Implement Optimized NewFromFloatWithExponent

decimal.go

@@ -188,14 +188,22 @@ func NewFromFloat32(f float32) Decimal {
 	return newFromDecimal(decimal.NewFromFloat32(f))
 }
 
-// fallback:
+// optimized:
 // NewFromFloatWithExponent converts a float64 to Decimal, with an arbitrary
 // number of fractional digits.
 //
 // Example:
 //
 //	NewFromFloatWithExponent(123.456, -2).String() // output: "123.46"
+//
+// NOTE: this will panic on NaN, +/-inf
 func NewFromFloatWithExponent(value float64, exp int32) Decimal {
+	decimalPlaces := -exp // decimalPlaces could be 0..12 here, inclusive
+	formatted := strconv.FormatFloat(value, 'f', int(decimalPlaces), 64)
+	if fixed, ok := parseFixed([]byte(formatted)); ok {
+		return Decimal{fixed: fixed}
+	}
+
 	return newFromDecimal(decimal.NewFromFloatWithExponent(value, exp))
 }
 

decimal_test.go

@@ -176,12 +176,54 @@ func TestDecimal(t *testing.T) {
 	})
 
 	t.Run("NewFromFloatWithExponent", func(t *testing.T) {
-		input := 123.456
+		{
+			input := 123.456
 
-		x := alpacadecimal.NewFromFloatWithExponent(input, -2)
-		y := decimal.NewFromFloatWithExponent(input, -2)
+			x := alpacadecimal.NewFromFloatWithExponent(input, -2)
+			y := decimal.NewFromFloatWithExponent(input, -2)
 
-		require.Equal(t, x.String(), y.String())
+			require.Equal(t, x.String(), y.String())
+			require.True(t, x.IsOptimized())
+		}
+
+		{
+			input := 123.4567890192214
+
+			x := alpacadecimal.NewFromFloatWithExponent(input, -12)
+			y := decimal.NewFromFloatWithExponent(input, -12)
+
+			require.Equal(t, x.String(), y.String())
+			require.True(t, x.IsOptimized())
+		}
+
+		{
+			input := 123.4567890192214
+
+			x := alpacadecimal.NewFromFloatWithExponent(input, -14)
+			y := decimal.NewFromFloatWithExponent(input, -14)
+
+			require.Equal(t, x.String(), y.String())
+			require.False(t, x.IsOptimized())
+		}
+
+		{
+			input := 1_123_123.4567890192214
+
+			x := alpacadecimal.NewFromFloatWithExponent(input, -12)
+			y := decimal.NewFromFloatWithExponent(input, -12)
+
+			require.Equal(t, x.String(), y.String())
+			require.True(t, x.IsOptimized())
+		}
+		{
+			input := 9_323_123.4567890192214 // max support decimal is 9_223_372.000_000_000_000
+
+			x := alpacadecimal.NewFromFloatWithExponent(input, -12)
+			y := decimal.NewFromFloatWithExponent(input, -12)
+
+			require.Equal(t, x.String(), y.String())
+			require.False(t, x.IsOptimized())
+		}
 	})
 
 	t.Run("NewFromFormattedString", func(t *testing.T) {