Function apply_unified_diff

patch_engine/src/lib.rs:203–258

(existing_code: &str, diff: &str)

}

fn apply_unified_diff(existing_code: &str, diff: &str) -> Result<String> {
    let original_lines = split_lines_preserving_empty(existing_code);
    let mut out = Vec::new();
    let mut current_original_index = 0usize;
    let mut in_hunk = false;

    for raw_line in diff.lines() {
        if raw_line.starts_with("--- ") || raw_line.starts_with("+++ ") {
            continue;
        }
        if let Some((old_start, _old_len, _new_start, _new_len)) = parse_hunk_header(raw_line)? {
            let target_index = old_start.saturating_sub(1);
            if target_index > original_lines.len() {
                bail!("hunk starts past end of file");
            }
            out.extend_from_slice(&original_lines[current_original_index..target_index]);
            current_original_index = target_index;
            in_hunk = true;
            continue;
        }
        if !in_hunk {
            continue;
        }
        let (prefix, content) = raw_line
            .chars()
            .next()
            .map(|ch| (ch, &raw_line[ch.len_utf8()..]))
            .ok_or_else(|| anyhow!("malformed diff line"))?;
        match prefix {
            ' ' => {
                let original = original_lines
                    .get(current_original_index)
                    .ok_or_else(|| anyhow!("context line exceeds original file"))?;
                if original != &content {
                    bail!("context mismatch while applying patch");
                }
                out.push(content.to_string());
                current_original_index += 1;
            }
            '-' => {
                let original = original_lines
                    .get(current_original_index)
                    .ok_or_else(|| anyhow!("deletion exceeds original file"))?;
                if original != &content {
                    bail!("deletion mismatch while applying patch");
                }
                current_original_index += 1;
            }
            '+' => out.push(content.to_string()),
            _ => bail!("unsupported diff line prefix '{prefix}'"),
        }
    }

    out.extend_from_slice(&original_lines[current_original_index..]);
    Ok(join_lines_like_source(existing_code, &out))
}

fn parse_hunk_header(line: &str) -> Result<Option<(usize, usize, usize, usize)>> {