feat: add MSET "hashing" function spec

specs/mset.md

@@ -0,0 +1,44 @@
+# MSET is a "hashing" function that encode repating sets of bytes
+
+The name is inspired by the memset function.
+
+It is similar to identity as it isn't a hash, but a complete representation of the data.
+
+The main goal of MSET is not to be used for effective data compression.
+The goal is instead to compress the trivial cases of data padding.
+
+## Digest decoding
+
+```
+<varuint count - 2><pattern>
+```
+
+First, you read a varuint, this is the number of time the pattern must be repeated minus two (so you add two to get the true value).
+
+Everything left in your buffer is the pattern to repeat.
+
+If there is nothing left in the buffer (that mean that the count is the ONLY thing in the digest), then the pattern is `0x00`.
+
+The varuint count MUST be minimal and complete, if it's not that an invalid MSET hash.
+
+The pattern size SHOULD be a power of two (implementations could likely use faster vectorized loops then).
+
+## Examples
+
+- `0x0242` -> `0x42424242`; repeat a `uint8` equal to `0x42` 4 times
+- `0x001234` -> `0x12341234`; repeat a `uint16` equal to `0x1234` 2 times
+- `0x7e42` -> `0x42 * 128`; repeat a `uint8` equal to `0x42` 128 times
+- `0x7f1234` -> `0x1234 * 129`; repeat a `uint16` equal to `0x1234` 129 times
+- `0x800242` -> `0x42 * 256`; repeat a `uint8` equal to `0x42` 258 times
+- `0x01123456` -> `0x123456123456123456`; repeat a `uint24` equal to `0x123456123456123456` 3 times
+- `0x03` -> `0x0000000000`; zerofill 5 bytes.
+
+## Rational
+
+- Varuint count minus two.
+  The varuint count is minus two because counts of 0 and 1 are better served by an identity CIDs, it doesn't make sense to encode them here then.
+  Subbing by two allows powers 128 to be stored in one less byte.
+- No pattern equal zerofill.
+  An empty pattern would lead a multiplication by 0 of the size which would be empty data, however that is better served by identity CIDs.
+  We instead reuse this shorter value for something more usefull.
+  NUL bytes is the most popular padding value used in most apps, it make sense to grant them this one byte shorter opportunity.

table.csv

@@ -152,6 +152,7 @@ ed25519-priv,                   key,            0x1300,         draft,     Ed255
 secp256k1-priv,                 key,            0x1301,         draft,     Secp256k1 private key
 x25519-priv,                    key,            0x1302,         draft,     Curve25519 private key
 kangarootwelve,                 multihash,      0x1d01,         draft,     KangarooTwelve is an extendable-output hash function based on Keccak-p
+mset,                           multihash,      0x3488,         draft,     MSET "hashing" function; see specs/mset.md file
 sm3-256,                        multihash,      0x534d,         draft,
 blake2b-8,                      multihash,      0xb201,         draft,     Blake2b consists of 64 output lengths that give different hashes
 blake2b-16,                     multihash,      0xb202,         draft,