fix: allow parsing empty context lines at the very end of unified diffs

[rtritto]

When parsing unified diff files, empty context lines (which should technically consist of a single space " ") are often inadvertently truncated to completely empty strings ("") due to code editors trimming trailing whitespace or formatting tools removing extraneous spaces.

Currently, the parsePatch (specifically parseHunk) function successfully forgives and maps these empty strings back to a space token (' '), except when the empty string happens to be the last line in the diffstr array:

// Before
const operation = (diffstr[i].length == 0 && i != (diffstr.length - 1)) ? ' ' : diffstr[i][0];

If a patch file ends with a trailing newline, uniDiff.split(/\n/) yields "" as the last element of the array. If a hunk still expects one last context line at that exact end-of-file position, i points to this final "" string. Because of the i != (diffstr.length - 1) check, it falls back to diffstr[i][0], which evaluates to undefined.

Since undefined is not a valid operation (+, -, , \), parseHunk throws an opaque error:
Error: Hunk at line X contained invalid line

Changes Made:

Removed the boundary exception && i != (diffstr.length - 1) condition. Empty strings are now globally and safely interpreted as empty context lines regardless of their index in the diff string.

// After
const operation = diffstr[i].length == 0 ? ' ' : diffstr[i][0];

Why this fixes real-world use cases:

This minor tweak prevents runtime exceptions when using applyPatch() on valid patch files generated by package managers (like yarn patch or pnpm patch) or saved via code editors, where a hunk requires an empty contextual line extending to the very end of the file.

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Adjusts how parsePatch determines the hunk line “operation” for empty strings while iterating through a unified diff.

Changes:

• Simplifies operation detection by treating empty lines as context (' ') unconditionally.

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[rtritto]

FYI @ExplodingCabbage