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

package engine

import (
	"fmt"
	"strconv"
	"strings"
	"time"
)

var monthNames = map[string]time.Month{
	"jan": time.January, "january": time.January,
	"feb": time.February, "february": time.February,
	"mar": time.March, "march": time.March,
	"apr": time.April, "april": time.April,
	"may": time.May,
	"jun": time.June, "june": time.June,
	"jul": time.July, "july": time.July,
	"aug": time.August, "august": time.August,
	"sep": time.September, "september": time.September,
	"oct": time.October, "october": time.October,
	"nov": time.November, "november": time.November,
	"dec": time.December, "december": time.December,
}

// DateParser handles all date/time/duration parsing with configurable options
type DateParser struct {
	base      time.Time
	weekStart time.Weekday
}

// NewDateParser creates a new DateParser with the given base time and week start
func NewDateParser(base time.Time, weekStart time.Weekday) *DateParser {
	return &DateParser{
		base:      base,
		weekStart: weekStart,
	}
}

// NewDefaultDateParser creates a DateParser with current time and Monday week start
func NewDefaultDateParser() *DateParser {
	return &DateParser{
		base:      timeNow(),
		weekStart: time.Monday,
	}
}

// ParseDate is the main entry point for date parsing
// Handles: ISO dates, weekdays, month names, day+month, period boundaries, durations, time of day
func (p *DateParser) ParseDate(s string) (time.Time, error) {
	s = strings.ToLower(strings.TrimSpace(s))

	// Check for time of day component (e.g., "mon:15:35" or "21jan:0800")
	if dateStr, timeStr, hasTime := p.splitDateTime(s); hasTime {
		dateVal, err := p.parseDateOnly(dateStr)
		if err != nil {
			return time.Time{}, err
		}
		return p.parseTimeOfDay(dateVal, timeStr)
	}

	return p.parseDateOnly(s)
}

// parseDateOnly parses just the date component without time
func (p *DateParser) parseDateOnly(s string) (time.Time, error) {
	// Empty string means "today" (used when parsing just time like "15:35")
	if s == "" {
		return time.Date(p.base.Year(), p.base.Month(), p.base.Day(), 0, 0, 0, 0, p.base.Location()), nil
	}

	// Try ISO format first
	if t, err := time.ParseInLocation("2006-01-02", s, p.base.Location()); err == nil {
		return t, nil
	}

	// Relative dates
	if t, ok := p.parseRelative(s); ok {
		return t, nil
	}

	// Weekday names
	if t, ok := p.parseWeekday(s); ok {
		return t, nil
	}

	// Month names (jan, january, feb, etc.)
	if t, ok := p.parseMonthName(s); ok {
		return t, nil
	}

	// Day+month format (21jan, Jan21, etc.)
	if t, ok := p.parseDayMonth(s); ok {
		return t, nil
	}

	// Period boundaries (sod, eod, sow, eow, etc.)
	if t, ok := p.parsePeriodBoundary(s); ok {
		return t, nil
	}

	// Special keywords (later, someday)
	if t, ok := p.parseSpecialKeyword(s); ok {
		return t, nil
	}

	// Duration as date offset (2d, 3w, etc.)
	if t, ok := p.parseDurationAsDate(s); ok {
		return t, nil
	}

	return time.Time{}, fmt.Errorf("unable to parse date: %s", s)
}

// splitDateTime checks if the string contains a time component
// Returns dateStr, timeStr, hasTime
func (p *DateParser) splitDateTime(s string) (string, string, bool) {
	// Look for time patterns: HH:MM or HHMM at the end
	// Examples: "mon:15:35", "21jan:0800", "15:35" (just time)

	parts := strings.Split(s, ":")
	if len(parts) < 2 {
		return s, "", false
	}

	lastPart := parts[len(parts)-1]

	// Format: "mon:15:35" or "tomorrow:15:35" (HH:MM with 3+ parts)
	if len(parts) >= 3 {
		// Check if last two parts form a valid time
		secondLast := parts[len(parts)-2]
		if h, err := strconv.Atoi(secondLast); err == nil && h >= 0 && h <= 23 {
			if m, err := strconv.Atoi(lastPart); err == nil && m >= 0 && m <= 59 {
				// HH:MM format
				dateStr := strings.Join(parts[:len(parts)-2], ":")
				if dateStr == "" {
					dateStr = "today" // Just time means today
				}
				timeStr := secondLast + ":" + lastPart
				return dateStr, timeStr, true
			}
		}
	}

	// Format with 2 parts: could be "15:35" (just time) or "mon:0800" (date+HHMM)
	if len(parts) == 2 {
		// Check if lastPart is HHMM format (4 digits)
		if len(lastPart) == 4 {
			if _, err := strconv.Atoi(lastPart); err == nil {
				h, _ := strconv.Atoi(lastPart[0:2])
				m, _ := strconv.Atoi(lastPart[2:4])
				if h >= 0 && h <= 23 && m >= 0 && m <= 59 {
					// Valid HHMM time
					// If first part is also numeric, it's a date part (like "21jan:0800" but already lowercase)
					// Otherwise it's a keyword like "tomorrow:0800"
					return parts[0], lastPart, true
				}
			}
		}

		// Check if it's "HH:MM" format (both parts are 2-digit numbers)
		if len(parts[0]) <= 2 && len(lastPart) <= 2 {
			if h, err := strconv.Atoi(parts[0]); err == nil && h >= 0 && h <= 23 {
				if m, err := strconv.Atoi(lastPart); err == nil && m >= 0 && m <= 59 {
					// Just time, no date
					return "", parts[0] + ":" + lastPart, true
				}
			}
		}
	}

	return s, "", false
}

// parseTimeOfDay applies time to a date
func (p *DateParser) parseTimeOfDay(date time.Time, timeStr string) (time.Time, error) {
	var hour, minute int
	var err error

	if strings.Contains(timeStr, ":") {
		// HH:MM format
		parts := strings.Split(timeStr, ":")
		if len(parts) != 2 {
			return time.Time{}, fmt.Errorf("invalid time format: %s", timeStr)
		}
		hour, err = strconv.Atoi(parts[0])
		if err != nil {
			return time.Time{}, fmt.Errorf("invalid hour: %s", parts[0])
		}
		minute, err = strconv.Atoi(parts[1])
		if err != nil {
			return time.Time{}, fmt.Errorf("invalid minute: %s", parts[1])
		}
	} else {
		// HHMM format
		if len(timeStr) != 4 {
			return time.Time{}, fmt.Errorf("invalid time format: %s (expected HHMM)", timeStr)
		}
		hour, err = strconv.Atoi(timeStr[0:2])
		if err != nil {
			return time.Time{}, fmt.Errorf("invalid hour: %s", timeStr[0:2])
		}
		minute, err = strconv.Atoi(timeStr[2:4])
		if err != nil {
			return time.Time{}, fmt.Errorf("invalid minute: %s", timeStr[2:4])
		}
	}

	// Validate ranges
	if hour < 0 || hour > 23 {
		return time.Time{}, fmt.Errorf("hour must be 0-23: %d", hour)
	}
	if minute < 0 || minute > 59 {
		return time.Time{}, fmt.Errorf("minute must be 0-59: %d", minute)
	}

	return time.Date(date.Year(), date.Month(), date.Day(), hour, minute, 0, 0, date.Location()), nil
}

// parseRelative handles relative date keywords
func (p *DateParser) parseRelative(s string) (time.Time, bool) {
	switch s {
	case "now":
		return p.base, true
	case "today":
		return time.Date(p.base.Year(), p.base.Month(), p.base.Day(), 0, 0, 0, 0, p.base.Location()), true
	case "tomorrow":
		tomorrow := p.base.AddDate(0, 0, 1)
		return time.Date(tomorrow.Year(), tomorrow.Month(), tomorrow.Day(), 0, 0, 0, 0, tomorrow.Location()), true
	case "yesterday":
		yesterday := p.base.AddDate(0, 0, -1)
		return time.Date(yesterday.Year(), yesterday.Month(), yesterday.Day(), 0, 0, 0, 0, yesterday.Location()), true
	}
	return time.Time{}, false
}

// parseWeekday handles weekday names (mon, monday, etc.)
func (p *DateParser) parseWeekday(s string) (time.Time, bool) {
	weekdays := map[string]time.Weekday{
		"sun": time.Sunday, "sunday": time.Sunday,
		"mon": time.Monday, "monday": time.Monday,
		"tue": time.Tuesday, "tuesday": time.Tuesday,
		"wed": time.Wednesday, "wednesday": time.Wednesday,
		"thu": time.Thursday, "thursday": time.Thursday,
		"fri": time.Friday, "friday": time.Friday,
		"sat": time.Saturday, "saturday": time.Saturday,
	}

	if targetWeekday, ok := weekdays[s]; ok {
		return p.nextWeekday(targetWeekday), true
	}
	return time.Time{}, false
}

// parseMonthName handles month names (jan, january, feb, february, etc.)
func (p *DateParser) parseMonthName(s string) (time.Time, bool) {
	month, ok := monthNames[s]
	if !ok {
		return time.Time{}, false
	}

	// Set to first of month
	// If that date has passed this year, use next year
	year := p.base.Year()
	firstOfMonth := time.Date(year, month, 1, 0, 0, 0, 0, p.base.Location())

	if firstOfMonth.Before(p.base) || firstOfMonth.Equal(p.base) {
		// First of month has passed, use next year
		year++
		firstOfMonth = time.Date(year, month, 1, 0, 0, 0, 0, p.base.Location())
	}

	return firstOfMonth, true
}

// parseDayMonth handles day+month formats (21jan, Jan21, 21January, January21)
func (p *DateParser) parseDayMonth(s string) (time.Time, bool) {
	// Try pattern: digits followed by month name (21jan, 21January)
	for i := 0; i < len(s); i++ {
		if s[i] >= '0' && s[i] <= '9' {
			continue
		}
		// Found first non-digit
		if i > 0 && i < len(s) {
			dayStr := s[:i]
			monthStr := strings.ToLower(s[i:])
			if day, month, ok := p.parseDayAndMonth(dayStr, monthStr); ok {
				return p.constructDate(day, month), true
			}
		}
		break
	}

	// Try pattern: month name followed by digits (jan21, January21)
	for i := 0; i < len(s); i++ {
		if s[i] >= 'a' && s[i] <= 'z' || s[i] >= 'A' && s[i] <= 'Z' {
			continue
		}
		// Found first non-letter
		if i > 0 && i < len(s) {
			monthStr := strings.ToLower(s[:i])
			dayStr := s[i:]
			if day, month, ok := p.parseDayAndMonth(dayStr, monthStr); ok {
				return p.constructDate(day, month), true
			}
		}
		break
	}

	return time.Time{}, false
}

// parseDayAndMonth extracts day and month from string parts
func (p *DateParser) parseDayAndMonth(dayStr, monthStr string) (int, time.Month, bool) {
	day, err := strconv.Atoi(dayStr)
	if err != nil || day < 1 || day > 31 {
		return 0, 0, false
	}

	month, ok := monthNames[monthStr]
	if !ok {
		return 0, 0, false
	}

	return day, month, true
}

// constructDate creates a date with year logic (current year if future, next year if past)
func (p *DateParser) constructDate(day int, month time.Month) time.Time {
	year := p.base.Year()
	date := time.Date(year, month, day, 0, 0, 0, 0, p.base.Location())

	// Validate day is valid for month (handles Feb 30, etc.)
	if date.Month() != month {
		// Invalid day for month, return zero time (will be caught as parse error)
		return time.Time{}
	}

	if date.Before(p.base) || date.Equal(p.base) {
		// Date has passed, use next year
		year++
		date = time.Date(year, month, day, 0, 0, 0, 0, p.base.Location())
	}

	return date
}

// parsePeriodBoundary handles period boundary keywords (sod, eod, sow, eow, etc.)
func (p *DateParser) parsePeriodBoundary(s string) (time.Time, bool) {
	now := p.base
	loc := now.Location()

	switch s {
	case "sod":
		// Start of day - today at 00:00:00
		return time.Date(now.Year(), now.Month(), now.Day(), 0, 0, 0, 0, loc), true

	case "eod":
		// End of day - today at 23:59:59
		return time.Date(now.Year(), now.Month(), now.Day(), 23, 59, 59, 0, loc), true

	case "sow":
		// Start of week - next occurrence of week start day at 00:00:00
		return p.nextWeekday(p.weekStart), true

	case "eow":
		// End of week - next occurrence of week end day at 23:59:59
		weekEnd := p.weekStart - 1
		if weekEnd < 0 {
			weekEnd = 6
		}
		endDate := p.nextWeekday(weekEnd)
		return time.Date(endDate.Year(), endDate.Month(), endDate.Day(), 23, 59, 59, 0, loc), true

	case "som":
		// Start of month - 1st of current month at 00:00:00
		// If we're past the 1st, use next month
		firstOfMonth := time.Date(now.Year(), now.Month(), 1, 0, 0, 0, 0, loc)
		if firstOfMonth.Before(now) || firstOfMonth.Equal(now) {
			// Already past the 1st, use next month
			firstOfMonth = time.Date(now.Year(), now.Month()+1, 1, 0, 0, 0, 0, loc)
		}
		return firstOfMonth, true

	case "eom":
		// End of month - last day of current month at 23:59:59
		// Get first day of next month, then subtract one day
		firstOfNextMonth := time.Date(now.Year(), now.Month()+1, 1, 0, 0, 0, 0, loc)
		lastOfMonth := firstOfNextMonth.AddDate(0, 0, -1)
		return time.Date(lastOfMonth.Year(), lastOfMonth.Month(), lastOfMonth.Day(), 23, 59, 59, 0, loc), true

	case "soy":
		// Start of year - Jan 1 of current year at 00:00:00
		// If we're past Jan 1, use next year
		firstOfYear := time.Date(now.Year(), time.January, 1, 0, 0, 0, 0, loc)
		if firstOfYear.Before(now) || firstOfYear.Equal(now) {
			firstOfYear = time.Date(now.Year()+1, time.January, 1, 0, 0, 0, 0, loc)
		}
		return firstOfYear, true

	case "eoy":
		// End of year - Dec 31 of current year at 23:59:59
		return time.Date(now.Year(), time.December, 31, 23, 59, 59, 0, loc), true

	default:
		return time.Time{}, false
	}
}

// parseSpecialKeyword handles special keywords (later, someday)
func (p *DateParser) parseSpecialKeyword(s string) (time.Time, bool) {
	switch s {
	case "later", "someday":
		// Far future: 2150-01-01 at 00:00:00
		return time.Date(2150, time.January, 1, 0, 0, 0, 0, p.base.Location()), true
	default:
		return time.Time{}, false
	}
}

// parseDurationAsDate handles duration as date offset (2d, 3w, etc.) and named durations
func (p *DateParser) parseDurationAsDate(s string) (time.Time, bool) {
	// Named duration aliases
	namedDurations := map[string]string{
		"daily":   "1d",
		"weekly":  "1w",
		"monthly": "30d",
		"yearly":  "1y",
	}

	// Check for named duration
	if pattern, ok := namedDurations[s]; ok {
		s = pattern
	}

	// Try parsing as duration
	duration, err := ParseRecurrencePattern(s)
	if err != nil {
		return time.Time{}, false
	}

	// Add duration to base time
	return p.base.Add(duration), true
}

// ParseDuration parses duration strings (1d, 2w, 5min, etc.)
func (p *DateParser) ParseDuration(s string) (time.Duration, error) {
	// Use existing ParseRecurrencePattern for now (will expand in Phase 2)
	return ParseRecurrencePattern(s)
}

// nextWeekday returns the next occurrence of the target weekday
// Smart logic: if today is Thursday and target is Sunday, returns this Sunday
// If today is Sunday and target is Sunday, returns next Sunday
func (p *DateParser) nextWeekday(target time.Weekday) time.Time {
	from := p.base
	// Calculate days until target
	daysUntil := int(target - from.Weekday())

	if daysUntil <= 0 {
		daysUntil += 7 // Next week
	}

	next := from.AddDate(0, 0, daysUntil)
	return time.Date(next.Year(), next.Month(), next.Day(), 0, 0, 0, 0, next.Location())
}

// ParseDate is the public API that uses config settings
func ParseDate(s string) (time.Time, error) {
	cfg, err := GetConfig()
	weekStart := time.Monday // default
	if err == nil {
		weekStart = cfg.GetWeekStart()
	}

	parser := NewDateParser(timeNow(), weekStart)
	return parser.ParseDate(s)
}