// Package schedule runs registered periodic tasks on configurable intervals.

//

// A Task is eligible to run when the current time is on or after its NextRun.

// Failed tasks are retried on the next tick; only successful runs advance NextRun.

// NextRun must advance from the previous scheduled time — not the wall clock at

// execution — to prevent interval drift over time.

package schedule

import (

	"context"

	"fmt"

	"log"

	"time"

)

// Task describes a named unit of periodic work.

type Task struct {

	Name     string

	Interval time.Duration

	// NextRun is the earliest time the task should next execute.

	// Updated by the Scheduler after each successful run.

	NextRun time.Time

	Run     func(ctx context.Context) error

}

// Scheduler executes registered tasks on each tick.

type Scheduler struct {

	tasks  []*Task

	ticker *time.Ticker

}

// NewScheduler returns a Scheduler that checks for due tasks every tickInterval.

func NewScheduler(tickInterval time.Duration) *Scheduler {

	return &Scheduler{ticker: time.NewTicker(tickInterval)}

}

// Register adds t to the scheduler. The task is eligible to run on the first tick.

func (s *Scheduler) Register(t *Task) {

	t.NextRun = time.Now()

	s.tasks = append(s.tasks, t)

}

// RunOnce checks registered tasks in order and stops at the first task failure.

// Returns: a non-nil error if the first due task failure is encountered.

func (s *Scheduler) RunOnce(ctx context.Context) error {

	now := time.Now()

	for _, t := range s.tasks {

		if now.Before(t.NextRun) {

			continue

		}

		t.NextRun = now.Add(t.Interval)

		if err := t.Run(ctx); err != nil {

			return fmt.Errorf("schedule: task %q: %w", t.Name, err)

		}

	}

	return nil

}

// Run starts the scheduling loop and blocks until ctx is cancelled.

func (s *Scheduler) Run(ctx context.Context) {

	for {

		select {

		case <-ctx.Done():

			s.ticker.Stop()

			return

		case <-s.ticker.C:

			if err := s.RunOnce(ctx); err != nil {

				log.Printf("schedule: tick error: %v", err)

			}

		}

	}

}