Fix/zcash zip317 nu6.1 fees

[Thorian1te]

Summary by CodeRabbit

• Bug Fixes
  • Resolved memo fee double-counting issue in Zcash transactions, eliminating unexpected fee inflation when transaction memos are included
  • Unified fee calculation methodology across all transaction creation pathways to provide consistent, accurate, and predictable transaction fees

[coderabbitai]

Caution

Review failed

The pull request is closed.

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: 1a93e6fb-daf1-44a2-976f-e4f4a7656906

📥 Commits

Reviewing files that changed from the base of the PR and between 06826b5 and 6df2034.

📒 Files selected for processing (4)

• .changeset/fix-zcash-memo-fees.md
• packages/xchain-zcash/__tests__/client.test.ts
• packages/xchain-zcash/src/client.ts
• packages/zcash-js/src/builder.ts

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📝 Walkthrough

Walkthrough

This PR fixes memo fee double-counting in Zcash by standardizing how memo output slots are calculated. It increases the marginal fee constant, refactors fee calculation logic to use Math.max(inCount, outCount) instead of summing both with a grace floor, and updates both the builder and client to use fixed output counts while delegating memo slot accounting to the getFee function.

Changes

Cohort / File(s)	Summary
Release Notes .changeset/fix-zcash-memo-fees.md	Added changeset documenting patch release fixing memo fee double-counting and standardizing memo output slot calculation in Zcash packages.
Zcash Client packages/xchain-zcash/__tests__/client.test.ts, packages/xchain-zcash/src/client.ts	Updated fee test expectations to reflect new calculations; modified prepareMaxTx to use fixed output count (1), delegating memo slot accounting to getFee.
Zcash Builder packages/zcash-js/src/builder.ts	Increased MARGINAL_FEE from 5000 to 10000; refactored calculateFee to use Math.max(inCount, outCount) instead of input+output sum; standardized buildTx and buildMaxTx to use fixed output counts with getFee handling memo slots.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~22 minutes

Possibly related PRs

• feat(zcash): add buildMaxTx for sweep transactions #1611: Modifies the same Zcash builder and client fee/max-tx logic and interactions.
• fix(zcash): update ZIP-317 fee calculation for NU6.1 compatibility #1671: Implements identical code-level changes to ZIP-317 fee logic and memo slot standardization across builder and client.

Suggested reviewers

• underthesun49
• Naq302

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/zcash-zip317-nu6.1-fees

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