compare with EqualFold instead of ToLower

[pierrre]

I did a profiling of my application, and noticed that msgauth/dkim is comparing strings with ToLower.

go-msgauth/dkim/header.go

Lines 152 to 158 in e98a2ee

	key = strings.ToLower(key)
	at := p.picked[key]
	for i := len(p.h) - 1; i >= 0; i-- {
	kv := p.h[i]
	k, _ := parseHeaderField(kv)
	if strings.ToLower(k) != key {

go-msgauth/dkim/verify.go

Line 244 in e98a2ee

if strings.ToLower(k) == "from" {

Each time this function is called, it allocates a new string, which creates more work for the garbage collector.
I think there is an "allocation free" alternative: EqualFold.

WDYT ?
I can submit a benchmark if needed.

[emersion]

Sure, EqualFold LGTM

[pierrre]

For reference, here is a very simple micro-benchmark:

package main

import (
	"strings"
	"testing"
)

func BenchmarkToLower(b *testing.B) {
	for i := 0; i < b.N; i++ {
		if strings.ToLower("ABCDEFGHIJ") != "abcdefghij" {
			b.Fatal("not equal")
		}
	}
}

func BenchmarkEqualFold(b *testing.B) {
	for i := 0; i < b.N; i++ {
		if !strings.EqualFold("ABCDEFGHIJ", "abcdefghij") {
			b.Fatal("not equal")
		}
	}
}

The result

BenchmarkToLower
BenchmarkToLower-12      	26062184	        42.60 ns/op	      16 B/op	       1 allocs/op
BenchmarkEqualFold
BenchmarkEqualFold-12    	41775878	        27.07 ns/op	       0 B/op	       0 allocs/op

I tried to integrate the benchmark to msgauth, but I don't know a simple way to write it.