optimize cid header inspection

Author: turbocrime

.gas-snapshot

@@ -81,11 +81,11 @@ ReadBignum_Test:test_UInt256_middle() (gas: 5686)
 ReadBignum_Test:test_UInt256_multi() (gas: 4074)
 ReadBignum_Test:test_UInt256_random(uint256) (runs: 256, μ: 4567, ~: 4567)
 ReadBignum_Test:test_UInt256_single() (gas: 4367)
-ReadCidSha256_Test:test_Cid() (gas: 853)
-ReadCidSha256_Test:test_Cid_multicodec_raw() (gas: 799)
-ReadCidSha256_Test:test_Cid_random(uint256) (runs: 256, μ: 4260, ~: 4260)
+ReadCidSha256_Test:test_Cid() (gas: 590)
+ReadCidSha256_Test:test_Cid_multicodec_raw() (gas: 518)
+ReadCidSha256_Test:test_Cid_random(uint256) (runs: 256, μ: 4003, ~: 4003)
 ReadCidSha256_Test:test_NullableCid_nullCbor() (gas: 3440)
-ReadCidSha256_Test:test_NullableCid_random(uint256) (runs: 256, μ: 4121, ~: 4129)
+ReadCidSha256_Test:test_NullableCid_random(uint256) (runs: 256, μ: 3868, ~: 3872)
 SimpleTest:test_decodeFalse() (gas: 3305)
 SimpleTest:test_decodeTrue() (gas: 3311)
 SimpleTest:test_skipNull() (gas: 427)

lcov.info

@@ -472,30 +472,30 @@ BRH:12
 end_of_record
 TN:
 SF:src/tags/ReadCidSha256.sol
-DA:67,512
-FN:67,ReadCidSha256.Cid
+DA:66,512
+FN:66,ReadCidSha256.Cid
 FNDA:512,ReadCidSha256.Cid
-DA:68,512
-DA:71,513
-FN:71,ReadCidSha256.Cid
+DA:67,512
+DA:70,513
+FN:70,ReadCidSha256.Cid
 FNDA:513,ReadCidSha256.Cid
+DA:75,513
 DA:76,513
-DA:77,513
-DA:78,513
-DA:83,513
-DA:84,513
+DA:79,513
 DA:85,513
-DA:88,513
-BRDA:88,0,0,1
-BRDA:88,0,1,512
-DA:89,512
-BRDA:89,1,0,2
-BRDA:89,1,1,510
-DA:91,510
-DA:101,258
-FN:101,ReadCidSha256.NullableCid
+DA:86,513
+DA:87,513
+DA:90,513
+BRDA:90,0,0,1
+BRDA:90,0,1,512
+DA:91,512
+BRDA:91,1,0,2
+BRDA:91,1,1,510
+DA:93,510
+DA:103,258
+FN:103,ReadCidSha256.NullableCid
 FNDA:258,ReadCidSha256.NullableCid
-DA:102,258
+DA:104,258
 FNF:3
 FNH:3
 LF:14

src/tags/ReadCidSha256.sol

@@ -1,6 +1,7 @@
 // SPDX-License-Identifier: Apache-2.0
 pragma solidity ^0.8.28;
 
+import "forge-std/console.sol";
 import "../ReadCbor.sol";
 
 // we will only encounter CID v1 dag-cbor sha256, which perfectly fits uint256.
@@ -45,10 +46,8 @@ library ReadCidSha256 {
     //       12    │ multihash type sha-256
     //       20    │ multihash size 32 bytes
     //    ─────────┴─────────
-    bytes6 private constant cborTag42_cborBytes37_multibaseCidV1 = hex"D82A58250001";
-    bytes1 private constant multicodecDagCbor = hex"71";
-    bytes1 private constant multicodecRaw = hex"55";
-    bytes2 private constant multihashSha256_multihashBytes32 = hex"1220";
+    bytes9 private constant cborTag42_cborBytes37_multibaseCidV1_multicodecZERO_multihashSha256_multihashBytes32 =
+        hex"D82A58250001001220";
 
     /**
      * @notice Reads a CID from CBOR encoded data at the specified byte index
@@ -65,27 +64,30 @@ library ReadCidSha256 {
      * @return CidSha256 The representative hash
      */
     function Cid(bytes memory cbor, uint32 i) internal pure returns (uint32, CidSha256) {
-        return Cid(cbor, i, multicodecDagCbor);
+        return Cid(cbor, i, 0x71);
     }
 
     function Cid(bytes memory cbor, uint32 i, bytes1 multicodec)
         internal
         pure
         returns (uint32 n, CidSha256 cidSha256)
     {
-        assert(multicodec == multicodecDagCbor || multicodec == multicodecRaw);
-        bytes9 cborMultibase;
-        bytes9 expect =
-            bytes9(bytes.concat(cborTag42_cborBytes37_multibaseCidV1, multicodec, multihashSha256_multihashBytes32));
-
+        bytes9 expect;
+        bytes9 cborHeader;
         assembly ("memory-safe") {
+            // expected head
+            expect :=
+                or(
+                    cborTag42_cborBytes37_multibaseCidV1_multicodecZERO_multihashSha256_multihashBytes32,
+                    shr(0x30, multicodec)
+                )
             // cbor header at index
-            cborMultibase := mload(add(cbor, add(0x20, i)))
+            cborHeader := mload(add(cbor, add(0x20, i)))
             cidSha256 := mload(add(cbor, add(0x29, i)))
             n := add(i, 41) // 4 bytes cbor header + 5 bytes multibase header + 32 bytes hash
         }
 
-        require(cborMultibase == expect, "Expected CBOR tag 42 and 37-byte CIDv1");
+        require(expect == cborHeader, "Expected CBOR tag 42 and 37-byte CIDv1");
         require(!cidSha256.isNull(), "Expected non-zero CID value");
 
         cbor.requireRange(n);