fn compute_replacements()

in codex-rs/apply-patch/src/lib.rs [412:495]

• 59 lines of code
• 10 McCabe index (conditional complexity)

fn compute_replacements(
    original_lines: &[String],
    path: &Path,
    chunks: &[UpdateFileChunk],
) -> std::result::Result<Vec<(usize, usize, Vec<String>)>, ApplyPatchError> {
    let mut replacements: Vec<(usize, usize, Vec<String>)> = Vec::new();
    let mut line_index: usize = 0;

    for chunk in chunks {
        // If a chunk has a `change_context`, we use seek_sequence to find it, then
        // adjust our `line_index` to continue from there.
        if let Some(ctx_line) = &chunk.change_context {
            if let Some(idx) =
                seek_sequence::seek_sequence(original_lines, &[ctx_line.clone()], line_index, false)
            {
                line_index = idx + 1;
            } else {
                return Err(ApplyPatchError::ComputeReplacements(format!(
                    "Failed to find context '{}' in {}",
                    ctx_line,
                    path.display()
                )));
            }
        }

        if chunk.old_lines.is_empty() {
            // Pure addition (no old lines). We'll add them at the end or just
            // before the final empty line if one exists.
            let insertion_idx = if original_lines.last().is_some_and(|s| s.is_empty()) {
                original_lines.len() - 1
            } else {
                original_lines.len()
            };
            replacements.push((insertion_idx, 0, chunk.new_lines.clone()));
            continue;
        }

        // Otherwise, try to match the existing lines in the file with the old lines
        // from the chunk. If found, schedule that region for replacement.
        // Attempt to locate the `old_lines` verbatim within the file.  In many
        // real‑world diffs the last element of `old_lines` is an *empty* string
        // representing the terminating newline of the region being replaced.
        // This sentinel is not present in `original_lines` because we strip the
        // trailing empty slice emitted by `split('\n')`.  If a direct search
        // fails and the pattern ends with an empty string, retry without that
        // final element so that modifications touching the end‑of‑file can be
        // located reliably.

        let mut pattern: &[String] = &chunk.old_lines;
        let mut found =
            seek_sequence::seek_sequence(original_lines, pattern, line_index, chunk.is_end_of_file);

        let mut new_slice: &[String] = &chunk.new_lines;

        if found.is_none() && pattern.last().is_some_and(|s| s.is_empty()) {
            // Retry without the trailing empty line which represents the final
            // newline in the file.
            pattern = &pattern[..pattern.len() - 1];
            if new_slice.last().is_some_and(|s| s.is_empty()) {
                new_slice = &new_slice[..new_slice.len() - 1];
            }

            found = seek_sequence::seek_sequence(
                original_lines,
                pattern,
                line_index,
                chunk.is_end_of_file,
            );
        }

        if let Some(start_idx) = found {
            replacements.push((start_idx, pattern.len(), new_slice.to_vec()));
            line_index = start_idx + pattern.len();
        } else {
            return Err(ApplyPatchError::ComputeReplacements(format!(
                "Failed to find expected lines {:?} in {}",
                chunk.old_lines,
                path.display()
            )));
        }
    }

    Ok(replacements)
}