package main

import (
	"context"
	"fmt"
	"time"

	"github.com/google/uuid"
)

type Queue struct {
	driver Driver
}

func NewQueue(ctx context.Context, driver Driver) (Queue, error) {
	_, err := driver.ExecContext(ctx, `
      DROP TABLE IF EXISTS queue;
      CREATE TABLE IF NOT EXISTS queue (
         id INTEGER PRIMARY KEY,
         updated INTEGER NOT NULL,
         reservation TEXT,
         payload TEXT
      );
   `)
	return Queue{driver: driver}, err
}

func (q Queue) Enqueue(ctx context.Context, b []byte) error {
	_, err := q.driver.ExecContext(ctx, `
      INSERT INTO queue (updated, payload) VALUES (?, ?)
   `,
		time.Now().Unix(),
		b,
	)
	return err
}

func (q Queue) Dequeue(ctx context.Context) (string, []byte, error) {
	for {
		reservation, m, err := q.dequeue(ctx)
		if reservation != nil || err != nil {
			return string(reservation), m, err
		}

		select {
		case <-ctx.Done():
			return "", nil, ctx.Err()
		case <-time.After(time.Second):
		}
	}
}

func (q Queue) dequeue(ctx context.Context) ([]byte, []byte, error) {
	now := time.Now().Unix()
	reservation := []byte(uuid.New().String())
	var payload []byte
	if result, err := q.driver.ExecContext(ctx, `
      UPDATE queue
      SET
         updated = ?, reservation = ?
      WHERE
         id IN (
            SELECT id
            FROM queue
            WHERE
               reservation IS NULL
               OR ? - updated > 60
            LIMIT 1
         )
   `, now, reservation, now); err != nil {
		return nil, nil, fmt.Errorf("failed to assign reservation: %w", err)
	} else if n, err := result.RowsAffected(); err != nil {
		return nil, nil, fmt.Errorf("failed to assign reservation: no count: %w", err)
	} else if n == 0 {
		return nil, nil, fmt.Errorf("failed to assign reservation: zero updates")
	}

	rows, err := q.driver.QueryContext(ctx, `
      SELECT payload
      FROM queue
      WHERE reservation==?
      LIMIT 1
   `, reservation)
	if err != nil {
		return nil, nil, fmt.Errorf("failed to query reservation: %w", err)
	}
	defer rows.Close()
	for rows.Next() {
		if err := rows.Scan(&payload); err != nil {
			return nil, nil, fmt.Errorf("failed to parse reservation: %w", err)
		}
	}
	if err := rows.Err(); err != nil {
		return nil, nil, fmt.Errorf("failed to page reservation: %w", err)
	}

	return reservation, payload, nil
}

func (q Queue) Ack(ctx context.Context, reservation string) error {
	_, err := q.driver.ExecContext(ctx, `
      DELETE FROM queue
      WHERE reservation==?
   `, reservation)
	return err
}