feat(spec): bit-exact PCG port of Go math/rand/v2

pkg/spec/src/pcg.ts

@@ -0,0 +1,96 @@
+// Bit-exact port of Go's math/rand/v2 PCG source wrapped in rand.Rand.
+//
+// This MUST match `rand.New(rand.NewPCG(hi, lo))` draw-for-draw, because every
+// random decision the action picker makes is shared between the goja verifier
+// and the V8 web runtime. Divergence here breaks reproducibility across engines.
+//
+// The golden fixture in test/fixtures/pcg-golden.json is generated by
+// internal/_oracle/main.go and pins the exact sequences asserted in pcg.test.ts.
+
+const MASK64 = (1n << 64n) - 1n;
+
+const MUL_HI = 2549297995355413924n;
+const MUL_LO = 4865540595714422341n;
+const INC_HI = 6364136223846793005n;
+const INC_LO = 1442695040888963407n;
+const CHEAP_MUL = 0xda942042e4dd58b5n;
+
+const POW53 = 1n << 53n;
+
+// mul64 returns the 128-bit product of two uint64 values split into high and
+// low 64-bit halves, matching Go's math/bits.Mul64.
+function mul64(a: bigint, b: bigint): { hi: bigint; lo: bigint } {
+  const product = a * b;
+  return { hi: (product >> 64n) & MASK64, lo: product & MASK64 };
+}
+
+export class Pcg {
+  private hi: bigint;
+  private lo: bigint;
+
+  constructor(hi: bigint, lo: bigint) {
+    this.hi = hi & MASK64;
+    this.lo = lo & MASK64;
+  }
+
+  // next advances the 128-bit LCG state and returns the new (hi, lo) pair.
+  private next(): { hi: bigint; lo: bigint } {
+    // 128-bit multiply of state by the 128-bit multiplier, keeping 128 bits.
+    let { hi, lo } = mul64(this.lo, MUL_LO);
+    hi = (hi + this.hi * MUL_LO + this.lo * MUL_HI) & MASK64;
+
+    // 128-bit add of the increment with carry propagation.
+    const loSum = lo + INC_LO;
+    const carry = loSum >> 64n;
+    lo = loSum & MASK64;
+    hi = (hi + INC_HI + carry) & MASK64;
+
+    this.lo = lo;
+    this.hi = hi;
+    return { hi, lo };
+  }
+
+  // uint64 mirrors PCG.Uint64: the DXSM output permutation over next().
+  uint64(): bigint {
+    let { hi, lo } = this.next();
+    hi ^= hi >> 32n;
+    hi = (hi * CHEAP_MUL) & MASK64;
+    hi ^= hi >> 48n;
+    hi = (hi * (lo | 1n)) & MASK64;
+    return hi & MASK64;
+  }
+
+  // float64 mirrors rand.Rand.Float64: float64(Uint64()<<11>>11) / (1<<53),
+  // which keeps the low 53 bits and divides by 2^53.
+  float64(): number {
+    const bits = this.uint64() & (POW53 - 1n);
+    return Number(bits) / Number(POW53);
+  }
+
+  // intN mirrors rand.Rand.IntN via uint64n (Lemire with a rejection threshold).
+  // The host arch is 64-bit, so the 32-bit fast path never applies.
+  intN(n: number | bigint): number {
+    const bound = typeof n === "bigint" ? n : BigInt(n);
+    if (bound <= 0n) {
+      throw new Error("invalid argument to intN");
+    }
+    return Number(this.uint64n(bound));
+  }
+
+  private uint64n(n: bigint): bigint {
+    if ((n & (n - 1n)) === 0n) {
+      // n is a power of two: mask the low bits.
+      return this.uint64() & (n - 1n);
+    }
+
+    let { hi, lo } = mul64(this.uint64(), n);
+    if (lo < n) {
+      // Go computes thresh := -n % n in uint64 math, where -n wraps to 2^64 - n.
+      const threshold = ((1n << 64n) - n) % n;
+      while (lo < threshold) {
+        ({ hi, lo } = mul64(this.uint64(), n));
+      }
+    }
+    return hi;
+  }
+}