luhndemo.gno

  1// Package luhndemo is a small gnoweb demo of the Luhn mod-10 checksum provided
  2// by the [p/moul/x/daily/luhn](/p/moul/x/daily/luhn/v0) library: put a number
  3// in the path and it tells you whether the check digit is right, and what it
  4// should have been.
  5//
  6// It contains no checksum logic of its own — validation, the check digit and
  7// the separator handling all come from the library. Read-only and stateless, so
  8// Render is fully deterministic.
  9package luhndemo
 10
 11import (
 12	"strconv"
 13	"strings"
 14
 15	"gno.land/p/g1magld5ptgzaktqcexxq0pgq5lmdf0kz6re8hlv/md/v0"
 16	"gno.land/p/g1magld5ptgzaktqcexxq0pgq5lmdf0kz6re8hlv/x/daily/luhn/v0"
 17)
 18
 19// samples are shown on the root page: a valid number, a typo'd one, and a
 20// transposition — the two error classes Luhn is designed to catch.
 21var samples = []struct {
 22	number string
 23	note   string
 24}{
 25	{"79927398713", "the textbook example"},
 26	{"79927398710", "same number, wrong check digit"},
 27	{"4539148803436467", "a test card number"},
 28	{"4539148803436647", "same card, two digits swapped"},
 29}
 30
 31// Render renders the checker for gnoweb.
 32//
 33//	Render("")        / Render("/") -> usage + worked samples
 34//	Render("/<digits>")             -> verdict for that number
 35func Render(path string) string {
 36	in := parseInput(path)
 37
 38	var b strings.Builder
 39	b.WriteString("# Luhn Checksum\n\n")
 40	b.WriteString("The mod-10 check behind card numbers and IMEIs, demoing the ")
 41	b.WriteString("[`p/moul/x/daily/luhn`](/p/moul/x/daily/luhn/v0) library.\n\n")
 42
 43	if in == "" {
 44		b.WriteString("Append a number to the path to check it — spaces and hyphens are ignored.\n\n")
 45		b.WriteString("## Samples\n\n")
 46		b.WriteString("| number | valid? | note |\n|---|---|---|\n")
 47		for _, s := range samples {
 48			b.WriteString("| `")
 49			b.WriteString(s.number)
 50			b.WriteString("` | ")
 51			b.WriteString(verdict(luhn.Valid(s.number)))
 52			b.WriteString(" | ")
 53			b.WriteString(s.note)
 54			b.WriteString(" |\n")
 55		}
 56		b.WriteString("\n> Luhn is a **typo check, not a security check**: it catches every ")
 57		b.WriteString("single-digit error and nearly every swap of adjacent digits, but ")
 58		b.WriteString("anyone can compute a valid number. Never authorize anything with it.\n")
 59		return b.String()
 60	}
 61
 62	b.WriteString("## " + md.InlineCode(in) + "\n\n")
 63
 64	if luhn.Valid(in) {
 65		b.WriteString("**Valid** — the check digit matches.\n")
 66		return b.String()
 67	}
 68
 69	b.WriteString("**Not valid.**\n\n")
 70	// Offer the fix: treat everything but the last digit as the payload.
 71	if payload, ok := trimLast(in); ok {
 72		if d, ok2 := luhn.CheckDigit(payload); ok2 {
 73			b.WriteString("Keeping the leading digits, the check digit should be **")
 74			b.WriteString(strconv.Itoa(d))
 75			b.WriteString("**")
 76			if fixed, ok3 := luhn.Append(payload); ok3 {
 77				b.WriteString(" — i.e. `")
 78				b.WriteString(fixed)
 79				b.WriteString("`")
 80			}
 81			b.WriteString(".\n")
 82			return b.String()
 83		}
 84	}
 85	b.WriteString("It has no usable digits to check.\n")
 86	return b.String()
 87}
 88
 89// parseInput extracts the first path segment.
 90func parseInput(path string) string {
 91	s := strings.TrimSpace(path)
 92	s = strings.TrimPrefix(s, "/")
 93	if i := strings.IndexByte(s, '/'); i >= 0 {
 94		s = s[:i]
 95	}
 96	return s
 97}
 98
 99// trimLast drops the final digit, ignoring separators, to recover the payload.
100func trimLast(s string) (string, bool) {
101	for i := len(s) - 1; i >= 0; i-- {
102		if s[i] >= '0' && s[i] <= '9' {
103			return s[:i], true
104		}
105	}
106	return "", false
107}
108
109func verdict(ok bool) string {
110	if ok {
111		return "✅ valid"
112	}
113	return "❌ invalid"
114}