Function apply_line_replacement

patch_engine/src/lib.rs:85–139

(
    existing_code: &str,
    replacement_range: LineRange,
    replacement_code: &str,
)

}

fn apply_line_replacement(
    existing_code: &str,
    replacement_range: LineRange,
    replacement_code: &str,
) -> Result<String> {
    if replacement_range.start_line == 0
        || replacement_range.end_line < replacement_range.start_line
    {
        bail!(
            "invalid replacement range {}..{}",
            replacement_range.start_line,
            replacement_range.end_line
        );
    }

    let mut lines: Vec<&str> = existing_code.split('\n').collect();
    let had_trailing_newline = existing_code.ends_with('\n');
    if had_trailing_newline && lines.last() == Some(&"") {
        lines.pop();
    }

    let line_count = lines.len();
    let normalized = replacement_code.replace("\r\n", "\n");
    if line_count == 0 && replacement_range.start_line == 1 && replacement_range.end_line == 1 {
        let mut updated = normalized;
        if had_trailing_newline && !updated.ends_with('\n') {
            updated.push('\n');
        }
        return Ok(updated);
    }

    if replacement_range.end_line > line_count {
        bail!(
            "replacement range {}..{} exceeds file length {}",
            replacement_range.start_line,
            replacement_range.end_line,
            line_count
        );
    }

    let start_idx = replacement_range.start_line - 1;
    let end_idx = replacement_range.end_line;
    let replacement_lines = split_lines_preserving_empty(&normalized);

    let mut out = Vec::new();
    out.extend_from_slice(&lines[..start_idx]);
    out.extend(replacement_lines.iter().map(|line| line.as_str()));
    out.extend_from_slice(&lines[end_idx..]);

    let mut updated = out.join("\n");
    if had_trailing_newline {
        updated.push('\n');
    }
    Ok(updated)
}

fn build_unified_diff(file_path: &str, old: &str, new: &str) -> String {
    if old == new {