README: describe the Foo[] fold as nil-hash seeded in every passage

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Closes #21

What

README.md "Handling pointers": the Foo[] fold is now described once, as the nil-hash-seeded fold, in both passages that walk through it, and test/HashPatternFold.t.sol pins every claim those passages make.

• L432-437 step-by-step rewritten: start from the nil hash N, A = hash(foos_[0]), B = hash(N + A), C = hash(foos_[1]), D = hash(B + C), etc.
• "Nil hash prefix" gains one sentence (L451-452): the seed is what separates [x], which hashes to hash(nil + hash(x)), from x, which hashes to hash(x).
• test/HashPatternFold.t.sol (new, one contract HashPatternFoldTest): the letters N, A, B, C, D over two fuzzed Foos; every length 0 to 4 against a builtin-only fold; the empty Foo[] folding to the nil hash; [x] folding to hash(nil + hash(x)) and not to hash(x).

Why the seeded fold is the pattern

Three passages of the README already pin it; only the step-by-step passage disagreed.

1. The general rule ("compute the hash of all data up to the pointer, compute the hash of all data referenced by the pointer, hash these two hashes together"): for a bare Foo[] nothing precedes the first pointer, so the first "hash of all data up to the pointer" is keccak256 of nothing, i.e. the nil hash. The seeded fold is that rule applied literally.
2. "If the array is 0 length then the hash will be the nil hash": the seeded fold gives this with no special case; the unseeded fold needs an if (len == 0) return nil branch that no passage mentions.
3. "Security of composition" needs hash(x) and hash(hash(a) + hash(b)) not to collide. The unseeded fold makes fold([x]) == hash(x) by construction, so a Foo and a one-element Foo[] collide and the README's own argument fails at length 1; the seeded fold gives hash(nil + hash(x)) != hash(x).

The README diff is confined to L432-437 and L451-452. The gas sentence at L438-440 (#25), the "Security of composition" section (#22) and the assembly examples (#20) belong to sibling issues and are untouched.

QA

• Discriminating tests: testFoldLetters, testFoldMatchesBuiltins, testFoldSingletonIsNotItem, testFoldEmptyIsNilHash (test/HashPatternFold.t.sol) — each fails on base's reading: base README L434-438 described the unseeded fold, mutant M1 below is that reading implemented verbatim in foldPattern, and the first three tests fail under it (probe run below). testFoldEmptyIsNilHash passes under both readings, the empty array being the one input on which they agree; it pins the "Nil hash prefix" empty-array clause and M2 shows it discriminates the seed value.
• Mutations applied (mutation-probe over an untracked mutants.toml, suite = CI's forge test in the rainix dbcd9d3 sol-shell; baseline green 26 passed; 7/7 KILLED, 0 survived, 0 no-run, 0 harness errors):
  • test/HashPatternFold.t.sol L37-39 → M1 seed dropped: if (foos.length == 0) return HASH_NIL; acc = hashFoo(foos[0]); for (i = 1; …) (base's fold) → KILLED by testFoldLetters, testFoldMatchesBuiltins, testFoldSingletonIsNotItem
  • test/HashPatternFold.t.sol L37 → M2 acc = HASH_NIL → acc = bytes32(0) → KILLED by testFoldEmptyIsNilHash, testFoldLetters, testFoldMatchesBuiltins, testFoldSingletonIsNotItem
  • test/HashPatternFold.t.sol L39 → M3 combineHashes(acc, hashFoo(foos[i])) → combineHashes(hashFoo(foos[i]), acc) → KILLED by testFoldLetters, testFoldMatchesBuiltins, testFoldSingletonIsNotItem
  • test/HashPatternFold.t.sol L38 → M4 i < foos.length → i + 1 < foos.length (last item skipped) → KILLED by testFoldLetters, testFoldMatchesBuiltins, testFoldSingletonIsNotItem
  • test/HashPatternFold.t.sol L48 → M5 oracle abi.encodePacked(expected, hashFoo(foos[i])) → abi.encodePacked(hashFoo(foos[i]), expected) → KILLED by testFoldLetters, testFoldMatchesBuiltins
  • src/LibHashNoAlloc.sol L104 → M6 (structural, size) keccak256(0, 0x40) → keccak256(0, 0x20) → KILLED by testFoldLetters, testFoldMatchesBuiltins, testFoldSingletonIsNotItem, testCombineHashes, testCombineHashesNoAlloc
  • src/LibHashNoAlloc.sol L102 → M7 (structural, offset) mstore(0, a) → mstore(0x20, a) → KILLED by testFoldLetters, testFoldMatchesBuiltins, testFoldSingletonIsNotItem, testCombineHashes, testCombineHashesNoAlloc
• Oracle: builtins only. foldOracle is expected = HASH_NIL; for each item: expected = keccak256(abi.encodePacked(expected, hashFoo(item))); hashFoo is the README's A-E composition as keccak256(abi.encode(a, b)), keccak256(abi.encodePacked(c)), keccak256(d) and keccak256(abi.encodePacked(·, ·)); the empty fold is checked against keccak256("") and [x] against keccak256(abi.encodePacked(HASH_NIL, hashFoo(x))). LibHashNoAlloc.combineHashes appears only on the side under test, as the "write both to scratch and hash" step. cast keccak "" = 0xc5d2460186f7233c927e7db2dcc703c0e500b653ca82273b7bfad8045d85a470 = HASH_NIL.
• Category check: issue asks (A) state one fold, (B) rewrite the step-by-step passage to match it, (C) name the [x]/x collision the choice avoids; covered A (seeded, with the three README reasons above), B (L432-437), C (L451-452). Ruling that every README layout/hashing claim gets a committed test: the letters (testFoldLetters), the fold for every length 0-4 (testFoldMatchesBuiltins), the empty-array clause (testFoldEmptyIsNilHash) and the [x] sentence (testFoldSingletonIsNotItem).

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Summary by CodeRabbit

• Documentation

  • Clarified that list hashing starts with the nil hash and processes items sequentially.
  • Documented that this seed distinguishes a single-item list from its item hash.

• Tests

  • Added coverage comparing folded list hashes with a keccak-based reference.
  • Verified empty folds produce the nil hash and singleton folds remain distinct from their item hash.

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Walkthrough

The README now defines pointer-list folding with HASH_NIL as the initial accumulator. A new Solidity test contract compares this fold with a builtin oracle and checks empty, singleton, and multi-item cases.

Changes

Nil-seeded fold

Layer / File(s)	Summary
Fold contract README.md	The fold description now starts with the nil hash and explains that the seed prevents a singleton array from hashing like its item.
Fold validation test/HashPatternFold.t.sol	New helpers and tests compare LibHashNoAlloc.combineHashes with builtin hashing for arrays of lengths 0 through 4. The tests also verify the empty and singleton cases.

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: ⚪ Minimal · up to e7011

The README consistently documents nil-seeded list hashing and explains the singleton distinction. No merge-blocking risk remains.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly identifies the main change: documenting the Foo[] fold as nil-hash seeded throughout the README.
Linked Issues check	✅ Passed	The README now consistently documents the nil-hash-seeded fold, explains the [x] versus x distinction, and preserves the nil-hash result for empty arrays. The added tests verify these requirements fro…
Out of Scope Changes check	✅ Passed	The changes remain in scope for issue #21. The README edits address the fold ambiguity, and the added tests validate the documented behavior without unrelated code changes.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0…

Full details: Linked Issues check

Explanation

The README now consistently documents the nil-hash-seeded fold, explains the [x] versus x distinction, and preserves the nil-hash result for empty arrays. The added tests verify these requirements from issue #21.

Full details: Docstring Coverage

Explanation

No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0 files. (1 skipped: 1 unsupported.)

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Inline comments:
In `@README.md`:
- Line 433: Update the README wording to hyphenate the compound modifiers:
change “fold/reduce style accumulator” to “fold/reduce-style accumulator” and
the corresponding “one item array” wording to “one-item array.”

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