volume_param_gain.h

#pragma once
#include <algorithm>
#include <cstdint>
#include <limits>


constexpr int32_t kVolumeParamUnityPos = 536870912;
constexpr int32_t kVolumeParamMaxPos = 1073741823;
 
struct VolumeParamGain {
    int32_t param;
    int32_t remainingGainPos;
};

constexpr int32_t volumeParamToGainPos(int32_t volumeParam) {
    return (volumeParam >> 2) + kVolumeParamUnityPos;
}

constexpr VolumeParamGain applyGainToVolumeParam(int32_t volumeParam, int32_t gainPos) {
    int64_t pos = static_cast<int64_t>(volumeParamToGainPos(volumeParam));
    int64_t scaledPos = (pos * static_cast<int64_t>(gainPos)) >> 29;
 
    if (scaledPos > kVolumeParamMaxPos) {
        // Saturated. Report how much gain we actually delivered so the caller can carry the rest elsewhere.
        int64_t remaining = (scaledPos << 29) / kVolumeParamMaxPos;
        return {std::numeric_limits<int32_t>::max(),
                static_cast<int32_t>(std::min<int64_t>(remaining, std::numeric_limits<int32_t>::max()))};
    }
 
    int64_t param = (scaledPos - kVolumeParamUnityPos) << 2;
    param = std::clamp<int64_t>(param, std::numeric_limits<int32_t>::min(), std::numeric_limits<int32_t>::max());
    return {static_cast<int32_t>(param), kVolumeParamUnityPos};
}