Use offset and count to request slices of lists, maps, and typed_data.

runtime/vm/json_stream.cc

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "platform/assert.h"
 
 #include "vm/dart_entry.h"
 #include "vm/debugger.h"
 #include "vm/json_stream.h"
 #include "vm/message.h"
@@ skipping 12 matching lines @@
 JSONStream::JSONStream(intptr_t buf_size)
     : open_objects_(0),
       buffer_(buf_size),
       default_id_zone_(),
       id_zone_(&default_id_zone_),
       reply_port_(ILLEGAL_PORT),
       seq_(NULL),
       method_(""),
       param_keys_(NULL),
       param_values_(NULL),
-      num_params_(0) {
+      num_params_(0),
+      offset_(0),
+      count_(-1) {
   ObjectIdRing* ring = NULL;
   Isolate* isolate = Isolate::Current();
   if (isolate != NULL) {
     ring = isolate->object_id_ring();
   }
   default_id_zone_.Init(ring, ObjectIdRing::kAllocateId);
 }
 
 
 JSONStream::~JSONStream() {
@@ skipping 194 matching lines @@
 
 
 bool JSONStream::ParamIs(const char* key, const char* value) const {
   ASSERT(key);
   ASSERT(value);
   const char* key_value = LookupParam(key);
   return (key_value != NULL) && (strcmp(key_value, value) == 0);
 }
 
 
+void JSONStream::ComputeOffsetAndCount(intptr_t length,
+                                       intptr_t* offset,
+                                       intptr_t* count) {
+  // This function is written to avoid adding (count + offset) in case
+  // that triggers an integer overflow.
+  *offset = offset_;
+  if (*offset > length) {
+    *offset = length;
+  }
+  intptr_t remaining = length - *offset;
+  *count = count_;
+  if (*count < 0 || *count > remaining) {
+    *count = remaining;
+  }
+}
+
 void JSONStream::Clear() {
   buffer_.Clear();
   open_objects_ = 0;
 }
 
 
 void JSONStream::OpenObject(const char* property_name) {
   PrintCommaIfNeeded();
   open_objects_++;
   if (property_name != NULL) {
@@ skipping 548 matching lines @@
   intptr_t len2 = OS::VSNPrint(p, len+1, format, args);
   va_end(args);
   ASSERT(len == len2);
   stream_->buffer_.AddChar('"');
   stream_->AddEscapedUTF8String(p);
   stream_->buffer_.AddChar('"');
   free(p);
 }
 
 }  // namespace dart