package deadlinks

import (
	"context"
	"database/sql"
	"errors"
	"fmt"
	"strings"
	"time"

	"code.betamike.com/mediocregopher/mediocre-go-lib/miter"

	_ "github.com/mattn/go-sqlite3"
	migrate "github.com/rubenv/sql-migrate"
)

// Store keeps track of the current status of all discovered Resources, and
// links between them. A Resource which is neither pinned nor linked to from
// another Resource is considered to not exist.
//
// An implementation of Store must be thread-safe.
type Store interface {
	// GetByStatus returns all Resources with the given Status.
	GetByStatus(ResourceStatus) miter.Iterator[Resource]

	// GetURLsByLastChecked returns the URLs of all Resources with LastChecked
	// values older than the given timestamp, or which have never been checked.
	GetURLsByLastChecked(olderThan time.Time) miter.Iterator[URL]

	// SetPinned overwrites the set of pinned URLs with the given one.
	SetPinned(context.Context, []URL) error

	// Update updates the Resource identified by the given URL with the given
	// arguments.
	//
	// Update returns an error if the URL has not been pinned nor referenced as
	// an outgoing URL of a different Resource.
	Update(
		ctx context.Context,
		now time.Time,
		url URL,
		status ResourceStatus,
		errorString string,
		outgoing []URL,
	) error

	// GC will garbage collect the store, removing any orphaned Resources.
	GC(context.Context) error
}

var migrations = &migrate.MemoryMigrationSource{Migrations: []*migrate.Migration{
	{
		Id: "1",
		Up: []string{
			`CREATE TABLE urls (
				id  INTEGER NOT NULL PRIMARY KEY,
				url TEXT    NOT NULL,
				UNIQUE(url)
			)`,

			`CREATE TABLE resources (
				url_id       INTEGER NOT NULL PRIMARY KEY,
				status       INTEGER NOT NULL DEFAULT 0,
				pinned       INTEGER NOT NULL,
				last_checked INTEGER NOT NULL DEFAULT 0,
				error_string TEXT    NOT NULL DEFAULT '',
				FOREIGN KEY(url_id) REFERENCES urls(id) ON DELETE CASCADE
			)`,

			`CREATE TABLE links (
				from_url_id INTEGER NOT NULL,
				to_url_id   INTEGER NOT NULL,
				FOREIGN KEY(from_url_id) REFERENCES urls(id) ON DELETE CASCADE,
				FOREIGN KEY(to_url_id) REFERENCES urls(id) ON DELETE CASCADE,
				PRIMARY KEY(from_url_id, to_url_id)
			)`,

			`CREATE INDEX links_outgoing_idx ON links (to_url_id)`,
		},
	},
}}

// SQLiteSQLiteStoreOpts are optional fields which can be provided to
// NewSQLiteStore. A nil SQLiteSQLiteStoreOpts is equivalent to an empty one.
type SQLiteStoreOpts struct {
	// Path to the database file to use.
	//
	// Defaults to ":memory:", indicating an in-memory database will be used.
	Path string
}

func (o *SQLiteStoreOpts) withDefaults() *SQLiteStoreOpts {
	if o == nil {
		o = new(SQLiteStoreOpts)
	}

	if o.Path == "" {
		o.Path = ":memory:"
	}

	return o
}

type SQLiteStore struct {
	db *sql.DB
}

var _ Store = (*SQLiteStore)(nil)

// NewSQLiteStore returns a Store implementation which uses an in-memory SQLite
// db.
func NewSQLiteStore(o *SQLiteStoreOpts) *SQLiteStore {
	o = o.withDefaults()

	db, err := sql.Open("sqlite3", o.Path+"?_foreign_keys=1")
	if err != nil {
		panic(fmt.Errorf("opening sqlite in memory: %w", err))
	}

	// go-sqlite doesn't support multiple go-routines, this is equivalent to
	// wrapping each call to the db in a mutex.
	db.SetMaxOpenConns(1)

	if _, err := migrate.Exec(db, "sqlite3", migrations, migrate.Up); err != nil {
		panic(fmt.Errorf("running migrations: %w", err))
	}

	return &SQLiteStore{db}
}

// Close cleans up all resources held by the SQLiteStore store, if any. It must
// be the final method call to the SQLiteStore.
func (s *SQLiteStore) Close() error {
	return s.db.Close()
}

// iterate is a helper which can be used to read the results of a query in
// chunks, producing a single unified Iterator.
//
// iterate assumes that the rows are being scanned ordered by their row ID,
// which must be a number, and that each query call has some kind of limit
// applied.
func iterate[T any](
	db interface {
		QueryContext(context.Context, string, ...any) (*sql.Rows, error)
	},
	query string,
	mkArgs func(minRowID int) []any,
	scan func(*sql.Rows) (T, int, error), // returns scanned value and its rowID
) miter.Iterator[T] {
	var (
		zero         T
		minRowID     = -1
		res, resBase []T

		pop = func() T {
			r := res[0]
			res = res[1:]
			return r
		}
	)

	return miter.FromFunc(func(ctx context.Context) (T, error) {
		if len(res) > 0 {
			return pop(), nil
		}

		res = resBase

		rows, err := db.QueryContext(ctx, query, mkArgs(minRowID)...)
		if err != nil {
			return zero, fmt.Errorf("executing query: %w", err)
		}
		defer rows.Close()

		for rows.Next() {
			var r T
			if r, minRowID, err = scan(rows); err != nil {
				return zero, fmt.Errorf("scanning row: %w", err)
			}

			res = append(res, r)
		}

		resBase = res[:0]

		if len(res) == 0 {
			return zero, miter.ErrEnd
		}

		return pop(), nil
	})
}

const getByStatusLimit = 16

// GetByStatus implements the method for the Store interface.
func (s *SQLiteStore) GetByStatus(status ResourceStatus) miter.Iterator[Resource] {
	const query = `
		WITH
			incoming(url_id, urls) AS (
				SELECT
					to_url_id,
					GROUP_CONCAT(url, char(0))
				FROM links
				JOIN urls ON (urls.id = links.from_url_id)
				GROUP BY to_url_id
			),
			outgoing(url_id, urls) AS (
				SELECT
					from_url_id,
					GROUP_CONCAT(url, char(0))
				FROM links
				JOIN urls ON (urls.id = links.to_url_id)
				GROUP BY from_url_id
			)
		SELECT
			resources.url_id,
			url,
			status,
			pinned,
			last_checked,
			error_string,
			incoming.urls,
			outgoing.urls
		FROM resources
		JOIN urls ON (urls.id = resources.url_id)
		LEFT JOIN incoming ON (incoming.url_id = resources.url_id)
		LEFT JOIN outgoing ON (outgoing.url_id = resources.url_id)
		WHERE status = ?
		AND (pinned OR incoming.urls IS NOT NULL)
		AND resources.url_id > ?
		ORDER BY resources.url_id ASC
		LIMIT ?`

	return iterate(
		s.db,
		query,
		func(minRowID int) []any {
			return []any{status, minRowID, getByStatusLimit}
		},
		func(rows *sql.Rows) (Resource, int, error) {
			var (
				r                  Resource
				rowID              int
				err                error
				lastChecked        int64
				incoming, outgoing sql.NullString
			)

			if err := rows.Scan(
				&rowID,
				&r.URL,
				&r.Status,
				&r.Pinned,
				&lastChecked,
				&r.ErrorString,
				&incoming,
				&outgoing,
			); err != nil {
				return Resource{}, 0, fmt.Errorf("calling Scan: %w", err)
			}

			if lastChecked != 0 {
				r.LastChecked = time.Unix(lastChecked, 0).UTC()
			}

			if incoming.String != "" {
				if r.IncomingLinkURLs, err = parseURLs(
					strings.Split(incoming.String, "\x00"),
				); err != nil {
					return Resource{}, 0, fmt.Errorf(
						"parsing incoming links: %w", err,
					)
				}
			}

			if outgoing.String != "" {
				if r.OutgoingLinkURLs, err = parseURLs(
					strings.Split(outgoing.String, "\x00"),
				); err != nil {
					return Resource{}, 0, fmt.Errorf(
						"parsing outgoing links: %w", err,
					)
				}
			}

			return r, rowID, nil
		},
	)
}

const getURLsByLastCheckedLimit = 64

// GetURLsByLastChecked implements the method for the Store interface.
func (s *SQLiteStore) GetURLsByLastChecked(
	olderThan time.Time,
) miter.Iterator[URL] {
	const query = `
		WITH
			incoming(url_id, urls) AS (
				SELECT to_url_id, COUNT(1)
				FROM links
				GROUP BY to_url_id
			)
		SELECT resources.url_id, url
		FROM resources
		JOIN urls ON (urls.id = resources.url_id)
		LEFT JOIN incoming ON (incoming.url_id = resources.url_id)
		WHERE last_checked < ?
		AND (pinned OR incoming.urls IS NOT NULL)
		AND resources.url_id > ?
		ORDER BY resources.url_id ASC
		LIMIT ?`

	return iterate(
		s.db,
		query,
		func(minRowID int) []any {
			return []any{
				olderThan.Unix(), minRowID, getURLsByLastCheckedLimit,
			}
		},
		func(rows *sql.Rows) (URL, int, error) {
			var (
				urlID  int
				urlStr string
			)

			if err := rows.Scan(&urlID, &urlStr); err != nil {
				return "", 0, fmt.Errorf("scanning url: %w", err)
			}

			url, err := ParseURL(urlStr)
			if err != nil {
				return "", 0, fmt.Errorf(
					"parsing url %q from db: %w", urlStr, err,
				)
			}

			return url, urlID, nil
		},
	)
}

func (s *SQLiteStore) touch(ctx context.Context, urls []URL, pinned bool) (
	[]int, error,
) {
	var (
		urlsQueryParams      = make([]any, len(urls))
		resourcesQueryParams = make([]any, 0, (len(urls)*2)+1)
		ids                  = make([]int, 0, len(urls))
	)

	for i := range urls {
		urlsQueryParams[i] = urls[i]
	}

	urlsQuery := `
		INSERT INTO urls (url)
		VALUES ` + joinRepeated("(?)", ",", len(urls)) + `
		ON CONFLICT DO UPDATE SET url=url
		RETURNING id`

	rows, err := s.db.QueryContext(ctx, urlsQuery, urlsQueryParams...)
	if err != nil {
		return nil, fmt.Errorf("inserting into urls: %w", err)
	}

	for range urls {
		if !rows.Next() {
			return nil, errors.Join(
				errors.New("expected a returned row"), rows.Close(),
			)
		}

		var id int
		if err := rows.Scan(&id); err != nil {
			return nil, errors.Join(
				fmt.Errorf("scanning return from insert into urls: %w", err),
				rows.Close(),
			)
		}

		resourcesQueryParams = append(resourcesQueryParams, id, pinned)
		ids = append(ids, id)
	}

	rows.Close()

	resourcesQuery := `
		INSERT INTO resources (url_id, pinned)
		VALUES ` + joinRepeated("(?,?)", ",", len(urls)) + `
		ON CONFLICT DO UPDATE SET pinned = ?`

	resourcesQueryParams = append(resourcesQueryParams, pinned)

	_, err = s.db.ExecContext(ctx, resourcesQuery, resourcesQueryParams...)
	if err != nil {
		return nil, fmt.Errorf("inserting into resources: %w", err)
	}

	return ids, nil
}

// SetPinned implements the method for the Store interface.
func (s *SQLiteStore) SetPinned(ctx context.Context, urls []URL) error {
	_, err := s.db.ExecContext(ctx, `UPDATE resources SET pinned = 0`)
	if err != nil {
		return fmt.Errorf("unsetting pinned on all resources: %w", err)
	}

	if _, err := s.touch(ctx, urls, true); err != nil {
		return fmt.Errorf("pinning resources: %w", err)
	}

	return err
}

// Update implements the method for the Store interface.
func (s *SQLiteStore) Update(
	ctx context.Context,
	now time.Time,
	url URL,
	status ResourceStatus,
	errorString string,
	outgoing []URL,
) error {
	const resourcesQuery = `
		UPDATE resources
		SET
			status = ?,
			last_checked = ?,
			error_string = ?
		WHERE url_id = (SELECT id FROM urls WHERE url = ?)
		RETURNING url_id`

	var urlID int

	err := s.db.QueryRowContext(
		ctx, resourcesQuery, status, now.Unix(), errorString, url,
	).Scan(&urlID)
	if err != nil {
		return fmt.Errorf("inserting into resources: %w", err)
	}

	_, err = s.db.ExecContext(
		ctx, `DELETE FROM links WHERE from_url_id = ?`, urlID,
	)
	if err != nil {
		return fmt.Errorf("deleting from links: %w", err)
	}

	if len(outgoing) == 0 {
		return nil
	}

	outgoingIDs, err := s.touch(ctx, outgoing, false)
	if err != nil {
		return fmt.Errorf("touching outgoing links: %w", err)
	}

	linksQueryParams := make([]any, 0, len(outgoingIDs)*2)
	for i := range outgoingIDs {
		linksQueryParams = append(linksQueryParams, urlID, outgoingIDs[i])
	}

	linksQuery := `
		INSERT INTO links (from_url_id, to_url_id)
		VALUES ` + joinRepeated("(?,?)", ",", len(outgoing)) + `
		ON CONFLICT DO NOTHING`

	_, err = s.db.ExecContext(ctx, linksQuery, linksQueryParams...)
	if err != nil {
		return fmt.Errorf("inserting into links: %w", err)
	}

	return nil
}

// GC implements the method for the Store interface.
func (s *SQLiteStore) GC(ctx context.Context) error {
	const query = `
		WITH orphans AS (
			SELECT url_id FROM resources
			LEFT JOIN links ON (links.to_url_id = resources.url_id)
			WHERE pinned = 0 AND from_url_id IS NULL
		)
		DELETE FROM urls WHERE id IN orphans
	`

	for {
		res, err := s.db.ExecContext(ctx, query)
		if err != nil {
			return fmt.Errorf("performing delete: %w", err)
		} else if n, err := res.RowsAffected(); err != nil {
			return fmt.Errorf("determining rows affected: %w", err)
		} else if n == 0 {
			return nil
		}
	}
}

func joinRepeated(str, sep string, n int) string {
	res := strings.Repeat(str+sep, n)
	return res[:len(res)-len(sep)]
}