '_')) return false;
    }
    return true;
}

/* Strings are [u8 length][bytes]; a name is an ASCII identifier of 1..31 bytes. */
PURE_COLD static bool name_at(const unsigned char *strings, uint32_t size, uint32_t offset,
                    bool identifier, char out[PURE_MODULE_NAME]) {
    if (offset >= size) return false;
    uint32_t length = strings[offset];
    if (!length || length >= PURE_MODULE_NAME || length > size - offset - 1) return false;
    for (uint32_t i = 0; i < length; ++i) {
        unsigned char c = strings[offset + 1 + i];
        bool letter = (c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z') || c == '_';
        if (!letter && !(i && c >= '0' && c <= '9') && (identifier || c < 32 || c > 126)) return false;
    }
    memcpy(out, strings + offset + 1, length);
    out[length] = 0;
    return true;
}

/* The input whose handler this is, or -1 for frame. */
static int event_input(uint32_t event) { return event ? (int)((event - 1) / 2) : -1; }

/* The state an argument names must have this type. */
static bool state_of(const PureModule *m, uint32_t state, uint8_t type) {
    return state < m->states && m->state_type[state] == type;
}

/* The list an item operation names: its low 16 bits (the field is above). */
static uint32_t list_of(uint32_t arg) { return arg & 0xFFFF; }
static uint32_t item_field(uint32_t arg) { return arg >> 16; }

static unsigned char field_type(const PureModule *m, uint32_t list, uint32_t field) {
    return m->record_field_text[m->record[m->state_ref[list]].first + field] ? T_TEXT : T_NUM;
}

PURE_COLD static bool names_arg(const PureModule *m, unsigned kind, uint32_t arg, const PureInputShape *shape, int row) {
    switch (kind) {
    case A_ITEM:
        return state_of(m, list_of(arg), PURE_STATE_LIST) && item_field(arg) < m->record[m->state_ref[list_of(arg)]].count;
    case A_FIELD: return arg < shape->field_count;
    case A_COLLECTION: return arg < m->collections;
    case A_CONSTANT: return arg < m->constants;
    case A_UNIT: return arg <= PURE_UNIT_LINE;
    case A_ROW: return (int)arg == row;
    case A_FOLD: return (arg & 0xFF) <= PURE_FOLD_MIN && state_of(m, arg >> 8 & 0xFF, PURE_STATE_LIST) && arg >> 16;
    case A_NONE: case A_TARGET: case A_DEPTH: return true;
    default: return state_of(m, arg, (uint8_t)(kind - A_STATE));
    }
}

/* The stack a branch brings to its target, while the target is ahead. */
typedef struct { uint32_t target; int height; unsigned char types[PURE_MODULE_STACK], owners[PURE_MODULE_STACK]; } Join;

static bool same_stack(const Join *join, int height, const unsigned char *types, const unsigned char *owners) {
    return join->height == height && !memcmp(join->types, types, (size_t)height) &&
           !memcmp(join->owners, owners, (size_t)height);
}

/* Types and heights are checked per instruction; a cursor's type records
 * which text it belongs to and an index's type which list (other values
 * belong to nothing). Branches go forward within the block. Every path into
 * a join brings the same stack (types and owners), so a value may cross a
 * join, as in a conditional expression; at most 8 joins with a non-empty
 * stack are ahead at once. Handlers end empty, views with one value of their
 * type. A column (row is its list) may read the row it is evaluated for.
 * A fold's block is verified as a column of its list (reads only, no fold
 * in it); the block around treats it as one instruction and may not branch
 * into it. Returns why the block is refused, with at moved to the
 * instruction when one is at fault, or NULL. */
PURE_COLD static const char *verify_block(const PureModule *m, PureBlock block, int view, int row, uint32_t event,
                                uint32_t *at) {
#define REFUSE(why) do { *at = pc; return why; } while (0)
    if (block.start > m->code_count || block.length > m->code_count - block.start) return "code range outside module";
    if (view >= 0 && !block.length) return "empty view";
    int input = view >= 0 ? -1 : event_input(event);
    const PureInputShape *shape = input >= 0 ? &m->input[input] : NULL;
    unsigned where = view >= 0 ? IN_VIEW | (row >= 0 ? IN_ROW : 0) : IN_HANDLER;
    if (shape && shape->kind == PURE_KIND_MESSAGE) where |= IN_MESSAGE | (shape->keyed ? IN_KEYED : 0);
    if (shape && shape->kind == PURE_KIND_TEXT) where |= IN_TEXT;
    unsigned char types[PURE_MODULE_STACK], owners[PURE_MODULE_STACK], target[PURE_MODULE_CODE + 1] = {0};
    Join joins[PURE_MODULE_JOINS];
    unsigned open = 0;
    int height = 0;
    bool reachable = true;
    for (uint32_t i = 0; i <= block.length; ++i) {
        uint32_t pc = block.start + i;
        /* Branches here brought an empty stack (target) or the stack of a join. */
        unsigned join = open;
        for (unsigned k = 0; k < open; ++k) if (joins[k].target == pc) join = k;
        if (join < open) {
            if (target[i] || (reachable && !same_stack(&joins[join], height, types, owners)))
                REFUSE("join with different stacks");
            height = joins[join].height;
            memcpy(types, joins[join].types, (size_t)height);
            memcpy(owners, joins[join].owners, (size_t)height);
            joins[join] = joins[--open];
            reachable = true;
        } else if (target[i]) {
            if (reachable && height) REFUSE("join with different stacks");
            height = 0;
            reachable = true;
        }
        if (i == block.length) break;
        if (!reachable) REFUSE("unreachable instruction");
        const PureOp *op = &m->code[pc];
        if (op->op >= PURE_OP_COUNT) REFUSE("unknown opcode");
        unsigned allowed = signature[op->op].where, kind = signature[op->op].arg, of = signature[op->op].of;
        if (!(allowed & where)) {
            if (allowed & IN_ROW) REFUSE("row outside a column");
            if (view == FILTER) REFUSE("filter with an effect");
            if (view >= FOLD_ONE) REFUSE("fold with an effect");
            if (view >= 0) REFUSE("view with an effect");
            if (allowed & IN_KEYED) REFUSE("document operation outside a keyed message handler");
            if (allowed & IN_TEXT) REFUSE("text input outside a text handler");
            REFUSE("message operation outside a message handler");
        }
        uint32_t arg = op->arg;
        if ((kind == A_NONE && arg) || (op->op == PURE_OP_CONST ? !isfinite(op->value) : op->value != 0))
            REFUSE("non-canonical operand");
        if (!names_arg(m, kind, arg, shape, row)) REFUSE("operand names no such state, field, constant or unit");
        /* PICK copies the value arg below the top; SLIDE drops arg values under the top. */
        if (kind == A_DEPTH) {
            bool pick = op->op == PURE_OP_PICK;
            if (pick ? arg >= (uint32_t)height : !arg || arg >= (uint32_t)height) REFUSE("stack underflow");
            if (pick && height == PURE_MODULE_STACK) REFUSE("stack overflow");
            int from = height - 1 - (int)arg, to = pick ? height++ : from;
            types[to] = pick ? types[from] : types[height - 1];
            owners[to] = pick ? owners[from] : owners[height - 1];
            if (!pick) height = from + 1;
            continue;
        }
        uint32_t skip = 0;
        if (kind == A_FOLD) {
            uint32_t inner = pc;
            skip = arg >> 16;
            if (view >= FOLD_ONE) REFUSE(view == FILTER ? "fold inside a filter" : "fold inside a fold");
            if (skip > block.length - i - 1) REFUSE("fold block outside its block");
            const char *why = verify_block(m, (PureBlock){pc + 1, skip}, (arg & 0xFF) == PURE_FOLD_COUNT ? FOLD_ONE : FOLD_TWO,
                                           (int)(arg >> 8 & 0xFF), 0, &inner);
            if (why) { *at = inner; return why; }
            for (uint32_t k = i + 1; k <= i + skip; ++k) {
                if (target[k]) REFUSE("branch into a fold");
                for (unsigned j = 0; j < open; ++j) if (joins[j].target == block.start + k) REFUSE("branch into a fold");
            }
        }
        /* Operand types, bottom to top; record operations take their fields. */
        unsigned char need[PURE_RECORD_FIELDS + 2];
        unsigned pops = signature[op->op].pops;
        memcpy(need, signature[op->op].in, sizeof(signature[op->op].in));
        if (op->op == PURE_OP_LIST_APPEND || op->op == PURE_OP_LIST_INSERT) {
            const PureRecord *record = &m->record[m->state_ref[arg]];
            for (unsigned f = 0; f < record->count; ++f) need[pops + f] = field_type(m, arg, f);
            pops += record->count;
        } else if (op->op == PURE_OP_ITEM_SET) {
            need[pops++] = field_type(m, list_of(arg), item_field(arg));
        }
        if (height < (int)pops) REFUSE("stack underflow");
        height -= (int)pops;
        for (unsigned k = 0; k < pops; ++k)
            if (types[height + (int)k] != need[k]) REFUSE("operand type");
        unsigned owner = of == O_ARG ? arg : of == O_REF ? m->state_ref[arg] : of == O_LIST ? list_of(arg) : 0;
        for (unsigned k = 0; of >= O_ARG && k < pops; ++k) {
            unsigned char type = types[height + (int)k];
            if ((type == T_CURSOR || type == T_INDEX) && owners[height + (int)k] != owner)
                REFUSE(type == T_CURSOR ? "cursor of another text" : "index of another list");
        }
        unsigned char out = signature[op->op].out;
        if (out == T_FIELD) out = m->field[shape->field_start + arg].type == PURE_FIELD_U32 ? T_NUM : T_BYTES;
        if (out == T_ITEM) out = field_type(m, list_of(arg), item_field(arg));
        if (out) {
            if (height == PURE_MODULE_STACK) REFUSE("stack overflow");
            owners[height] = (unsigned char)(out != T_CURSOR && out != T_INDEX ? 0 : of == O_OPERAND ? owners[height] : owner);
            types[height++] = out;
        }
        if (kind == A_TARGET) {
            if (arg <= pc || arg > block.start + block.length) REFUSE("branch not forward within its block");
            join = open;
            for (unsigned k = 0; k < open; ++k) if (joins[k].target == arg) join = k;
            if (!height) {
                if (join < open) REFUSE("join with different stacks");
                target[arg - block.start] = 1;
            } else if (join < open) {
                if (!same_stack(&joins[join], height, types, owners)) REFUSE("join with different stacks");
            } else {
                if (target[arg - block.start]) REFUSE("join with different stacks");
                if (open == PURE_MODULE_JOINS) REFUSE("more than 8 joins ahead");
                joins[open] = (Join){.target = arg, .height = height};
                memcpy(joins[open].types, types, (size_t)height);
                memcpy(joins[open++].owners, owners, (size_t)height);
            }
            if (op->op == PURE_OP_JMP) reachable = false;
        }
        i += skip;
    }
    if (!reachable) return "block end is unreachable";
    if (view == FILTER) return height != 1 || types[0] != T_NUM ? "filter must leave one number" : NULL;
    if (view >= FOLD_ONE) {
        int want = view - FOLD_ONE + 1;
        return height != want || types[0] != T_NUM || types[want - 1] != T_NUM ? "fold block must leave its numbers" : NULL;
    }
    unsigned char result = view == PURE_VIEW_TEXT ? T_TEXT : T_NUM;
    if (view >= 0 ? height != 1 || types[0] != result : height != 0)
        return view >= 0 ? "view must leave one value of its type" : "handler must leave no value";
    return NULL;
#undef REFUSE
}

PURE_COLD PureResult pure_module_load(const void *bytes, size_t size, PureModule *out,
                            char *error, size_t error_size) {
    const unsigned char *p = bytes;
    const char *why;
    uint32_t bad = 0;
    PureResult result = PURE_INVALID;
#define REFUSE(message, index) do { why = message; bad = (uint32_t)(index); goto refused; } while (0)
    if (error_size) error[0] = 0;
    memset(out, 0, sizeof(*out));
    if (!p || size < PURE_MODULE_HEADER || memcmp(p, magic, sizeof(magic))) REFUSE("not a Pure module v5", 0);
    out->states = (uint16_t)read16(p + 8);
    out->inputs = (uint16_t)read16(p + 10);
    out->views = (uint16_t)read16(p + 12);
    out->handlers = (uint16_t)read16(p + 14);
    out->code_count = read32(p + 16);
    uint32_t strings = read32(p + 20);
    out->fields = (uint16_t)read16(p + 24);
    out->collections = (uint16_t)read16(p + 26);
    out->constants = (uint16_t)read16(p + 28);
    out->records = (uint16_t)read16(p + 30);
    uint32_t constant_bytes = read32(p + 32);
    out->record_fields = (uint16_t)read16(p + 36);
    if (read16(p + 38)) REFUSE("reserved header bytes", 38);
    if (out->states > PURE_MODULE_STATES || out->inputs > PURE_MODULE_INPUTS ||
        out->views > PURE_MODULE_VIEWS || out->handlers > PURE_MODULE_HANDLERS ||
        out->code_count > PURE_MODULE_CODE || strings > PURE_MODULE_STRINGS ||
        out->fields > PURE_MODULE_FIELDS || out->collections > PURE_MODULE_COLLECTIONS ||
        out->constants > PURE_MODULE_CONSTANTS || constant_bytes > PURE_MODULE_CONSTANT_BYTES ||
        out->records > PURE_MODULE_RECORDS || out->record_fields > PURE_MODULE_RECORD_FIELDS) {
        result = PURE_LIMIT;
        REFUSE("module exceeds a limit", 8);
    }
    size_t expected = PURE_MODULE_HEADER + (size_t)out->states * STATE_BYTES +
        (size_t)out->collections * COLLECTION_BYTES + (size_t)out->inputs * INPUT_BYTES +
        (size_t)out->fields * FIELD_BYTES + (size_t)out->constants * CONSTANT_BYTES +
        (size_t)out->records * RECORD_BYTES + (size_t)out->record_fields * RECORD_FIELD_BYTES +
        (size_t)out->views * VIEW_BYTES + (size_t)out->handlers * HANDLER_BYTES +
        (size_t)out->code_count * OP_BYTES + strings + constant_bytes;
    if (size != expected) REFUSE("module size", expected);
    const unsigned char *at = p + PURE_MODULE_HEADER;
    const unsigned char *text = p + size - constant_bytes - strings;
    memcpy(out->constant_data, p + size - constant_bytes, constant_bytes);
    for (uint32_t i = 0; i < out->states; ++i, at += STATE_BYTES) {
        out->state_type[i] = (uint8_t)read32(at + 4);
        if (!name_at(text, strings, read32(at), true, out->state_names[i])) REFUSE("state name", i);
        if (read32(at + 4) > PURE_STATE_INDEX) REFUSE("state type", i);
        if (out->state_type[i] == PURE_STATE_NUM) {
            out->initial[i] = read_double(at + 8);
            if (!isfinite(out->initial[i])) REFUSE("non-finite initial value", i);
        } else {
            uint32_t flags = read32(at + 12);
            if ((flags && (out->state_type[i] != PURE_STATE_CURSOR || flags != PURE_CURSOR_STAYS)) ||
                read32(at + 8) > UINT16_MAX) REFUSE("state reference", i);
            out->state_ref[i] = (uint16_t)read32(at + 8);
            out->state_flags[i] = (uint8_t)flags;
        }
    }
    for (uint32_t i = 0; i < out->collections; ++i, at += COLLECTION_BYTES) {
        if (!name_at(text, strings, read32(at), true, out->collection_names[i])) REFUSE("collection name", i);
        if (read32(at + 4) != 1 || read32(at + 8) != 1) REFUSE("collection must hold bytes by key", i);
    }
    uint32_t next_field = 0;
    const unsigned char *fields = at + (size_t)out->inputs * INPUT_BYTES;
    for (uint32_t i = 0; i < out->inputs; ++i, at += INPUT_BYTES) {
        PureInputShape *shape = &out->input[i];
        shape->kind = at[4];
        shape->keyed = at[5];
        shape->field_start = (uint16_t)read16(at + 12);
        shape->field_count = (uint16_t)read16(at + 14);
        if (!name_at(text, strings, read32(at), true, out->input_names[i])) REFUSE("input name", i);
        if (read16(at + 6) || at[5] > 1) REFUSE("input flags", i);
        if (shape->kind == PURE_KIND_KEY) {
            if (shape->keyed || shape->field_start || shape->field_count ||
                !name_at(text, strings, read32(at + 8), false, out->input_keys[i]) ||
                !key_name(out->input_keys[i])) REFUSE("input key", i);
            for (uint32_t k = 0; k < i; ++k)
                if (out->input[k].kind == PURE_KIND_KEY && !strcmp(out->input_keys[k], out->input_keys[i]))
                    REFUSE("duplicate key", i);
            continue;
        }
        if (shape->kind == PURE_KIND_TEXT) {
            if (shape->keyed || shape->field_start || shape->field_count || read32(at + 8))
                REFUSE("text input shape", i);
            continue;
        }
        if (shape->kind != PURE_KIND_MESSAGE || read32(at + 8) || shape->field_start != next_field ||
            shape->field_count > out->fields - next_field) REFUSE("message shape", i);
        /* Fixed u32le fields in order, then at most one bytes tail. */
        for (uint32_t f = 0; f < shape->field_count; ++f) {
            const unsigned char *entry = fields + (size_t)(next_field + f) * FIELD_BYTES;
            PureField *field = &out->field[next_field + f];
            field->type = (uint8_t)read32(entry + 4);
            if (!name_at(text, strings, read32(entry), true, field->name) ||
                (field->type != PURE_FIELD_U32 && field->type != PURE_FIELD_BYTES) || read32(entry + 4) > 2 ||
                (field->type == PURE_FIELD_BYTES && f + 1 != shape->field_count))
                REFUSE("message field", next_field + f);
            for (uint32_t k = 0; k < f; ++k)
                if (!strcmp(out->field[next_field + k].name, field->name)) REFUSE("duplicate field", next_field + f);
            field->offset = shape->prefix;
            if (field->type == PURE_FIELD_U32) shape->prefix += 4;
        }
        next_field += shape->field_count;
    }
    if (next_field != out->fields) REFUSE("unused message fields", next_field);
    at = fields + (size_t)out->fields * FIELD_BYTES;
    for (uint32_t i = 0; i < out->constants; ++i, at += CONSTANT_BYTES) {
        out->constant_offset[i] = read32(at);
        out->constant_length[i] = read32(at + 4);
        if (out->constant_offset[i] > constant_bytes || out->constant_length[i] > constant_bytes - out->constant_offset[i] ||
            !pure_utf8_valid((PureSpan){out->constant_data + out->constant_offset[i], out->constant_length[i]}))
            REFUSE("text constant", i);
    }
    /* Records: fields in order, numbered from first; a text default is a constant. */
    const unsigned char *fields_at = at + (size_t)out->records * RECORD_BYTES;
    uint32_t next_record_field = 0;
    for (uint32_t i = 0; i < out->records; ++i, at += RECORD_BYTES) {
        PureRecord *record = &out->record[i];
        record->first = (uint16_t)read16(at + 4);
        record->count = (uint16_t)read16(at + 6);
        if (!name_at(text, strings, read32(at), true, out->record_names[i]) || record->first != next_record_field ||
            !record->count || record->count > PURE_RECORD_FIELDS || record->count > out->record_fields - next_record_field)
            REFUSE("record", i);
        for (uint32_t f = record->first; f < record->first + record->count; ++f) {
            const unsigned char *entry = fields_at + (size_t)f * RECORD_FIELD_BYTES;
            uint32_t type = read32(entry + 4);
            out->record_field_text[f] = type == 1;
            if (!name_at(text, strings, read32(entry), true, out->record_field_names[f]) || type > 1)
                REFUSE("record field", f);
            for (uint32_t k = record->first; k < f; ++k)
                if (!strcmp(out->record_field_names[k], out->record_field_names[f])) REFUSE("duplicate record field", f);
            out->record_field_default[f] = type ? read32(entry + 8) : read_double(entry + 8);
            if (type ? read32(entry + 12) || read32(entry + 8) >= out->constants : !isfinite(out->record_field_default[f]))
                REFUSE("record field default", f);
        }
        next_record_field += record->count;
    }
    if (next_record_field != out->record_fields) REFUSE("unused record fields", next_record_field);
    at = fields_at + (size_t)out->record_fields * RECORD_FIELD_BYTES;
    for (uint32_t i = 0; i < out->states; ++i)
        if ((out->state_type[i] == PURE_STATE_TEXT && out->state_ref[i] >= out->constants) ||
            (out->state_type[i] == PURE_STATE_CURSOR && !state_of(out, out->state_ref[i], PURE_STATE_TEXT)) ||
            (out->state_type[i] == PURE_STATE_LIST && out->state_ref[i] >= out->records) ||
            (out->state_type[i] == PURE_STATE_INDEX && !state_of(out, out->state_ref[i], PURE_STATE_LIST)))
            REFUSE("state reference", i);
    /* A table names its list in place of code; its columns follow it. */
    for (uint32_t i = 0; i < out->views; ++i, at += VIEW_BYTES) {
        if (!name_at(text, strings, read32(at), true, out->view_names[i])) REFUSE("view name", i);
        out->view_type[i] = at[4];
        out->view_table[i] = (uint16_t)read16(at + 6);
        out->view_code[i] = (PureBlock){read32(at + 8), read32(at + 12)};
        uint32_t table = out->view_table[i];
        if (at[4] > PURE_VIEW_TABLE || (at[5] && at[4] != PURE_VIEW_TABLE) ||
            (table && (table > i || out->view_type[table - 1] != PURE_VIEW_TABLE || at[4] == PURE_VIEW_TABLE)))
            REFUSE("view type", i);
        /* A table names its list in start, or, filtered, in the byte after its
         * type (list + 1) with its filter's block in start and length. */
        if (at[4] == PURE_VIEW_TABLE) {
            uint32_t list = at[5] ? at[5] - 1u : out->view_code[i].start;
            if (!state_of(out, list, PURE_STATE_LIST) || !at[5] != !out->view_code[i].length) REFUSE("table list", i);
            out->view_list[i] = (uint16_t)list;
        } else if (table) out->view_list[i] = out->view_list[table - 1];
    }
    for (uint32_t i = 0; i < out->handlers; ++i, at += HANDLER_BYTES) {
        uint32_t event = read32(at);
        out->handler[i] = (PureHandler){event, {read32(at + 4), read32(at + 8)}};
        int input = event_input(event);
        if (event > 2u * out->inputs || (input >= 0 && out->input[input].kind != PURE_KIND_KEY && !(event & 1)))
            REFUSE("handler event", i);
    }
    for (uint32_t i = 0; i < out->code_count; ++i, at += OP_BYTES) {
        if (at[1] || at[2] || at[3]) REFUSE("instruction padding", i);
        out->code[i] = (PureOp){.op = at[0], .arg = read32(at + 4), .value = read_double(at + 8)};
    }
    /* States, records, collections, inputs and views share one namespace. */
    const char *names[PURE_MODULE_STATES + PURE_MODULE_RECORDS + PURE_MODULE_COLLECTIONS +
                      PURE_MODULE_INPUTS + PURE_MODULE_VIEWS];
    size_t count = 0;
    for (uint32_t i = 0; i < out->states; ++i) names[count++] = out->state_names[i];
    for (uint32_t i = 0; i < out->records; ++i) names[count++] = out->record_names[i];
    for (uint32_t i = 0; i < out->collections; ++i) names[count++] = out->collection_names[i];
    for (uint32_t i = 0; i < out->inputs; ++i) names[count++] = out->input_names[i];
    for (uint32_t i = 0; i < out->views; ++i) if (!out->view_table[i]) names[count++] = out->view_names[i];
    for (size_t i = 0; i < count; ++i)
        for (size_t k = 0; k < i; ++k)
            if (!strcmp(names[i], names[k])) REFUSE("duplicate name", i);
    /* Column names are unique within their table. */
    for (uint32_t i = 0; i < out->views; ++i)
        for (uint32_t k = 0; k < i; ++k)
            if (out->view_table[i] && out->view_table[k] == out->view_table[i] &&
                !strcmp(out->view_names[i], out->view_names[k])) REFUSE("duplicate column", i);
    for (uint32_t i = 0; i < out->views; ++i) {
        bad = i;
        bool table = out->view_type[i] == PURE_VIEW_TABLE;
        if ((!table || out->view_code[i].length) &&
            (why = verify_block(out, out->view_code[i], table ? FILTER : out->view_type[i],
                                table || out->view_table[i] ? out->view_list[i] : -1, 0, &bad))) goto refused;
    }
    for (uint32_t i = 0; i < out->handlers; ++i) {
        bad = i;
        if ((why = verify_block(out, out->handler[i].code, -1, -1, out->handler[i].event, &bad))) goto refused;
    }
    return PURE_OK;
refused:
    if (error_size) snprintf(error, error_size, "%s (%u)", why, bad);
    return result;
#undef REFUSE
}

static int find(const char (*names)[PURE_MODULE_NAME], unsigned count, const char *name) {
    for (unsigned i = 0; i < count; ++i)
        if (!strcmp(names[i], name)) return (int)i;
    return -1;
}

int pure_module_state(const PureModule *m, const char *name) { return find(m->state_names, m->states, name); }
int pure_module_input(const PureModule *m, const char *name) { return find(m->input_names, m->inputs, name); }
int pure_module_view(const PureModule *m, const char *name) {
    for (uint32_t i = 0; i < m->views; ++i)
        if (!m->view_table[i] && !strcmp(m->view_names[i], name)) return (int)i;
    return -1;
}

int pure_module_position(const PureModule *m, uint32_t view) {
    if (view >= m->views || m->view_type[view] != PURE_VIEW_NUM || m->view_table[view]) return -1;
    const PureBlock *block = &m->view_code[view];
    const PureOp *code = &m->code[block->start];
    return block->length == 2 && code[0].op == PURE_OP_INDEX_LOAD && code[1].op == PURE_OP_INDEX_NUM ? (int)code[0].arg : -1;
}

int pure_module_column(const PureModule *m, uint32_t table, const char *name) {
    for (uint32_t i = 0; i < m->views; ++i)
        if (m->view_table[i] == table + 1 && !strcmp(m->view_names[i], name)) return (int)i;
    return -1;
}

/* ---- Execution ------------------------------------------------------ */

/* IEEE 754-2019 minimum/maximum on finite values: -0 orders below +0. */
static double minimum(double a, double b) { return a < b || (a == b && signbit(a)) ? a : b; }
static double maximum(double a, double b) { return a > b || (a == b && !signbit(a)) ? a : b; }

typedef struct { double num; PureBytes bytes; } Slot;
typedef struct {
    PureApp *app;
    const PureInputShape *shape; /* message, or NULL */
    const char *key;             /* a keyed message's key */
    int64_t base;                /* the revision a message's edit was made against, or -1 */
    PureArena *arena;
    double dt;
    size_t row; /* the row a column is evaluated for */
    bool modified, exists;
} Event;

static PureResult fault(PureApp *app, PureFault kind, uint32_t pc) {
    app->fault = kind;
    app->fault_pc = pc;
    return kind == PURE_FAULT_LIMIT ? PURE_LIMIT : kind == PURE_FAULT_NOT_FOUND ? PURE_NOT_FOUND : PURE_INVALID;
}

static bool reserve(PureArena *arena, size_t more) {
    if (more <= arena->capacity - arena->used) return true;
    size_t capacity = arena->capacity ? arena->capacity : 256;
    while (capacity - arena->used < more) {
        if (capacity > SIZE_MAX / 2) return false;
        capacity *= 2;
    }
    unsigned char *bytes = realloc(arena->bytes, capacity);
    if (!bytes) return false;
    arena->bytes = bytes;
    arena->capacity = capacity;
    return true;
}

/* Visit a value's bytes in order, from wherever they live. */
static PureResult visit_value(const PureApp *app, const PureArena *arena, PureBytes v,
                              bool (*visit)(void *, PureSpan), void *context) {
    if (!v.length) return PURE_OK;
    const unsigned char *direct = NULL;
    switch (v.source) {
    case FROM_PAYLOAD: direct = app->payload.data + v.offset; break;
    case FROM_COPY: direct = app->copied + v.offset; break;
    case FROM_CONSTANT: direct = app->module->constant_data + v.offset; break;
    case FROM_ARENA: direct = arena->bytes + v.offset; break;
    case FROM_TEXT:
        return pure_draft_read_version(app->texts[v.owner], v.version, (size_t)v.offset, (size_t)v.length,
                                       visit, context);
    default:
        return app->data.read(app->data.context, app->document, v.version, (size_t)v.offset,
                              (size_t)v.length, visit, context);
    }
    visit(context, (PureSpan){direct, (size_t)v.length});
    return PURE_OK;
}

static bool append_span(void *context, PureSpan span) {
    PureArena *arena = context;
    memmove(arena->bytes + arena->used, span.data, span.size);
    arena->used += span.size;
    return true;
}

/* Copy a value to the end of the arena. Room is reserved first, so reading
 * an arena value while appending never follows a moved buffer. */
static PureResult append_value(const PureApp *app, PureArena *arena, PureBytes v) {
    if (!reserve(arena, (size_t)v.length)) return PURE_ALLOCATION;
    return visit_value(app, arena, v, append_span, arena);
}

/* A contiguous span of a value, copying into the arena when it is spread out. */
static PureResult contiguous(const PureApp *app, PureArena *arena, PureBytes v, PureSpan *out) {
    if (v.source == FROM_PAYLOAD || v.source == FROM_COPY || v.source == FROM_CONSTANT) {
        const unsigned char *base = v.source == FROM_PAYLOAD ? app->payload.data :
                                    v.source == FROM_COPY ? app->copied : app->module->constant_data;
        *out = (PureSpan){v.length ? base + v.offset : NULL, (size_t)v.length};
        return PURE_OK;
    }
    if (v.source == FROM_ARENA) {
        *out = (PureSpan){arena->bytes + v.offset, (size_t)v.length};
        return PURE_OK;
    }
    size_t start = arena->used;
    PureResult result = append_value(app, arena, v);
    *out = (PureSpan){arena->bytes + start, (size_t)v.length};
    return result;
}

typedef struct { size_t count; unsigned unit; unsigned utf8; bool valid; } Measure;

static bool measure(void *context, PureSpan span) {
    Measure *m = context;
    if (m->unit == 2) m->valid = m->valid && pure_utf8_step(&m->utf8, span);
    else m->count += m->unit ? pure_utf8_newlines(span) : pure_utf8_scalars(span);
    return true;
}

static PureBytes text_of(const PureApp *app, uint32_t state) {
    PureVersion version = pure_draft_version(app->texts[state]);
    return (PureBytes){FROM_TEXT, (uint16_t)state, version, 0, version.bytes};
}

static void move_cursors(const PureModule *m, double *values, uint32_t text, size_t at,
                         size_t removed, size_t inserted) {
    for (uint32_t c = 0; c < m->states; ++c) {
        if (m->state_type[c] != PURE_STATE_CURSOR || m->state_ref[c] != text) continue;
        double position = values[c];
        if (removed) {
            if (position >= at + removed) position -= (double)removed;
            else if (position > at) position = (double)at;
        } else if (position > at || (position == at && !(m->state_flags[c] & PURE_CURSOR_STAYS)))
            position += (double)inserted;
        values[c] = position;
    }
}

/* Indices follow their item across an insert or remove; an index whose item
 * was removed names the next item (or the last, or 0 in an empty list). */
static void move_indices(const PureModule *m, double *values, uint32_t list, size_t count,
                         size_t at, bool inserted) {
    double last = count ? (double)(count - 1) : 0;
    for (uint32_t i = 0; i < m->states; ++i) {
        if (m->state_type[i] != PURE_STATE_INDEX || m->state_ref[i] != list) continue;
        if (inserted) {
            if (count > 1 && values[i] >= (double)at) ++values[i];
        } else {
            if (values[i] > (double)at) --values[i];
            values[i] = minimum(values[i], last);
        }
    }
}

static bool text_field(const PureModule *m, uint32_t list, uint32_t field) {
    return m->record_field_text[m->record[m->state_ref[list]].first + field];
}

/* Append bytes to the arena as a value of it. */
static PureResult keep_bytes(PureArena *arena, const void *bytes, size_t length, PureBytes *out) {
    if (!reserve(arena, length)) return PURE_ALLOCATION;
    if (length) memcpy(arena->bytes + arena->used, bytes, length);
    *out = (PureBytes){FROM_ARENA, 0, {0, 0}, arena->used, length};
    arena->used += length;
    return PURE_OK;
}

/* Run a verified block. Types and stack bounds were proven at load. A change
 * the data refuses (a size limit, a missing item) fails the event with its
 * fault; other failures pass through. */
static PureResult run(const PureModule *m, double *values, PureBlock block, Event *ev, Slot *result) {
    Slot stack[PURE_MODULE_STACK];
    size_t sp = 0;
    PureApp *app = ev->app;
    PureArena *arena = ev->arena;
    PureResult status;
    PureFault kind;
    const PureOp *op = NULL;
    for (uint32_t pc = block.start, end = block.start + block.length; pc < end;) {
        op = &m->code[pc++];
        uint32_t arg = op->arg;
        Slot *top = sp ? &stack[sp - 1] : stack;
        switch (op->op) {
        case PURE_OP_CONST: stack[sp++].num = op->value; continue;
        case PURE_OP_LOAD: stack[sp++].num = values[arg]; continue;
        case PURE_OP_DT: stack[sp++].num = ev->dt; continue;
        case PURE_OP_STORE: case PURE_OP_CUR_STORE: values[arg] = stack[--sp].num; continue;
        case PURE_OP_JZ: if (stack[--sp].num == 0) pc = arg; continue;
        case PURE_OP_JMP: pc = arg; continue;
        case PURE_OP_NEG: top->num = -top->num; continue;
        case PURE_OP_NOT: top->num = top->num == 0; continue;
        case PURE_OP_CLAMP: {
            double high = stack[--sp].num, low = stack[--sp].num;
            if (low > high) { kind = PURE_FAULT_CLAMP; goto failed; }
            stack[sp - 1].num = maximum(low, minimum(stack[sp - 1].num, high));
            continue;
        }
        case PURE_OP_FIELD: {
            const PureField *field = &m->field[ev->shape->field_start + arg];
            if (field->type == PURE_FIELD_U32) stack[sp++].num = read32(app->payload.data + field->offset);
            else stack[sp++].bytes = (PureBytes){FROM_PAYLOAD, 0, {0, 0}, field->offset, app->payload.size - field->offset};
            continue;
        }
        case PURE_OP_DOC: case PURE_OP_DOC_SIZE: {
            if (op->op == PURE_OP_DOC && !ev->exists) { kind = PURE_FAULT_NOT_FOUND; goto failed; }
            PureVersion version = app->data.version(app->data.context, app->document);
            if (op->op == PURE_OP_DOC) stack[sp++].bytes = (PureBytes){FROM_DOCUMENT, 0, version, 0, version.bytes};
            else stack[sp++].num = (double)version.bytes;
            continue;
        }
        case PURE_OP_LEN: case PURE_OP_TEXT_BYTES: top->num = (double)top->bytes.length; continue;
        case PURE_OP_REPLACE: {
            PureBytes insert = stack[--sp].bytes;
            double last = stack[--sp].num, first = stack[--sp].num;
            double size = (double)app->data.version(app->data.context, app->document).bytes;
            if (first != floor(first) || last != floor(last) || first < 0 || last < first || last > size) {
                kind = PURE_FAULT_LIMIT;
                goto failed;
            }
            PureSpan span;
            size_t mark = arena->used;
            if ((status = contiguous(app, arena, insert, &span)) == PURE_OK)
                status = app->data.splice(app->data.context, app->document, (size_t)first, (size_t)(last - first), span);
            arena->used = mark;
            if (status != PURE_OK) goto refused;
            ev->modified = ev->exists = true;
            continue;
        }
        /* A reply outlives the arena mark of later statements: keep it by value. */
        case PURE_OP_REPLY: app->reply = stack[--sp].bytes; app->has_reply = true; continue;
        case PURE_OP_REQUIRE: if (stack[--sp].num == 0) { kind = PURE_FAULT_LIMIT; goto failed; } continue;
        case PURE_OP_TEXT_CONST:
            stack[sp++].bytes = (PureBytes){FROM_CONSTANT, 0, {0, 0}, m->constant_offset[arg], m->constant_length[arg]};
            continue;
        case PURE_OP_TEXT_LOAD: stack[sp++].bytes = text_of(app, arg); continue;
        case PURE_OP_TEXT_INPUT: stack[sp++].bytes = (PureBytes){FROM_PAYLOAD, 0, {0, 0}, 0, app->payload.size}; continue;
        case PURE_OP_NUM_TEXT: {
            char digits[32];
            if ((status = keep_bytes(arena, digits, pure_format_number(top->num, digits), &top->bytes)) != PURE_OK)
                return status;
            continue;
        }
        case PURE_OP_CONCAT: {
            PureBytes right = stack[--sp].bytes, left = stack[sp - 1].bytes;
            size_t start = arena->used;
            if ((status = append_value(app, arena, left)) != PURE_OK ||
                (status = append_value(app, arena, right)) != PURE_OK) return status;
            stack[sp - 1].bytes = (PureBytes){FROM_ARENA, 0, {0, 0}, start, left.length + right.length};
            continue;
        }
        case PURE_OP_CHARS: case PURE_OP_LINES: case PURE_OP_TEXT_FROM: {
            Measure count = {0, op->op == PURE_OP_LINES ? 1 : op->op == PURE_OP_TEXT_FROM ? 2 : 0, 0, true};
            if ((status = visit_value(app, arena, top->bytes, measure, &count)) != PURE_OK) return status;
            if (op->op != PURE_OP_TEXT_FROM) top->num = (double)(count.count + (op->op == PURE_OP_LINES));
            else if (!count.valid || !pure_utf8_complete(count.utf8)) { kind = PURE_FAULT_ENCODING; goto failed; }
            continue;
        }
        /* Positions: a cursor or an index, with the text or list it is in. */
        case PURE_OP_CUR_LOAD: case PURE_OP_INDEX_LOAD:
            stack[sp].num = values[arg];
            stack[sp++].bytes.owner = m->state_ref[arg];
            continue;
        case PURE_OP_CUR_START: case PURE_OP_CUR_END: case PURE_OP_INDEX_FIRST: case PURE_OP_INDEX_LAST:
        case PURE_OP_ROW: {
            size_t last = op->op == PURE_OP_CUR_END ? pure_draft_size(app->texts[arg]) :
                          op->op == PURE_OP_INDEX_LAST ? pure_list_count(app->lists[arg]) : 0;
            stack[sp].num = op->op == PURE_OP_ROW ? (double)ev->row :
                            op->op == PURE_OP_INDEX_LAST && last ? (double)(last - 1) : (double)last;
            stack[sp++].bytes.owner = (uint16_t)arg;
            continue;
        }
        case PURE_OP_CUR_MOVE: case PURE_OP_INDEX_MOVE: {
            double count = stack[--sp].num;
            Slot *at = &stack[sp - 1];
            if (count != floor(count)) { kind = PURE_FAULT_LIMIT; goto failed; }
            if (op->op == PURE_OP_INDEX_MOVE) {
                size_t items = pure_list_count(app->lists[at->bytes.owner]);
                at->num = maximum(0, minimum(at->num + count, items ? (double)(items - 1) : 0));
                continue;
            }
            PureDraft *text = app->texts[at->bytes.owner];
            PureVersion version = pure_draft_version(text);
            at->num = (double)(arg == PURE_UNIT_CHAR ? pure_text_step(text, version, (size_t)at->num, count)
                                                     : pure_text_line_step(text, version, (size_t)at->num, count));
            continue;
        }
        case PURE_OP_CUR_COUNT: {
            PureDraft *text = app->texts[top->bytes.owner];
            top->num = (double)pure_text_count(text, pure_draft_version(text), (size_t)top->num, arg);
            continue;
        }
        case PURE_OP_TEXT_INSERT: case PURE_OP_TEXT_SET: {
            PureBytes insert = stack[--sp].bytes;
            size_t at = op->op == PURE_OP_TEXT_INSERT ? (size_t)stack[--sp].num : 0;
            size_t removed = op->op == PURE_OP_TEXT_SET ? pure_draft_size(app->texts[arg]) : 0;
            PureSpan span;
            size_t mark = arena->used;
            if ((status = contiguous(app, arena, insert, &span)) == PURE_OK)
                status = pure_draft_splice(app->texts[arg], at, removed, span);
            arena->used = mark;
            if (status != PURE_OK) goto refused;
            if (op->op == PURE_OP_TEXT_SET) {
                for (uint32_t c = 0; c < m->states; ++c)
                    if (m->state_type[c] == PURE_STATE_CURSOR && m->state_ref[c] == arg) values[c] = 0;
            } else move_cursors(m, values, arg, at, 0, span.size);
            continue;
        }
        case PURE_OP_TEXT_REMOVE: {
            double y = stack[--sp].num, x = stack[--sp].num;
            size_t first = (size_t)minimum(x, y), last = (size_t)maximum(x, y);
            if ((status = pure_draft_splice(app->texts[arg], first, last - first, (PureSpan){0})) != PURE_OK) goto refused;
            if (last > first) move_cursors(m, values, arg, first, last - first, 0);
            continue;
        }
        case PURE_OP_TEXT_LINE: {
            PureDraft *text = app->texts[arg];
            PureVersion version = pure_draft_version(text);
            size_t first = pure_text_line_start(text, version, (size_t)top->num);
            size_t last = pure_text_line_end(text, version, (size_t)top->num);
            top->bytes = (PureBytes){FROM_TEXT, (uint16_t)arg, version, first, last - first};
            continue;
        }
        case PURE_OP_BYTES_FROM: case PURE_OP_INDEX_NUM: continue; /* the same bytes or position, retyped */
        case PURE_OP_LIST_COUNT: stack[sp++].num = (double)pure_list_count(app->lists[arg]); continue;
        case PURE_OP_LIST_APPEND: case PURE_OP_LIST_INSERT: {
            PureList *list = app->lists[arg];
            const PureRecord *record = &m->record[m->state_ref[arg]];
            size_t base = sp - record->count, count = pure_list_count(list), at = count;
            sp = base;
            if (op->op == PURE_OP_LIST_INSERT && (at = (size_t)stack[--sp].num) > count) {
                kind = PURE_FAULT_NOT_FOUND;
                goto failed;
            }
            /* Room for every text first, so no span moves while the next is copied. */
            size_t mark = arena->used, total = 0;
            for (unsigned f = 0; f < record->count; ++f)
                if (text_field(m, arg, f)) total += (size_t)stack[base + f].bytes.length;
            if (!reserve(arena, total)) return PURE_ALLOCATION;
            PureCell cells[PURE_RECORD_FIELDS];
            for (unsigned f = 0; f < record->count; ++f) {
                PureSpan span = {0};
                if (text_field(m, arg, f) && (status = contiguous(app, arena, stack[base + f].bytes, &span)) != PURE_OK)
                    return status;
                cells[f] = (PureCell){stack[base + f].num, (unsigned char *)span.data, span.size};
            }
            status = pure_list_insert(list, at, cells, &app->undo);
            arena->used = mark;
            if (status != PURE_OK) goto refused;
            move_indices(m, values, arg, pure_list_count(list), at, true);
            continue;
        }
        case PURE_OP_LIST_REMOVE: {
            size_t at = (size_t)stack[--sp].num;
            if ((status = pure_list_remove(app->lists[arg], at, &app->undo)) != PURE_OK) goto refused;
            move_indices(m, values, arg, pure_list_count(app->lists[arg]), at, false);
            continue;
        }
        case PURE_OP_ITEM_GET: {
            uint32_t list = list_of(arg), field = item_field(arg);
            size_t at = (size_t)top->num;
            if (at >= pure_list_count(app->lists[list])) { kind = PURE_FAULT_NOT_FOUND; goto failed; }
            PureCell cell = pure_list_get(app->lists[list], at, field);
            /* A text is copied: the item may change or go before the value is used. */
            if (!text_field(m, list, field)) top->num = cell.num;
            else if ((status = keep_bytes(arena, cell.text, cell.size, &top->bytes)) != PURE_OK) return status;
            continue;
        }
        case PURE_OP_ITEM_SET: {
            uint32_t list = list_of(arg), field = item_field(arg);
            Slot value = stack[--sp];
            size_t at = (size_t)stack[--sp].num, mark = arena->used;
            PureSpan span = {0};
            status = text_field(m, list, field) ? contiguous(app, arena, value.bytes, &span) : PURE_OK;
            if (status == PURE_OK)
                status = pure_list_set(app->lists[list], at, field,
                                       (PureCell){value.num, (unsigned char *)span.data, span.size}, &app->undo);
            arena->used = mark;
            if (status != PURE_OK) goto refused;
            continue;
        }
        case PURE_OP_INDEX_STORE: {
            /* An index from before a remove may be past the end. */
            double position = stack[--sp].num;
            if (position && position >= (double)pure_list_count(app->lists[m->state_ref[arg]])) {
                kind = PURE_FAULT_NOT_FOUND;
                goto failed;
            }
            values[arg] = position;
            continue;
        }
        case PURE_OP_FOLD: {
            /* The block that follows runs once per item, as a column of the list. */
            unsigned how = arg & 0xFF;
            PureBlock inner = {pc, arg >> 16};
            PureList *list = app->lists[arg >> 8 & 0xFF];
            pc += inner.length;
            double total = 0;
            bool any = false;
            Event per = *ev;
            for (size_t row = 0, items = pure_list_count(list); row < items; ++row) {
                Slot out[2];
                size_t mark = arena->used;
                per.row = row;
                status = run(m, values, inner, &per, out);
                arena->used = mark;
                if (status != PURE_OK) return status;
                double value = out[0].num;
                if (how == PURE_FOLD_COUNT ? value == 0 : out[1].num == 0) continue;
                total = how == PURE_FOLD_COUNT ? total + 1 : how == PURE_FOLD_SUM ? total + value :
                        !any ? value : how == PURE_FOLD_MAX ? maximum(total, value) : minimum(total, value);
                any = true;
            }
            if (!any && how >= PURE_FOLD_MAX) { kind = PURE_FAULT_NOT_FOUND; goto failed; }
            if (!isfinite(total)) { kind = PURE_FAULT_NON_FINITE; goto failed; }
            stack[sp++].num = total;
            continue;
        }
        case PURE_OP_PICK: stack[sp] = stack[sp - 1 - arg]; ++sp; continue;
        case PURE_OP_SLIDE: stack[sp - 1 - arg] = stack[sp - 1]; sp -= arg; continue;
        default: break;
        }
        double b = stack[--sp].num, a = stack[sp - 1].num, value;
        switch (op->op) {
        case PURE_OP_ADD: value = a + b; break;
        case PURE_OP_SUB: value = a - b; break;
        case PURE_OP_MUL: value = a * b; break;
        case PURE_OP_DIV:
            if (b == 0) { kind = PURE_FAULT_DIVIDE; goto failed; }
            value = a / b;
            break;
        case PURE_OP_LT: value = a < b; break;
        case PURE_OP_LE: value = a <= b; break;
        case PURE_OP_GT: value = a > b; break;
        case PURE_OP_GE: value = a >= b; break;
        case PURE_OP_EQ: value = a == b; break;
        case PURE_OP_NE: value = a != b; break;
        case PURE_OP_AND: value = a != 0 && b != 0; break;
        case PURE_OP_OR: value = a != 0 || b != 0; break;
        case PURE_OP_MIN: value = minimum(a, b); break;
        default: value = maximum(a, b); break;
        }
        if (!isfinite(value)) { kind = PURE_FAULT_NON_FINITE; goto failed; }
        stack[sp - 1].num = value;
    }
    if (result) memcpy(result, stack, sp * sizeof(*stack)); /* a view's value, or a fold block's numbers */
    return PURE_OK;
refused:
    kind = status == PURE_LIMIT ? PURE_FAULT_LIMIT : status == PURE_NOT_FOUND ? PURE_FAULT_NOT_FOUND : PURE_FAULT_NONE;
    if (kind == PURE_FAULT_NONE) return status;
failed:
    return fault(app, kind, (uint32_t)(op - m->code));
}

PURE_COLD static PureResult new_text(PureDraft **out, const PureModule *m, uint32_t constant) {
    *out = pure_draft_new();
    if (!*out) return PURE_ALLOCATION;
    PureSpan initial = {m->constant_data + m->constant_offset[constant], m->constant_length[constant]};
    PureResult result = pure_draft_splice(*out, 0, 0, initial);
    if (result != PURE_OK) { pure_draft_close(*out); *out = NULL; }
    return result;
}

PURE_COLD static PureList *new_list(const PureModule *m, uint32_t record) {
    return pure_list_new(m->record[record].count, m->record_field_text + m->record[record].first);
}

static bool pending(const PureApp *app, const void *document) {
    for (unsigned i = 0; i < app->pending_count; ++i) if (app->pending[i].document == document) return true;
    return false;
}

void pure_app_finish(PureApp *app) {
    /* A document in the unsaved burst stays open until the save. */
    if (app->document && !pending(app, app->document)) app->data.close(app->data.context, app->document);
    free(app->copied);
    app->document = NULL;
    app->copied = NULL;
    app->has_reply = false;
    app->payload = (PureSpan){0};
    app->arena.used = 0;
}

/* The start of a burst: what a failed save returns to. Text and stored list
 * drafts stay held until the save, so every version since stays readable. */
static void open_burst(PureApp *app) {
    if (app->open) return;
    const PureModule *m = app->module;
    memcpy(app->durable_values, app->values, sizeof(app->values));
    app->durable_ticks = app->ticks;
    app->durable_events = app->events;
    for (uint32_t i = 0; i < m->states; ++i) {
        if (app->texts[i]) {
            pure_draft_hold(app->texts[i], true);
            app->durable_texts[i] = pure_draft_version(app->texts[i]);
        }
        if (app->list_drafts[i]) {
            pure_draft_hold(app->list_drafts[i], true);
            app->durable_lists[i] = pure_draft_version(app->list_drafts[i]);
        }
    }
    app->open = true;
}

/* End a burst, keeping it (saved) or undoing all of it. The documents it held
 * close, except the current event's, which pure_app_finish closes. */
static void close_burst(PureApp *app, bool keep) {
    for (unsigned i = 0; i < app->pending_count; ++i)
        if (app->pending[i].document != app->document) app->data.close(app->data.context, app->pending[i].document);
    app->pending_count = 0;
    if (!app->open) return;
    const PureModule *m = app->module;
    for (uint32_t i = 0; i < m->states; ++i) {
        if (app->texts[i]) {
            if (!keep) pure_draft_restore(app->texts[i], app->durable_texts[i]);
            pure_draft_hold(app->texts[i], false);
        }
        if (app->list_drafts[i]) {
            if (!keep) pure_draft_restore(app->list_drafts[i], app->durable_lists[i]);
            pure_draft_hold(app->list_drafts[i], false);
        }
    }
    if (keep) pure_undo_commit(&app->undo);
    else {
        pure_undo_rollback(&app->undo, 0);
        memcpy(app->values, app->durable_values, sizeof(app->values));
        app->ticks = app->durable_ticks;
        app->events = app->durable_events;
        ++app->changes;
    }
    app->open = false;
}

PureResult pure_app_save(PureApp *app) {
    void *documents[PURE_APP_PENDING];
    int64_t revisions[PURE_APP_PENDING];
    size_t count = 0;
    for (unsigned i = 0; i < app->pending_count; ++i)
        if (app->pending[i].edited) documents[count++] = app->pending[i].document;
    PureResult result = pure_save_commit(app, app->values, documents, count, revisions);
    close_burst(app, result == PURE_OK);
    return result;
}

bool pure_app_defer(PureApp *app, bool deferred) {
    if (deferred && !pure_save_enabled(app)) return false;
    app->deferred = deferred;
    return true;
}

void pure_app_stop(PureApp *app) {
    pure_app_finish(app);
    close_burst(app, true); /* unsaved changes are dropped with the app */
    for (uint32_t i = 0; i < PURE_MODULE_STATES; ++i) {
        pure_draft_close(app->texts[i]);
        pure_list_free(app->lists[i]);
        pure_draft_close(app->list_drafts[i]);
    }
    pure_draft_close(app->stored_values);
    free(app->pending);
    pure_undo_free(&app->undo);
    for (unsigned i = 0; i < app->row_tables; ++i) free(app->rows[i].items);
    free(app->rows);
    free(app->arena.bytes);
    free(app->view_arena.bytes);
    memset(app, 0, sizeof(*app));
}

/* Where one event began inside the burst: a failed event returns there. */
typedef struct { PureVersion texts[PURE_MODULE_STATES], lists[PURE_MODULE_STATES]; size_t undo; bool opened; } Mark;

static void mark_event(PureApp *app, Mark *mark) {
    mark->opened = !app->open;
    open_burst(app);
    for (uint32_t i = 0; i < app->module->states; ++i) {
        if (app->texts[i]) mark->texts[i] = pure_draft_version(app->texts[i]);
        if (app->list_drafts[i]) mark->lists[i] = pure_draft_version(app->list_drafts[i]);
    }
    mark->undo = app->undo.count;
}

static void undo_event(PureApp *app, const Mark *mark) {
    for (uint32_t i = 0; i < app->module->states; ++i) {
        if (app->texts[i]) pure_draft_restore(app->texts[i], mark->texts[i]);
        if (app->list_drafts[i]) pure_draft_restore(app->list_drafts[i], mark->lists[i]);
    }
    pure_undo_rollback(&app->undo, mark->undo);
    if (mark->opened) close_burst(app, true); /* nothing happened: release the holds */
}

static PureResult keep_reply(PureApp *app);

/* One event: every handler for it runs in module order against pending
 * states, all or nothing. A key, text or message event saves before it
 * returns unless saving is deferred; a frame leaves that to the next event
 * (frames come 60 times a second). A message's document joins the burst. */
static PureResult dispatch(PureApp *app, uint32_t event, Event *ev) {
    const PureModule *m = app->module;
    double pending[PURE_MODULE_STATES];
    Mark mark;
    mark_event(app, &mark);
    memcpy(pending, app->values, sizeof(app->values));
    app->fault = PURE_FAULT_NONE;
    app->fault_pc = UINT32_MAX;
    PureResult result = PURE_OK;
    for (uint32_t i = 0; i < m->handlers && result == PURE_OK; ++i)
        if (m->handler[i].event == event) result = run(m, pending, m->handler[i].code, ev, NULL);
    /* An edit needs the revision it was made against. A save rebases an
     * edited document and releasing texts may reclaim earlier text versions:
     * keep such a reply by copying it first. */
    if (result == PURE_OK && ev->modified && ev->base < 0) result = PURE_INVALID;
    if (result == PURE_OK && app->has_reply &&
        ((app->reply.source == FROM_DOCUMENT && ev->modified) || app->reply.source == FROM_TEXT))
        result = keep_reply(app);
    if (result != PURE_OK) {
        undo_event(app, &mark);
        return result;
    }
    memcpy(app->values, pending, sizeof(pending));
    ++app->events;
    ++app->changes;
    if (!event) ++app->ticks;
    if (app->document) {
        app->pending[app->pending_count].document = app->document;
        app->pending[app->pending_count].edited = ev->modified;
        memcpy(app->pending[app->pending_count++].key, ev->key, strlen(ev->key) + 1);
        app->revision += ev->modified; /* a save moves an edited document one revision */
    }
    return event && !app->deferred ? pure_app_save(app) : PURE_OK;
}

PureResult pure_app_frame(PureApp *app) {
    pure_app_finish(app);
    Event ev = {.app = app, .arena = &app->arena, .dt = 1.0 / 60.0};
    return dispatch(app, 0, &ev);
}

PureResult pure_app_input(PureApp *app, uint32_t input, PureEdge edge) {
    if (input >= app->module->inputs || app->module->input[input].kind != PURE_KIND_KEY ||
        (edge != PURE_EDGE_DOWN && edge != PURE_EDGE_UP)) return PURE_INVALID;
    pure_app_finish(app);
    Event ev = {.app = app, .arena = &app->arena};
    return dispatch(app, 1 + 2 * input + (edge == PURE_EDGE_UP), &ev);
}

PureResult pure_app_point(PureApp *app, uint32_t state, size_t position) {
    const PureModule *m = app->module;
    pure_app_finish(app);
    if (state >= m->states || m->state_type[state] != PURE_STATE_INDEX) return PURE_INVALID;
    if (position >= pure_list_count(app->lists[m->state_ref[state]])) return PURE_NOT_FOUND;
    open_burst(app); /* what a failed save returns to */
    app->fault = PURE_FAULT_NONE;
    app->fault_pc = UINT32_MAX;
    app->values[state] = (double)position;
    ++app->events;
    ++app->changes;
    return app->deferred ? PURE_OK : pure_app_save(app);
}

PureResult pure_app_text(PureApp *app, uint32_t input, PureSpan text) {
    pure_app_finish(app);
    if (input >= app->module->inputs || app->module->input[input].kind != PURE_KIND_TEXT ||
        (text.size && !text.data) || !pure_utf8_valid(text)) return PURE_INVALID;
    app->payload = text;
    Event ev = {.app = app, .arena = &app->arena};
    PureResult result = dispatch(app, 1 + 2 * input, &ev);
    app->payload = (PureSpan){0};
    return result;
}

static PureResult keep_reply(PureApp *app) {
    size_t length = (size_t)app->reply.length, start = app->arena.used;
    unsigned char *copy = malloc(length ? length : 1);
    if (!copy) return PURE_ALLOCATION;
    PureResult result = append_value(app, &app->arena, app->reply);
    if (result != PURE_OK) { free(copy); return result; }
    if (length) memcpy(copy, app->arena.bytes + start, length);
    app->arena.used = start;
    app->copied = copy;
    app->reply = (PureBytes){FROM_COPY, 0, {0, 0}, 0, length};
    return PURE_OK;
}

PureResult pure_app_message(PureApp *app, uint32_t input, const char *key, int64_t base, PureSpan payload) {
    pure_app_finish(app);
    const PureModule *m = app->module;
    app->fault = PURE_FAULT_NONE;
    app->fault_pc = UINT32_MAX;
    app->revision = 0;
    if (input >= m->inputs || m->input[input].kind != PURE_KIND_MESSAGE || base < -1 ||
        (payload.size && !payload.data) || (m->input[input].keyed ? !document_key(key) : key != NULL))
        return PURE_INVALID;
    const PureInputShape *shape = &m->input[input];
    if (payload.size < shape->prefix) return fault(app, PURE_FAULT_LIMIT, UINT32_MAX);
    if (shape->keyed && m->collections) {
        if (!app->data.open) return PURE_INVALID;
        /* A key joins a burst once, so its revision moves once per save. */
        bool seen = app->pending_count == PURE_APP_PENDING;
        for (unsigned i = 0; i < app->pending_count && !seen; ++i) seen = !strcmp(app->pending[i].key, key);
        if (seen) {
            PureResult saved = pure_app_save(app);
            if (saved != PURE_OK) return saved;
        }
        if (app->pending_count == app->pending_capacity) {
            unsigned capacity = app->pending_capacity ? 2 * app->pending_capacity : 16;
            struct PurePending *grown = realloc(app->pending, capacity * sizeof(*grown));
            if (!grown) return PURE_ALLOCATION;
            app->pending = grown;
            app->pending_capacity = capacity;
        }
        PureResult opened = app->data.open(app->data.context, 0, key, &app->document, &app->revision);
        if (opened != PURE_OK) { app->document = NULL; return opened; }
        if (base >= 0 && base != app->revision) { pure_app_finish(app); return PURE_CONFLICT; }
    }
    app->payload = payload;
    Event ev = {.app = app, .shape = shape, .key = key, .base = base, .arena = &app->arena, .exists = app->revision > 0};
    PureResult result = dispatch(app, 1 + 2 * input, &ev);
    if (result != PURE_OK) pure_app_finish(app);
    return result;
}

PureResult pure_app_reply(const PureApp *app, bool (*visit)(void *, PureSpan), void *context) {
    if (!app->has_reply) return PURE_OK;
    return visit_value(app, &app->arena, app->reply, visit, context);
}

/* A view that fails reports it by its result; the fault stays the last event's. */
static PureResult evaluate(PureApp *app, uint32_t view, size_t row, Slot *out) {
    double values[PURE_MODULE_STATES];
    memcpy(values, app->values, sizeof(values));
    app->view_arena.used = 0;
    Event ev = {.app = app, .arena = &app->view_arena, .row = row};
    PureFault fault = app->fault;
    uint32_t fault_pc = app->fault_pc;
    PureResult result = run(app->module, values, app->module->view_code[view], &ev, out);
    app->fault = fault;
    app->fault_pc = fault_pc;
    return result;
}

/* A filtered table's items, as fresh as the app's states: the filter runs
 * once per item after each change (an item it fails on is left out), not
 * once per cell. NULL for a table without a filter, or without room. */
static struct PureRows *filtered(PureApp *app, uint32_t table) {
    const PureModule *m = app->module;
    if (!m->view_code[table].length) return NULL;
    struct PureRows *rows = NULL;
    for (unsigned i = 0; i < app->row_tables && !rows; ++i) if (app->rows[i].table == table) rows = &app->rows[i];
    if (!rows) {
        struct PureRows *grown = realloc(app->rows, (app->row_tables + 1) * sizeof(*grown));
        if (!grown) return NULL;
        app->rows = grown;
        rows = &app->rows[app->row_tables++];
        *rows = (struct PureRows){.table = table, .changes = UINT64_MAX};
    }
    if (rows->changes == app->changes) return rows;
    rows->count = 0;
    for (size_t item = 0, items = pure_list_count(app->lists[m->view_list[table]]); item < items; ++item) {
        Slot keep;
        if (evaluate(app, table, item, &keep) != PURE_OK || keep.num == 0) continue;
        if (rows->count == rows->capacity) {
            size_t capacity = rows->capacity ? 2 * rows->capacity : 16;
            size_t *grown = realloc(rows->items, capacity * sizeof(*grown));
            if (!grown) return NULL;
            rows->items = grown;
            rows->capacity = capacity;
        }
        rows->items[rows->count++] = item;
    }
    rows->changes = app->changes;
    return rows;
}

bool pure_app_row_item(PureApp *app, uint32_t table, size_t row, size_t *item) {
    const PureModule *m = app->module;
    if (table >= m->views || m->view_type[table] != PURE_VIEW_TABLE) return false;
    if (!m->view_code[table].length) {
        *item = row;
        return row < pure_list_count(app->lists[m->view_list[table]]);
    }
    struct PureRows *rows = filtered(app, table);
    if (!rows || row >= rows->count) return false;
    *item = rows->items[row];
    return true;
}

bool pure_app_item_row(PureApp *app, uint32_t table, size_t item, size_t *row) {
    const PureModule *m = app->module;
    if (table >= m->views || m->view_type[table] != PURE_VIEW_TABLE) return false;
    if (!m->view_code[table].length) {
        *row = item;
        return item < pure_list_count(app->lists[m->view_list[table]]);
    }
    struct PureRows *rows = filtered(app, table);
    size_t low = 0, high = rows ? rows->count : 0; /* the items are in order */
    while (low < high) {
        size_t middle = low + (high - low) / 2;
        if (rows->items[middle] < item) low = middle + 1;
        else high = middle;
    }
    if (low == (rows ? rows->count : 0) || rows->items[low] != item) return false;
    *row = low;
    return true;
}

/* A view outside a table (column false), or a column and a row of its table. */
static PureResult view_of(PureApp *app, uint32_t view, bool column, size_t row, uint8_t type, Slot *slot) {
    const PureModule *m = app->module;
    if (view >= m->views || m->view_type[view] != type || (m->view_table[view] != 0) != column) return PURE_INVALID;
    if (column && !pure_app_row_item(app, m->view_table[view] - 1u, row, &row)) return PURE_NOT_FOUND;
    return evaluate(app, view, row, slot);
}

bool pure_app_view(PureApp *app, uint32_t view, double *out) {
    Slot slot;
    if (view_of(app, view, false, 0, PURE_VIEW_NUM, &slot) != PURE_OK) return false;
    *out = slot.num;
    return true;
}

PureResult pure_app_view_text(PureApp *app, uint32_t view, bool (*visit)(void *, PureSpan), void *context) {
    Slot slot;
    PureResult result = view_of(app, view, false, 0, PURE_VIEW_TEXT, &slot);
    return result == PURE_OK ? visit_value(app, &app->view_arena, slot.bytes, visit, context) : result;
}

size_t pure_app_rows(PureApp *app, uint32_t table) {
    const PureModule *m = app->module;
    if (table >= m->views || m->view_type[table] != PURE_VIEW_TABLE) return 0;
    struct PureRows *rows = filtered(app, table);
    return rows ? rows->count : m->view_code[table].length ? 0 : pure_list_count(app->lists[m->view_list[table]]);
}

bool pure_app_cell(PureApp *app, uint32_t column, size_t row, double *out) {
    Slot slot;
    if (view_of(app, column, true, row, PURE_VIEW_NUM, &slot) != PURE_OK) return false;
    *out = slot.num;
    return true;
}

PureResult pure_app_cell_text(PureApp *app, uint32_t column, size_t row, bool (*visit)(void *, PureSpan), void *context) {
    Slot slot;
    PureResult result = view_of(app, column, true, row, PURE_VIEW_TEXT, &slot);
    return result == PURE_OK ? visit_value(app, &app->view_arena, slot.bytes, visit, context) : result;
}

/* A list with record's fields: each takes the field of `from` with the same
 * name and kind, or its default. */
PURE_COLD PureResult pure_list_migrate(PureList **out, const PureList *from, const char *const *names,
                             const uint8_t *text, unsigned count, const PureModule *m, uint32_t record) {
    const PureRecord *r = &m->record[record];
    int source[PURE_RECORD_FIELDS];
    for (unsigned f = 0; f < r->count; ++f) {
        source[f] = -1;
        for (unsigned k = 0; k < count; ++k)
            if (!strcmp(m->record_field_names[r->first + f], names[k]) && m->record_field_text[r->first + f] == text[k])
                source[f] = (int)k;
    }
    if (!(*out = new_list(m, record))) return PURE_ALLOCATION;
    PureResult result = PURE_OK;
    for (size_t at = 0; result == PURE_OK && at < pure_list_count(from); ++at) {
        PureCell cells[PURE_RECORD_FIELDS];
        for (unsigned f = 0; f < r->count; ++f) {
            uint32_t field = r->first + f, constant = (uint32_t)m->record_field_default[field];
            cells[f] = source[f] >= 0 ? pure_list_get(from, at, (unsigned)source[f]) :
                       m->record_field_text[field] ?
                           (PureCell){0, (unsigned char *)m->constant_data + m->constant_offset[constant],
                                      m->constant_length[constant]} :
                           (PureCell){m->record_field_default[field], NULL, 0};
        }
        result = pure_list_insert(*out, at, cells, NULL);
    }
    if (result != PURE_OK) { pure_list_free(*out); *out = NULL; }
    return result;
}

/* A kept list keeps its items; with the same record it is the same list. */
PURE_COLD static PureResult keep_list(PureList **out, PureList *from, const PureModule *old, uint32_t old_record,
                            const PureModule *m, uint32_t record) {
    const PureRecord *r = &m->record[record], *o = &old->record[old_record];
    const char *names[PURE_RECORD_FIELDS];
    bool same = r->count == o->count;
    for (unsigned k = 0; k < o->count; ++k) {
        names[k] = old->record_field_names[o->first + k];
        same = same && !strcmp(names[k], m->record_field_names[r->first + k]) &&
               old->record_field_text[o->first + k] == m->record_field_text[r->first + k];
    }
    if (same) { *out = from; return PURE_OK; }
    return pure_list_migrate(out, from, names, old->record_field_text + o->first, o->count, m, record);
}

/* Open a module in an app. States keep their values by name and kind (texts
 * their content, cursors their position in it, lists their items, moved by
 * field name when the record changed); every other state starts from its
 * stored value, or its initial one. A list whose record changed is stored
 * again, and the next event or save publishes the module's states; a state
 * the module drops stays stored as it was. Starting an app opens its module
 * over none. */
PURE_COLD static PureResult open_module(PureApp *app, const PureModule *module) {
    const PureModule *old = app->module;
    bool stored = pure_save_enabled(app);
    double values[PURE_MODULE_STATES] = {0};
    PureDraft *texts[PURE_MODULE_STATES] = {0}, *list_drafts[PURE_MODULE_STATES] = {0};
    PureList *lists[PURE_MODULE_STATES] = {0};
    PureVersion saved[PURE_MODULE_STATES] = {{0, 0}}, before[PURE_MODULE_STATES] = {{0, 0}};
    int kept[PURE_MODULE_STATES];
    bool rewrite[PURE_MODULE_STATES] = {false};
    PureResult result = PURE_OK;
    for (uint32_t i = 0; i < module->states; ++i) {
        int previous = old ? pure_module_state(old, module->state_names[i]) : -1;
        kept[i] = previous >= 0 && old->state_type[previous] == module->state_type[i] ? previous : -1;
        if (kept[i] >= 0) saved[i] = app->saved[kept[i]];
        if (module->state_type[i] == PURE_STATE_TEXT) {
            if (kept[i] >= 0) texts[i] = app->texts[kept[i]];
            else if (result == PURE_OK)
                result = stored ? pure_save_open_text(app, module, i, &texts[i], &saved[i]) :
                         new_text(&texts[i], module, module->state_ref[i]);
        } else if (module->state_type[i] == PURE_STATE_LIST) {
            if (result != PURE_OK) continue;
            if (kept[i] >= 0) {
                result = keep_list(&lists[i], app->lists[kept[i]], old, old->state_ref[kept[i]], module,
                                   module->state_ref[i]);
                list_drafts[i] = app->list_drafts[kept[i]];
                rewrite[i] = result == PURE_OK && list_drafts[i] && lists[i] != app->lists[kept[i]];
            } else if (stored) {
                result = pure_save_open_list(app, module, i, &lists[i], &list_drafts[i], &saved[i]);
            } else if (!(lists[i] = new_list(module, module->state_ref[i]))) result = PURE_ALLOCATION;
        } else values[i] = kept[i] >= 0 ? app->values[kept[i]] : module->initial[i];
    }
    /* A cursor or index keeps its position only while its text or list is the
     * one it belonged to. */
    for (uint32_t i = 0; i < module->states; ++i) {
        if (module->state_type[i] != PURE_STATE_CURSOR && module->state_type[i] != PURE_STATE_INDEX) continue;
        uint32_t owner = module->state_ref[i];
        bool same = kept[i] >= 0 && kept[owner] >= 0 && old->state_ref[kept[i]] == (unsigned)kept[owner];
        values[i] = same ? app->values[kept[i]] : 0;
    }
    if (result == PURE_OK && stored) result = pure_save_open_values(app, module, values, kept, texts, lists);
    /* Store the lists whose record changed last, so that a failure can put
     * every draft back as it was. */
    for (uint32_t i = 0; i < module->states; ++i)
        if (rewrite[i]) {
            before[i] = pure_draft_version(list_drafts[i]);
            pure_draft_hold(list_drafts[i], true);
        }
    for (uint32_t i = 0; i < module->states && result == PURE_OK; ++i)
        if (rewrite[i]) result = pure_save_rewrite_list(module, i, lists[i], list_drafts[i]);
    for (uint32_t i = 0; i < module->states; ++i)
        if (rewrite[i]) {
            if (result != PURE_OK) pure_draft_restore(list_drafts[i], before[i]);
            pure_draft_hold(list_drafts[i], false);
        }
    if (result != PURE_OK) {
        for (uint32_t i = 0; i < module->states; ++i) {
            if (kept[i] >= 0) {
                if (lists[i] && lists[i] != app->lists[kept[i]]) pure_list_free(lists[i]);
                continue;
            }
            pure_draft_close(texts[i]);
            pure_list_free(lists[i]);
            pure_draft_close(list_drafts[i]);
        }
        return result;
    }
    for (uint32_t i = 0; old && i < old->states; ++i) {
        bool reused = false, draft_reused = false;
        for (uint32_t k = 0; k < module->states; ++k) {
            reused = reused || (texts[k] && texts[k] == app->texts[i]) || (lists[k] && lists[k] == app->lists[i]);
            draft_reused = draft_reused || (list_drafts[k] && list_drafts[k] == app->list_drafts[i]);
        }
        if (!reused) {
            pure_draft_close(app->texts[i]);
            pure_list_free(app->lists[i]);
        }
        if (!draft_reused) pure_draft_close(app->list_drafts[i]);
    }
    memcpy(app->values, values, sizeof(values));
    memcpy(app->texts, texts, sizeof(texts));
    memcpy(app->lists, lists, sizeof(lists));
    memcpy(app->list_drafts, list_drafts, sizeof(list_drafts));
    memcpy(app->saved, saved, sizeof(saved));
    app->module = module;
    ++app->changes;
    return PURE_OK;
}

PURE_COLD PureResult pure_app_start(PureApp *app, const PureModule *module, PureData data) {
    *app = (PureApp){.data = data, .fault_pc = UINT32_MAX};
    PureResult result = open_module(app, module);
    if (result == PURE_OK) result = pure_app_save(app);
    if (result != PURE_OK) pure_app_stop(app);
    return result;
}

PURE_COLD PureResult pure_app_replace(PureApp *app, const PureModule *module) {
    pure_app_finish(app);
    if (app->open || app->pending_count) {
        PureResult saved = pure_app_save(app); /* the burst belongs to the module that made it */
        if (saved != PURE_OK) return saved;
    }
    return open_module(app, module);
}