"""Task-attempt ledger for the step-5 metrics loop (audit gap 3).

Choice made here, with the trade-off stated rather than hidden:

- This module adds a SEPARATE ``task_attempts`` table; it does NOT
  add ``attempts``/``defects`` columns to ``tasks``. Columns would be
  simpler to query (one row per task, no join), but they collapse
  history into two counters: a task's per-attempt story (which
  attempt failed, with what defect category, in what order) would be
  unrecoverable, and widening the hot ``tasks`` table on production
  Postgres is a heavier migration than adding a new, empty table.
  The ledger costs one aggregate query at report time, and that
  query is exactly what ``aggregate_attempts`` exists to pin down.
- The ledger is append-only in intent: ``record_attempt`` only ever
  inserts a new ``(task_id, attempt_no)`` row (``attempt_no`` is
  ``MAX + 1`` per task, never reused). There is no update path, so a
  recorded attempt cannot be silently rewritten into a first-pass
  success after the fact.

Semantics (pinned by tests, not left to each caller):

- One row = one real attempt at the task. ``outcome`` is one of
  ``success`` / ``failure`` / ``interrupted``.
- ``interrupted`` rows are kept for audit but NEVER count toward
  ``attempts`` or ``defects``: a platform interruption that
  auto-resumes as a fresh run is a platform event, not an agent
  failure, and the step-5 rule that ``interrupted`` is not a
  terminal task status would be meaningless if the interruption
  still inflated the attempt count. A task whose ledger holds only
  interrupted rows has NO counted attempt, so it is omitted from
  the aggregate entirely -- the endpoint then still reports its
  rates as unknown (``None``), never as 0.
- ``is_defect`` is set explicitly by the recorder, never inferred
  from ``outcome``: a failed attempt is not automatically a defect
  (a retry can fail for a transient reason without being a
  defect round), and a defect round is a judgement the recording
  caller makes. ``defect_category`` requires ``is_defect``; an
  interrupted row can never be a defect. Both contradictions raise
  ``ValueError`` instead of being stored as ambiguous data.
- ``defect_category`` for a task (the single category that
  ``compute_metrics`` carries per task) is the category of its
  FIRST defect row in attempt order. Later defect rows still count
  toward ``defects``; only the category label is first-wins, so it
  is deterministic.
- Old tasks have no ledger rows, and that is NOT backfilled: the
  aggregate simply has no entry for them, the endpoint leaves their
  ``attempts``/``defects`` as ``None``, and ``compute_metrics``
  excludes them from rate denominators (they are neither counted
  as first-pass nor as defect-free). Fabricating a row for an old
  task (e.g. ``attempts=1, defects=0`` from "it completed") would
  be inventing history.

Persistence notes:

- SQLite: ``ensure_task_attempts_table`` runs
  ``CREATE TABLE IF NOT EXISTS`` -- idempotent, safe to call on
  every write, and a no-op on a database that already has the
  table (including one that already has rows).
- Postgres: the production table is NOT created by
  ``init_postgres_schema`` (deliberately: that schema is covered by
  the migrations test asserting ``MIGRATION_TABLES`` equals the
  schema set, and this ledger must not silently widen that set).
  ``PG_DDL`` below is the exact statement to run manually on
  production Postgres and verify (``\\d task_attempts`` or the
  ``information_schema`` equivalent) before any Postgres-backed
  endpoint is expected to return real rates. The DDL uses only
  Postgres-valid types (``TEXT``/``INTEGER``/``DOUBLE PRECISION``),
  a composite primary key, and no ``AUTOINCREMENT``/identity
  column, so it needs no sequence handling; the parameterized
  ``INSERT`` uses ``?`` placeholders, which the ``cloud.db``
  Postgres adapter translates to ``%s`` (see the adapter test in
  ``test_attempt_ledger.py``). Until that manual step runs, the
  endpoint treats a missing table exactly like "no ledger rows":
  unknown rates, not an error -- the same fail-soft shape already
  used for missing ``cost_records``.
- The read path (``GET /metrics-report``) never creates the table:
  a read-only endpoint must not mutate schema. Table creation
  happens only on the write path, inside ``record_attempt``.
- No lifecycle hook writes to this ledger yet. Hooking a writer
  into the worker's retry/resume loops would risk exactly the
  misclassification this module refuses to make (platform
  auto-resume counted as an attempt, transient retry counted as a
  defect). The writer -- which lifecycle event counts as an
  attempt, and who judges ``is_defect`` -- is a separate decision,
  listed as an open item; until it lands, the ledger fills only
  through explicit ``record_attempt`` calls, and production rates
  stay honestly unknown rather than auto-filled.

This module imports no ``cloud.*`` module and opens no database
connection itself: callers pass a connection in, so the layer rules
(``arch_conformance``: no direct driver connection outside
``cloud.db``) are untouched -- there is no ``sqlite3.connect`` and
no ``psycopg*`` import here at all.
"""

import time

TABLE = "task_attempts"

OUTCOMES = frozenset({"success", "failure", "interrupted"})

# One DDL serves both backends: every type and the composite key are
# valid in SQLite and in Postgres, and there is no AUTOINCREMENT or
# identity column for either backend's sequence handling to chase.
SQLITE_DDL = (
    "CREATE TABLE IF NOT EXISTS task_attempts("
    "task_id TEXT NOT NULL, "
    "attempt_no INTEGER NOT NULL, "
    "outcome TEXT NOT NULL, "
    "is_defect INTEGER NOT NULL DEFAULT 0, "
    "defect_category TEXT, "
    "created_at DOUBLE PRECISION, "
    "PRIMARY KEY (task_id, attempt_no))"
)

# Production Postgres: run this verbatim by hand and verify the
# table exists before expecting real rates from a Postgres-backed
# endpoint. It is intentionally NOT part of POSTGRES_SCHEMA (see
# the module docstring: migrations coverage must not widen silently).
PG_DDL = SQLITE_DDL


def ensure_task_attempts_table(conn):
    """Create the ledger table if absent. Idempotent; does not
    touch existing rows. The caller owns the commit (``record_attempt``
    commits; a bare ensure inside a larger transaction does not)."""
    conn.execute(SQLITE_DDL)


def record_attempt(conn, task_id, outcome, is_defect=False,
                   defect_category=None, created_at=None):
    """Append one attempt row for ``task_id``. Returns ``attempt_no``.

    Validates before writing: unknown outcome, a defect flag on an
    interrupted row, and a defect category without a defect flag
    all raise ``ValueError`` and write nothing.
    """
    if not task_id:
        raise ValueError("task_id is required")
    if outcome not in OUTCOMES:
        raise ValueError("outcome must be one of %s, got %r"
                         % (sorted(OUTCOMES), outcome))
    if outcome == "interrupted" and is_defect:
        raise ValueError("an interrupted attempt is never a defect")
    if defect_category is not None and not is_defect:
        raise ValueError("defect_category requires is_defect=True")
    ensure_task_attempts_table(conn)
    row = conn.execute(
        "SELECT COALESCE(MAX(attempt_no), 0) FROM task_attempts "
        "WHERE task_id = ?",
        (task_id,),
    ).fetchone()
    attempt_no = int(row[0]) + 1
    conn.execute(
        "INSERT INTO task_attempts(task_id, attempt_no, outcome, "
        "is_defect, defect_category, created_at) "
        "VALUES(?, ?, ?, ?, ?, ?)",
        (task_id, attempt_no, outcome, 1 if is_defect else 0,
         defect_category,
         time.time() if created_at is None else created_at),
    )
    conn.commit()
    return attempt_no


def read_attempt_rows(conn):
    """All ledger rows in attempt order. Raises the backend's
    OperationalError when the table does not exist yet; the
    endpoint caller treats that as "no ledger", not as a failure."""
    return conn.execute(
        "SELECT task_id, attempt_no, outcome, is_defect, "
        "defect_category FROM task_attempts "
        "ORDER BY task_id, attempt_no").fetchall()


def _field(row, name):
    # sqlite3.Row, dict, and the Postgres adapter's rows all support
    # key access; a plain tuple row is not accepted (callers pass
    # rows, not raw tuples, so a wrong shape fails loudly here).
    return row[name]


def aggregate_attempts(rows):
    """Pure aggregation: ledger rows -> ``{task_id: record}`` where
    ``record`` is the ``attempts``/``defects``/``defect_category``
    dict that ``compute_metrics`` consumes.

    Interrupted rows are excluded from both counts, and a task
    with only interrupted rows is omitted entirely (no counted
    attempt => its rates stay unknown, never 0).
    """
    per_task = {}
    ordered = sorted(
        rows,
        key=lambda r: (_field(r, "task_id"), _field(r, "attempt_no")))
    for row in ordered:
        task_id = _field(row, "task_id")
        if _field(row, "outcome") == "interrupted":
            continue
        entry = per_task.setdefault(
            task_id, {"attempts": 0, "defects": 0,
                      "defect_category": None})
        entry["attempts"] += 1
        if _field(row, "is_defect"):
            entry["defects"] += 1
            if entry["defect_category"] is None:
                entry["defect_category"] = _field(row, "defect_category")
    return per_task


def apply_to_records(records, aggregates):
    """Return a new record list with ledger values merged in.

    Records for tasks with no aggregate entry keep whatever
    ``attempts``/``defects`` they already carried (the endpoint
    passes ``None`` there, i.e. old tasks stay unknown). Input
    records are not modified.
    """
    merged = []
    for record in records:
        out = dict(record)
        agg = aggregates.get(record.get("id"))
        if agg is not None:
            out["attempts"] = agg["attempts"]
            out["defects"] = agg["defects"]
            out["defect_category"] = agg["defect_category"]
        merged.append(out)
    return merged