package store

import (
	"context"
	"database/sql"
	"errors"
	"fmt"
)

// metaKeyEmbedderID names the embedder that wrote the stored vectors.
//
// Why one value for the whole DB and not a column on every vector row: the
// vectors are only ever rewritten all at once (one backfill re-embeds every
// note and fact together), so a per-row marker would hold the same string in
// every row and cost a column on two tables for nothing.
const metaKeyEmbedderID = "embedder_id"

// Meta reads a single value from the meta table. Missing key ⇒ empty string.
func (s *Store) Meta(ctx context.Context, key string) (string, error) {
	var v string
	err := s.db.QueryRowContext(ctx, `SELECT value FROM meta WHERE key = ?`, key).Scan(&v)
	if errors.Is(err, sql.ErrNoRows) {
		return "", nil
	}
	if err != nil {
		return "", fmt.Errorf("read meta %s: %w", key, err)
	}
	return v, nil
}

// SetMeta writes (or overwrites) a single meta value.
func (s *Store) SetMeta(ctx context.Context, key, value string) error {
	_, err := s.db.ExecContext(ctx,
		`INSERT INTO meta (key, value) VALUES (?,?)
		 ON CONFLICT(key) DO UPDATE SET value = excluded.value`, key, value)
	if err != nil {
		return fmt.Errorf("write meta %s: %w", key, err)
	}
	return nil
}

// CheckEmbedder compares the embedder now configured against the one that
// wrote the stored vectors.
//
// A DB that has never recorded one is claimed for the current embedder: either
// it is fresh (nothing stored yet, nothing to fix) or it predates this marker.
// Returns the stored id and whether it differs from the current one.
//
// Vectors from two different models live in different spaces, so cosine
// between them is noise rather than a low score — and both of our models are
// 384-dimensional, so nothing else catches it.
//
// TODO(#378): on a mismatch, run the one-shot backfill here — re-embed every
// stored note and fact text with the current embedder (EmbedPassage side),
// write the vectors back, then SetMeta the current id.
func (s *Store) CheckEmbedder(ctx context.Context, currentID string) (stored string, mismatch bool, err error) {
	stored, err = s.Meta(ctx, metaKeyEmbedderID)
	if err != nil {
		return "", false, err
	}
	if stored == "" {
		return currentID, false, s.SetMeta(ctx, metaKeyEmbedderID, currentID)
	}
	return stored, stored != currentID, nil
}