kache 1.2.1

#!/usr/bin/env python3
"""Write the Rust diff the changed-line mutation gate mutates.

`git diff` shows code moved to another place as removed and added again, so
splitting a large file would put every moved function in the gate. This
writes the same diff, but each added line that git marks as moved, unchanged
apart from its indentation, becomes a context line. Its text stays in the
diff, which cargo-mutants checks against the tree; only lines the change
wrote or edited count as changed.

A context line counts on both sides of a hunk, so each moved line also
lengthens the old side. Later hunks in the same file start that much further
down on the old side, which keeps the hunks in order and apart: cargo-mutants
refuses a diff whose hunks overlap, and reads only the new side against the
tree.

Moved lines come from git's own detection (`--color-moved=blocks`): a moved
block needs at least 20 alphanumeric characters, so a lone brace or a common
one-liner that happens to appear elsewhere is still treated as new. When the
colored and plain diffs do not line up, or the rewritten diff is not well
formed, the plain diff is written unchanged, so the gate never mutates less
than it did.

Usage: mutation-diff.py BASE HEAD > pr.diff
"""

import re
import subprocess
import sys

DIFF_ARGS = (
    "diff",
    "--no-ext-diff",
    "--diff-algorithm=histogram",
    "--unified=1",
)
# A color no other part of the diff uses, so a moved added line is known by
# the escape it starts with.
MOVED_COLOR = "magenta ul"
HUNK = re.compile(r"^@@ -(\d+)(?:,(\d+))? \+(\d+)(?:,(\d+))? @@(.*)$")
ESCAPE = re.compile(r"\x1b\[[0-9;]*m")


def git(*args):
    return subprocess.run(
        ("git", *args), check=True, capture_output=True, text=True
    ).stdout


def moved_escape():
    return git("config", "--get-color", "", MOVED_COLOR)


def colored_args():
    moved = [
        f"color.diff.{slot}={MOVED_COLOR}"
        for slot in (
            "newMoved",
            "newMovedAlternative",
            "newMovedDimmed",
            "newMovedAlternativeDimmed",
        )
    ]
    config = [part for setting in moved for part in ("-c", setting)]
    return (
        *config,
        *DIFF_ARGS,
        "--color=always",
        "--color-moved=blocks",
        "--color-moved-ws=allow-indentation-change",
    )


def moved_added_lines(plain, colored, escape):
    """Indexes of the plain diff's added lines that git colored as moved, or
    None when the two diffs do not describe the same lines."""
    if len(plain) != len(colored):
        return None
    moved = set()
    for index, (line, painted) in enumerate(zip(plain, colored)):
        if ESCAPE.sub("", painted) != line:
            return None
        if line.startswith("+") and not line.startswith("+++") and painted.startswith(escape):
            moved.add(index)
    return moved


def rewrite(plain, moved):
    """The plain diff with moved added lines as context, hunk counts redone,
    and hunks or files left with nothing changed dropped."""
    out = []
    header = []
    hunk = None
    # Moved lines written so far in this file as context, which lengthen the
    # old side of every later hunk.
    shift = 0

    def flush_hunk():
        nonlocal hunk, shift
        if hunk is None:
            return
        start, lines, converted = hunk
        hunk = None
        if not any(line[:1] in "+-" for line in lines):
            return
        old = sum(1 for line in lines if line[:1] in " -")
        new = sum(1 for line in lines if line[:1] in " +")
        old_start, new_start, tail = start
        if header:
            out.extend(header)
            header.clear()
        out.append(f"@@ -{int(old_start) + shift},{old} +{new_start},{new} @@{tail}")
        out.extend(lines)
        shift += converted

    for index, line in enumerate(plain):
        if line.startswith("diff --git "):
            flush_hunk()
            header[:] = [line]
            shift = 0
            continue
        match = HUNK.match(line)
        if match:
            flush_hunk()
            hunk = ((match.group(1), match.group(3), match.group(5)), [], 0)
            continue
        if hunk is None:
            header.append(line)
            continue
        if index in moved:
            line = " " + line[1:]
            hunk = (hunk[0], hunk[1], hunk[2] + 1)
        hunk[1].append(line)
    flush_hunk()
    return out


def well_formed(diff):
    """Every hunk counts exactly the lines under it, and within a file each
    hunk ends before the next starts on both sides. This is the order check
    cargo-mutants' diff parser applies, on the header numbers as written."""
    lines = list(diff)
    previous = None
    for index, line in enumerate(lines):
        if line.startswith("diff --git "):
            previous = None
            continue
        match = HUNK.match(line)
        if not match:
            continue
        old_start, new_start = int(match.group(1)), int(match.group(3))
        old_count, new_count = int(match.group(2) or 1), int(match.group(4) or 1)
        body = []
        for following in lines[index + 1 :]:
            if following.startswith(("@@", "diff --git ")):
                break
            body.append(following)
        if old_count != sum(1 for b in body if b[:1] in " -"):
            return False
        if new_count != sum(1 for b in body if b[:1] in " +"):
            return False
        if previous is not None and (previous[0] > old_start or previous[1] > new_start):
            return False
        previous = (old_start + old_count, new_start + new_count)
    return True


def main(argv):
    if len(argv) != 3:
        sys.exit(__doc__.strip().splitlines()[-1])
    base, head = argv[1], argv[2]
    paths = ("--", "*.rs")
    plain = git(*DIFF_ARGS, "--no-color", base, head, *paths).splitlines()
    colored = git(*colored_args(), base, head, *paths).splitlines()
    moved = moved_added_lines(plain, colored, moved_escape())
    if moved is None:
        print("mutation-diff: colored diff did not line up; keeping every line", file=sys.stderr)
        moved = set()
    added = sum(1 for line in plain if line.startswith("+") and not line.startswith("+++"))
    print(f"mutation-diff: {len(moved)} of {added} added lines are moved", file=sys.stderr)
    out = rewrite(plain, moved)
    if not well_formed(out):
        print("mutation-diff: rewritten diff is not well formed; keeping every line", file=sys.stderr)
        out = plain
    sys.stdout.writelines(line + "\n" for line in out)


if __name__ == "__main__":
    main(sys.argv)