fix(strutil): keep center-aligned Resize within the requested length

The center branch of resize() added the odd-padding space whenever the
input was non-empty, because the parity check omitted the source length
(wantLen - padLn*2 instead of wantLen - sLen - padLn*2). This made Resize,
Align and their utf8 variants overflow wantLen by one for every non-empty
string with an even amount of padding. Subtract sLen so the extra space is
only added when the total padding is actually odd.

strutil/padding.go

@@ -164,7 +164,9 @@ func resize(s string, sLen, wantLen int, align comdef.Align, cutOverflow bool) s
 
 	if align == comdef.Center {
 		padLn := (wantLen - sLen) / 2
-		if diff = wantLen - padLn*2; diff > 0 {
+		// add a single extra space only when the total padding is odd,
+		// otherwise the result overflows the requested length.
+		if diff = wantLen - sLen - padLn*2; diff > 0 {
 			s += " "
 		}
 		padStr := string(RepeatBytes(' ', padLn))

strutil/padding_test.go

@@ -38,6 +38,29 @@ func TestPadding(t *testing.T) {
 	assert.Eq(t, "hi,你好000", strutil.Utf8Padding(s, "0", 10, strutil.PosRight))
 }
 
+func TestResize_center_length(t *testing.T) {
+	// Center alignment must always produce a string of exactly the requested
+	// length; the extra padding space is only needed when the total padding
+	// (wantLen - len(s)) is odd.
+	tests := []struct {
+		give string
+		len  int
+		want string
+	}{
+		{"ab", 6, "  ab  "},   // even padding (4): 2 + 2
+		{"ab", 4, " ab "},     // even padding (2): 1 + 1
+		{"ab", 5, " ab  "},    // odd padding (3): 1 + 2
+		{"abc", 7, "  abc  "}, // even padding (4): 2 + 2
+		{"a", 4, " a  "},      // odd padding (3): 1 + 2
+	}
+
+	for _, tt := range tests {
+		got := strutil.Resize(tt.give, tt.len, comdef.Center)
+		assert.Eq(t, tt.len, len(got), "Resize(%q,%d,Center)=%q", tt.give, tt.len, got)
+		assert.Eq(t, tt.want, got)
+	}
+}
+
 func TestResize(t *testing.T) {
 	tests := []struct {
 		want, give string