gno.land/r/moul/x/daily/luhndemo/v0⧉

1// Package luhndemo is a small gnoweb demo of the Luhn mod-10 checksum provided2// by the [p/moul/x/daily/luhn](/p/moul/x/daily/luhn/v0) library: put a number3// in the path and it tells you whether the check digit is right, and what it4// should have been.5//6// It contains no checksum logic of its own — validation, the check digit and7// the separator handling all come from the library. Read-only and stateless, so8// Render is fully deterministic.9package luhndemo1011import (12	"strconv"13	"strings"1415	"gno.land/p/moul/x/daily/luhn/v0"16)1718// samples are shown on the root page: a valid number, a typo'd one, and a19// transposition — the two error classes Luhn is designed to catch.20var samples = []struct {21	number string22	note   string23}{24	{"79927398713", "the textbook example"},25	{"79927398710", "same number, wrong check digit"},26	{"4539148803436467", "a test card number"},27	{"4539148803436647", "same card, two digits swapped"},28}2930// Render renders the checker for gnoweb.31//32//	Render("")        / Render("/") -> usage + worked samples33//	Render("/<digits>")             -> verdict for that number34func Render(path string) string {35	in := parseInput(path)3637	var b strings.Builder38	b.WriteString("# Luhn Checksum\n\n")39	b.WriteString("The mod-10 check behind card numbers and IMEIs, demoing the ")40	b.WriteString("[`p/moul/x/daily/luhn`](/p/moul/x/daily/luhn/v0) library.\n\n")4142	if in == "" {43		b.WriteString("Append a number to the path to check it — spaces and hyphens are ignored.\n\n")44		b.WriteString("## Samples\n\n")45		b.WriteString("| number | valid? | note |\n|---|---|---|\n")46		for _, s := range samples {47			b.WriteString("| `")48			b.WriteString(s.number)49			b.WriteString("` | ")50			b.WriteString(verdict(luhn.Valid(s.number)))51			b.WriteString(" | ")52			b.WriteString(s.note)53			b.WriteString(" |\n")54		}55		b.WriteString("\n> Luhn is a **typo check, not a security check**: it catches every ")56		b.WriteString("single-digit error and nearly every swap of adjacent digits, but ")57		b.WriteString("anyone can compute a valid number. Never authorize anything with it.\n")58		return b.String()59	}6061	b.WriteString("## `")62	b.WriteString(in)63	b.WriteString("`\n\n")6465	if luhn.Valid(in) {66		b.WriteString("**Valid** — the check digit matches.\n")67		return b.String()68	}6970	b.WriteString("**Not valid.**\n\n")71	// Offer the fix: treat everything but the last digit as the payload.72	if payload, ok := trimLast(in); ok {73		if d, ok2 := luhn.CheckDigit(payload); ok2 {74			b.WriteString("Keeping the leading digits, the check digit should be **")75			b.WriteString(strconv.Itoa(d))76			b.WriteString("**")77			if fixed, ok3 := luhn.Append(payload); ok3 {78				b.WriteString(" — i.e. `")79				b.WriteString(fixed)80				b.WriteString("`")81			}82			b.WriteString(".\n")83			return b.String()84		}85	}86	b.WriteString("It has no usable digits to check.\n")87	return b.String()88}8990// parseInput extracts the first path segment.91func parseInput(path string) string {92	s := strings.TrimSpace(path)93	s = strings.TrimPrefix(s, "/")94	if i := strings.IndexByte(s, '/'); i >= 0 {95		s = s[:i]96	}97	return s98}99100// trimLast drops the final digit, ignoring separators, to recover the payload.101func trimLast(s string) (string, bool) {102	for i := len(s) - 1; i >= 0; i-- {103		if s[i] >= '0' && s[i] <= '9' {104			return s[:i], true105		}106	}107	return "", false108}109110func verdict(ok bool) string {111	if ok {112		return "✅ valid"113	}114	return "❌ invalid"115}116