src: move BE/LE buffer conversion to StringSlice()

Move the big endian to little endian conversion logic for UCS2 input
from src/string_bytes.cc to src/node_buffer.cc; StringSlice() is the
only function that actually needs it and with this commit, a second
copy is avoided on big endian architectures.

src/node.h

@@ -274,6 +274,7 @@ NODE_EXTERN v8::Local<v8::Value> Encode(v8::Isolate* isolate,
                                         size_t len,
                                         enum encoding encoding = BINARY);
 
+// The input buffer should be in host endianness.
 NODE_EXTERN v8::Local<v8::Value> Encode(v8::Isolate* isolate,
                                         const uint16_t* buf,
                                         size_t len);

src/node_buffer.cc

@@ -276,7 +276,11 @@ void StringSlice<UCS2>(const FunctionCallbackInfo<Value>& args) {
   const uint16_t* buf;
   bool release = false;
 
-  if (reinterpret_cast<uintptr_t>(data) % sizeof(*buf) == 0) {
+  // Node's "ucs2" encoding expects LE character data inside a Buffer, so we
+  // need to reorder on BE platforms.  See http://nodejs.org/api/buffer.html
+  // regarding Node's "ucs2" encoding specification.
+  const bool aligned = (reinterpret_cast<uintptr_t>(data) % sizeof(*buf) == 0);
+  if (IsLittleEndian() && aligned) {
     buf = reinterpret_cast<const uint16_t*>(data);
   } else {
     // Make a copy to avoid unaligned accesses in v8::String::NewFromTwoByte().

src/string_bytes.cc

@@ -777,19 +777,6 @@ Local<Value> StringBytes::Encode(Isolate* isolate,
                                  size_t buflen) {
   const uint16_t* src = buf;
 
-  // Node's "ucs2" encoding expects LE character data inside a
-  // Buffer, so we need to reorder on BE platforms.  See
-  // http://nodejs.org/api/buffer.html regarding Node's "ucs2"
-  // encoding specification.
-  if (IsBigEndian()) {
-    // Inefficient, see StringSlice<UCS2>() in src/node_buffer.cc;
-    // this is potentially the second copy of the actual input.
-    uint16_t* copy = new uint16_t[buflen];
-    for (size_t i = 0; i < buflen; i += 1)
-      copy[i] = buf[i] << 8 | buf[i] >> 8;
-    src = copy;
-  }
-
   Local<String> val;
   if (buflen < EXTERN_APEX) {
     val = String::NewFromTwoByte(isolate,

src/string_bytes.h

@@ -77,6 +77,7 @@ class StringBytes {
                                      size_t buflen,
                                      enum encoding encoding);
 
+  // The input buffer should be in host endianness.
   static v8::Local<v8::Value> Encode(v8::Isolate* isolate,
                                      const uint16_t* buf,
                                      size_t buflen);