"""Workflow-event and conversation timeline model (#637, Phase 1).

Operators cannot browse a unified timeline of workflow events, decisions,
tool calls, and handoffs: the evidence is scattered across control-plane
events, Gitea canonical handoff comments, and local logs. This module defines
one durable, versioned event schema and per-source adapters that normalise
those scattered records into a single ``WorkflowEvent`` stream, plus a
read-only query layer (filter by issue / PR / session, stable ordering,
pagination) that the ``/api/v1/timeline`` route serves.

Design rules honoured here:

- **Read-only.** Sources are read; nothing is mutated. The control-plane
  database is opened through a ``mode=ro`` URI so a missing or unwritable DB
  degrades to a reason instead of creating directories or running migrations.
- **Fail-soft per source.** An unavailable source degrades to a status with a
  reason rather than raising, and a source that could not run is never
  rendered as an empty-and-healthy timeline.
- **Answerable filters only.** Each source declares which filter dimensions it
  can actually answer. A filter dimension no source that ran can carry is
  refused with an explicit reason rather than silently matching nothing: an
  empty page from an unanswerable filter reads to an operator as "no such
  activity", which is a different — and false — statement.
- **Redaction at the boundary, fail closed.** Every free-text field (event
  messages, redacted tool arguments, decision/proof text) is run through the
  console redaction policy before it leaves this module, and *before* any
  structured value is derived from it — evidence references are extracted from
  redacted text, then independently revalidated before serialization. An
  unredactable value becomes the placeholder, and a value that cannot be proven
  safe is dropped — an unredacted payload is never emitted, and a generation
  error never drops raw data to a caller or a log.
- **Stable ordering.** Events sort by ``(timestamp, source_rank, event_key)``
  with a deterministic tiebreak, so pagination is stable across calls and
  events with equal or missing timestamps keep a fixed order.

Non-goals (from the issue): no full chat replay, no mutation of historical
events, no unredacted tool-argument storage.
"""

from __future__ import annotations

import re
import sqlite3
from dataclasses import dataclass, replace
from datetime import datetime, timezone
from typing import Any, Callable, Iterable

import control_plane_db
from webui import console_redaction

# The schema is versioned so consumers can branch on shape. Bump on any
# breaking change to WorkflowEvent's serialized form.
TIMELINE_SCHEMA_VERSION = 1

# Known event sources and their deterministic ordering rank. When two events
# carry the same timestamp, the source rank breaks the tie before the
# per-source event key, so a control-plane event and a handoff comment minted
# in the same second always sort in a fixed order.
SOURCE_CONTROL_PLANE = "control_plane"
SOURCE_GITEA_HANDOFF = "gitea_handoff"
_SOURCE_RANK = {
    SOURCE_CONTROL_PLANE: 0,
    SOURCE_GITEA_HANDOFF: 1,
}

# The filter dimensions the query layer accepts.
FILTER_ISSUE = "issue"
FILTER_PR = "pr"
FILTER_SESSION = "session"

# Which dimensions each source can actually answer. This is a property of the
# underlying records, not of the query code: the control-plane ``events`` table
# is (event_id, work_item_id, event_type, message, created_at) and carries no
# session identity at all, so no control-plane event can ever match a session
# filter. A CTH handoff comment can declare its session as a field, so the
# handoff source answers all three. Filtering on a dimension the surviving
# sources cannot carry is refused in ``load_timeline`` rather than answered
# with an empty page.
_SOURCE_FILTER_SUPPORT: dict[str, tuple[str, ...]] = {
    SOURCE_CONTROL_PLANE: (FILTER_ISSUE, FILTER_PR),
    SOURCE_GITEA_HANDOFF: (FILTER_ISSUE, FILTER_PR, FILTER_SESSION),
}

# Why a source cannot answer a dimension, for the refusal reason an operator reads.
_SOURCE_FILTER_LIMITS: dict[tuple[str, str], str] = {
    (SOURCE_CONTROL_PLANE, FILTER_SESSION): (
        "control-plane events carry no session identity "
        "(the events table has no session column)"
    ),
}

# A timestamp far in the future so events with no parseable timestamp sort
# last (after everything real) instead of first, without raising.
_MISSING_TS_SORT = "9999-12-31T23:59:59Z"


def _parse_ts(value: str | None) -> str | None:
    """Normalise a timestamp to ``...Z`` UTC, or None when unparseable."""
    if not value:
        return None
    text = str(value).strip()
    if not text:
        return None
    candidate = text[:-1] + "+00:00" if text.endswith("Z") else text
    try:
        parsed = datetime.fromisoformat(candidate)
    except ValueError:
        return None
    if parsed.tzinfo is None:
        parsed = parsed.replace(tzinfo=timezone.utc)
    return parsed.astimezone(timezone.utc).replace(microsecond=0).isoformat().replace("+00:00", "Z")


def _redact(value: Any) -> Any:
    """Redact a single free-text field, failing closed to the placeholder."""
    if value is None:
        return None
    return console_redaction.redact_text(str(value))


@dataclass(frozen=True)
class WorkflowEvent:
    """One normalised timeline event.

    Every field is optional except ``source``/``event_type``/``event_key``
    because sources carry different subsets. The class is frozen so an adapted
    event is an immutable record; a consumer that needs a variant builds a new
    one rather than mutating history.
    """

    source: str
    event_type: str
    event_key: str
    timestamp: str | None = None
    actor: str | None = None
    role: str | None = None
    issue_number: int | None = None
    pr_number: int | None = None
    session_id: str | None = None
    tool_name: str | None = None
    decision: str | None = None
    message: str | None = None
    correlation_id: str | None = None
    evidence_refs: tuple[str, ...] = ()
    sensitive: bool = False

    def sort_key(self) -> tuple[str, int, str]:
        return (
            self.timestamp or _MISSING_TS_SORT,
            _SOURCE_RANK.get(self.source, 99),
            self.event_key,
        )

    def to_dict(self) -> dict[str, Any]:
        return {
            "source": self.source,
            "event_type": self.event_type,
            "event_key": self.event_key,
            "timestamp": self.timestamp,
            "actor": self.actor,
            "role": self.role,
            "issue_number": self.issue_number,
            "pr_number": self.pr_number,
            "session_id": self.session_id,
            "tool_name": self.tool_name,
            "decision": self.decision,
            "message": self.message,
            "correlation_id": self.correlation_id,
            "evidence_refs": list(self.evidence_refs),
            "sensitive": self.sensitive,
        }


# --------------------------------------------------------------------------- #
# Adapters — pure functions from a source's raw records to WorkflowEvents.     #
# Each is total: a malformed record is skipped, never raised on.               #
# --------------------------------------------------------------------------- #

# Event types whose payload is treated as sensitive and always redaction-hard
# (they can carry lease/session provenance or tool arguments).
_SENSITIVE_EVENT_HINTS = ("lease", "capability", "token", "auth", "secret")

# Reference tokens (issue/PR/comment ids) and SHAs parsed out of proof text.
_EVIDENCE_REF_RE = re.compile(r"(?:#|PR\s*#?|issue\s*#?|comment\s*#?)(\d+)", re.IGNORECASE)

# A commit reference is only recognised when the text *declares* it as one.
# A bare lowercase hex run is not evidence of anything: at 40 characters it is
# exactly the shape of a Gitea personal access token, and at 7 it also matches
# ordinary words such as "defaced". Requiring an anchoring keyword keeps real
# references ("commit abc1234", "at head a209756...", "base caaae9b6") usable
# while refusing to lift an undeclared secret-shaped run out of free text.
_SHA_RE = re.compile(
    r"(?i:\b(?:commit|sha|head|base|parent|revision|rev|merge[- ]base)\b[\s:=@#]*)"
    r"([0-9a-f]{7,40})\b"
)

# Shapes a serialized evidence reference is allowed to take. Anything else is
# dropped rather than emitted.
_REF_ISSUE_SHAPE = re.compile(r"^#[0-9]{1,9}$")
_REF_SHA_SHAPE = re.compile(r"^[0-9a-f]{7,40}$")

# A long undelimited hex run with no declaring context is treated as credential
# material wherever it appears, never as an identifier.
_BARE_SECRET_SHAPE = re.compile(r"^[0-9a-f]{32,}$")

# An event type reads like an identifier, but a stored one is externally
# influenced: any producer that writes the control-plane ``events`` table
# chooses the string. It reaches ``to_dict`` verbatim, so it is validated here
# rather than trusted because of where it came from.
_CP_EVENT_TYPE_SHAPE = re.compile(r"^[A-Za-z][A-Za-z0-9._:+-]{0,63}$")

# Emitted in place of a value that cannot be proven safe. Deliberately not a
# plausible workflow type: an unsafe value is refused, never quietly rewritten
# into a different valid-looking one that would misdescribe the record.
UNSAFE_EVENT_TYPE = "unsafe:redacted"

# Emitted for a CTH heading that is not a declared member of ``CTH_TYPES``. The
# contract is enforced on write (``format_cth_body``) and on assess; the read
# path the timeline uses enforces it too rather than assuming it was.
UNKNOWN_HANDOFF_EVENT_TYPE = "handoff:unrecognized"

# A source record id is a plain integer in both sources it comes from: the
# control-plane ``events`` primary key and a Gitea comment id. ``event_key`` is
# serialized verbatim and is the pagination tiebreak, so anything else is
# refused rather than interpolated into it.
_RECORD_ID_SHAPE = re.compile(r"^[0-9]{1,19}$")


def _kind_to_numbers(kind: str | None, number: int | None) -> tuple[int | None, int | None]:
    """Map a control-plane work-item (kind, number) to (issue_no, pr_no)."""
    if number is None:
        return (None, None)
    if kind == "pr":
        return (None, int(number))
    if kind == "issue":
        return (int(number), None)
    return (None, None)


def _correlation_for(kind: str | None, number: int | None) -> str | None:
    if number is None or kind not in ("issue", "pr"):
        return None
    return f"{kind}#{number}"


def _extract_evidence_refs(*texts: str | None) -> tuple[str, ...]:
    """Extract issue/PR and declared-commit references from **redacted** text.

    Callers must pass text that has already been through :func:`_redact`; this
    function derives a structured field from its input, so extracting ahead of
    redaction would republish whatever redaction was about to remove. Every
    reference is revalidated by :func:`_validated_evidence_refs` before it is
    serialized.
    """
    refs: list[str] = []
    for text in texts:
        if not text:
            continue
        for match in _EVIDENCE_REF_RE.finditer(text):
            token = f"#{match.group(1)}"
            if token not in refs:
                refs.append(token)
        for match in _SHA_RE.finditer(text):
            token = match.group(1)
            if token not in refs:
                refs.append(token)
    return tuple(refs)


def _validated_evidence_refs(refs: Iterable[str]) -> tuple[tuple[str, ...], bool]:
    """Independently revalidate references immediately before serialization.

    Extraction is not trusted on its own. A reference survives only when it has
    a known reference shape and is unchanged by a second redaction pass — a
    value the redaction policy would alter is credential material that must not
    be emitted as a structured field. A full 40-character SHA stays usable
    because extraction only accepts a hex run the source text explicitly
    declared as a commit. Returns ``(safe_refs, dropped_any)``; ``dropped_any``
    marks the event sensitive so the drop is visible rather than silent.
    """
    safe: list[str] = []
    dropped = False
    for ref in refs or ():
        try:
            token = str(ref).strip()
            if not token:
                continue
            recognised = bool(_REF_ISSUE_SHAPE.match(token) or _REF_SHA_SHAPE.match(token))
            if not recognised:
                dropped = True
                continue
            if _redact(token) != token:
                dropped = True
                continue
            if token not in safe:
                safe.append(token)
        except Exception:
            # Fail closed: a reference that cannot be proven safe is dropped.
            dropped = True
            continue
    return (tuple(safe), dropped)


def _safe_session_id(value: Any) -> str | None:
    """Return a session identifier only when it is safe to emit.

    The value is authoritative source data — a session the record names for
    itself — but it is still free text. It is dropped when redaction alters it
    or when it is a bare secret-shaped hex run, so a credential parked in a
    session field can never reach the payload or be echoed back by a filter.
    """
    if value is None:
        return None
    text = str(value).strip()
    if not text:
        return None
    if _BARE_SECRET_SHAPE.match(text):
        return None
    return text if _redact(text) == text else None


def _safe_record_id(value: Any) -> str | None:
    """Return a source record id only when it is a plain numeric identifier.

    ``event_key`` is serialized verbatim and is the deterministic pagination
    tiebreak, so an id is interpolated into it only when it has the shape both
    real sources actually produce. A record whose identity cannot be trusted is
    refused by the caller rather than keyed on.
    """
    if value is None or isinstance(value, bool):
        return None
    if isinstance(value, int):
        return str(value)
    text = str(value).strip()
    return text if _RECORD_ID_SHAPE.match(text) else None


def _safe_cp_event_type(value: Any) -> tuple[str, bool]:
    """Validate a stored control-plane event type. Returns ``(type, unsafe)``.

    The stored value is externally influenced — whichever producer wrote the
    ``events`` row chose the string — and ``to_dict`` serializes it verbatim, so
    it passes a boundary of its own instead of relying on the one ``message``
    passes. A value survives only when it is an ordinary identifier, is not a
    bare secret-shaped hex run, and is unchanged by a redaction pass. Anything
    else fails closed to :data:`UNSAFE_EVENT_TYPE`: the record stays visible as
    an audit entry, but the value itself is never republished — not verbatim,
    not partially sanitized, and not rewritten into some other valid-looking
    type that would misdescribe what happened.
    """
    text = ("" if value is None else str(value)).strip()
    if not text:
        return ("", False)
    if _BARE_SECRET_SHAPE.match(text):
        return (UNSAFE_EVENT_TYPE, True)
    if not _CP_EVENT_TYPE_SHAPE.match(text):
        return (UNSAFE_EVENT_TYPE, True)
    if _redact(text) != text:
        return (UNSAFE_EVENT_TYPE, True)
    return (text, False)


def _safe_echo(value: Any) -> Any:
    """Guard a scalar that is echoed back rather than derived from a record.

    Query scope and filter values are caller-supplied and are reflected in the
    response so an operator can see what was asked. Reflection is still
    emission: a value redaction would alter, or a bare secret-shaped hex run, is
    replaced by the placeholder instead of being echoed verbatim. Ordinary
    scope and filter values pass through untouched.
    """
    if value is None or isinstance(value, (int, bool)):
        return value
    text = str(value)
    if _BARE_SECRET_SHAPE.match(text.strip()):
        return console_redaction.REDACTED
    return _redact(text)


def adapt_cp_events(rows: Iterable[dict[str, Any]]) -> list[WorkflowEvent]:
    """Adapt control-plane ``events`` rows (joined to work_items) into events.

    Each row is expected to carry ``event_id``, ``event_type``, ``message``,
    ``created_at`` and the joined work-item ``kind``/``number``. Rows missing
    an id or type are skipped so a partially written table never raises.
    """
    events: list[WorkflowEvent] = []
    for row in rows or []:
        try:
            event_id = _safe_record_id(row.get("event_id"))
            raw_event_type = (row.get("event_type") or "").strip()
            if event_id is None or not raw_event_type:
                continue
            # The stored type is source data, not a trusted constant: validate
            # it before it is serialized, exactly as `message` below is redacted
            # before it is serialized.
            event_type, event_type_unsafe = _safe_cp_event_type(raw_event_type)
            kind = row.get("kind")
            number = row.get("number")
            issue_no, pr_no = _kind_to_numbers(kind, number)
            sensitive = event_type_unsafe or any(
                hint in raw_event_type.lower() for hint in _SENSITIVE_EVENT_HINTS
            )
            events.append(
                WorkflowEvent(
                    source=SOURCE_CONTROL_PLANE,
                    event_type=event_type,
                    event_key=f"cp:{event_id}",
                    timestamp=_parse_ts(row.get("created_at")),
                    issue_number=issue_no,
                    pr_number=pr_no,
                    # No session_id: the control-plane events table is
                    # (event_id, work_item_id, event_type, message, created_at)
                    # and records no session. Inventing one from the work item
                    # or the message text would be a guess, so this source
                    # declares the session dimension unsupported instead
                    # (_SOURCE_FILTER_SUPPORT) and the query layer refuses a
                    # session filter it cannot honestly answer.
                    message=_redact(row.get("message")),
                    correlation_id=_correlation_for(kind, number),
                    sensitive=sensitive,
                )
            )
        except Exception:
            # A single malformed row must not sink the whole adaptation.
            continue
    return events


def adapt_cth_comments(
    comments: Iterable[dict[str, Any]],
    *,
    kind: str,
    number: int,
) -> list[WorkflowEvent]:
    """Adapt Gitea Canonical Thread Handoff (CTH) comments into events.

    Only comments that parse as a CTH (``canonical_thread_handoff.parse_cth_comment``)
    become events; ordinary comments are ignored. ``kind``/``number`` scope the
    events to the issue or PR the comments belong to.
    """
    # Imported lazily so this module has no import-time dependency on the
    # handoff parser when only the control-plane adapter is used.
    from canonical_thread_handoff import is_known_cth_type, parse_cth_comment

    # ``kind``/``number`` are interpolated into event_key and correlation_id, so
    # they are normalised once here. A scope this adapter cannot express is
    # refused outright rather than serialized into an identifier.
    kind = (kind or "").strip().lower()
    if kind not in ("issue", "pr"):
        return []
    try:
        number = int(number)
    except (TypeError, ValueError):
        return []

    issue_no, pr_no = _kind_to_numbers(kind, number)
    correlation = _correlation_for(kind, number)
    events: list[WorkflowEvent] = []
    for comment in comments or []:
        try:
            body = comment.get("body") or ""
            parsed = parse_cth_comment(body)
            if not parsed:
                continue
            fields = parsed.get("fields") or {}
            cth_type = parsed.get("cth_type") or ""
            comment_id = _safe_record_id(comment.get("id"))
            if comment_id is None:
                continue
            # The CTH heading is free text: the parser accepts whatever follows
            # "## CTH:", and only the write and assess paths check it against
            # the contract. Check it here too — an unrecognised heading is
            # reported as such rather than serialized into event_type, so
            # arbitrary, malformed, or secret-shaped heading content has no way
            # through. Declared types are preserved exactly.
            cth_type_known = is_known_cth_type(cth_type)
            # Redaction runs first, and every derived value is taken from the
            # redacted text — deriving evidence refs from the raw proof would
            # re-emit exactly what redaction was about to remove.
            decision = _redact(fields.get("decision"))
            proof = _redact(fields.get("proof"))
            next_action = _redact(fields.get("next action"))
            refs, refs_dropped = _validated_evidence_refs(
                _extract_evidence_refs(proof, decision)
            )
            events.append(
                WorkflowEvent(
                    source=SOURCE_GITEA_HANDOFF,
                    event_type=(
                        f"handoff:{cth_type.strip()}"
                        if cth_type_known
                        else UNKNOWN_HANDOFF_EVENT_TYPE
                    ),
                    event_key=f"cth:{kind}:{number}:{comment_id}",
                    timestamp=_parse_ts(comment.get("created_at")),
                    actor=_redact((comment.get("user") or {}).get("login")),
                    role=_redact(fields.get("next owner")),
                    issue_number=issue_no,
                    pr_number=pr_no,
                    # A CTH names its own session when the producer records one;
                    # it is read from that declared field, never inferred from
                    # unrelated text.
                    session_id=_safe_session_id(fields.get("session")),
                    decision=decision,
                    message=next_action or _redact(fields.get("status")),
                    correlation_id=correlation,
                    evidence_refs=refs,
                    sensitive=refs_dropped or not cth_type_known,
                )
            )
        except Exception:
            continue
    return events


# --------------------------------------------------------------------------- #
# Read-only control-plane event source.                                       #
# --------------------------------------------------------------------------- #

_CP_EVENTS_QUERY = """
SELECT e.event_id   AS event_id,
       e.event_type AS event_type,
       e.message    AS message,
       e.created_at AS created_at,
       w.kind       AS kind,
       w.number     AS number
FROM events e
JOIN work_items w ON e.work_item_id = w.work_item_id
WHERE w.remote = ? AND w.org = ? AND w.repo = ?
"""


@dataclass(frozen=True)
class SourceStatus:
    """Fail-soft status for one timeline source.

    ``supported_filters`` states which filter dimensions this source's records
    can carry; ``unsupported_filters`` names the requested dimensions it cannot,
    so an operator can see *why* a source contributed nothing rather than being
    left to read an empty list as an absence of activity.
    """

    name: str
    ok: bool
    reason: str | None = None
    count: int = 0
    supported_filters: tuple[str, ...] = ()
    unsupported_filters: tuple[str, ...] = ()

    def to_dict(self) -> dict[str, Any]:
        return {
            "name": self.name,
            "ok": self.ok,
            "reason": self.reason,
            "count": self.count,
            "supported_filters": list(self.supported_filters),
            "unsupported_filters": list(self.unsupported_filters),
        }


def _cp_status(*, ok: bool, reason: str | None = None, count: int = 0) -> SourceStatus:
    return SourceStatus(
        SOURCE_CONTROL_PLANE,
        ok=ok,
        # A failure reason is serialized like any other field and is often an
        # exception string carrying a path or a transport error, so it crosses
        # the redaction boundary too. Static reasons pass through unchanged.
        reason=_redact(reason),
        count=count,
        supported_filters=_SOURCE_FILTER_SUPPORT[SOURCE_CONTROL_PLANE],
    )


def _handoff_status(*, ok: bool, reason: str | None = None, count: int = 0) -> SourceStatus:
    return SourceStatus(
        SOURCE_GITEA_HANDOFF,
        ok=ok,
        # Same boundary as the control-plane status: this reason can quote an
        # error raised by a live authenticated fetch.
        reason=_redact(reason),
        count=count,
        supported_filters=_SOURCE_FILTER_SUPPORT[SOURCE_GITEA_HANDOFF],
    )


def read_cp_events(
    *,
    remote: str,
    org: str,
    repo: str,
    db_path: str | None = None,
) -> tuple[list[WorkflowEvent], SourceStatus]:
    """Read scoped control-plane events read-only. Never creates the DB.

    Opens the SQLite file through a ``mode=ro`` URI: a health/timeline read
    must never create directories or run the schema migration that
    ``ControlPlaneDB()`` performs on construction. A missing or unreadable DB
    degrades to a status with a reason.
    """
    path = (db_path or control_plane_db.default_db_path()).strip()
    conn: sqlite3.Connection | None = None
    try:
        conn = sqlite3.connect(f"file:{path}?mode=ro", uri=True)
        conn.row_factory = sqlite3.Row
        cursor = conn.execute(_CP_EVENTS_QUERY, (remote, org, repo))
        rows = [dict(r) for r in cursor.fetchall()]
    except sqlite3.OperationalError as exc:
        return ([], _cp_status(ok=False, reason=f"control-plane DB unavailable: {exc}"))
    except sqlite3.Error as exc:
        return ([], _cp_status(ok=False, reason=f"control-plane read failed: {exc}"))
    finally:
        if conn is not None:
            conn.close()
    events = adapt_cp_events(rows)
    return (events, _cp_status(ok=True, count=len(events)))


# --------------------------------------------------------------------------- #
# Filter, sort, paginate.                                                     #
# --------------------------------------------------------------------------- #


def filter_events(
    events: Iterable[WorkflowEvent],
    *,
    issue: int | None = None,
    pr: int | None = None,
    session: str | None = None,
) -> list[WorkflowEvent]:
    """Filter events by issue number, PR number, and/or session id.

    Filters are conjunctive. A filter that names a dimension an event does not
    carry excludes that event (an issue filter excludes PR-only events).
    """
    out: list[WorkflowEvent] = []
    for ev in events:
        if issue is not None and ev.issue_number != issue:
            continue
        if pr is not None and ev.pr_number != pr:
            continue
        if session is not None and ev.session_id != session:
            continue
        out.append(ev)
    return out


def sort_events(events: Iterable[WorkflowEvent]) -> list[WorkflowEvent]:
    """Return events in stable timeline order (ascending)."""
    return sorted(events, key=lambda ev: ev.sort_key())


@dataclass(frozen=True)
class TimelinePage:
    """One page of the sorted, filtered timeline."""

    events: tuple[WorkflowEvent, ...]
    total: int
    limit: int
    offset: int

    @property
    def next_offset(self) -> int | None:
        nxt = self.offset + len(self.events)
        return nxt if nxt < self.total else None

    def to_dict(self) -> dict[str, Any]:
        return {
            "events": [ev.to_dict() for ev in self.events],
            "pagination": {
                "total": self.total,
                "limit": self.limit,
                "offset": self.offset,
                "returned": len(self.events),
                "next_offset": self.next_offset,
                "has_more": self.next_offset is not None,
            },
        }


_MAX_LIMIT = 500
_DEFAULT_LIMIT = 50


def _coerce_bounds(limit: int | None, offset: int | None) -> tuple[int, int]:
    try:
        lim = int(limit) if limit is not None else _DEFAULT_LIMIT
    except (TypeError, ValueError):
        lim = _DEFAULT_LIMIT
    try:
        off = int(offset) if offset is not None else 0
    except (TypeError, ValueError):
        off = 0
    lim = max(1, min(lim, _MAX_LIMIT))
    off = max(0, off)
    return (lim, off)


def paginate(events: list[WorkflowEvent], *, limit: int | None, offset: int | None) -> TimelinePage:
    lim, off = _coerce_bounds(limit, offset)
    window = events[off : off + lim]
    return TimelinePage(events=tuple(window), total=len(events), limit=lim, offset=off)


# --------------------------------------------------------------------------- #
# Composition — load_timeline aggregates all sources, fail-soft.              #
# --------------------------------------------------------------------------- #

# A comment source is a callable that, given (kind, number), returns the raw
# Gitea comment list for that issue/PR. The route supplies a live fail-soft
# fetcher; tests supply a fixture. When None, the handoff source is reported as
# not-run (never silently empty-and-healthy).
CommentSource = Callable[[str, int], list[dict[str, Any]]]


@dataclass(frozen=True)
class TimelineSnapshot:
    """One answered timeline query.

    ``ok`` is False when the query could not be answered as asked — currently
    when a requested filter dimension no surviving source can carry was
    supplied. The page is then empty *and* the snapshot says so, because an
    ``ok`` empty page is a claim that no such activity exists.
    """

    schema_version: int
    remote: str
    org: str
    repo: str
    filters: dict[str, Any]
    page: TimelinePage
    sources: tuple[SourceStatus, ...]
    ok: bool = True
    error: dict[str, Any] | None = None

    def to_dict(self) -> dict[str, Any]:
        return {
            "ok": self.ok,
            "error": self.error,
            "schema_version": self.schema_version,
            # Scope and filters are echoed caller input, not derived record
            # data. Reflecting a value is still emitting it, so both cross the
            # same boundary; ordinary scope and filter values are unchanged.
            "scope": {
                "remote": _safe_echo(self.remote),
                "org": _safe_echo(self.org),
                "repo": _safe_echo(self.repo),
            },
            "filters": {key: _safe_echo(value) for key, value in self.filters.items()},
            "sources": [s.to_dict() for s in self.sources],
            **self.page.to_dict(),
        }


def _unanswerable_reasons(
    statuses: Iterable[SourceStatus], unanswerable: Iterable[str]
) -> list[dict[str, str]]:
    """Explain, per source, why each unanswerable dimension went unanswered."""
    out: list[dict[str, str]] = []
    for status in statuses:
        for dim in unanswerable:
            if dim not in status.supported_filters:
                reason = _SOURCE_FILTER_LIMITS.get(
                    (status.name, dim), f"this source's records carry no {dim} identity"
                )
            elif not status.ok:
                reason = (
                    f"this source can carry {dim} but did not run: "
                    f"{status.reason or 'unavailable'}"
                )
            else:
                continue
            out.append({"source": status.name, "filter": dim, "reason": reason})
    return out


def load_timeline(
    *,
    remote: str,
    org: str,
    repo: str,
    issue: int | None = None,
    pr: int | None = None,
    session: str | None = None,
    limit: int | None = None,
    offset: int | None = None,
    db_path: str | None = None,
    comment_source: CommentSource | None = None,
) -> TimelineSnapshot:
    """Aggregate every timeline source into one filtered, paginated snapshot.

    Sources are read independently and fail soft: an unavailable source
    contributes a ``SourceStatus`` with ``ok=False`` and a reason, and never
    collapses the whole timeline. The handoff source only runs when a specific
    issue or PR is requested (a handoff comment belongs to one thread) and a
    ``comment_source`` is available; otherwise it is reported as ``not run``
    rather than as an empty-and-healthy source.

    A filter dimension that no surviving source can carry — a ``session``
    filter when the only source that ran is the control plane, whose events
    record no session — is refused with ``ok=False`` and a structured error
    instead of being answered with an empty page.
    """
    all_events: list[WorkflowEvent] = []
    statuses: list[SourceStatus] = []

    cp_events, cp_status = read_cp_events(remote=remote, org=org, repo=repo, db_path=db_path)
    all_events.extend(cp_events)
    statuses.append(cp_status)

    # Gitea handoff comments are thread-scoped: only fetch when the caller
    # narrowed to one issue or PR, and only when a source was provided.
    handoff_target: tuple[str, int] | None = None
    if pr is not None:
        handoff_target = ("pr", pr)
    elif issue is not None:
        handoff_target = ("issue", issue)

    if handoff_target is None:
        statuses.append(
            _handoff_status(
                ok=False,
                reason="not run: handoff comments are thread-scoped; filter by issue or pr to include them",
            )
        )
    elif comment_source is None:
        statuses.append(
            _handoff_status(
                ok=False,
                reason="not run: no comment source configured for this timeline read",
            )
        )
    else:
        kind, number = handoff_target
        try:
            comments = comment_source(kind, number) or []
            handoff_events = adapt_cth_comments(comments, kind=kind, number=number)
            all_events.extend(handoff_events)
            statuses.append(_handoff_status(ok=True, count=len(handoff_events)))
        except Exception as exc:  # fail soft: a fetch/parse error degrades this source only
            statuses.append(_handoff_status(ok=False, reason=f"handoff source failed: {exc}"))

    requested = tuple(
        name
        for name, value in ((FILTER_ISSUE, issue), (FILTER_PR, pr), (FILTER_SESSION, session))
        if value is not None
    )
    statuses = [
        replace(
            status,
            unsupported_filters=tuple(
                dim for dim in requested if dim not in status.supported_filters
            ),
        )
        for status in statuses
    ]
    filters = {"issue": issue, "pr": pr, "session": session}

    # A dimension is answerable only if a source that actually ran can carry it.
    # If none can, refuse: an empty page would assert "no such activity", which
    # is a claim this timeline is not in a position to make.
    answerable: set[str] = set()
    for status in statuses:
        if status.ok:
            answerable.update(status.supported_filters)
    unanswerable = tuple(dim for dim in requested if dim not in answerable)

    if unanswerable:
        return TimelineSnapshot(
            schema_version=TIMELINE_SCHEMA_VERSION,
            remote=remote,
            org=org,
            repo=repo,
            filters=filters,
            page=paginate([], limit=limit, offset=offset),
            sources=tuple(statuses),
            ok=False,
            error={
                "code": "filter_not_supported",
                "unsupported_filters": list(unanswerable),
                "detail": (
                    "no timeline source that ran can answer "
                    + ", ".join(f"'{dim}'" for dim in unanswerable)
                    + "; the result is refused rather than returned empty"
                ),
                "sources": _unanswerable_reasons(statuses, unanswerable),
            },
        )

    filtered = filter_events(all_events, issue=issue, pr=pr, session=session)
    ordered = sort_events(filtered)
    page = paginate(ordered, limit=limit, offset=offset)

    return TimelineSnapshot(
        schema_version=TIMELINE_SCHEMA_VERSION,
        remote=remote,
        org=org,
        repo=repo,
        filters=filters,
        page=page,
        sources=tuple(statuses),
    )


def snapshot_to_dict(snapshot: TimelineSnapshot) -> dict[str, Any]:
    return snapshot.to_dict()
