Snip/backend/internal/service/shortcode.go

package service

import (
	"crypto/rand"
	"math/big"
	"regexp"
	"strings"

	"github.com/snip/backend/internal/domain"
)

const base62 = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"

// ReservedCodes can never be claimed as custom aliases, because these top-level
// paths are owned by the frontend / api commands.
var ReservedCodes = map[string]struct{}{
	"":            {},
	"api":         {},
	"login":       {},
	"dashboard":   {},
	"assets":      {},
	"static":      {},
	"healthz":     {},
	"favicon.svg": {},
	"favicon.ico": {},
	"robots.txt":  {},
	"unlock":      {},
}

// IsReserved reports whether a code collides with an app-owned path.
func IsReserved(code string) bool {
	_, ok := ReservedCodes[strings.ToLower(code)]
	return ok
}

func encodeBase62(n int64) string {
	if n == 0 {
		return string(base62[0])
	}
	var b strings.Builder
	for n > 0 {
		b.WriteByte(base62[n%62])
		n /= 62
	}
	// reverse
	s := []byte(b.String())
	for i, j := 0, len(s)-1; i < j; i, j = i+1, j-1 {
		s[i], s[j] = s[j], s[i]
	}
	return string(s)
}

// scramble spreads sequential ids so codes don't look enumerable, while staying
// short. Reversible multiplicative hash over a 31-bit space.
func scramble(id int64) int64 {
	const prime = 2654435761
	const mod = 0x7fffffff
	return ((id + 1) * prime) % mod
}

// CodeFromSequence turns a DB sequence value into the shortest unique base62
// code (no collision checks needed — the sequence guarantees uniqueness).
func CodeFromSequence(seq int64) string {
	return encodeBase62(scramble(seq + 100000))
}

var aliasRe = regexp.MustCompile(`^[a-zA-Z0-9_-]{3,32}$`)

// ValidateAlias checks a user-supplied custom code.
func ValidateAlias(alias string) error {
	if !aliasRe.MatchString(alias) {
		return domain.ErrInvalidAlias
	}
	if IsReserved(alias) {
		return domain.ErrReserved
	}
	return nil
}

var urlRe = regexp.MustCompile(`(?i)^https?://[^\s.]+\.[^\s]{2,}$`)

// NormalizeURL ensures a scheme is present.
func NormalizeURL(raw string) string {
	s := strings.TrimSpace(raw)
	if s == "" {
		return ""
	}
	low := strings.ToLower(s)
	if strings.HasPrefix(low, "http://") || strings.HasPrefix(low, "https://") {
		return s
	}
	return "https://" + s
}

// ValidURL validates a normalized destination.
func ValidURL(raw string) bool {
	return urlRe.MatchString(raw)
}

var pinRe = regexp.MustCompile(`^\d{6}$`)

// ValidPin reports whether a string is exactly 6 digits.
func ValidPin(pin string) bool { return pinRe.MatchString(pin) }

func pick(arr []string) string {
	n, _ := rand.Int(rand.Reader, big.NewInt(int64(len(arr))))
	return arr[n.Int64()]
}

// MemorableSlug builds an easy-to-say "adjective-noun-verb" code.
func MemorableSlug() string {
	return pick(adjectives) + "-" + pick(nouns) + "-" + pick(verbs)
}

// newID mints an opaque link id.
func newID() string {
	const n = 10
	b := make([]byte, n)
	for i := range b {
		idx, _ := rand.Int(rand.Reader, big.NewInt(62))
		b[i] = base62[idx.Int64()]
	}
	return "l_" + string(b)
}