package chlog

import (
	"fmt"
	"regexp"
	"strconv"
	"time"
)

var durationRe = regexp.MustCompile(`^(\d+)([smhdw])$`)

var durationUnits = map[string]time.Duration{
	"s": time.Second,
	"m": time.Minute,
	"h": time.Hour,
	"d": 24 * time.Hour,
	"w": 7 * 24 * time.Hour,
}

// ParseTimeSpec accepts a relative duration (15m, 1h, 2d, 1w) meaning "that
// long before now", or an absolute RFC3339 timestamp.
func ParseTimeSpec(spec string, now time.Time) (time.Time, error) {
	if spec == "" {
		return time.Time{}, fmt.Errorf("empty time spec")
	}
	if m := durationRe.FindStringSubmatch(spec); m != nil {
		n, err := strconv.ParseInt(m[1], 10, 64)
		if err != nil {
			return time.Time{}, fmt.Errorf("invalid duration %q: %w", spec, err)
		}
		return now.Add(-time.Duration(n) * durationUnits[m[2]]), nil
	}
	t, err := time.Parse(time.RFC3339, spec)
	if err != nil {
		return time.Time{}, fmt.Errorf("invalid time %q: use a duration (15m, 1h, 2d) or RFC3339", spec)
	}
	return t.UTC(), nil
}