package core

import (
	"database/sql"
	"fmt"
	"net/url"
	"strings"
	"time"

	// Pure-Go SQLite driver. No cgo means CGO_ENABLED=0 static binaries and
	// trivial cross-compilation for arm64.
	_ "modernc.org/sqlite"
)

// pragmas applied to every pooled connection, via the DSN.
//
// These are passed in the DSN rather than run as statements after connecting
// because database/sql pools connections: a one-off PRAGMA would configure
// only whichever single connection happened to serve it.
var sqlitePragmas = []string{
	"journal_mode(WAL)", // concurrent readers during a scan
	// busy_timeout is deliberately generous. Chunked scans hold the single
	// write lock in short bursts, so a concurrent write is never blocked for
	// long — but it is blocked, and measured tail latency on a first scan of
	// 15,000 new files reached 3.3s. At the old 5s ceiling that left almost no
	// margin, and on slower storage a progress save would fail outright with
	// SQLITE_BUSY and the user would lose their place. Waiting is strictly
	// better than failing: the write still succeeds, just later.
	"busy_timeout(15000)",
	"foreign_keys(1)",     // enforce the schema's referential integrity
	"synchronous(NORMAL)", // safe under WAL, far faster than FULL
	"cache_size(-32000)",  // ~32 MB page cache
}

// OpenDB opens the SQLite database with the pragmas Vellum depends on.
func OpenDB(path string) (*sql.DB, error) {
	db, err := sql.Open("sqlite", buildDSN(path, sqlitePragmas))
	if err != nil {
		return nil, fmt.Errorf("open sqlite %s: %w", path, err)
	}

	// SQLite is a single-writer database. A large pool just produces
	// SQLITE_BUSY contention; a small pool with a busy_timeout is faster.
	db.SetMaxOpenConns(8)
	db.SetMaxIdleConns(8)
	db.SetConnMaxLifetime(0)
	db.SetConnMaxIdleTime(5 * time.Minute)

	if err := db.Ping(); err != nil {
		_ = db.Close()
		return nil, fmt.Errorf("ping sqlite %s: %w", path, err)
	}
	return db, nil
}

// buildDSN assembles a modernc.org/sqlite DSN with repeated _pragma params.
func buildDSN(path string, pragmas []string) string {
	var b strings.Builder
	b.WriteString("file:")
	b.WriteString(path)
	b.WriteString("?_time_format=sqlite")
	for _, p := range pragmas {
		b.WriteString("&_pragma=")
		b.WriteString(url.QueryEscape(p))
	}
	return b.String()
}