fmt: remove math package dependency and avoid float operations

Remove floating point comparisons and rely only on the information
directly provided by appendFloat.
Make restoring the zero padding flag explicit instead of using a defer.
Rearrange some case distinctions to remove duplicated code.
Add more test cases for zero padded floating point numbers with sign.

benchmark                   old ns/op     new ns/op     delta
BenchmarkSprintfFloat-4     187           180           -3.74%

Change-Id: Ifa2ae85257909f40b1b18118c92b516933271729
Reviewed-on: https://go-review.googlesource.com/19721Reviewed-by: Rob Pike <r@golang.org>

src/fmt/fmt_test.go

@@ -259,6 +259,12 @@ var fmtTests = []struct {
 	{"%+.3F", float32(-1.0), "-1.000"},
 	{"%+07.2f", 1.0, "+001.00"},
 	{"%+07.2f", -1.0, "-001.00"},
+	{"%-07.2f", 1.0, "1.00   "},
+	{"%-07.2f", -1.0, "-1.00  "},
+	{"%+-07.2f", 1.0, "+1.00  "},
+	{"%+-07.2f", -1.0, "-1.00  "},
+	{"%-+07.2f", 1.0, "+1.00  "},
+	{"%-+07.2f", -1.0, "-1.00  "},
 	{"%+10.2f", +1.0, "     +1.00"},
 	{"%+10.2f", -1.0, "     -1.00"},
 	{"% .3E", -1.0, "-1.000E+00"},

src/fmt/format.go

@@ -5,7 +5,6 @@
 package fmt
 
 import (
-	"math"
 	"strconv"
 	"unicode/utf8"
 )
@@ -405,42 +404,34 @@ func doPrec(f *fmt, def int) int {
 // formatFloat formats a float64; it is an efficient equivalent to  f.pad(strconv.FormatFloat()...).
 func (f *fmt) formatFloat(v float64, verb byte, prec, n int) {
 	// Format number, reserving space for leading + sign if needed.
-	num := strconv.AppendFloat(f.intbuf[0:1], v, verb, prec, n)
+	num := strconv.AppendFloat(f.intbuf[:1], v, verb, prec, n)
 	if num[1] == '-' || num[1] == '+' {
 		num = num[1:]
 	} else {
 		num[0] = '+'
 	}
-	// Special handling for infinity, which doesn't look like a number so shouldn't be padded with zeros.
-	if math.IsInf(v, 0) {
-		if f.zero {
-			defer func() { f.zero = true }()
-			f.zero = false
-		}
-	}
-	// num is now a signed version of the number.
-	// If we're zero padding, want the sign before the leading zeros.
-	// Achieve this by writing the sign out and then padding the unsigned number.
-	if f.zero && f.widPresent && f.wid > len(num) {
-		if f.space && v >= 0 {
-			f.buf.WriteByte(' ') // This is what C does: even with zero, f.space means space.
-			f.wid--
-		} else if f.plus || v < 0 {
-			f.buf.WriteByte(num[0])
-			f.wid--
-		}
-		f.pad(num[1:])
-		return
-	}
 	// f.space says to replace a leading + with a space.
 	if f.space && num[0] == '+' {
 		num[0] = ' '
+	}
+	// Special handling for "+Inf" and "-Inf",
+	// which don't look like a number so shouldn't be padded with zeros.
+	if num[1] == 'I' {
+		oldZero := f.zero
+		f.zero = false
 		f.pad(num)
+		f.zero = oldZero
 		return
 	}
-	// Now we know the sign is attached directly to the number, if present at all.
-	// We want a sign if asked for, if it's negative, or if it's infinity (+Inf vs. -Inf).
-	if f.plus || num[0] == '-' || math.IsInf(v, 0) {
+	// We want a sign if asked for and if the sign is not positive.
+	if f.plus || num[0] != '+' {
+		// If we're zero padding we want the sign before the leading zeros.
+		// Achieve this by writing the sign out and then padding the unsigned number.
+		if f.zero && f.widPresent && f.wid > len(num) {
+			f.buf.WriteByte(num[0])
+			f.wid--
+			num = num[1:]
+		}
 		f.pad(num)
 		return
 	}