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

package engine

import (
	"fmt"
	"strconv"
	"time"

	"github.com/google/uuid"
)

// ParseRecurrencePattern converts duration strings to time.Duration
// Supports: 1d, 2w, 3m, 1y, 5min, 5hrs, 30sec
// Also supports: days, weeks, months, years, minutes, hours, seconds
// And common forms: daily, weekly, monthly, yearly
func ParseRecurrencePattern(pattern string) (time.Duration, error) {
	if len(pattern) == 0 {
		return 0, fmt.Errorf("empty duration pattern")
	}

	// Handle word-based durations (hour, hours, day, days, etc.)
	wordDurations := map[string]time.Duration{
		"second": time.Second, "seconds": time.Second, "sec": time.Second,
		"minute": time.Minute, "minutes": time.Minute, "min": time.Minute,
		"hour": time.Hour, "hours": time.Hour, "hrs": time.Hour, "hr": time.Hour,
		"day": 24 * time.Hour, "days": 24 * time.Hour, "daily": 24 * time.Hour,
		"week": 7 * 24 * time.Hour, "weeks": 7 * 24 * time.Hour, "weekly": 7 * 24 * time.Hour,
		"month": 30 * 24 * time.Hour, "months": 30 * 24 * time.Hour, "monthly": 30 * 24 * time.Hour,
		"year": 365 * 24 * time.Hour, "years": 365 * 24 * time.Hour, "yearly": 365 * 24 * time.Hour,
	}

	if duration, ok := wordDurations[pattern]; ok {
		return duration, nil
	}

	// Find where number ends and unit begins
	splitIdx := -1
	for i, ch := range pattern {
		if (ch < '0' || ch > '9') && ch != '-' {
			splitIdx = i
			break
		}
	}

	if splitIdx == -1 || splitIdx == 0 {
		return 0, fmt.Errorf("invalid duration pattern: %s", pattern)
	}

	numStr := pattern[:splitIdx]
	unit := pattern[splitIdx:]

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

	// Single letter units
	if len(unit) == 1 {
		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: %s (use d/w/m/y)", unit)
		}
	}

	// Multi-character units
	if baseDuration, ok := wordDurations[unit]; ok {
		return time.Duration(num) * baseDuration, nil
	}

	return 0, fmt.Errorf("unknown duration unit: %s", 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)
}

// CreateRecurringTask creates a recurring task template and its first instance.
// It validates the recurrence pattern and due date, creates the template with
// StatusRecurring, then creates the first pending instance linked to it.
// Returns the first instance task.
func CreateRecurringTask(description string, mod *Modifier) (*Task, error) {
	recurPattern := mod.SetAttributes["recur"]
	if recurPattern == nil {
		return nil, fmt.Errorf("no recurrence pattern specified")
	}

	if mod.SetAttributes["due"] == nil {
		return nil, fmt.Errorf("recurring tasks require a due date (use due:YYYY-MM-DD or due:monday)")
	}

	duration, err := ParseRecurrencePattern(*recurPattern)
	if err != nil {
		return nil, fmt.Errorf("invalid recurrence pattern: %w", err)
	}

	now := time.Now()
	template := &Task{
		UUID:               uuid.New(),
		Status:             StatusRecurring,
		Description:        description,
		Priority:           PriorityDefault,
		Created:            now,
		Modified:           now,
		RecurrenceDuration: &duration,
		Tags:               []string{},
	}

	// Build modifier without the recur attribute
	tempMod := &Modifier{
		SetAttributes:  make(map[string]*string),
		AttributeOrder: []string{},
		AddTags:        mod.AddTags,
		RemoveTags:     mod.RemoveTags,
	}
	for _, key := range mod.AttributeOrder {
		if key != "recur" {
			tempMod.SetAttributes[key] = mod.SetAttributes[key]
			tempMod.AttributeOrder = append(tempMod.AttributeOrder, key)
		}
	}

	if err := tempMod.ApplyToNew(template); err != nil {
		return nil, fmt.Errorf("failed to apply modifiers to template: %w", err)
	}

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

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

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

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

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

	return instance, nil
}

// 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
}