airkjw
0210c14858
PCG32 seeded RNG; immutable 8x8 GridState with occupancy bitmask; placement legality + anyPlacementExists; simultaneous row/col clears with single-count gem credit; combo scoring with one-move grace; weighted-bag generator with pity bias and depth-3 solvability nudge. All TDD, 51 tests green. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
49 lines
1.6 KiB
Dart
49 lines
1.6 KiB
Dart
/// Deterministic seedable PRNG for the game engine.
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///
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/// dart:math's Random is implementation-unspecified across platforms; piece
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/// sequences must be byte-identical on iOS, Android, and in tests, so we own
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/// the algorithm (PCG32, https://www.pcg-random.org).
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class SeededRng {
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SeededRng(int seed, [int sequence = 0])
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: _state = 0,
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_inc = ((sequence << 1) | 1) {
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_nextRaw();
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_state = _state + seed;
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_nextRaw();
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}
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// Dart native ints are 64-bit with wrapping arithmetic, which is exactly
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// what the PCG32 state transition needs.
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int _state;
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final int _inc;
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static const _multiplier = 6364136223846793005;
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static const _mask32 = 0xFFFFFFFF;
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/// One PCG32 step: 32-bit output via xorshift-high + random rotation.
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int _nextRaw() {
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final old = _state;
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_state = old * _multiplier + _inc;
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final xorshifted = (((old >>> 18) ^ old) >>> 27) & _mask32;
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final rot = old >>> 59;
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return ((xorshifted >>> rot) | (xorshifted << ((-rot) & 31))) & _mask32;
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}
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/// Uniform integer in [0, max), bias-free via rejection sampling.
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int nextInt(int max) {
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assert(max > 0);
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final threshold = (0x100000000 - (0x100000000 % max)) & _mask32;
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if (threshold == 0) return _nextRaw() % max;
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while (true) {
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final r = _nextRaw();
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if (r < threshold) return r % max;
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}
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}
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/// Uniform double in [0, 1).
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double nextDouble() => _nextRaw() / 4294967296.0;
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/// Derives an independent deterministic stream (e.g. per retry attempt).
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SeededRng fork(int streamId) => SeededRng(_nextRaw() ^ streamId, streamId);
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}
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