More efficient BitReader::SkipBits for large numbers of bits.

media/base/bit_reader_core.cc

 // Copyright 2014 The Chromium 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 "media/base/bit_reader_core.h"
 
 #include <base/port.h>
 #include <base/sys_byteorder.h>
 
 namespace {
@@ skipping 33 matching lines @@
 
 int BitReaderCore::PeekBitsMsbAligned(int num_bits, uint64* out) {
   // Try to have at least |num_bits| in the bit register.
   if (nbits_ < num_bits)
     Refill(num_bits);
 
   *out = reg_;
   return nbits_;
 }
 
-bool BitReaderCore::SkipBits(int num_bits) {
-  // TODO(dalecurtis): Rewrite to be efficient, see http://crbug.com/376450
+bool BitReaderCore::SkipBitsSmall(int num_bits) {
   DCHECK_GE(num_bits, 0);
-  DVLOG_IF(1, num_bits > 100)
-      << "BitReader::SkipBits inefficient for large skips";
-
   uint64 dummy;
   while (num_bits >= kRegWidthInBits) {
     if (!ReadBitsInternal(kRegWidthInBits, &dummy))
       return false;
     num_bits -= kRegWidthInBits;
   }
   return ReadBitsInternal(num_bits, &dummy);
 }
 
+bool BitReaderCore::SkipBits(int num_bits) {
+  DCHECK_GE(num_bits, 0);
+
+  const int remaining_bits = nbits_ + nbits_next_;
+  if (remaining_bits >= num_bits)
+    return SkipBitsSmall(num_bits);
+
+  // Skip first the remaining available bits.
+  num_bits -= remaining_bits;
+  bits_read_ += remaining_bits;
+  nbits_ = 0;
+  reg_ = 0;
+  nbits_next_ = 0;
+  reg_next_ = 0;
+
+  // Next, skip an integer number of bytes.
+  const int nbytes = num_bits / 8;
+  if (nbytes > 0) {
+    const uint8* byte_stream_window;
+    int window_size =

[DaleCurtis, 2014/09/24 18:35:43]

const since you're doing so above.

[damienv1, 2014/09/25 00:50:06]

Done.

+        byte_stream_provider_->GetBytes(nbytes, &byte_stream_window);
+    DCHECK_GE(window_size, 0);
+    DCHECK_LE(window_size, nbytes);
+    if (window_size < nbytes)
+      return false;
+    num_bits -= 8 * nbytes;
+    bits_read_ += 8 * nbytes;
+  }
+
+  // Skip the remaining bits.
+  return SkipBitsSmall(num_bits);
+}
+
 int BitReaderCore::bits_read() const {
   return bits_read_;
 }
 
 bool BitReaderCore::ReadBitsInternal(int num_bits, uint64* out) {
   DCHECK_GE(num_bits, 0);
 
   if (num_bits == 0) {
     *out = 0;
     return true;
@@ skipping 72 matching lines @@
     nbits_next_ = 0;
     return;
   }
 
   nbits_ += free_nbits;
   reg_next_ <<= free_nbits;
   nbits_next_ -= free_nbits;
 }
 
 }  // namespace media