// SPDX-FileCopyrightText: 2026 Echolot contributors
// SPDX-License-Identifier: GPL-3.0-or-later

// Package canarydns serves the authoritative canary zone (spec §6.1). The
// reference records below are FROZEN by the protocol spec — names, TTLs, and
// RDATA are ground truth the client compares against, so they must never
// change without a spec revision and a matching update in the app. All
// addresses are documentation-range (RFC 5737 192.0.2.0/24, RFC 3849
// 2001:db8::/32).
package canarydns

import "net/netip"

// refRecord is one frozen reference name (relative to the zone) with its
// per-type answers. A zero value in a field means "no record of that type".
type refRecord struct {
	label string
	ttl   uint32
	a     []netip.Addr // A / AAAA answers (order preserved)
	txt   []string     // one string per TXT record
}

// referenceRecords are the spec §6.1 fixed records. The RDATA constants are
// frozen HERE (the spec calls this file the source of truth) and mirrored in
// the app. Order within many-rr is part of the test (order/stripping check).
var referenceRecords = []refRecord{
	{label: "ttl-5", ttl: 5,
		a:   []netip.Addr{netip.MustParseAddr("192.0.2.5"), netip.MustParseAddr("2001:db8::5")},
		txt: []string{"echolot-ref ttl=5"}},
	{label: "ttl-60", ttl: 60,
		a:   []netip.Addr{netip.MustParseAddr("192.0.2.60"), netip.MustParseAddr("2001:db8::60")},
		txt: []string{"echolot-ref ttl=60"}},
	{label: "ttl-3600", ttl: 3600,
		a:   []netip.Addr{netip.MustParseAddr("192.0.2.36"), netip.MustParseAddr("2001:db8::3600")},
		txt: []string{"echolot-ref ttl=3600"}},
	{label: "ttl-86400", ttl: 86400,
		a:   []netip.Addr{netip.MustParseAddr("192.0.2.86"), netip.MustParseAddr("2001:db8::8640")},
		txt: []string{"echolot-ref ttl=86400"}},
	// many-rr: exactly 8 A records in defined order.
	{label: "many-rr", ttl: 300, a: []netip.Addr{
		netip.MustParseAddr("192.0.2.101"), netip.MustParseAddr("192.0.2.102"),
		netip.MustParseAddr("192.0.2.103"), netip.MustParseAddr("192.0.2.104"),
		netip.MustParseAddr("192.0.2.105"), netip.MustParseAddr("192.0.2.106"),
		netip.MustParseAddr("192.0.2.107"), netip.MustParseAddr("192.0.2.108"),
	}},
	// big-txt: ~1800 bytes, exercises EDNS bufsize / TCP fallback.
	{label: "big-txt", ttl: 300, txt: bigTxt()},
}

// bigTxt builds a deterministic ~1800-byte TXT payload as a sequence of
// 255-byte character-strings (the DNS TXT chunk limit). The content is fixed
// so the client can verify integrity, not just length.
func bigTxt() []string {
	const total = 1800
	const pattern = "echolot-big-txt-reference-0123456789abcdef-"
	buf := make([]byte, 0, total)
	for len(buf) < total {
		buf = append(buf, pattern...)
	}
	buf = buf[:total]
	var out []string
	for len(buf) > 0 {
		n := min(255, len(buf))
		out = append(out, string(buf[:n]))
		buf = buf[n:]
	}
	return out
}

// findReference returns the reference record for a label, or nil.
func findReference(label string) *refRecord {
	for i := range referenceRecords {
		if referenceRecords[i].label == label {
			return &referenceRecords[i]
		}
	}
	return nil
}