fix: fix bug of Comma, issue #248

formatter/formatter.go

@@ -26,16 +26,25 @@ func Comma[T constraints.Float | constraints.Integer | string](value T, symbol s
 		return ""
 	}
 
+	isNegative := strings.HasPrefix(numString, "-")
+	if isNegative {
+		numString = numString[1:]
+	}
+
 	index := strings.Index(numString, ".")
 	if index == -1 {
 		index = len(numString)
 	}
 
 	for index > 3 {
-		index = index - 3
+		index -= 3
 		numString = numString[:index] + "," + numString[index:]
 	}
 
+	if isNegative {
+		numString = "-" + numString
+	}
+
 	return symbol + numString
 }
 

formatter/formatter_test.go

@@ -28,6 +28,10 @@ func TestComma(t *testing.T) {
 	assert.Equal("12,345.6789", Comma(+12345.6789, ""))
 	assert.Equal("12,345,678.9", Comma(12345678.9, ""))
 	assert.Equal("123,456,789,000", Comma(123456789000, ""))
+
+	assert.Equal("-999", Comma(-999, ""))
+	assert.Equal("-1,000", Comma(-1000, ""))
+	assert.Equal("-1,234,567", Comma(-1234567, ""))
 }
 
 func TestPretty(t *testing.T) {