internal/itoa, os: move os.uitox to itoa.Uitox

This packages already contains other similar functions. Also add a test
for it.

Change-Id: Iafa8c14f5cb1f5ef89a0e16ccc855c568a3b5727
Reviewed-on: https://go-review.googlesource.com/c/go/+/518317
Reviewed-by: Michael Knyszek <[email protected]>
Run-TryBot: Tobias Klauser <[email protected]>
Run-TryBot: Ian Lance Taylor <[email protected]>
Auto-Submit: Tobias Klauser <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

src/internal/itoa/itoa.go

@@ -31,3 +31,27 @@ func Uitoa(val uint) string {
 	buf[i] = byte('0' + val)
 	return string(buf[i:])
 }
+
+const hex = "0123456789abcdef"
+
+// Uitox converts val (a uint) to a hexadecimal string.
+func Uitox(val uint) string {
+	if val == 0 { // avoid string allocation
+		return "0x0"
+	}
+	var buf [20]byte // big enough for 64bit value base 16 + 0x
+	i := len(buf) - 1
+	for val >= 16 {
+		q := val / 16
+		buf[i] = hex[val%16]
+		i--
+		val = q
+	}
+	// val < 16
+	buf[i] = hex[val%16]
+	i--
+	buf[i] = 'x'
+	i--
+	buf[i] = '0'
+	return string(buf[i:])
+}

src/internal/itoa/itoa_test.go

@@ -38,3 +38,14 @@ func TestUitoa(t *testing.T) {
 		}
 	}
 }
+
+func TestUitox(t *testing.T) {
+	tests := []uint{0, 1, 15, 100, 999, math.MaxUint32, uint(maxUint64)}
+	for _, tt := range tests {
+		got := itoa.Uitox(tt)
+		want := fmt.Sprintf("%#x", tt)
+		if want != got {
+			t.Fatalf("Uitox(%x) = %s, want %s", tt, got, want)
+		}
+	}
+}

src/os/exec_posix.go

@@ -105,7 +105,7 @@ func (p *ProcessState) String() string {
 	case status.Exited():
 		code := status.ExitStatus()
 		if runtime.GOOS == "windows" && uint(code) >= 1<<16 { // windows uses large hex numbers
-			res = "exit status " + uitox(uint(code))
+			res = "exit status " + itoa.Uitox(uint(code))
 		} else { // unix systems use small decimal integers
 			res = "exit status " + itoa.Itoa(code) // unix
 		}