Add a new incremental parsing API

Change-Id: I6b921766836d58df0281fb23b2add3f62a478e14

webm_parser/src/var_int_parser.cc

@@ -0,0 +1,71 @@
+// Copyright (c) 2016 The WebM 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 in the root of the source
+// tree. An additional intellectual property rights grant can be found
+// in the file PATENTS.  All contributing project authors may
+// be found in the AUTHORS file in the root of the source tree.
+#include "src/var_int_parser.h"
+
+#include <cassert>
+#include <cstdint>
+#include <limits>
+
+#include "src/bit_utils.h"
+#include "src/parser_utils.h"
+#include "webm/reader.h"
+#include "webm/status.h"
+
+namespace webm {
+
+// Spec references:
+// http://matroska.org/technical/specs/index.html#EBML_ex
+// https://github.com/Matroska-Org/ebml-specification/blob/master/specification.markdown#variable-size-integer
+Status VarIntParser::Feed(Callback* callback, Reader* reader,
+                          std::uint64_t* num_bytes_read) {
+  assert(callback != nullptr);
+  assert(reader != nullptr);
+  assert(num_bytes_read != nullptr);
+  assert(num_bytes_remaining_ != 0);
+
+  *num_bytes_read = 0;
+
+  if (num_bytes_remaining_ == -1) {
+    std::uint8_t first_byte;
+    const Status status = ReadByte(reader, &first_byte);
+    if (!status.completed_ok()) {
+      return status;
+    }
+    ++*num_bytes_read;
+
+    // The first byte must have a marker bit set to indicate how many octets are
+    // used.
+    if (first_byte == 0) {
+      return Status(Status::kInvalidElementValue);
+    }
+
+    total_data_bytes_ = CountLeadingZeros(first_byte);
+    num_bytes_remaining_ = total_data_bytes_;
+
+    value_ = first_byte;
+  }
+
+  std::uint64_t local_num_bytes_read;
+  const Status status = AccumulateIntegerBytes(num_bytes_remaining_, reader,
+                                               &value_, &local_num_bytes_read);
+  *num_bytes_read += local_num_bytes_read;
+  num_bytes_remaining_ -= static_cast<int>(local_num_bytes_read);
+
+  if (!status.completed_ok()) {
+    return status;
+  }
+
+  // Clear the marker bit.
+  constexpr std::uint64_t all_bits = std::numeric_limits<std::uint64_t>::max();
+  const std::uint64_t data_bits = all_bits >> (57 - 7 * total_data_bytes_);
+  value_ &= data_bits;
+
+  return Status(Status::kOkCompleted);
+}
+
+}  // namespace webm