"""PO persistence: merge-writes to the purchase-orders DynamoDB table.

Owns the SET-only merge path (_write_fields / _merge_update) with the sticky
"Cancelled" ConditionExpression guard, the collapsed _save_merge behind the
save_new_po/save_revision public wrappers, and save_cancellation. Uses a lazily
built, cached DynamoDB resource kept under the public name ``dynamodb`` so the
tests' setattr(persistence, "dynamodb", fake) patch surface is unchanged.
"""

import logging
import os
from datetime import datetime, timezone

import boto3

logger = logging.getLogger()
logger.setLevel(logging.INFO)

PO_TABLE = os.environ.get("PO_TABLE", "purchase-orders")

# "Cancelled" is a sticky, authoritative status: once a PO reaches it, a later
# new_po/revision may enrich other fields but must never move it back to a
# non-cancelled status.
CANCELLED_STATUS = "Cancelled"

# Lazily-built, cached DynamoDB resource. Public name ``dynamodb`` is preserved
# so the monkeypatch attribute is unchanged; building at first CALL (not import)
# keeps the moto-before-handler invariant and honors any patched fake.
dynamodb = None


def _get_dynamodb():
    global dynamodb
    if dynamodb is None:
        dynamodb = boto3.resource("dynamodb")
    return dynamodb


def _write_fields(po_number: str, fields: dict, *, guard_cancelled: bool):
    """SET the given non-null fields on a PO record via update_item.

    Only the fields supplied are written; absent fields are left untouched, so a
    partial payload can never delete data that an earlier email established. The
    record is created if it does not exist (DynamoDB update_item upsert).

    When ``guard_cancelled`` is True the write carries a ConditionExpression that
    only permits it while the record is not already Cancelled. The condition is
    evaluated atomically by DynamoDB at write time, so a cancellation that lands
    first always wins — there is no read-then-write TOCTOU window. A failed guard
    raises ConditionalCheckFailedException for the caller to handle.
    """
    table = _get_dynamodb().Table(PO_TABLE)

    set_parts = []
    attr_names = {}
    attr_values = {}
    for key, value in fields.items():
        if value is None or key == "po_number":
            continue
        name_ph = f"#{key}"
        val_ph = f":{key}"
        attr_names[name_ph] = key
        attr_values[val_ph] = value
        set_parts.append(f"{name_ph} = {val_ph}")

    if not set_parts:
        return

    params = {
        "Key": {"po_number": po_number},
        "UpdateExpression": "SET " + ", ".join(set_parts),
        "ExpressionAttributeNames": attr_names,
        "ExpressionAttributeValues": attr_values,
    }
    if guard_cancelled:
        params["ExpressionAttributeValues"][":__cancelled_marker"] = CANCELLED_STATUS
        params["ConditionExpression"] = (
            "attribute_not_exists(po_status) OR po_status <> :__cancelled_marker"
        )

    table.update_item(**params)


def _merge_update(po_number: str, fields: dict):
    """Merge (SET-only) the given fields onto a PO record, keeping Cancelled sticky.

    Only the fields supplied are written; absent fields are left untouched. The
    record is created if it does not exist (DynamoDB update_item upsert).

    "Cancelled" is a sticky, authoritative status. When the incoming payload
    carries a non-cancelled ``po_status``, the write is guarded by a
    ConditionExpression so the status is only applied while the record is not
    already Cancelled — enforced atomically at write time, eliminating the
    read-then-write TOCTOU where a concurrently-landing cancellation could be
    silently un-cancelled. If the guard fails (the PO is already Cancelled), the
    same fields are re-written WITHOUT po_status/cancelled_at and
    unconditionally, so the other fields still merge while the Cancelled status
    stays intact.

    A payload with no ``po_status``, or one whose status is already "Cancelled",
    needs no guard — a plain merge is correct. This is what keeps legitimate
    status updates (non-cancelled PO) and status-less revisions from ever being
    dropped: the status is only ever suppressed on a true un-cancel transition.
    """
    incoming_status = fields.get("po_status")
    if incoming_status is None or incoming_status == CANCELLED_STATUS:
        _write_fields(po_number, fields, guard_cancelled=False)
        return

    try:
        _write_fields(po_number, fields, guard_cancelled=True)
    except _get_dynamodb().meta.client.exceptions.ConditionalCheckFailedException:
        logger.info(
            f"PO {po_number} is Cancelled; suppressing incoming "
            f"po_status={incoming_status!r} and merging remaining fields"
        )
        enrich_fields = {
            k: v for k, v in fields.items() if k not in ("po_status", "cancelled_at")
        }
        _write_fields(po_number, enrich_fields, guard_cancelled=False)


def _save_merge(parsed: dict, log_verb: str):
    po_number = parsed["po_number"]
    fields = {k: v for k, v in parsed.items() if v is not None}
    _merge_update(po_number, fields)
    logger.info(f"{log_verb} PO {po_number}")


def save_new_po(parsed: dict):
    """Create a PO, merging into any pre-existing record.

    Uses a merge update rather than a conditional put so that an out-of-order
    cancellation (which leaves a Cancelled skeleton) is filled in with the full
    PO data instead of the new_po being silently dropped. "Cancelled" is a sticky
    status enforced atomically inside _merge_update: if the PO was already
    cancelled, the new_po backfills its remaining fields (supplier, line_items,
    amounts) but never un-cancels it.
    """
    _save_merge(parsed, "Created/merged")


def save_revision(parsed: dict):
    """Merge revised data into an existing PO without deleting omitted fields.

    A revision email often omits unchanged sections (line_items, supplier). The
    previous full-overwrite put_item permanently dropped those. This SETs only the
    fields present in the revision, leaving everything else intact.

    "Cancelled" is a sticky status: a revision may enrich a cancelled PO's fields
    but must never move it to a non-cancelled status. That invariant is enforced
    atomically inside _merge_update and applies ONLY to the un-cancel transition —
    a revision that carries no status change, or one targeting a non-cancelled PO,
    updates po_status normally.
    """
    _save_merge(parsed, "Revised")


def save_cancellation(parsed: dict):
    """Mark a PO Cancelled, creating a minimal skeleton if it doesn't exist yet.

    If the cancellation arrives before the new_po, the skeleton it creates is
    later backfilled by save_new_po (which preserves this Cancelled status), so no
    PO data is lost on out-of-order delivery.
    """
    table = _get_dynamodb().Table(PO_TABLE)

    table.update_item(
        Key={"po_number": parsed["po_number"]},
        UpdateExpression="SET po_status = :status, cancelled_at = :cancelled_at, raw_s3_key = :s3_key",
        ExpressionAttributeValues={
            ":status": "Cancelled",
            ":cancelled_at": parsed.get(
                "processed_at", datetime.now(timezone.utc).isoformat()
            ),
            ":s3_key": parsed.get("raw_s3_key", ""),
        },
    )
    logger.info(f"Cancelled PO {parsed['po_number']}")