"""A rule's edit history — the snapshot taken when an edit overwrites text.

Sibling of `note_versions`, and the differences are the whole design.

WHAT A VERSION IS FOR. It records what a rule USED TO SAY, because an update
overwrites that and nothing can recompute it. It is not a record of the rule's
lifecycle: a soft delete keeps the rule row whole and restorable, so there is
nothing to preserve at delete time, and anything that outlived a purge would be
data the operator explicitly asked to be gone.

WHAT IS DELIBERATELY NOT COPIED FROM note_versions (milestone 323). Both of its
guards defend against note autosave, which fires every 60 seconds and would
otherwise burn every slot. Rules have no autosave — every version here comes
from a deliberate `update_rule` — so both guards would only ever discard a real
edit:

- No `MIN_VERSION_INTERVAL_SECONDS`. Two deliberate edits four minutes apart
  are two edits.
- No `MAX_VERSIONS` and no pruning. A rule is edited a handful of times in its
  life; a cap invites losing the one edit somebody needed.

TEXT ONLY. A rule's topic, its Systems tags and its typed edges are not here.
They have their own lifecycle, and folding them in would make one word,
"version", mean two things — the rule's wording, and the rule's place in the
graph.
"""
from sqlalchemy import select

from scribe.models import async_session
from scribe.models.rule_version import RuleVersion

# The columns a version carries, and therefore the ones whose change is worth
# a snapshot. `order_index` and `arose_from_id` are editable through
# update_rule and are deliberately absent: reordering a rulebook is not an
# edit to what any rule says, and a run of "moved rule 4 above rule 3"
# snapshots would bury the edits somebody is actually looking for.
SNAPSHOT_FIELDS = (
    "title", "statement", "why", "how_to_apply", "when_to_apply",
    "tier", "verify_with", "expires_when",
)


def snapshot(rule) -> dict:
    """The rule's text right now, as a plain dict.

    Plain values rather than the ORM object because the caller has to hold
    this ACROSS the mutation — a live `rule` would show the new text by the
    time it was read, which is the one thing the snapshot must not do.
    """
    return {f: getattr(rule, f) for f in SNAPSHOT_FIELDS}


def record_if_changed(session, rule, actor_user_id: int | None, before: dict):
    """Add a version holding `before`, unless the edit changed no text.

    Git semantics, and not a nicety: `update_rule` is reached by callers that
    resend every field, so without this a form saved twice would file a second
    identical snapshot and the history would stop reading as a list of edits.

    Session-BOUND, unlike `note_versions.create_version` which opens its own.
    The version and the edit that caused it commit together or not at all;
    a separate session could leave a snapshot behind after the update it
    describes had failed, which is a history entry for an edit that never
    happened.
    """
    if snapshot(rule) == before:
        return None
    version = RuleVersion(
        rule_id=rule.id, user_id=actor_user_id, **before,
    )
    session.add(version)
    return version


async def list_versions(rule_id: int) -> list[RuleVersion]:
    """Newest first — the reader's question is "what did this just say?"."""
    async with async_session() as session:
        return list((await session.execute(
            select(RuleVersion)
            .where(RuleVersion.rule_id == rule_id)
            .order_by(RuleVersion.created_at.desc(), RuleVersion.id.desc())
        )).scalars().all())
