package cal

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

// parseDuration parses an RFC 5545 duration: [+-]P[nW][nD][T[nH][nM][nS]].
// A leading '-' means before the reference time, so "-PT15M" is -15m.
func parseDuration(s string) (time.Duration, error) {
	orig := s
	neg := false
	switch {
	case strings.HasPrefix(s, "-"):
		neg, s = true, s[1:]
	case strings.HasPrefix(s, "+"):
		s = s[1:]
	}
	if !strings.HasPrefix(s, "P") {
		return 0, fmt.Errorf("not a duration: %q", orig)
	}
	s = s[1:]

	var d time.Duration
	inTime := false
	num := ""
	for _, r := range s {
		switch {
		case r >= '0' && r <= '9':
			num += string(r)
			continue
		case r == 'T':
			inTime = true
			continue
		}
		if num == "" {
			return 0, fmt.Errorf("unit %q with no count in %q", r, orig)
		}
		n, err := strconv.Atoi(num)
		if err != nil {
			return 0, fmt.Errorf("bad count in %q", orig)
		}
		num = ""
		var unit time.Duration
		switch r {
		case 'W':
			unit = 7 * 24 * time.Hour
		case 'D':
			unit = 24 * time.Hour
		case 'H':
			if !inTime {
				return 0, fmt.Errorf("H outside time part in %q", orig)
			}
			unit = time.Hour
		case 'M':
			if !inTime {
				return 0, fmt.Errorf("M outside time part in %q", orig)
			}
			unit = time.Minute
		case 'S':
			if !inTime {
				return 0, fmt.Errorf("S outside time part in %q", orig)
			}
			unit = time.Second
		default:
			return 0, fmt.Errorf("unknown unit %q in %q", r, orig)
		}
		d += time.Duration(n) * unit
	}
	if num != "" {
		return 0, fmt.Errorf("trailing count in %q", orig)
	}
	if neg {
		d = -d
	}
	return d, nil
}