Make the separation between target and interferer scenario depend on microphone spacing in NonlinearBeamformer

Depends on this CL: https://codereview.webrtc.org/1378973003/

Review URL: https://codereview.webrtc.org/1388033002

Cr-Commit-Position: refs/heads/master@{#10330}

webrtc/modules/audio_processing/beamformer/array_util.cc

@@ -0,0 +1,32 @@
+/*
+ *  Copyright (c) 2015 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_processing/beamformer/array_util.h"
+
+#include <algorithm>
+#include <limits>
+
+#include "webrtc/base/checks.h"
+
+namespace webrtc {
+
+float GetMinimumSpacing(const std::vector<Point>& array_geometry) {
+  RTC_CHECK_GT(array_geometry.size(), 1u);
+  float mic_spacing = std::numeric_limits<float>::max();
+  for (size_t i = 0; i < (array_geometry.size() - 1); ++i) {
+    for (size_t j = i + 1; j < array_geometry.size(); ++j) {
+      mic_spacing =
+          std::min(mic_spacing, Distance(array_geometry[i], array_geometry[j]));
+    }
+  }
+  return mic_spacing;
+}
+
+}  // namespace webrtc