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

package engine

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

// 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) {
	// Try ISO format first
	if t, err := time.Parse("2006-01-02", s); 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
	}

	// Check last part for time pattern (2-4 digits)
	lastPart := parts[len(parts)-1]
	if len(lastPart) == 2 || len(lastPart) == 4 {
		// Could be minutes (HH:MM) or HHMM format
		if _, err := strconv.Atoi(lastPart); err == nil {
			// Last part is numeric, could be time
			if len(parts) == 2 {
				// Format: "15:35" (just time, no date)
				return "", s, true
			} else if len(parts) >= 3 {
				// Format: "mon:15:35" or similar
				secondLast := parts[len(parts)-2]
				if _, err := strconv.Atoi(secondLast); err == nil {
					// HH:MM format
					dateStr := strings.Join(parts[:len(parts)-2], ":")
					timeStr := secondLast + ":" + lastPart
					return dateStr, timeStr, true
				} else if len(lastPart) == 4 {
					// HHMM format
					dateStr := strings.Join(parts[:len(parts)-1], ":")
					return dateStr, 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) {
	// Placeholder for Phase 2
	return time.Time{}, false
}

// parseDayMonth handles day+month formats (21jan, Jan21, etc.)
func (p *DateParser) parseDayMonth(s string) (time.Time, bool) {
	// Placeholder for Phase 2
	return time.Time{}, false
}

// parsePeriodBoundary handles period boundary keywords (sod, eod, sow, eow, etc.)
func (p *DateParser) parsePeriodBoundary(s string) (time.Time, bool) {
	// Placeholder for Phase 2
	return time.Time{}, false
}

// parseSpecialKeyword handles special keywords (later, someday)
func (p *DateParser) parseSpecialKeyword(s string) (time.Time, bool) {
	// Placeholder for Phase 2
	return time.Time{}, false
}

// parseDurationAsDate handles duration as date offset (2d, 3w, etc.)
func (p *DateParser) parseDurationAsDate(s string) (time.Time, bool) {
	// Placeholder for Phase 2
	return time.Time{}, false
}

// 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 default settings
func ParseDate(s string) (time.Time, error) {
	parser := NewDefaultDateParser()
	return parser.ParseDate(s)
}