avoid double alloc in NewCidV1

We allocate once via "make([]byte, len)",
and again when that buffer is converted to a string.

Thankfully, since Go 1.10 we have strings.Builder,
designed specifically for this use case.

In a downstream benchmark in go-car,
which needs to reconstruct many CID values,
we see small but nice gains:

    name           old time/op    new time/op    delta
    ReadBlocks-16    1.09ms ± 4%    1.06ms ± 5%   -3.33%  (p=0.007 n=11+11)

    name           old speed      new speed      delta
    ReadBlocks-16   478MB/s ± 4%   494MB/s ± 5%   +3.46%  (p=0.007 n=11+11)

    name           old alloc/op   new alloc/op   delta
    ReadBlocks-16    1.30MB ± 0%    1.25MB ± 0%   -3.86%  (p=0.000 n=12+12)

    name           old allocs/op  new allocs/op  delta
    ReadBlocks-16     9.50k ± 0%     8.45k ± 0%  -11.05%  (p=0.000 n=12+12)

.github/workflows/go-check.yml

@@ -27,7 +27,7 @@ jobs:
             echo "RUNGOGENERATE=true" >> $GITHUB_ENV
           fi
       - name: Install staticcheck
-        run: go install honnef.co/go/tools/cmd/staticcheck@c8caa92bad8c27ae734c6725b8a04932d54a147b # 2021.1.2 (v0.2.2)
+        run: go install honnef.co/go/tools/cmd/staticcheck@df71e5d0e0ed317ebf43e6e59cf919430fa4b8f2 # 2021.1.1 (v0.2.1)
       - name: Check that go.mod is tidy
         uses: protocol/multiple-go-modules@v1.2
         with:

.github/workflows/go-test.yml

@@ -26,24 +26,13 @@ jobs:
         run: |
           go version
           go env
-      - name: Use msys2 on windows
-        if: ${{ matrix.os == 'windows' }}
-        shell: bash
-        # The executable for msys2 is also called bash.cmd
-        #   https://github.com/actions/virtual-environments/blob/main/images/win/Windows2019-Readme.md#shells
-        # If we prepend its location to the PATH
-        #   subsequent 'shell: bash' steps will use msys2 instead of gitbash
-        run: echo "C:/msys64/usr/bin" >> $GITHUB_PATH
       - name: Run repo-specific setup
         uses: ./.github/actions/go-test-setup
         if: hashFiles('./.github/actions/go-test-setup') != ''
       - name: Run tests
         uses: protocol/multiple-go-modules@v1.2
         with:
-          # Use -coverpkg=./..., so that we include cross-package coverage.
-          # If package ./A imports ./B, and ./A's tests also cover ./B,
-          # this means ./B's coverage will be significantly higher than 0%.
-          run: go test -v -coverprofile=module-coverage.txt -coverpkg=./... ./...
+          run: go test -v -coverprofile module-coverage.txt ./...
       - name: Run tests (32 bit)
         if: ${{ matrix.os != 'macos' }} # can't run 32 bit tests on OSX.
         uses: protocol/multiple-go-modules@v1.2
@@ -60,7 +49,7 @@ jobs:
         shell: bash
         run: echo "COVERAGES=$(find . -type f -name 'module-coverage.txt' | tr -s '\n' ',' | sed 's/,$//')" >> $GITHUB_ENV
       - name: Upload coverage to Codecov
-        uses: codecov/codecov-action@f32b3a3741e1053eb607407145bc9619351dc93b # v2.1.0
+        uses: codecov/codecov-action@51d810878be5422784e86451c0e7c14e5860ec47 # v2.0.2
         with:
           files: '${{ env.COVERAGES }}'
           env_vars: OS=${{ matrix.os }}, GO=${{ matrix.go }}

.github/workflows/release-check.yml

@@ -1,11 +0,0 @@
-# File managed by web3-bot. DO NOT EDIT.
-# See https://github.com/protocol/.github/ for details.
-
-name: Release Checker
-on:
-  pull_request:
-    paths: [ 'version.json' ]
-
-jobs:
-  release-check:
-    uses: protocol/.github/.github/workflows/release-check.yml@master

.github/workflows/releaser.yml

@@ -1,11 +0,0 @@
-# File managed by web3-bot. DO NOT EDIT.
-# See https://github.com/protocol/.github/ for details.
-
-name: Releaser
-on:
-  push:
-    paths: [ 'version.json' ]
-
-jobs:
-  releaser:
-    uses: protocol/.github/.github/workflows/releaser.yml@master

.github/workflows/tagpush.yml

@@ -1,12 +0,0 @@
-# File managed by web3-bot. DO NOT EDIT.
-# See https://github.com/protocol/.github/ for details.
-
-name: Tag Push Checker
-on:
-  push:
-    tags:
-      - v*
-
-jobs:
-  releaser:
-    uses: protocol/.github/.github/workflows/tagpush.yml@master

benchmark_test.go

@@ -1,61 +0,0 @@
-package cid_test
-
-import (
-	"math/rand"
-	"testing"
-
-	"github.com/ipfs/go-cid"
-	"github.com/multiformats/go-multihash"
-)
-
-// BenchmarkIdentityCheck benchmarks two ways of checking whether a CIDv1 has multihash.IDENTITY
-// code:
-//  1. Cid.Prefix(), and
-//  2. decoding the Cid.Hash().
-//
-// This benchmark illustrates that using Cid.Prefix is more efficient than multihash.Decode.
-// Users wishing to perform such a check should use Cid.Prefix.
-//
-// Consider that `Cid.Prefix` is already efficient enough and introducing a dedicated API for
-// performing this check will likely result in small gains.
-func BenchmarkIdentityCheck(b *testing.B) {
-	rng := rand.New(rand.NewSource(1413))
-
-	data := make([]byte, rng.Intn(100)+1024)
-	if _, err := rng.Read(data); err != nil {
-		b.Fatal(err)
-	}
-	mh, err := multihash.Sum(data, multihash.IDENTITY, -1)
-	if err != nil {
-		b.Fatal(err)
-	}
-	cv1 := cid.NewCidV1(cid.Raw, mh)
-
-	b.SetBytes(int64(cv1.ByteLen()))
-	b.ReportAllocs()
-	b.ResetTimer()
-
-	b.Run("Prefix", func(b *testing.B) {
-		b.RunParallel(func(pb *testing.PB) {
-			for pb.Next() {
-				if cv1.Prefix().MhType != multihash.IDENTITY {
-					b.Fatal("expected IDENTITY CID")
-				}
-			}
-		})
-	})
-
-	b.Run("MultihashDecode", func(b *testing.B) {
-		b.RunParallel(func(pb *testing.PB) {
-			for pb.Next() {
-				dmh, err := multihash.Decode(cv1.Hash())
-				if err != nil {
-					b.Fatal(err)
-				}
-				if dmh.Code != multihash.IDENTITY {
-					b.Fatal("expected IDENTITY CID")
-				}
-			}
-		})
-	})
-}

go.mod

@@ -2,7 +2,7 @@ module github.com/ipfs/go-cid
 
 require (
 	github.com/multiformats/go-multibase v0.0.3
-	github.com/multiformats/go-multihash v0.1.0
+	github.com/multiformats/go-multihash v0.0.15
 	github.com/multiformats/go-varint v0.0.6
 	golang.org/x/crypto v0.0.0-20210506145944-38f3c27a63bf // indirect
 )

version.json

@@ -1,3 +0,0 @@
-{
-  "version": "v0.1.0"
-}