cmd/gofmt: normalize number prefixes and exponents

Rewrite non-decimal number prefixes to always use a lower-case base
("0X" -> "0x", etc.), and rewrite exponents to use a lower-case 'e'
or 'p'. Leave hexadecimal digits and 0-octals alone.

Comparing the best time of 3 runs of `time go test -run All` with
the time for a gofmt that doesn't do the rewrite shows no increase
in runtime for this bulk gofmt application (in fact on my machine
I see a small decline, probably due to cache effects).

R=Go1.13

Updates #12711.
Updates #19308.
Updates #29008.

Change-Id: I9c6ebed2ffa0a6a001c59412a73382090955f5a9
Reviewed-on: https://go-review.googlesource.com/c/160184
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/gofmt/gofmt.go

@@ -112,6 +112,8 @@ func processFile(filename string, in io.Reader, out io.Writer, stdin bool) error
 		simplify(file)
 	}
 
+	ast.Inspect(file, normalizeNumbers)
+
 	res, err := format(fileSet, file, sourceAdj, indentAdj, src, printer.Config{Mode: printerMode, Tabwidth: tabWidth})
 	if err != nil {
 		return err
@@ -326,3 +328,48 @@ func backupFile(filename string, data []byte, perm os.FileMode) (string, error)
 
 	return bakname, err
 }
+
+// normalizeNumbers rewrites base prefixes and exponents to
+// use lower-case letters. It leaves hexadecimal digits alone.
+func normalizeNumbers(n ast.Node) bool {
+	lit, _ := n.(*ast.BasicLit)
+	if lit == nil {
+		return true
+	}
+	if len(lit.Value) < 2 {
+		return false // only one digit - nothing to do
+	}
+	// lit.Value >= 2
+
+	switch lit.Kind {
+	case token.INT:
+		switch lit.Value[:2] {
+		case "0X":
+			lit.Value = "0x" + lit.Value[2:]
+		case "0O":
+			lit.Value = "0o" + lit.Value[2:]
+		case "0B":
+			lit.Value = "0b" + lit.Value[2:]
+		}
+
+	case token.FLOAT:
+		switch lit.Value[:2] {
+		default:
+			if i := strings.LastIndexByte(lit.Value, 'E'); i >= 0 {
+				lit.Value = lit.Value[:i] + "e" + lit.Value[i+1:]
+			}
+		case "0x":
+			if i := strings.LastIndexByte(lit.Value, 'P'); i >= 0 {
+				lit.Value = lit.Value[:i] + "p" + lit.Value[i+1:]
+			}
+		case "0X":
+			if i := strings.LastIndexByte(lit.Value, 'P'); i >= 0 {
+				lit.Value = "0x" + lit.Value[2:i] + "p" + lit.Value[i+1:]
+			} else {
+				lit.Value = "0x" + lit.Value[2:]
+			}
+		}
+	}
+
+	return false
+}

src/cmd/gofmt/testdata/go2numbers.golden

@@ -1,6 +1,7 @@
 package p
 
 const (
+	// 0-octals
 	_ = 0
 	_ = 0123
 	_ = 0123456
@@ -21,39 +22,39 @@ const (
 	_ = 0x1234
 	_ = 0xcafef00d
 
-	_ = 0X0
-	_ = 0X1234
-	_ = 0XCAFEf00d
+	_ = 0x0
+	_ = 0x1234
+	_ = 0xCAFEf00d
 
-	_ = 0X_0
-	_ = 0X_1234
-	_ = 0X_CAFE_f00d
+	_ = 0x_0
+	_ = 0x_1234
+	_ = 0x_CAFE_f00d
 
 	// octals
 	_ = 0o0
 	_ = 0o1234
 	_ = 0o01234567
 
-	_ = 0O0
-	_ = 0O1234
-	_ = 0O01234567
+	_ = 0o0
+	_ = 0o1234
+	_ = 0o01234567
 
 	_ = 0o_0
 	_ = 0o_1234
 	_ = 0o0123_4567
 
-	_ = 0O_0
-	_ = 0O_1234
-	_ = 0O0123_4567
+	_ = 0o_0
+	_ = 0o_1234
+	_ = 0o0123_4567
 
 	// binaries
 	_ = 0b0
 	_ = 0b1011
 	_ = 0b00101101
 
-	_ = 0B0
-	_ = 0B1011
-	_ = 0B00101101
+	_ = 0b0
+	_ = 0b1011
+	_ = 0b00101101
 
 	_ = 0b_0
 	_ = 0b10_11
@@ -70,26 +71,26 @@ const (
 
 	_ = 0e0
 	_ = 123e+0
-	_ = 0123E-1
+	_ = 0123e-1
 
 	_ = 0e-0
-	_ = 123E+0
-	_ = 0123E123
+	_ = 123e+0
+	_ = 0123e123
 
 	_ = 0.e+1
-	_ = 123.E-10
+	_ = 123.e-10
 	_ = 0123.e123
 
 	_ = .0e-1
-	_ = .123E+10
-	_ = .0123E123
+	_ = .123e+10
+	_ = .0123e123
 
 	_ = 0.0
 	_ = 123.123
 	_ = 0123.0123
 
 	_ = 0.0e1
-	_ = 123.123E-10
+	_ = 123.123e-10
 	_ = 0123.0123e+456
 
 	_ = 1_2_3.
@@ -100,42 +101,43 @@ const (
 	_ = 0_123e0
 
 	_ = 0e-0_0
-	_ = 1_2_3E+0
-	_ = 0123E1_2_3
+	_ = 1_2_3e+0
+	_ = 0123e1_2_3
 
 	_ = 0.e+1
-	_ = 123.E-1_0
+	_ = 123.e-1_0
 	_ = 01_23.e123
 
 	_ = .0e-1
-	_ = .123E+10
-	_ = .0123E123
+	_ = .123e+10
+	_ = .0123e123
 
 	_ = 1_2_3.123
 	_ = 0123.01_23
 
 	// hexadecimal floats
 	_ = 0x0.p+0
-	_ = 0Xdeadcafe.p-10
-	_ = 0x1234.P123
+	_ = 0xdeadcafe.p-10
+	_ = 0x1234.p123
 
 	_ = 0x.1p-0
-	_ = 0X.deadcafep2
-	_ = 0x.1234P+10
+	_ = 0x.deadcafep2
+	_ = 0x.1234p+10
 
 	_ = 0x0p0
-	_ = 0Xdeadcafep+1
-	_ = 0x1234P-10
+	_ = 0xdeadcafep+1
+	_ = 0x1234p-10
 
 	_ = 0x0.0p0
-	_ = 0Xdead.cafep+1
-	_ = 0x12.34P-10
+	_ = 0xdead.cafep+1
+	_ = 0x12.34p-10
 
-	_ = 0Xdead_cafep+1
-	_ = 0x_1234P-10
+	_ = 0xdead_cafep+1
+	_ = 0x_1234p-10
 
-	_ = 0X_dead_cafe.p-10
-	_ = 0x12_34.P1_2_3
+	_ = 0x_dead_cafe.p-10
+	_ = 0x12_34.p1_2_3
+	_ = 0x1_2_3_4.p-1_2_3
 
 	// imaginaries
 	_ = 0i

src/cmd/gofmt/testdata/go2numbers.input

@@ -1,6 +1,7 @@
 package p
 
 const (
+	// 0-octals
 	_ = 0
 	_ = 0123
 	_ = 0123456
@@ -136,6 +137,7 @@ const (
 
 	_ = 0X_dead_cafe.p-10
 	_ = 0x12_34.P1_2_3
+	_ = 0X1_2_3_4.P-1_2_3
 
 	// imaginaries
 	_ = 0i