aws/signer/v4: Optimize stripExcessSpaces

Optimizes stripExcessSpaces by using a hand rolled trim space avoiding the overhead of strings.TrimSpace and ensuring we only do a single pass on the string for both search and copy when stripping multiple spaces.

This improves the performance on the test machine from 2378 ns/op to 1528 ns/op while keeping allocs the same.

aws/signer/v4/v4.go

@@ -55,7 +55,6 @@
 package v4
 
 import (
-	"bytes"
 	"crypto/hmac"
 	"crypto/sha256"
 	"encoding/hex"
@@ -717,47 +716,46 @@ func makeSha256Reader(reader io.ReadSeeker) []byte {
 	return hash.Sum(nil)
 }
 
-const doubleSpaces = "  "
-
-var doubleSpaceBytes = []byte(doubleSpaces)
+const doubleSpace = "  "
 
 // stripExcessSpaces will rewrite the passed in slice's string values to not
 // contain muliple side-by-side spaces.
 func stripExcessSpaces(vals []string) {
+	var j, k, l, m, spaces int
 	for i, str := range vals {
-		// Trim leading and trailing spaces
-		trimmed := strings.TrimSpace(str)
+		// Trim trailing spaces
+		for j = len(str) - 1; j >= 0 && str[j] == ' '; j-- {
+		}
 
-		idx := strings.Index(trimmed, doubleSpaces)
-		if idx < 0 {
-			vals[i] = trimmed
-			continue
+		// Trim leading spaces
+		for k = 0; k < j && str[k] == ' '; k++ {
 		}
+		str = str[k : j+1]
 
-		buf := []byte(trimmed)
-		for idx > -1 {
-			idx++ // Start on the second space
+		// Strip multiple spaces.
+		j = strings.Index(str, doubleSpace)
+		if j < 0 {
+			vals[i] = str
+			continue
+		}
 
-			stripped := false
-			for j := idx; j < len(buf); j++ {
-				if buf[j] != ' ' {
-					buf = append(buf[:idx], buf[j:]...)
-					stripped = true
-					break
+		buf := []byte(str)
+		for k, m, l = j, j, len(buf); k < l; k++ {
+			if buf[k] == ' ' {
+				if spaces == 0 {
+					// First space.
+					buf[m] = buf[k]
+					m++
 				}
-			}
-			if !stripped {
-				break
-			}
-
-			// Find next double space
-			origIdx := idx
-			idx = bytes.Index(buf[idx:], doubleSpaceBytes)
-			if idx > 0 {
-				idx += origIdx
+				spaces++
+			} else {
+				// End of multiple spaces.
+				spaces = 0
+				buf[m] = buf[k]
+				m++
 			}
 		}
 
-		vals[i] = string(buf)
+		vals[i] = string(buf[:m])
 	}
 }

aws/signer/v4/v4_test.go

@@ -20,8 +20,10 @@ import (
 
 func TestStripExcessHeaders(t *testing.T) {
 	vals := []string{
+		"",
 		"123",
 		"1 2 3",
+		"1 2 3 ",
 		"  1 2 3",
 		"1  2 3",
 		"1  23",
@@ -35,10 +37,12 @@ func TestStripExcessHeaders(t *testing.T) {
 	}
 
 	expected := []string{
+		"",
 		"123",
 		"1 2 3",
 		"1 2 3",
 		"1 2 3",
+		"1 2 3",
 		"1 23",
 		"1 2 3",
 		"1 2",