fix:modified utils.py to remove merkle sibling

[Arpitsh7]

Fix: Replace XOR Sibling Index with Explicit Parity Check in generate_merkle_proof()

fixes #45

What this PR does

As described in the issue, generate_merkle_proof() and compute_merkle_root() used two different strategies for handling odd-length tree levels:

Function	Old odd-node strategy	New odd-node strategy
compute_merkle_root()	Inline: right = left if i+1 >= len(leaves)	Unchanged
generate_merkle_proof()	Pre-mutates: leaves.append(leaves[-1])	Now matches compute_merkle_root()

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Changes Made

# BEFORE — fragile XOR dependent on prior array mutation
if len(leaves) % 2 == 1:
    leaves.append(leaves[-1])

sibling_index = index ^ 1
sibling = leaves[sibling_index]
is_left = sibling_index < index

next_level = []
for i in range(0, len(leaves), 2):
    combined = leaves[i] + leaves[i + 1]
    ...

# AFTER — explicit parity check, no hidden dependencies
if index % 2 == 0:
    sibling_index = index + 1
    if sibling_index >= len(leaves):
        sibling_index = index  # odd level — no real sibling, duplicate self
    is_left = False
else:
    sibling_index = index - 1
    is_left = True

next_level = []
for i in range(0, len(leaves), 2):
    left = leaves[i]
    right = leaves[i + 1] if (i + 1) < len(leaves) else left  # unified with compute_merkle_root()
    combined = left + right
    ...

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Why this is safe

• Both functions now use identical odd-node logic
• verify_merkle_proof() is unchanged — existing proofs remain verifiable
• No other functions were modified

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Testing Checklist

• Proof generates correctly for even number of chunks
• Proof generates correctly for odd number of chunks
• verify_merkle_proof() returns True for valid proof against root
• verify_merkle_proof() returns False for tampered chunk

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Labels: bug, merkle-tree, cryptography

Checklist

• My code follows the project's code style and conventions
• I have made corresponding changes to the documentation
• My changes generate no new warnings or errors
• I have joined the Discord server and I will share a link to this PR with the project maintainers there
• I have read the Contributing Guidelines

Summary by CodeRabbit

• Refactor
  • Improved the internal verification tree computation algorithm for more reliable and consistent handling of validation operations, with better support for edge cases.

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ℹ️ Recent review info

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📥 Commits

Reviewing files that changed from the base of the PR and between c258e31 and 5dcd7c2.

📒 Files selected for processing (1)

• openverifiablellm/utils.py

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Walkthrough

Refactored generate_merkle_proof() to replace XOR-based sibling index calculation with explicit parity checks, and replaced leaf pre-padding with inline odd-node handling. This aligns the sibling selection logic with compute_merkle_root() for improved maintainability and eliminates fragile dependencies on array mutations.

Changes

Cohort / File(s)	Summary
Merkle Proof Logic Refactor openverifiablellm/utils.py	Replaced XOR sibling selection (index ^ 1) with explicit parity-based approach; removed pre-padding logic that duplicated the last leaf; updated is_left flag derivation to use parity instead of sibling_index; modified next-level hash construction to handle odd leaves inline (right defaults to left when missing).

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Suggested labels

Python Lang

Poem

Hoppity hoppity, leaf by leaf we go,
Parity paints the siblings' flow,
No fragile XOR tricks remain,
Merkle trees now march in sync's domain,
🐰 Consistency blooms, the proof is sound!

🚥 Pre-merge checks | ✅ 3 | ❌ 1

❌ Failed checks (1 inconclusive)

Check name	Status	Explanation	Resolution
Title check	❓ Inconclusive	The title is partially related to the changeset, referring to a real aspect of the change (modifying utils.py and Merkle sibling logic) but is vague and imprecise with unclear phrasing like 'remove merkle sibling'.	Clarify the title to better describe the specific change: consider 'fix: unify odd-node handling in Merkle proof generation' or 'fix: replace XOR sibling calculation with explicit parity check'.

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Linked Issues check	✅ Passed	The PR successfully addresses all coding requirements from issue #45: unifies odd-node handling strategies, replaces XOR sibling calculation with explicit parity checks, guards against out-of-bounds access, and ensures proofs remain verifiable.
Out of Scope Changes check	✅ Passed	All changes in generate_merkle_proof are directly scoped to addressing issue #45; no unrelated modifications or out-of-scope changes detected beyond the stated objectives.

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