"""Commit journal (spec 13.1): append-only, hash-chained, fail-closed.

Each line is ``{"seq", "chunk", "content_id", "file_sha256", "prev",
"ts_utc"}``; ``prev`` is the hash of the previous line's exact UTF-8 text
(SHA-256 of the canonical JSON text of the previous line, no trailing
newline). seq 0 uses SHA-256 of the empty string as prev. ``journal_tail``
recorded in the checkpoint is (seq, hash of last line).
"""

import os

from .util import canonical_json, sha256_bytes, atomic_write_bytes

EMPTY_HASH = sha256_bytes(b"")


class JournalError(Exception):
    pass


def _line_for(record):
    return canonical_json(record)


def _line_hash(line_text):
    return sha256_bytes(line_text)


def append_commit(journal_path, seq, chunk_index, content_id, file_sha256,
                  ts_utc, prev_hash):
    """Append one commit line atomically (fail-closed: fsync)."""
    record = {
        "seq": seq,
        "chunk": chunk_index,
        "content_id": content_id,
        "file_sha256": file_sha256,
        "prev": prev_hash,
        "ts_utc": ts_utc,
    }
    line = _line_for(record)
    os.makedirs(os.path.dirname(os.path.abspath(journal_path)), exist_ok=True)
    with open(journal_path, "ab") as fh:
        fh.write(line.encode("utf-8"))
        fh.flush()
        os.fsync(fh.fileno())
    # Line hash is defined over the line text WITHOUT the trailing newline
    # (read_journal recomputes it the same way).
    return _line_hash(line.rstrip("\n"))


def read_journal(journal_path):
    """Read and validate the chain. Returns (records, tail_hash).
    Raises JournalError on chain break or malformed line."""
    if not os.path.exists(journal_path):
        return [], EMPTY_HASH
    records = []
    prev = EMPTY_HASH
    tail = EMPTY_HASH
    with open(journal_path, "rb") as fh:
        for raw in fh:
            text = raw.rstrip(b"\r\n")
            if not text:
                continue
            if raw.endswith(b"\r\n"):
                line = raw[:-2]
            elif raw.endswith(b"\n"):
                line = raw[:-1]
            else:
                line = raw
            try:
                import json
                record = json.loads(line.decode("utf-8"))
            except Exception as e:
                raise JournalError("journal line unparseable: %s" % e)
            if record.get("prev") != prev:
                raise JournalError("journal chain broken at seq %r"
                                   % record.get("seq"))
            lh = _line_hash(line.decode("utf-8"))
            records.append(record)
            prev = lh
            tail = lh
    return records, tail


def seq_after(records):
    """Next seq = last seq + 1 (0 when journal is empty)."""
    if not records:
        return 0
    return records[-1].get("seq", -1) + 1