fix: correct after_line=-1 prepend position in append_file #16

[four-bytes-robby]

Closes #16

Bug

When mode=prepend and after_line=-1, content inserted at the wrong position (end of file instead of before the last line).

Root Causes

1. split('\n') trailing newline inflation — Files ending with \n produce a trailing empty string in the split array, making lines.length one larger than the true line count
2. Negative index formula — The old Math.max(0, lines.length - 2) only worked for files without trailing newlines, and only for -1 specifically

Fix

• Strip the trailing empty string from split('\n') before computation, then restore it after
• Use Python-style negative indexing: -1 = before last line, -2 = before second-to-last, etc.
• insertPos = Math.max(0, lines.length + afterLine) for negative values

Tests Added

• after_line=-1 on a 3-line file (before last line)
• after_line=-1 on a 2-line file (edge case where old formula broke)

All 8 tests pass.

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Summary by cubic

Fix incorrect insert position in append_file when mode=prepend and after_line=-1. Now inserts before the last line (and supports other negative indexes) while preserving trailing newlines.

• Bug Fixes
  • Support Python-style negative after_line values (-1 before last line, -2 before second-to-last, etc.).
  • Normalize trailing newline: remove the extra empty split element before computing, then restore it.
  • Added tests for 3-line and 2-line files to cover after_line=-1.

^{Written for commit d55f68f. Summary will update on new commits.}

Summary by CodeRabbit

• Bug Fixes

  • Enhanced file prepend operations to correctly handle Python-style negative indexing and properly preserve file formatting.

• Tests

  • Added test coverage for prepend mode with negative line indexing edge cases.

[coderabbitai]

Caution

Review failed

The pull request is closed.

Recent review info

Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: b8733faf-f0c8-4c44-8424-af17bf7cf7e3

Commits

Reviewing files that changed from the base of the PR and between 993107a and d55f68f.

Files selected for processing (2)

• src/tools/append-file.ts
• tests/append-file.test.ts

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Walkthrough

Walkthrough

The appendFileTool prepend-mode insertion is refactored to support Python-style negative indexing for the after_line parameter. The implementation now strips a trailing newline before computing the insert position, clamps the derived index, and restores the newline after insertion. Two new tests cover the -1 case.

Changes

Prepend Mode Negative Index Fix

Layer / File(s)	Summary
Prepend insertion logic and argument description src/tools/append-file.ts	after_line description is extended to document negative indexing semantics. Prepend-mode logic is rewritten to detect and strip a trailing newline, compute an insert position via Python-like negative index clamping, insert content between the derived slices, and restore the trailing newline.
Prepend mode after_line=-1 test cases tests/append-file.test.ts	Two bun:test cases are added: one for a standard 3-line file and one for a 2-line edge case, each asserting the returned message contains prepend mode and that file contents reflect insertion before the last line.

Estimated code review effort

3 (Moderate) | ~15 minutes

Possibly related issues

• fix: append_file after_line=-1 inserts at wrong position in prepend mode #16: The PR directly fixes the wrong insertion position for after_line=-1 in prepend mode by mapping -1 to lines.length - 2, which is exactly the bug and fix described in that issue.
• feat: add append_file tool for simple file appending #14: The PR extends the negative indexing semantics for after_line in prepend mode beyond just -1, which aligns with the broader feature request for Python-style negative indexing in append_file.

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/16-append-file-after-line

Simplify code

• Create PR with simplified code
• Commit simplified code in branch fix/16-append-file-after-line

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