Abstract string building in JSON-stringifier into IncrementalStringBuilder.

src/string-builder.cc

+// Copyright 2014 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "src/string-builder.h"
+
+namespace v8 {
+namespace internal {
+
+MaybeHandle<String> ReplacementStringBuilder::ToString() {
+  Isolate* isolate = heap_->isolate();
+  if (array_builder_.length() == 0) {
+    return isolate->factory()->empty_string();
+  }
+
+  Handle<String> joined_string;
+  if (is_one_byte_) {
+    Handle<SeqOneByteString> seq;
+    ASSIGN_RETURN_ON_EXCEPTION(
+        isolate, seq, isolate->factory()->NewRawOneByteString(character_count_),
+        String);
+
+    DisallowHeapAllocation no_gc;
+    uint8_t* char_buffer = seq->GetChars();
+    StringBuilderConcatHelper(*subject_, char_buffer, *array_builder_.array(),
+                              array_builder_.length());
+    joined_string = Handle<String>::cast(seq);
+  } else {
+    // Two-byte.
+    Handle<SeqTwoByteString> seq;
+    ASSIGN_RETURN_ON_EXCEPTION(
+        isolate, seq, isolate->factory()->NewRawTwoByteString(character_count_),
+        String);
+
+    DisallowHeapAllocation no_gc;
+    uc16* char_buffer = seq->GetChars();
+    StringBuilderConcatHelper(*subject_, char_buffer, *array_builder_.array(),
+                              array_builder_.length());
+    joined_string = Handle<String>::cast(seq);
+  }
+  return joined_string;
+}
+
+
+IncrementalStringBuilder::IncrementalStringBuilder(Isolate* isolate)
+    : isolate_(isolate),
+      encoding_(String::ONE_BYTE_ENCODING),
+      overflowed_(false),
+      part_length_(kInitialPartLength),
+      current_index_(0) {
+  // Create an accumulator handle starting with the empty string.
+  accumulator_ = Handle<String>(isolate->heap()->empty_string(), isolate);
+  current_part_ =
+      factory()->NewRawOneByteString(part_length_).ToHandleChecked();
+}
+
+
+void IncrementalStringBuilder::Accumulate() {
+  // Only accumulate fully written strings. Shrink first if necessary.
+  DCHECK_EQ(current_index_, current_part()->length());
+  Handle<String> new_accumulator;
+  if (accumulator()->length() + current_part()->length() > String::kMaxLength) {
+    // Set the flag and carry on. Delay throwing the exception till the end.
+    new_accumulator = factory()->empty_string();
+    overflowed_ = true;
+  } else {
+    new_accumulator = factory()
+                          ->NewConsString(accumulator(), current_part())
+                          .ToHandleChecked();
+  }
+  set_accumulator(new_accumulator);
+}
+
+
+void IncrementalStringBuilder::Extend() {
+  Accumulate();
+  if (part_length_ <= kMaxPartLength / kPartLengthGrowthFactor) {
+    part_length_ *= kPartLengthGrowthFactor;
+  }
+  Handle<String> new_part;
+  if (encoding_ == String::ONE_BYTE_ENCODING) {
+    new_part = factory()->NewRawOneByteString(part_length_).ToHandleChecked();
+  } else {
+    new_part = factory()->NewRawTwoByteString(part_length_).ToHandleChecked();
+  }
+  // Reuse the same handle to avoid being invalidated when exiting handle scope.
+  set_current_part(new_part);
+  current_index_ = 0;
+}
+
+
+MaybeHandle<String> IncrementalStringBuilder::Finish() {
+  ShrinkCurrentPart();
+  Accumulate();
+  if (overflowed_) {
+    THROW_NEW_ERROR(isolate_, NewInvalidStringLengthError(), String);
+  }
+  return accumulator();
+}
+
+
+void IncrementalStringBuilder::AppendString(Handle<String> string) {
+  ShrinkCurrentPart();
+  part_length_ = kInitialPartLength;  // Allocate conservatively.
+  Extend();  // Attach current part and allocate new part.
+  Handle<String> concat =
+      factory()->NewConsString(accumulator(), string).ToHandleChecked();
+  set_accumulator(concat);
+}
+}
+}  // namespace v8::internal