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//! Disposable binding results and change history. No event state lives here.
const std = @import("std");
const Memo = struct {
value: f64 = 0,
revision: u64 = 0,
dependencies: u32 = 0,
start: u32 = 0,
length: u32 = 0,
};
const Change = struct { revision: u64 = 0, mask: u32 = 0 };
pub const Tables = struct {
memo: [32]Memo = @splat(.{}),
history: [8]Change = @splat(.{}),
};
pub const Evaluation = struct {
pub const max_cache = 32;
pub const max_history = 8;
revision: u64 = 1,
floor: u64 = 1,
cache_count: u32 = 0,
history_count: u32 = 8,
next_change: u32 = 0,
limit: u32 = 0,
work: u32 = 0,
instructions: u32 = 0,
hits: u32 = 0,
status: u32 = 0,
pub fn configure(self: *Evaluation, tables: ?*Tables, cache: u32, history: u32) bool {
if (cache > max_cache or history > max_history) return false;
self.cache_count = cache;
self.history_count = history;
self.forget(tables, 3);
return true;
}
pub fn forget(self: *Evaluation, tables: ?*Tables, flags: u32) void {
if (flags & 1 != 0) if (tables) |t| { t.memo = @splat(.{}); };
if (flags & 2 != 0) {
if (tables) |t| t.history = @splat(.{});
self.floor = self.revision;
self.next_change = 0;
}
}
pub fn changed(self: *Evaluation, tables: ?*Tables, mask: u32) void {
const t = tables orelse return;
if (mask == 0 or self.cache_count == 0) return;
if (self.revision == std.math.maxInt(u64)) {
self.revision = 1;
self.forget(t, 3);
}
self.revision += 1;
if (self.history_count == 0) {
self.floor = self.revision;
return;
}
const change = &t.history[self.next_change];
self.floor = @max(self.floor, change.revision);
change.* = .{ .revision = self.revision, .mask = mask };
self.next_change = (self.next_change + 1) % self.history_count;
}
pub fn begin(self: *Evaluation, limit: u32) void {
self.limit = limit;
self.work = 0;
self.instructions = 0;
self.hits = 0;
self.status = 0;
}
pub fn spend(self: *Evaluation) bool {
if (self.work == self.limit) {
self.status = 10;
return false;
}
self.work += 1;
return true;
}
fn slot(self: *const Evaluation, start: u32, length: u32) u32 {
// One candidate, including on misses; no scan proportional to cache size.
return (start ^ (start >> 5) ^ (length *% 0x9e3779b9)) % self.cache_count;
}
pub fn lookup(self: *Evaluation, tables: ?*Tables, start: u32, length: u32) ?f64 {
const t = tables orelse return null;
if (self.cache_count == 0) return null;
if (!self.spend()) return null;
const memo = &t.memo[self.slot(start, length)];
if (memo.revision == 0 or memo.start != start or memo.length != length) return null;
// Successful pure constants need no input-change evidence. A program
// replacement still clears them, just like every other binding memo.
if (memo.dependencies != 0 and memo.revision < self.floor) return null;
if (memo.dependencies != 0 and memo.revision != self.revision) {
for (t.history[0..self.history_count]) |change| {
if (!self.spend()) return null;
if (change.revision > memo.revision and change.mask & memo.dependencies != 0) return null;
}
}
memo.revision = self.revision;
self.hits += 1;
return memo.value;
}
pub fn remember(self: *Evaluation, tables: ?*Tables, start: u32, length: u32, value: f64, dependencies: u32) void {
const t = tables orelse return;
if (self.cache_count == 0) return;
t.memo[self.slot(start, length)] = .{ .start = start, .length = length,
.value = value, .dependencies = dependencies, .revision = self.revision };
}
};
test "record loss cannot authorize an old memo; unrelated changes can" {
var tables: Tables = .{};
var e: Evaluation = .{};
try std.testing.expect(e.configure(&tables, 1, 1));
e.begin(100);
e.remember(&tables, 0, 3, 12, 1);
e.changed(&tables, 2);
try std.testing.expectEqual(@as(?f64, 12), e.lookup(&tables, 0, 3));
e.changed(&tables, 2);
e.changed(&tables, 2); // The certificate now predates the surviving history.
try std.testing.expectEqual(@as(?f64, null), e.lookup(&tables, 0, 3));
e.remember(&tables, 0, 3, 12, 1);
e.changed(&tables, 1);
try std.testing.expectEqual(@as(?f64, null), e.lookup(&tables, 0, 3));
e.remember(&tables, 0, 3, 20, 1);
e.changed(&tables, 2);
e.forget(&tables, 2);
try std.testing.expectEqual(@as(?f64, null), e.lookup(&tables, 0, 3));
e.remember(&tables, 0, 3, 20, 1);
e.revision = std.math.maxInt(u64);
e.changed(&tables, 2);
try std.testing.expectEqual(@as(?f64, null), e.lookup(&tables, 0, 3));
}
test "budget, collisions, invalid settings, and disabled history are conservative" {
var tables: Tables = .{};
var e: Evaluation = .{};
try std.testing.expect(e.configure(&tables, 1, 0));
e.begin(100);
e.remember(&tables, 0, 3, 12, 1);
e.remember(&tables, 1, 3, 15, 2);
try std.testing.expectEqual(@as(?f64, null), e.lookup(&tables, 0, 3));
try std.testing.expectEqual(@as(?f64, 15), e.lookup(&tables, 1, 3));
e.changed(&tables, 0x80000000);
try std.testing.expectEqual(@as(?f64, null), e.lookup(&tables, 1, 3));
try std.testing.expect(!e.configure(&tables, 33, 0));
try std.testing.expectEqual(@as(u32, 1), e.cache_count);
e.begin(0);
try std.testing.expectEqual(@as(?f64, null), e.lookup(&tables, 1, 3));
try std.testing.expectEqual(@as(u32, 10), e.status);
try std.testing.expectEqual(@as(u32, 0), e.work);
e.begin(100);
e.remember(&tables, 2, 1, 99, 0);
e.changed(&tables, 1);
e.forget(&tables, 2);
try std.testing.expectEqual(@as(?f64, 99), e.lookup(&tables, 2, 1));
try std.testing.expectEqual(@as(u32, 1), e.work);
}
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