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/g1magld5ptgzaktqcexxq0pgq5lmdf0kz6re8hlv/md/v0"
14	"gno.land/p/g1magld5ptgzaktqcexxq0pgq5lmdf0kz6re8hlv/x/daily/crc32/v0"
15)
16
17var samples = []string{"", "a", "abc", "123456789", "hello world", "hello worle"}
18
19// Render renders the demo for gnoweb.
20//
21//	Render("")       / Render("/") -> the samples table
22//	Render("/<text>")              -> checksum that text
23func Render(path string) string {
24	var b strings.Builder
25	b.WriteString("# CRC-32\n\n")
26	b.WriteString("The IEEE checksum behind zip, gzip and PNG, demoing the ")
27	b.WriteString("[`p/moul/x/daily/crc32`](/p/moul/x/daily/crc32/v0) library.\n\n")
28
29	if in := parseArg(path); in != "" {
30		b.WriteString("## " + md.InlineCode(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			//gnovet:ignore render-path-in-code-span s ranges over a fixed sample list, never over the URL path
44			b.WriteString("`" + s + "`")
45		}
46		b.WriteString(" | `")
47		b.WriteString(crc32.ChecksumHex(s))
48		b.WriteString("` |\n")
49	}
50	b.WriteString("\n`123456789` → `cbf43926` is the standard CRC-32 check value, ")
51	b.WriteString("so this table doubles as a conformance test.\n\n")
52	b.WriteString("> The last two rows differ by one letter and share no digits: ")
53	b.WriteString("that avalanche is the point.\n\n")
54	b.WriteString("> A CRC detects **accidents**, not tampering — it is linear, so a ")
55	b.WriteString("matching message is easy to craft. Never authenticate with it.\n")
56	return b.String()
57}
58
59func parseArg(path string) string {
60	s := strings.TrimSpace(path)
61	s = strings.TrimPrefix(s, "/")
62	if i := strings.IndexByte(s, '/'); i >= 0 {
63		s = s[:i]
64	}
65	return s
66}