package tldr

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

const discordEpochMs = 1420070400000

var sinceRe = regexp.MustCompile(`^(\d+)([mhd])$`)

// ParseSince turns "30m", "6h" or "2d" into a duration, clamped to
// [1m, maxWindow]. time.ParseDuration has no day unit, hence the custom parser.
func ParseSince(s string) (time.Duration, error) {
	m := sinceRe.FindStringSubmatch(strings.ToLower(strings.TrimSpace(s)))
	if m == nil {
		return 0, fmt.Errorf("couldn't read %q, use something like 30m, 6h or 2d", s)
	}
	n, err := strconv.Atoi(m[1])
	if err != nil || n <= 0 {
		return 0, fmt.Errorf("couldn't read %q, use something like 30m, 6h or 2d", s)
	}

	unit := map[string]time.Duration{"m": time.Minute, "h": time.Hour, "d": 24 * time.Hour}[m[2]]
	if n > int(maxWindow/unit) {
		return maxWindow, nil
	}
	return max(time.Duration(n)*unit, time.Minute), nil
}

// SnowflakeFromTime returns the smallest Discord snowflake for t, usable as
// an "after" cursor to mean "messages posted after this moment".
func SnowflakeFromTime(t time.Time) string {
	ms := max(t.UnixMilli()-discordEpochMs, 0)
	return strconv.FormatInt(ms<<22, 10)
}

// CompareIDs compares two snowflakes numerically (-1, 0, 1).
func CompareIDs(a, b string) int {
	ai, aerr := strconv.ParseUint(a, 10, 64)
	bi, berr := strconv.ParseUint(b, 10, 64)
	if aerr != nil || berr != nil {
		return strings.Compare(a, b)
	}
	switch {
	case ai < bi:
		return -1
	case ai > bi:
		return 1
	}
	return 0
}