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+/*
+Copyright 2018 Google Inc. All Rights Reserved.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS-IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+#ifndef RESONANCE_AUDIO_AMBISONICS_UTILS_H_
+#define RESONANCE_AUDIO_AMBISONICS_UTILS_H_
+
+#include <cmath>
+
+#include "base/constants_and_types.h"
+#include "base/logging.h"
+#include "base/misc_math.h"
+
+namespace vraudio {
+
+// Returns ACN channel sequence from a degree and order of a spherical harmonic.
+inline int AcnSequence(int degree, int order) {
+  DCHECK_GE(degree, 0);
+  DCHECK_LE(-degree, order);
+  DCHECK_LE(order, degree);
+
+  return degree * degree + degree + order;
+}
+
+// Returns normalization factor for Schmidt semi-normalized spherical harmonics
+// used in AmbiX.
+inline float Sn3dNormalization(int degree, int order) {
+  DCHECK_GE(degree, 0);
+  DCHECK_LE(-degree, order);
+  DCHECK_LE(order, degree);
+  return std::sqrt((2.0f - ((order == 0) ? 1.0f : 0.0f)) *
+                   Factorial(degree - std::abs(order)) /
+                   Factorial(degree + std::abs(order)));
+}
+
+// Returns the number of spherical harmonics for a periphonic ambisonic sound
+// field of |ambisonic_order| at compile-time.
+// We have to use template metaprogramming because MSVC12 doesn't support
+// constexpr.
+template <size_t AmbisonicOrder>
+struct GetNumPeriphonicComponentsStatic {
+  enum { value = (AmbisonicOrder + 1) * (AmbisonicOrder + 1) };
+};
+
+// Returns the number of spherical harmonics for a periphonic ambisonic sound
+// field of |ambisonic_order|.
+inline size_t GetNumPeriphonicComponents(int ambisonic_order) {
+  return static_cast<size_t>((ambisonic_order + 1) * (ambisonic_order + 1));
+}
+
+// Returns the number of periphonic spherical harmonics (SHs) for a particular
+// Ambisonic order. E.g. number of 1st, 2nd or 3rd degree SHs in a 3rd order
+// sound field.
+inline size_t GetNumNthOrderPeriphonicComponents(int ambisonic_order) {
+  if (ambisonic_order == 0) return 1;
+  return static_cast<size_t>(GetNumPeriphonicComponents(ambisonic_order) -
+                             GetNumPeriphonicComponents(ambisonic_order - 1));
+}
+
+// Calculates the ambisonic order of a periphonic sound field with the given
+// number of spherical harmonics.
+inline int GetPeriphonicAmbisonicOrder(size_t num_components) {
+  DCHECK_GT(num_components, 0);
+  const int ambisonic_order = static_cast<int>(std::sqrt(num_components)) - 1;
+  // Detect when num_components is not square.
+  DCHECK_EQ((ambisonic_order + 1) * (ambisonic_order + 1),
+            static_cast<int>(num_components));
+  return ambisonic_order;
+}
+
+// Calculates the order of the current spherical harmonic channel as the integer
+// part of a square root of the channel number. Please note, that in Ambisonics
+// the terms 'order' (usually denoted as 'n') and 'degree' (usually denoted as
+// 'm') are used in the opposite meaning as in more traditional maths or physics
+// conventions:
+// [1] C. Nachbar, F. Zotter, E. Deleflie, A. Sontacchi, "AMBIX - A SUGGESTED
+//     AMBISONICS FORMAT", Proc. of the 2nd Ambisonics Symposium, June 2-3 2011,
+//     Lexington, KY, https://goo.gl/jzt4Yy.
+inline int GetPeriphonicAmbisonicOrderForChannel(size_t channel) {
+  return static_cast<int>(sqrtf(static_cast<float>(channel)));
+}
+
+// Calculates the degree of the current spherical harmonic channel. Please note,
+// that in Ambisonics the terms 'order' (usually denoted as 'n') and 'degree'
+// (usually denoted as 'm') are used in the opposite meaning as in more
+// traditional maths or physics conventions:
+// [1] C. Nachbar, F. Zotter, E. Deleflie, A. Sontacchi, "AMBIX - A SUGGESTED
+//     AMBISONICS FORMAT", Proc. of the 2nd Ambisonics Symposium, June 2-3 2011,
+//     Lexington, KY, https://goo.gl/jzt4Yy.
+inline int GetPeriphonicAmbisonicDegreeForChannel(size_t channel) {
+  const int order = GetPeriphonicAmbisonicOrderForChannel(channel);
+  return static_cast<int>(channel) - order * (order + 1);
+}
+
+// Returns whether the given |num_channels| corresponds to a valid ambisonic
+// order configuration.
+inline bool IsValidAmbisonicOrder(size_t num_channels) {
+  if (num_channels == 0) {
+    return false;
+  }
+  // Number of channels must be a square number for valid ambisonic orders.
+  const size_t sqrt_num_channels = static_cast<size_t>(std::sqrt(num_channels));
+  return num_channels == sqrt_num_channels * sqrt_num_channels;
+}
+
+}  // namespace vraudio
+
+#endif  // RESONANCE_AUDIO_AMBISONICS_UTILS_H_