git-reset-hunk

#!/usr/bin/env python3
import os, sys, subprocess
sys.path.insert(0, os.path.dirname(os.path.realpath(__file__)))
import git_hunk_utils

USAGE = """\
usage: git reset-hunk <file> <hunk-id>...

Selectively unstage hunks from the index.

Hunk IDs use the NEW file side of the @@ header: +offset,count
Example: @@ -10,5 +12,7 @@  →  hunk ID is +12,7 (NOT -10,5)

Run 'git diff --cached <file>' to see available hunk IDs."""


def main():
    if len(sys.argv) < 3:
        print(USAGE, file=sys.stderr)
        sys.exit(1)

    filepath = sys.argv[1]
    requested_ids = set(sys.argv[2:])

    text = git_hunk_utils.run_git_diff([filepath], cached=True)

    if not text.strip():
        print(f"no staged changes in {filepath}", file=sys.stderr)
        sys.exit(1)

    files = git_hunk_utils.parse_diff(text)
    if not files:
        print(f"no staged changes in {filepath}", file=sys.stderr)
        sys.exit(1)

    file_diff = files[0]
    available_ids = {h.id for h in file_diff.hunks}

    unknown = requested_ids - available_ids
    if unknown:
        for uid in sorted(unknown):
            print(f"error: unknown hunk ID: {uid}", file=sys.stderr)
            if uid.startswith("-"):
                print(f"  hint: '{uid}' is the OLD file side; use the NEW file side (+offset,count) instead", file=sys.stderr)
        print(f"\navailable hunk IDs for {filepath}:", file=sys.stderr)
        for h in file_diff.hunks:
            print(f"  {h.id}", file=sys.stderr)
        sys.exit(1)

    selected = [h for h in file_diff.hunks if h.id in requested_ids]
    patch = reassemble_patch(file_diff.header, selected)

    proc = subprocess.run(
        ["git", "apply", "--cached", "--reverse"],
        input=patch,
        text=True,
        capture_output=True,
    )
    if proc.returncode != 0:
        print(proc.stderr, file=sys.stderr, end="")
        sys.exit(proc.returncode)


def reassemble_patch(file_header, hunks):
    parts = [file_header]
    for h in hunks:
        parts.append(h.header_line + "\n")
        parts.append(h.body + "\n")
    return "".join(parts)


if __name__ == "__main__":
    main()