From e76e21e4c7e80bc09deba3e2082b325f977b9275 Mon Sep 17 00:00:00 2001
From: Aliaksandr Valialkin <valyala@gmail.com>
Date: Thu, 31 Oct 2019 13:24:47 +0200
Subject: [PATCH] lib/decimal: speed up FromFloat for common case with integers

---
 lib/decimal/decimal.go      | 94 ++++++++++++++++++++-----------------
 lib/decimal/decimal_test.go |  4 +-
 2 files changed, 54 insertions(+), 44 deletions(-)

diff --git a/lib/decimal/decimal.go b/lib/decimal/decimal.go
index 7bb739a98..8b02be0c8 100644
--- a/lib/decimal/decimal.go
+++ b/lib/decimal/decimal.go
@@ -265,64 +265,74 @@ var (
 // For instance, for f = -1.234 it returns v = -1234, e = -3.
 //
 // FromFloat doesn't work properly with NaN values, so don't pass them here.
-func FromFloat(f float64) (v int64, e int16) {
-	if math.IsInf(f, 0) {
-		// Special case for Inf
-		if math.IsInf(f, 1) {
-			return vInfPos, 0
-		}
-		return vInfNeg, 0
-	}
-
-	minus := false
-	if f < 0 {
-		f = -f
-		minus = true
-	}
+func FromFloat(f float64) (int64, int16) {
 	if f == 0 {
-		// Special case for 0.0 and -0.0
 		return 0, 0
 	}
-	v, e = positiveFloatToDecimal(f)
-	if minus {
-		v = -v
+	if math.IsInf(f, 0) {
+		return fromFloatInf(f)
 	}
-	if v == 0 {
-		e = 0
-	} else if v > vMax {
-		v = vMax
-	} else if v < vMin {
+	if f > 0 {
+		v, e := positiveFloatToDecimal(f)
+		if v > vMax {
+			v = vMax
+		}
+		return v, e
+	}
+	v, e := positiveFloatToDecimal(-f)
+	v = -v
+	if v < vMin {
 		v = vMin
 	}
 	return v, e
 }
 
+func fromFloatInf(f float64) (int64, int16) {
+	// Special case for Inf
+	if math.IsInf(f, 1) {
+		return vInfPos, 0
+	}
+	return vInfNeg, 0
+}
+
 func positiveFloatToDecimal(f float64) (int64, int16) {
-	var scale int16
+	// There is no need in checking for f == 0, since it should be already checked by the caller.
 	u := uint64(f)
-	if float64(u) == f {
-		// Fast path for integers.
-		for u >= 1<<55 {
-			// Remove trailing garbage bits left after float64->uint64 conversion,
-			// since float64 contains only 53 significant bits.
-			// See https://en.wikipedia.org/wiki/Double-precision_floating-point_format
-			u /= 10
-			scale++
-		}
-		if u%10 != 0 || u == 0 {
-			return int64(u), scale
-		}
-		// Minimize v by converting trailing zeros to scale.
+	if float64(u) != f {
+		return positiveFloatToDecimalSlow(f)
+	}
+	// Fast path for integers.
+	if u < 1<<55 && u%10 != 0 {
+		return int64(u), 0
+	}
+	return getDecimalAndScale(u)
+}
+
+func getDecimalAndScale(u uint64) (int64, int16) {
+	var scale int16
+	for u >= 1<<55 {
+		// Remove trailing garbage bits left after float64->uint64 conversion,
+		// since float64 contains only 53 significant bits.
+		// See https://en.wikipedia.org/wiki/Double-precision_floating-point_format
 		u /= 10
 		scale++
-		for u != 0 && u%10 == 0 {
-			u /= 10
-			scale++
-		}
+	}
+	if u%10 != 0 {
 		return int64(u), scale
 	}
+	// Minimize v by converting trailing zeros to scale.
+	u /= 10
+	scale++
+	for u != 0 && u%10 == 0 {
+		u /= 10
+		scale++
+	}
+	return int64(u), scale
+}
 
+func positiveFloatToDecimalSlow(f float64) (int64, int16) {
 	// Slow path for floating point numbers.
+	var scale int16
 	prec := conversionPrecision
 	if f > 1e6 || f < 1e-6 {
 		// Normalize f, so it is in the small range suitable
@@ -352,7 +362,7 @@ func positiveFloatToDecimal(f float64) (int64, int16) {
 		f *= 100
 		scale -= 2
 	}
-	u = uint64(f)
+	u := uint64(f)
 	if u%10 != 0 {
 		return int64(u), scale
 	}
diff --git a/lib/decimal/decimal_test.go b/lib/decimal/decimal_test.go
index ffb9d95b4..2f2cc53e6 100644
--- a/lib/decimal/decimal_test.go
+++ b/lib/decimal/decimal_test.go
@@ -18,7 +18,7 @@ func TestPositiveFloatToDecimal(t *testing.T) {
 			t.Fatalf("unexpected exponent for positiveFloatToDecimal(%f); got %d; want %d", f, exponent, exponentExpected)
 		}
 	}
-	f(0, 0, 0)
+	f(0, 0, 1) // The exponent is 1 is OK here. See comment in positiveFloatToDecimal.
 	f(1, 1, 0)
 	f(30, 3, 1)
 	f(12345678900000000, 123456789, 8)
@@ -206,7 +206,7 @@ func TestAppendFloatToDecimal(t *testing.T) {
 	// no-op
 	testAppendFloatToDecimal(t, []float64{}, nil, 0)
 	testAppendFloatToDecimal(t, []float64{0}, []int64{0}, 0)
-	testAppendFloatToDecimal(t, []float64{0, 1, -1, 12345678, -123456789}, []int64{0, 1, -1, 12345678, -123456789}, 0)
+	testAppendFloatToDecimal(t, []float64{0, -0, 1, -1, 12345678, -123456789}, []int64{0, 0, 1, -1, 12345678, -123456789}, 0)
 
 	// upExp
 	testAppendFloatToDecimal(t, []float64{-24, 0, 4.123, 0.3}, []int64{-24000, 0, 4123, 300}, -3)