major code rewrite

Makefile

@@ -1,4 +1,4 @@
 all: build-windows
-
+# Not powershell compatible. Meant to be used from unix shell
 build-windows:
 	env GOOS=windows GOARCH=amd64 go build -o ./build/universal-checksum-patcher.exe *.go

hex.go

@@ -1,11 +1,16 @@
 package main
 
-const limit = 14
+const (
+	limit       = 14
+	startLength = 3
+	endLength   = 6
+)
 
 var (
-	start1      = []byte{0x48, 0x8B, 0x12}
-	start2      = []byte{0x48, 0x8D, 0x0D}
-	start3      = []byte{0x48, 0x8B, 0xD0}
-	end         = []byte{0x85, 0xC0}
-	replacement = []byte{0x31, 0xC0}
+	start1 = []byte{0x48, 0x8B, 0x12}
+	start2 = []byte{0x48, 0x8D, 0x0D}
+	start3 = []byte{0x48, 0x8B, 0xD0}
+
+	end         = []byte{0x85, 0xC0, 0x0F, 0x94, 0xC3, 0xE8}
+	replacement = []byte{0x31, 0xC0, 0x0F, 0x94, 0xC3, 0xE8}
 )

main.go

@@ -43,5 +43,5 @@ func main() {
 	}
 
 	fmt.Println("Press enter to exit")
-	fmt.Scanln()
+	_, _ = fmt.Scanln()
 }

patch.go

@@ -43,23 +43,41 @@ func applyPatch(filename string) error {
 	return nil
 }
 
+func isStartCandidate(bytes []byte) bool {
+	return isByteSlicesEqual(bytes, start1) || isByteSlicesEqual(bytes, start2) || isByteSlicesEqual(bytes, start3)
+}
+
+func isEndCandidate(bytes []byte) bool {
+	return isByteSlicesEqual(bytes, end)
+}
+
 func modifyBytes(bytes []byte) error {
+	atLeastOnePatched := false
+
 	for i := 0; i < len(bytes); i++ {
-		if !isByteSlicesEqual(bytes[i:i+len(start1)], start1) &&
-			!isByteSlicesEqual(bytes[i:i+len(start2)], start2) &&
-			!isByteSlicesEqual(bytes[i:i+len(start3)], start3) {
+		if i > len(bytes)-limit {
+			break
+		}
+
+		if !isStartCandidate(bytes[i : i+startLength]) {
 			continue
 		}
 
-		for j := i + len(start1); j < i+len(start1)+limit && j < len(bytes); j++ {
-			if !isByteSlicesEqual(bytes[j:j+len(end)], end) {
+		for j := i + startLength; j < i+startLength+limit && j < len(bytes)-endLength; j++ {
+			if !isEndCandidate(bytes[j : j+endLength]) {
 				continue
 			}
 
-			copy(bytes[j:j+len(end)], replacement)
-			return nil
+			for k := 0; k < len(replacement); k++ {
+				bytes[j+k] = replacement[k]
+			}
+
+			atLeastOnePatched = true
 		}
 	}
 
+	if atLeastOnePatched {
+		return nil
+	}
 	return ErrNoMatch
 }