173 lines
7.1 KiB
Go
173 lines
7.1 KiB
Go
// Package cid centralizes CID-format helpers: decoding a multibase-base32
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// CIDv1 into its multicodec parts, ranking by utility for external
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// presentation, and picking which CID is the canonical externally-visible
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// identifier of a file.
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//
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// Sibling CIDs are stored as a bare-CID key-set (`cids/<cid> = "true"`); a
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// CID is self-describing — its algorithm is recoverable from the CIDv1
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// codec / multihash code, so there is no `<algorithm>:` prefix and no
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// per-algorithm field name. Ranking and selection parse the multicodec
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// rather than a string prefix.
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package cid
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import (
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"encoding/base32"
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"encoding/binary"
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"fmt"
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"strings"
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)
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// Multicodec / multihash codes used for ranking. Source of truth for the
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// digest codes is meta-hash/src/lib/hash-compute/MultiHashData.ts and
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// METADATA_KEYS.md §2.
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//
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// The tier every code maps to is pinned by the golden fixture
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// `cid-rank-vectors.json` (vendored next to this package). This is one of
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// seven implementations of the same ladder across Go, Rust and TypeScript;
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// the fixture is the only thing keeping them in agreement. If you add a code
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// here, add a vector — an unranked code silently falls to tier 0, which is
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// exactly how the nzb-release/url locators came to be ranked 20 in meta-share
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// and 0 here and in meta-search.
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const (
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CodecRaw = 0x55 // raw — IPFS content-addressed block
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CodecDagPB = 0x70 // dag-pb — IPFS content-addressed block
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CodeSha1 = 0x11 // multihash sha1
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CodeSha256 = 0x12 // multihash sha2-256
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CodeSha3_384 = 0x15 // multihash sha3-384
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CodeSha3_256 = 0x16 // multihash sha3-256
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CodeMd5 = 0xd5 // multihash md5
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CodeCrc32 = 0x0132 // multihash crc32
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CodeMidhash256 = 0x1000 // custom — SHA-256(size‖middle 1MB)
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CodeBtPiecesRoot = 0xb702 // bittorrent v2 pieces root
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CodeBtihV2 = 0x10B7 // custom — BitTorrent v2 info hash (BEP 52)
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CodeBtihV1File = 0x1001 // custom — per-file BitTorrent v1 CID
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CodeBtihV2File = 0x1002 // custom — per-file BitTorrent v2 CID
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// Opaque locators. These are NOT digests: the payload rides in an identity
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// multihash (0x00), so the code identifying them sits in the *content
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// codec* slot, never the multihash slot. Rank must match on the codec for
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// these — matching on mh would see 0x00 and rank them 0.
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CodeNzbRelease = 0x1005 // custom — self-describing Newznab release locator
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CodeURL = 0x1006 // custom — identity-multihash CID wrapping an http(s) URL
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)
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// Rank tiers, named so the ladder reads as a ladder.
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const (
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RankIPFS = 40 // dag-pb — broadest network reach
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RankDigest = 30 // sha2-256 / sha3-* — real content digests
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RankBtih = 20 // BitTorrent infohashes — swarm interop
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RankMidhash = 10 // the record address — internally meaningful, externally invisible
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RankLocator = 5 // opaque locators — never outrank a real digest
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RankUnknown = 0 // weak/unknown digests, unparseable input
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)
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// Decode parses a multibase-base32 CIDv1 string ("b…") into its content
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// codec and multihash function code. It is the inverse of the encoder in
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// internal/watcher/midhash.go. Returns an error for anything that isn't a
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// CIDv1 in base32lower (e.g. a CIDv0 "Qm…" or a non-CID string).
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func Decode(cidStr string) (codec uint64, mhCode uint64, err error) {
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if len(cidStr) < 2 || cidStr[0] != 'b' {
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return 0, 0, fmt.Errorf("cid %q: not a multibase base32 CIDv1", cidStr)
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}
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raw, err := base32.StdEncoding.WithPadding(base32.NoPadding).DecodeString(strings.ToUpper(cidStr[1:]))
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if err != nil {
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return 0, 0, fmt.Errorf("cid %q: base32 decode: %w", cidStr, err)
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}
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version, n := binary.Uvarint(raw)
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if n <= 0 {
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return 0, 0, fmt.Errorf("cid %q: bad version varint", cidStr)
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}
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raw = raw[n:]
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if version != 1 {
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return 0, 0, fmt.Errorf("cid %q: unsupported CID version %d", cidStr, version)
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}
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codec, n = binary.Uvarint(raw)
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if n <= 0 {
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return 0, 0, fmt.Errorf("cid %q: bad codec varint", cidStr)
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}
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raw = raw[n:]
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mhCode, n = binary.Uvarint(raw)
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if n <= 0 {
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return 0, 0, fmt.Errorf("cid %q: bad multihash-code varint", cidStr)
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}
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return codec, mhCode, nil
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}
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// Rank returns a score for a bare CID. Higher = more preferred as the
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// public-facing "canonical" identifier for a file. Unknown / undecodable
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// CIDs rank 0 so a new digest type can't accidentally outrank an
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// established one until it's classified here.
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//
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// We rank by the **multihash code** (the algorithm), with the custom
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// BitTorrent CID codecs checked alongside it. This is deliberate: MetaMesh's
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// two CID encoders disagree on the *content codec* — meta-hash/the watcher
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// set codec == algorithm code, while the fullhash plugin and gateway wrap
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// every digest with the raw codec (0x55) and put the algorithm in the
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// multihash code. The multihash code is the algorithm in all of them, so it
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// is the only reliable discriminator. (The BT info hash is given a distinct
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// multihash code — btih-v2 0x10B7 — so it can't be confused with a plain
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// sha2-256, see METADATA_KEYS.md §14.13.)
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//
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// Ordering rationale (utility-weighted, NOT cryptographic-strength):
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// - dag-pb (chunked IPFS file) wins: best retrieval over the IPFS network.
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// - sha2-256 / sha3 digests follow: standard content hashes (a raw IPFS
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// block of the file IS its sha2-256, so they share this tier).
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// - BitTorrent infohashes (btih v2, per-file v1/v2) outrank midhash256 for
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// swarm interop, not strength.
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// - midhash256 is the floor among real *digests*: internally meaningful,
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// externally invisible.
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// - opaque locators (nzb-release, url) sit BELOW midhash. They are not
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// content-derived — two URLs to the same bytes yield two different locator
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// CIDs — so a locator must never displace a real digest as the identity a
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// record is advertised under. Tier 5 means a locator wins the election only
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// when it is the record's sole CID, which is exactly the guarantee
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// METADATA_KEYS.md §2 states.
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// - md5 / crc32 / plain sha1 / pieces-root / unknown rank 0.
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func Rank(cidStr string) int {
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codec, mh, err := Decode(cidStr)
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if err != nil {
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return RankUnknown
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}
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if codec == CodecDagPB {
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return RankIPFS
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}
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// BitTorrent: distinct CID codecs (per-file v1/v2) or the btih-v2
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// multihash code, in either encoder convention.
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if codec == CodeBtihV1File || codec == CodeBtihV2File || codec == CodeBtihV2 ||
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mh == CodeBtihV1File || mh == CodeBtihV2File || mh == CodeBtihV2 {
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return RankBtih
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}
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// Opaque locators. Codec-only: their multihash is identity (0x00), so
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// there is nothing to match on the mh side.
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if codec == CodeNzbRelease || codec == CodeURL {
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return RankLocator
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}
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switch mh {
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case CodeSha256, CodeSha3_256, CodeSha3_384:
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return RankDigest
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case CodeMidhash256:
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return RankMidhash
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default:
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// md5, crc32, plain sha1, pieces-root, unknown.
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return RankUnknown
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}
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}
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// Better picks the higher-ranked of two CIDs. On ties, returns the
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// lexicographically smaller one — gives the reconciler a deterministic
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// answer so the chosen canonical doesn't churn when peers add the same
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// digest under a different ordering.
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func Better(a, b string) string {
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ra, rb := Rank(a), Rank(b)
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if ra > rb {
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return a
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}
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if rb > ra {
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return b
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}
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if a <= b {
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return a
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}
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return b
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}
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