"""Version-to-version comparison for artifacts (HUX-04).

Text artifacts get a unified diff from ``difflib``; anything that is not
valid UTF-8, or whose artifact type is binary, is described by sizes and
hashes only so the UI never tries to render bytes as text.
"""

from __future__ import annotations

import difflib
import hashlib
from typing import Any

TEXT_TYPES = frozenset({"markdown", "code", "html", "svg", "json", "csv"})


def is_text_type(artifact_type: str) -> bool:
    """True for artifact types whose content is served as UTF-8 text."""
    return artifact_type in TEXT_TYPES


def decode_text(data: bytes) -> str | None:
    """UTF-8 decode, or None when the bytes are not text."""
    try:
        return data.decode("utf-8")
    except UnicodeDecodeError:
        return None


def unified(artifact_type: str, older: bytes, newer: bytes, from_n: int, to_n: int) -> dict[str, Any]:
    """Diff record ``{"from", "to", "unified"|"binary"}`` for two versions."""
    record: dict[str, Any] = {"from": from_n, "to": to_n}
    old_text = decode_text(older) if is_text_type(artifact_type) else None
    new_text = decode_text(newer) if is_text_type(artifact_type) else None
    if old_text is None or new_text is None:
        record["binary"] = {
            "from_bytes": len(older),
            "to_bytes": len(newer),
            "from_hash": "sha256:" + hashlib.sha256(older).hexdigest(),
            "to_hash": "sha256:" + hashlib.sha256(newer).hexdigest(),
        }
        return record
    lines = difflib.unified_diff(
        old_text.splitlines(keepends=True),
        new_text.splitlines(keepends=True),
        fromfile=f"v{from_n}",
        tofile=f"v{to_n}",
    )
    record["unified"] = "".join(lines)
    return record