gems/opal-task/internal/engine/recurrence.go

package engine

import (
	"fmt"
	"strconv"
	"time"

	"github.com/google/uuid"
)

// ParseRecurrencePattern converts "1w", "2d", "1m" to time.Duration
func ParseRecurrencePattern(pattern string) (time.Duration, error) {
	if len(pattern) < 2 {
		return 0, fmt.Errorf("invalid recurrence pattern: %s", pattern)
	}

	numStr := pattern[:len(pattern)-1]
	unit := pattern[len(pattern)-1]

	num, err := strconv.Atoi(numStr)
	if err != nil {
		return 0, fmt.Errorf("invalid number in pattern: %s", pattern)
	}

	switch unit {
	case 'd':
		return time.Duration(num) * 24 * time.Hour, nil
	case 'w':
		return time.Duration(num) * 7 * 24 * time.Hour, nil
	case 'm':
		// Approximate: 30 days
		return time.Duration(num) * 30 * 24 * time.Hour, nil
	case 'y':
		// Approximate: 365 days
		return time.Duration(num) * 365 * 24 * time.Hour, nil
	default:
		return 0, fmt.Errorf("unknown unit: %c (use d/w/m/y)", unit)
	}
}

// FormatRecurrenceDuration converts time.Duration back to "1w", "2d" format
func FormatRecurrenceDuration(d time.Duration) string {
	days := int(d.Hours() / 24)

	if days%365 == 0 && days/365 > 0 {
		return fmt.Sprintf("%dy", days/365)
	}
	if days%30 == 0 && days/30 > 0 {
		return fmt.Sprintf("%dm", days/30)
	}
	if days%7 == 0 && days/7 > 0 {
		return fmt.Sprintf("%dw", days/7)
	}
	return fmt.Sprintf("%dd", days)
}

// CalculateNextDue calculates next due date based on current and recurrence
func CalculateNextDue(currentDue time.Time, recurrence time.Duration) time.Time {
	return currentDue.Add(recurrence)
}

// SpawnNextInstance creates a new task instance from completed recurring task
func SpawnNextInstance(completedInstance *Task) error {
	if completedInstance.ParentUUID == nil {
		return fmt.Errorf("task is not a recurring instance")
	}

	// Load template
	template, err := GetTask(*completedInstance.ParentUUID)
	if err != nil {
		return fmt.Errorf("failed to load template: %w", err)
	}

	if template.RecurrenceDuration == nil {
		return fmt.Errorf("template has no recurrence duration")
	}

	// Calculate next due date
	// Use End date (completion time) as base, fallback to Due date if End not set
	var baseDate time.Time
	if completedInstance.End != nil {
		baseDate = *completedInstance.End
	} else if completedInstance.Due != nil {
		baseDate = *completedInstance.Due
	} else {
		return fmt.Errorf("recurring instance has no due or end date")
	}

	next := CalculateNextDue(baseDate, *template.RecurrenceDuration)
	nextDue := &next

	// Check if we're past 'until' date
	if template.Until != nil && nextDue != nil && nextDue.After(*template.Until) {
		// Don't spawn, recurrence has expired
		return nil
	}

	// Create new instance
	now := time.Now()
	newInstance := &Task{
		UUID:        uuid.New(),
		Status:      StatusPending,
		Description: template.Description,
		Project:     template.Project,
		Priority:    template.Priority,
		Created:     now,
		Modified:    now,
		Due:         nextDue,
		Scheduled:   template.Scheduled,
		Wait:        template.Wait,
		Until:       template.Until,
		ParentUUID:  &template.UUID,
		Tags:        []string{},
	}

	if err := newInstance.Save(); err != nil {
		return fmt.Errorf("failed to save new instance: %w", err)
	}

	// Copy tags from template
	templateTags, err := template.GetTags()
	if err != nil {
		return fmt.Errorf("failed to get template tags: %w", err)
	}

	for _, tag := range templateTags {
		if err := newInstance.AddTag(tag); err != nil {
			return fmt.Errorf("failed to add tag: %w", err)
		}
	}

	return nil
}