// Package vectorstore stores brain note embeddings in pgvector on the
// shared postgres18 instance. One row per markdown path, cosine-distance
// indexed via HNSW for sub-millisecond top-k retrieval.
package vectorstore

import (
	"context"
	"errors"
	"fmt"
	"strings"
	"time"

	"github.com/jackc/pgx/v5"
	"github.com/jackc/pgx/v5/pgxpool"
)

// Hit is a single result from a cosine-distance search.
type Hit struct {
	Path     string
	Distance float64 // 0 = identical, 2 = opposite
}

// PGStore is a pgvector-backed embeddings store. Construct with New and
// call Init once to create the table + HNSW index. Use Close to release
// the underlying pool.
type PGStore struct {
	pool *pgxpool.Pool
}

// New opens a connection pool against dsn (a libpq-style URL). Caller
// owns the resulting *PGStore and must invoke Close.
func New(ctx context.Context, dsn string) (*PGStore, error) {
	pool, err := pgxpool.New(ctx, dsn)
	if err != nil {
		return nil, fmt.Errorf("pgxpool: %w", err)
	}
	if err := pool.Ping(ctx); err != nil {
		pool.Close()
		return nil, fmt.Errorf("ping: %w", err)
	}
	return &PGStore{pool: pool}, nil
}

// Close releases the underlying connection pool.
func (s *PGStore) Close() {
	if s.pool != nil {
		s.pool.Close()
	}
}

// Init creates the brain_embeddings table and its HNSW index if they
// don't already exist. Safe to call on every startup. Assumes the
// `vector` extension is already installed (one-time DBA setup; see
// scripts/brain-embeddings-init.sql).
func (s *PGStore) Init(ctx context.Context) error {
	const ddl = `
CREATE TABLE IF NOT EXISTS brain_embeddings (
    path       TEXT PRIMARY KEY,
    embedding  vector(768) NOT NULL,
    updated_at TIMESTAMPTZ NOT NULL DEFAULT now()
);
CREATE INDEX IF NOT EXISTS brain_embeddings_embedding_idx
    ON brain_embeddings USING hnsw (embedding vector_cosine_ops);
`
	_, err := s.pool.Exec(ctx, ddl)
	return err
}

// Upsert inserts or replaces the embedding for path. Embedding must be
// 768-dim (nomic-embed-text). Caller is responsible for normalising
// paths to forward-slash form.
func (s *PGStore) Upsert(ctx context.Context, path string, embedding []float32) error {
	if len(embedding) != 768 {
		return fmt.Errorf("expected 768-dim embedding, got %d", len(embedding))
	}
	_, err := s.pool.Exec(ctx, `
        INSERT INTO brain_embeddings (path, embedding, updated_at)
        VALUES ($1, $2, now())
        ON CONFLICT (path) DO UPDATE
        SET embedding = EXCLUDED.embedding, updated_at = now()
    `, path, vectorLiteral(embedding))
	return err
}

// Delete removes the row at path. No-op when the row doesn't exist.
func (s *PGStore) Delete(ctx context.Context, path string) error {
	_, err := s.pool.Exec(ctx, `DELETE FROM brain_embeddings WHERE path = $1`, path)
	return err
}

// Search returns the top-limit nearest paths by cosine distance.
func (s *PGStore) Search(ctx context.Context, query []float32, limit int) ([]Hit, error) {
	if len(query) != 768 {
		return nil, fmt.Errorf("expected 768-dim query, got %d", len(query))
	}
	if limit <= 0 {
		limit = 10
	}
	rows, err := s.pool.Query(ctx, `
        SELECT path, embedding <=> $1 AS distance
        FROM brain_embeddings
        ORDER BY embedding <=> $1
        LIMIT $2
    `, vectorLiteral(query), limit)
	if err != nil {
		return nil, fmt.Errorf("query: %w", err)
	}
	defer rows.Close()

	var hits []Hit
	for rows.Next() {
		var h Hit
		if err := rows.Scan(&h.Path, &h.Distance); err != nil {
			return nil, fmt.Errorf("scan: %w", err)
		}
		hits = append(hits, h)
	}
	if err := rows.Err(); err != nil && !errors.Is(err, pgx.ErrNoRows) {
		return nil, err
	}
	return hits, nil
}

// KnownPathsWithTime returns every embedded chunk path paired with the
// row's updated_at. Sync uses the timestamps to decide whether a file
// has been edited since its chunks were last embedded — when the file's
// mtime exceeds the oldest chunk's updated_at, the file is re-embedded.
func (s *PGStore) KnownPathsWithTime(ctx context.Context) (map[string]time.Time, error) {
	rows, err := s.pool.Query(ctx, `SELECT path, updated_at FROM brain_embeddings`)
	if err != nil {
		return nil, fmt.Errorf("query paths: %w", err)
	}
	defer rows.Close()
	out := make(map[string]time.Time)
	for rows.Next() {
		var (
			p string
			t time.Time
		)
		if err := rows.Scan(&p, &t); err != nil {
			return nil, err
		}
		out[p] = t
	}
	return out, rows.Err()
}

// vectorLiteral renders a Go float32 slice as the literal representation
// pgvector accepts as a parametric input: `[v1,v2,...,vN]`.
func vectorLiteral(v []float32) string {
	var b strings.Builder
	b.WriteByte('[')
	for i, x := range v {
		if i > 0 {
			b.WriteByte(',')
		}
		fmt.Fprintf(&b, "%g", x)
	}
	b.WriteByte(']')
	return b.String()
}