Avoiding overflow in cross correlation in NetEq.

webrtc/modules/audio_coding/neteq/cross_correlation.cc

+/*
+ *  Copyright (c) 2016 The WebRTC 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 "webrtc/modules/audio_coding/neteq/cross_correlation.h"
+
+#include <cstdlib>
+#include <limits>
+
+#include "webrtc/common_audio/signal_processing/include/signal_processing_library.h"
+
+namespace webrtc {
+
+// This function decides the overflow-protecting scaling and calls
+// WebRtcSpl_CrossCorrelation.
+int CrossCorrelationWithAutoShift(const int16_t* sequence_1,
+                                  const int16_t* sequence_2,
+                                  size_t sequence_1_length,
+                                  size_t cross_correlation_length,
+                                  int cross_correlation_step,
+                                  int32_t* cross_correlation) {
+  // Find the maximum absolute value of sequence_1 and 2.
+  const int16_t max_1 = WebRtcSpl_MaxAbsValueW16(sequence_1, sequence_1_length);
+  const int sequence_2_shift =
+      cross_correlation_step * (static_cast<int>(cross_correlation_length) - 1);
+  const int16_t* sequence_2_start =
+      sequence_2_shift >= 0 ? sequence_2 : sequence_2 + sequence_2_shift;
+  const size_t sequence_2_length =
+      sequence_1_length + std::abs(sequence_2_shift);
+  const int16_t max_2 =
+      WebRtcSpl_MaxAbsValueW16(sequence_2_start, sequence_2_length);
+
+  // In order to avoid overflow when computing the sum we should scale the
+  // samples so that (in_vector_length * max_1 * max_2) will not overflow.
+  // Expected scaling fulfills
+  // 1) sufficient:
+  //    sequence_1_length * (max_1 * max_2 >> scaling) <= 0x7fffffff;
+  // 2) necessary:
+  //    if (scaling > 0)
+  //      sequence_1_length * (max_1 * max_2 >> (scaling - 1)) > 0x7fffffff;
+  // The following calculation fulfills 1) and almost fulfills 2).
+  // There are some corner cases that 2) is not satisfied, e.g.,
+  // max_1 = 17, max_2 = 30848, sequence_1_length = 4095, in such case,
+  // optimal scaling is 0, while the following calculation results in 1.
+  const int32_t factor = (max_1 * max_2) / (std::numeric_limits<int32_t>::max()
+      / static_cast<int32_t>(sequence_1_length));
+  const int scaling = factor == 0 ? 0 : 31 - WebRtcSpl_NormW32(factor);
+
+  WebRtcSpl_CrossCorrelation(cross_correlation, sequence_1, sequence_2,
+                             sequence_1_length, cross_correlation_length,
+                             scaling, cross_correlation_step);
+
+  return scaling;
+}
+
+}  // namespace webrtc