package main

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

func main() {
	dates := []string{
		"today",
		"tomorrow",
		"tomorrow at 3am",
		"in 3 days",
		"in a month",
		"in 3 months",
		"next week",
		"next month",
		"in two weeks",
		"in 3 weeks",
		"2016-09-28",
		"3pm",
		"in 4 days at 10 pm",
	}
	for _, d := range dates {
		fmt.Printf("'%s': ", d)
		t, err := parseTime(d)
		if err != nil {
			fmt.Printf("%s\n", err)
		} else {
			fmt.Printf("%s\n", t)
		}
	}
}

var timeLayouts = []string{
	time.RFC3339,
	"2006-01-02",
}

func parseTime(s string) (time.Time, error) {
	now := time.Now().Round(time.Second)
	return parseTimeRelative(s, now)
}

func parseTimeRelative(s string, now time.Time) (time.Time, error) {
	var t time.Time

	parts := strings.Fields(s)
	// more indicates the parts index where there might be "more"
	// parsable data
	more := 0

	if len(parts) == 0 {
		return t, fmt.Errorf("empty date spec")
	}

	switch parts[0] {
	case "today":
		if len(parts) >= 1 {
			t = now
			more = 1
		}
	case "tomorrow":
		if len(parts) >= 1 {
			t = now.AddDate(0, 0, 1)
			more = 1
		}
	case "in":
		if len(parts) >= 3 {
			n, err := parseNumber(parts[1])
			if err != nil {
				return t, err
			}
			modifier, err := parseModifier(parts[2])
			if err != nil {
				return t, err
			}
			t = modifier(n, now)
			more = 3
		}
	case "next":
		if len(parts) >= 2 {
			modifier, err := parseModifier(parts[1])
			if err != nil {
				return t, err
			}
			t = modifier(1, now)
			more = 2
		}
	}

	if len(parts) == more {
		return t, nil
	} else if len(parts) == more+2 && parts[more] == "at" {
		h, err := parseHours(parts[more+1])
		if err != nil {
			return t, err
		}

		t = truncateHours(t)
		return t.Add(h), nil
	} else if len(parts) == more+3 && parts[more] == "at" &&
		(parts[more+2] == "am" || parts[more+2] == "pm") {
		h, err := parseHours(parts[more+1] + parts[more+2])
		if err != nil {
			return t, err
		}

		t = truncateHours(t)
		return t.Add(h), nil
	}

	l := len(parts)
	if (l == 1 || l == 2) && (strings.HasSuffix(parts[l-1], "am") || strings.HasSuffix(parts[l-1], "pm")) {
		s = parts[0]
		if l == 2 {
			s += parts[1]
		}

		h, err := parseHours(s)
		if err != nil {
			return t, err
		}

		t = truncateHours(now)
		return t.Add(h), nil
	}

	for _, layout := range timeLayouts {
		t, err := time.Parse(layout, s)
		if err == nil {
			return t, nil
		}
	}

	return t, fmt.Errorf("unknown date spec '%s' (unexpected)", s)
}

func parseNumber(n string) (int, error) {
	switch n {
	case "a", "an", "one":
		return 1, nil
	case "two":
		return 2, nil
	case "three":
		return 3, nil
	case "four":
		return 4, nil
	case "five":
		return 5, nil
	case "six":
		return 6, nil
	case "seven":
		return 7, nil
	case "eight":
		return 8, nil
	case "nine":
		return 9, nil
	case "ten":
		return 10, nil
	default:
		return strconv.Atoi(n)
	}
}

func parseModifier(m string) (func(int, time.Time) time.Time, error) {
	switch m {
	case "second", "seconds":
		return durationModifier(time.Second), nil
	case "minute", "minutes":
		return durationModifier(time.Minute), nil
	case "hour", "hours":
		return durationModifier(time.Hour), nil
	case "day", "days":
		return dateModifier(0, 0, 1), nil
	case "week", "weeks":
		return dateModifier(0, 0, 7), nil
	case "month", "months":
		return dateModifier(0, 1, 0), nil
	case "year", "years":
		return dateModifier(1, 0, 0), nil
	default:
		return nil, fmt.Errorf("unknown modifier '%s'", m)
	}
}

func durationModifier(d time.Duration) func(int, time.Time) time.Time {
	return func(n int, t time.Time) time.Time {
		return t.Add(time.Duration(n) * d)
	}
}

func dateModifier(years, months, days int) func(int, time.Time) time.Time {
	return func(n int, t time.Time) time.Time {
		return t.AddDate(n*years, n*months, n*days)
	}
}

func parseHours(s string) (time.Duration, error) {
	var d time.Duration

	isPm := false
	switch {
	case strings.HasSuffix(s, "am"):
		s = s[0 : len(s)-2]
	case strings.HasSuffix(s, "pm"):
		isPm = true
		s = s[0 : len(s)-2]
	default:
		return d, fmt.Errorf("unknown hours '%s'", s)
	}

	n, err := strconv.Atoi(s)
	if err != nil {
		return d, err
	}

	if n < 1 || n > 12 {
		return d, fmt.Errorf("invalid hour: %d (must be between 1 and 12)", n)
	}

	if isPm {
		n += 12
	}

	return time.Duration(n) * time.Hour, nil
}

func truncateHours(t time.Time) time.Time {
	return t.Truncate(time.Hour).Add(-time.Duration(t.Hour()) * time.Hour)
}