crc32demo.gno

 1// Package crc32demo is a small gnoweb demo of the CRC-32 checksum provided by
 2// the [p/moul/x/daily/crc32](/p/moul/x/daily/crc32/v0) library: it checksums a
 3// few strings and shows that a one-character change moves the result
 4// completely.
 5//
 6// It contains no checksum logic of its own. Stateless, so Render is
 7// deterministic.
 8package crc32demo
 9
10import (
11	"strings"
12
13	"gno.land/p/moul/x/daily/crc32/v0"
14)
15
16var samples = []string{"", "a", "abc", "123456789", "hello world", "hello worle"}
17
18// Render renders the demo for gnoweb.
19//
20//	Render("")       / Render("/") -> the samples table
21//	Render("/<text>")              -> checksum that text
22func Render(path string) string {
23	var b strings.Builder
24	b.WriteString("# CRC-32\n\n")
25	b.WriteString("The IEEE checksum behind zip, gzip and PNG, demoing the ")
26	b.WriteString("[`p/moul/x/daily/crc32`](/p/moul/x/daily/crc32/v0) library.\n\n")
27
28	if in := parseArg(path); in != "" {
29		b.WriteString("## `")
30		b.WriteString(in)
31		b.WriteString("`\n\n`")
32		b.WriteString(crc32.ChecksumHex(in))
33		b.WriteString("`\n")
34		return b.String()
35	}
36
37	b.WriteString("| input | crc32 |\n|---|---|\n")
38	for _, s := range samples {
39		b.WriteString("| ")
40		if s == "" {
41			b.WriteString("_(empty)_")
42		} else {
43			b.WriteString("`" + s + "`")
44		}
45		b.WriteString(" | `")
46		b.WriteString(crc32.ChecksumHex(s))
47		b.WriteString("` |\n")
48	}
49	b.WriteString("\n`123456789` → `cbf43926` is the standard CRC-32 check value, ")
50	b.WriteString("so this table doubles as a conformance test.\n\n")
51	b.WriteString("> The last two rows differ by one letter and share no digits: ")
52	b.WriteString("that avalanche is the point.\n\n")
53	b.WriteString("> A CRC detects **accidents**, not tampering — it is linear, so a ")
54	b.WriteString("matching message is easy to craft. Never authenticate with it.\n")
55	return b.String()
56}
57
58func parseArg(path string) string {
59	s := strings.TrimSpace(path)
60	s = strings.TrimPrefix(s, "/")
61	if i := strings.IndexByte(s, '/'); i >= 0 {
62		s = s[:i]
63	}
64	return s
65}