agent: comrak diff-fuzzer for the phase2-comrak-010 audit

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EcKPWQ3MQD4NdEHwar96u2

scripts/diff-fuzz-comrak.py

@@ -0,0 +1,90 @@
+#!/usr/bin/env python3
+"""Differential fuzz for comrak attempts: pristine vs candidate must
+produce byte-identical HTML (stdout), stderr, and exit code on mutated
+markdown. comrak is a single-threaded stdin->stdout filter, so raw byte
+comparison needs no canonicalization.
+
+    python3 scripts/diff-fuzz-comrak.py <workdir> <iters> <candidate-bin>
+
+Seeds: windows of the pinned Pro Git corpus. Mutations focus on inline/
+block structure the parser and teardown paths exercise: emphasis, code
+fences, links, HTML blocks, list markers, plus raw byte noise.
+"""
+
+import random
+import subprocess
+import sys
+from pathlib import Path
+
+ROOT = Path("/home/user/peltier.io")
+BASE = ROOT / "targets/comrak/baseline/release/comrak"
+CAND = ROOT / sys.argv[3]
+CORPUS = ROOT / "corpora/comrak/progit.md"
+WORK = Path(sys.argv[1])
+ITERS = int(sys.argv[2])
+
+TOKENS = [
+    b"**", b"*", b"__", b"_", b"`", b"```", b"~~~", b"~~", b"[", b"](", b")",
+    b"![", b"#", b"##", b">", b"- ", b"1. ", b"|", b"---", b"<div>", b"</div>",
+    b"<!--", b"-->", b"\\", b"&amp;", b"<http://x>", b"    ", b"\t",
+]
+
+
+def mutate(data: bytes, rng: random.Random) -> bytes:
+    b = bytearray(data)
+    for _ in range(rng.randrange(1, 8)):
+        if not b:
+            b = bytearray(rng.choice(TOKENS))
+            continue
+        op = rng.randrange(5)
+        pos = rng.randrange(len(b) + 1)
+        if op == 0:
+            b[pos:pos] = rng.choice(TOKENS)
+        elif op == 1 and len(b) > 2:
+            del b[pos:min(len(b), pos + rng.randrange(1, 16))]
+        elif op == 2 and pos < len(b):
+            b[pos] = rng.randrange(256)
+        elif op == 3:
+            end = min(len(b), pos + rng.randrange(1, 64))
+            b[end:end] = b[pos:end]
+        else:
+            b[pos:pos] = b"\n\n"
+    return bytes(b)
+
+
+def run(binary: Path, payload: bytes) -> tuple[bytes, bytes, int]:
+    r = subprocess.run([str(binary)], input=payload, capture_output=True, timeout=60)
+    return r.stdout, r.stderr, r.returncode
+
+
+def main() -> int:
+    rng = random.Random(0xC0FFEE)
+    corpus = CORPUS.read_bytes()
+    # Seed windows across the whole book, varied sizes.
+    seeds = [
+        corpus[o : o + n]
+        for o in range(0, len(corpus) - 65536, len(corpus) // 300)
+        for n in (512, 4096, 32768)
+    ]
+    WORK.mkdir(parents=True, exist_ok=True)
+    divergences = 0
+    for i in range(ITERS):
+        payload = mutate(rng.choice(seeds), rng)
+        a = run(BASE, payload)
+        b = run(CAND, payload)
+        if a != b:
+            divergences += 1
+            keep = WORK / f"divergence-{i}"
+            keep.mkdir()
+            (keep / "input.md").write_bytes(payload)
+            (keep / "baseline.out").write_bytes(a[0] + b"\n--stderr--\n" + a[1] + f"\nrc={a[2]}".encode())
+            (keep / "candidate.out").write_bytes(b[0] + b"\n--stderr--\n" + b[1] + f"\nrc={b[2]}".encode())
+            print(f"DIVERGENCE at iter {i}; kept in {keep}")
+        if (i + 1) % 1000 == 0:
+            print(f"{i + 1}/{ITERS}, {divergences} divergences", flush=True)
+    print(f"done: {ITERS} inputs, {divergences} divergences")
+    return 1 if divergences else 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())