unicode/utf16: delete dependence on package unicode

In the test, verify the copied constants are correct.
Also put the test into package utf16 rather than utf16_test;
the old location was probably due creating the test from
utf8, but the separation is not needed here.

R=golang-dev, bradfitz, rsc, rsc, r
CC=golang-dev
https://golang.org/cl/5752047

src/pkg/unicode/utf16/export_test.go

+// Copyright 2012 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package utf16
+
+// Extra names for constants so we can validate them during testing.
+const (
+	MaxRune         = maxRune
+	ReplacementChar = replacementChar
+)

src/pkg/unicode/utf16/utf16.go

@@ -5,7 +5,14 @@
 // Package utf16 implements encoding and decoding of UTF-16 sequences.
 package utf16
 
-import "unicode"
+// The conditions replacementChar==unicode.ReplacementChar and
+// maxRune==unicode.MaxRune are verified in the tests.
+// Defining them locally avoids this package depending on package unicode.
+
+const (
+	replacementChar = '\uFFFD'     // Unicode replacement character
+	maxRune         = '\U0010FFFF' // Maximum valid Unicode code point.
+)
 
 const (
 	// 0xd800-0xdc00 encodes the high 10 bits of a pair.
@@ -31,15 +38,15 @@ func DecodeRune(r1, r2 rune) rune {
 	if surr1 <= r1 && r1 < surr2 && surr2 <= r2 && r2 < surr3 {
 		return (rune(r1)-surr1)<<10 | (rune(r2) - surr2) + 0x10000
 	}
-	return unicode.ReplacementChar
+	return replacementChar
 }
 
 // EncodeRune returns the UTF-16 surrogate pair r1, r2 for the given rune.
 // If the rune is not a valid Unicode code point or does not need encoding,
 // EncodeRune returns U+FFFD, U+FFFD.
 func EncodeRune(r rune) (r1, r2 rune) {
-	if r < surrSelf || r > unicode.MaxRune || IsSurrogate(r) {
-		return unicode.ReplacementChar, unicode.ReplacementChar
+	if r < surrSelf || r > maxRune || IsSurrogate(r) {
+		return replacementChar, replacementChar
 	}
 	r -= surrSelf
 	return surr1 + (r>>10)&0x3ff, surr2 + r&0x3ff
@@ -58,8 +65,8 @@ func Encode(s []rune) []uint16 {
 	n = 0
 	for _, v := range s {
 		switch {
-		case v < 0, surr1 <= v && v < surr3, v > unicode.MaxRune:
-			v = unicode.ReplacementChar
+		case v < 0, surr1 <= v && v < surr3, v > maxRune:
+			v = replacementChar
 			fallthrough
 		case v < surrSelf:
 			a[n] = uint16(v)
@@ -89,7 +96,7 @@ func Decode(s []uint16) []rune {
 			n++
 		case surr1 <= r && r < surr3:
 			// invalid surrogate sequence
-			a[n] = unicode.ReplacementChar
+			a[n] = replacementChar
 			n++
 		default:
 			// normal rune

src/pkg/unicode/utf16/utf16_test.go

@@ -11,6 +11,16 @@ import (
 	. "unicode/utf16"
 )
 
+// Validate the constants redefined from unicode.
+func TestConstants(t *testing.T) {
+	if MaxRune != unicode.MaxRune {
+		t.Errorf("utf16.maxRune is wrong: %x should be %x", MaxRune, unicode.MaxRune)
+	}
+	if ReplacementChar != unicode.ReplacementChar {
+		t.Errorf("utf16.replacementChar is wrong: %x should be %x", ReplacementChar, unicode.ReplacementChar)
+	}
+}
+
 type encodeTest struct {
 	in  []rune
 	out []uint16

src/pkg/unicode/utf8/utf8_test.go

@@ -21,6 +21,16 @@ func init() {
 	}
 }
 
+// Validate the constants redefined from unicode.
+func TestConstants(t *testing.T) {
+	if MaxRune != unicode.MaxRune {
+		t.Errorf("utf8.MaxRune is wrong: %x should be %x", MaxRune, unicode.MaxRune)
+	}
+	if RuneError != unicode.ReplacementChar {
+		t.Errorf("utf8.RuneError is wrong: %x should be %x", RuneError, unicode.ReplacementChar)
+	}
+}
+
 type Utf8Map struct {
 	r   rune
 	str string