sound.cpp

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/*
    Copyright (C) 2003 - 2023
    by David White <dave@whitevine.net>
    Part of the Battle for Wesnoth Project https://www.wesnoth.org/
 
    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation; either version 2 of the License, or
    (at your option) any later version.
    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY.
 
    See the COPYING file for more details.
*/
 
#include "sound.hpp"
#include "config.hpp"
#include "filesystem.hpp"
#include "log.hpp"
#include "preferences/game.hpp"
#include "random.hpp"
#include "serialization/string_utils.hpp"
#include "sound_music_track.hpp"
 
#include <SDL2/SDL.h> // Travis doesn't like this, although it works on my machine -> '#include <SDL2/SDL_sound.h>
#include <SDL2/SDL_mixer.h>
 
#include <list>
#include <sstream>
#include <string>
 
static lg::log_domain log_audio("audio");
#define DBG_AUDIO LOG_STREAM(debug, log_audio)
#define LOG_AUDIO LOG_STREAM(info, log_audio)
#define ERR_AUDIO LOG_STREAM(err, log_audio)
 
#if (MIX_MAJOR_VERSION < 1) || (MIX_MAJOR_VERSION == 1) && ((MIX_MINOR_VERSION < 2) || (MIX_MINOR_VERSION == 2) && (MIX_PATCHLEVEL <= 11))
#error "Please upgrade to SDL mixer version >= 1.2.12, we don't support older versions anymore."
#endif
 
namespace sound
{
// Channel-chunk mapping lets us know, if we can safely free a given chunk
static std::vector<Mix_Chunk*> channel_chunks;
 
// Channel-id mapping for use with sound sources (to check if given source
// is playing on a channel for fading/panning)
static std::vector<int> channel_ids;
}
 
namespace
{
bool mix_ok = false;
int music_start_time = 0;
unsigned music_refresh = 0;
unsigned music_refresh_rate = 20;
bool want_new_music = false;
int fadingout_time = 5000;
bool no_fading = false;
bool unload_music = false;
 
// number of allocated channels,
const std::size_t n_of_channels = 32;
 
// we need 2 channels, because we it for timer as well
const std::size_t bell_channel = 0;
const std::size_t timer_channel = 1;
 
// number of channels reserved for sound sources
const std::size_t source_channels = 8;
const std::size_t source_channel_start = timer_channel + 1;
const std::size_t source_channel_last = source_channel_start + source_channels - 1;
const std::size_t UI_sound_channels = 2;
const std::size_t UI_sound_channel_start = source_channel_last + 1;
const std::size_t UI_sound_channel_last = UI_sound_channel_start + UI_sound_channels - 1;
const std::size_t n_reserved_channels = UI_sound_channel_last + 1; // sources, bell, timer and UI
 
// Max number of sound chunks that we want to cache
// Keep this above number of available channels to avoid busy-looping
unsigned max_cached_chunks = 256;
 
std::map<Mix_Chunk*, int> chunk_usage;
} // end anon namespace
 
static void increment_chunk_usage(Mix_Chunk* mcp)
{
    ++(chunk_usage[mcp]);
}
 
static void decrement_chunk_usage(Mix_Chunk* mcp)
{
    if(mcp == nullptr) {
        return;
    }
 
    std::map<Mix_Chunk*, int>::iterator this_usage = chunk_usage.find(mcp);
    assert(this_usage != chunk_usage.end());
    if(--(this_usage->second) == 0) {
        Mix_FreeChunk(mcp);
        chunk_usage.erase(this_usage);
    }
}
 
namespace
{
class sound_cache_chunk
{
public:
    sound_cache_chunk(const std::string& f)
        : group(sound::NULL_CHANNEL)
        , file(f)
        , data_(nullptr)
    {
    }
    sound_cache_chunk(const sound_cache_chunk& scc)
        : group(scc.group)
        , file(scc.file)
        , data_(scc.data_)
    {
        increment_chunk_usage(data_);
    }
 
    ~sound_cache_chunk()
    {
        decrement_chunk_usage(data_);
    }
 
    sound::channel_group group;
    std::string file;
 
    void set_data(Mix_Chunk* d)
    {
        increment_chunk_usage(d);
        decrement_chunk_usage(data_);
        data_ = d;
    }
 
    Mix_Chunk* get_data() const
    {
        return data_;
    }
 
    bool operator==(const sound_cache_chunk& scc) const
    {
        return file == scc.file;
    }
 
    bool operator!=(const sound_cache_chunk& scc) const
    {
        return !operator==(scc);
    }
 
    sound_cache_chunk& operator=(const sound_cache_chunk& scc)
    {
        file = scc.file;
        group = scc.group;
        set_data(scc.get_data());
        return *this;
    }
 
private:
    Mix_Chunk* data_;
};
 
std::list<sound_cache_chunk> sound_cache;
typedef std::list<sound_cache_chunk>::iterator sound_cache_iterator;
std::map<std::string, std::shared_ptr<Mix_Music>> music_cache;
 
std::vector<std::string> played_before;
 
//
// FIXME: the first music_track may be initialized before main()
// is reached. Using the logging facilities may lead to a SIGSEGV
// because it's not guaranteed that their objects are already alive.
//
// Use the music_track default constructor to avoid trying to
// invoke a log object while resolving paths.
//
std::vector<std::shared_ptr<sound::music_track>> current_track_list;
std::shared_ptr<sound::music_track> current_track;
unsigned int current_track_index = 0;
std::shared_ptr<sound::music_track> previous_track;
 
std::vector<std::shared_ptr<sound::music_track>>::const_iterator find_track(const sound::music_track& track)
{
    return std::find_if(current_track_list.begin(), current_track_list.end(),
        [&track](const std::shared_ptr<const sound::music_track>& ptr) { return *ptr == track; }
    );
}
 
} // end anon namespace
 
namespace sound
{
void flush_cache()
{
    sound_cache.clear();
    music_cache.clear();
}
 
std::optional<unsigned int> get_current_track_index()
{
    if(current_track_index >= current_track_list.size()){
        return {};
    }
    return current_track_index;
}
std::shared_ptr<music_track> get_current_track()
{
    return current_track;
}
std::shared_ptr<music_track> get_previous_music_track()
{
    return previous_track;
}
void set_previous_track(std::shared_ptr<music_track> track)
{
    previous_track = track;
}
 
unsigned int get_num_tracks()
{
    return current_track_list.size();
}
 
std::shared_ptr<music_track> get_track(unsigned int i)
{
    if(i < current_track_list.size()) {
        return current_track_list[i];
    }
 
    if(i == current_track_list.size()) {
        return current_track;
    }
 
    return nullptr;
}
 
void set_track(unsigned int i, const std::shared_ptr<music_track>& to)
{
    if(i < current_track_list.size() && find_track(*to) != current_track_list.end()) {
        current_track_list[i] = std::make_shared<music_track>(*to);
    }
}
 
void remove_track(unsigned int i)
{
    if(i >= current_track_list.size()) {
        return;
    }
 
    if(i == current_track_index) {
        // Let the track finish playing
        if(current_track){
            current_track->set_play_once(true);
        }
        // Set current index to the new size of the list
        current_track_index = current_track_list.size() - 1;
    } else if(i < current_track_index) {
        current_track_index--;
    }
 
    current_track_list.erase(current_track_list.begin() + i);
}
 
} // end namespace sound
 
static bool track_ok(const std::string& id)
{
    LOG_AUDIO << "Considering " << id;
 
    if(!current_track) {
        return true;
    }
 
    // If they committed changes to list, we forget previous plays, but
    // still *never* repeat same track twice if we have an option.
    if(id == current_track->file_path()) {
        return false;
    }
 
    if(current_track_list.size() <= 3) {
        return true;
    }
 
    // Timothy Pinkham says:
    // 1) can't be repeated without 2 other pieces have already played
    // since A was played.
    // 2) cannot play more than 2 times without every other piece
    // having played at least 1 time.
 
    // Dammit, if our musicians keep coming up with algorithms, I'll
    // be out of a job!
    unsigned int num_played = 0;
    std::set<std::string> played;
    std::vector<std::string>::reverse_iterator i;
 
    for(i = played_before.rbegin(); i != played_before.rend(); ++i) {
        if(*i == id) {
            ++num_played;
            if(num_played == 2) {
                break;
            }
        } else {
            played.insert(*i);
        }
    }
 
    // If we've played this twice, must have played every other track.
    if(num_played == 2 && played.size() != current_track_list.size() - 1) {
        LOG_AUDIO << "Played twice with only " << played.size() << " tracks between";
        return false;
    }
 
    // Check previous previous track not same.
    i = played_before.rbegin();
    if(i != played_before.rend()) {
        ++i;
        if(i != played_before.rend()) {
            if(*i == id) {
                LOG_AUDIO << "Played just before previous";
                return false;
            }
        }
    }
 
    return true;
}
 
static std::shared_ptr<sound::music_track> choose_track()
{
    assert(!current_track_list.empty());
 
    if(current_track_index >= current_track_list.size()) {
        current_track_index = 0;
    }
 
    if(current_track_list[current_track_index]->shuffle()) {
        unsigned int track = 0;
 
        if(current_track_list.size() > 1) {
            do {
                track = randomness::rng::default_instance().get_random_int(0, current_track_list.size()-1);
            } while(!track_ok(current_track_list[track]->file_path()));
        }
 
        current_track_index = track;
    }
 
    DBG_AUDIO << "Next track will be " << current_track_list[current_track_index]->file_path();
    played_before.push_back(current_track_list[current_track_index]->file_path());
    return current_track_list[current_track_index];
}
 
static std::string pick_one(const std::string& files)
{
    std::vector<std::string> ids = utils::square_parenthetical_split(files, ',', "[", "]");
 
    if(ids.empty()) {
        return "";
    }
 
    if(ids.size() == 1) {
        return ids[0];
    }
 
    // We avoid returning same choice twice if we can avoid it.
    static std::map<std::string, unsigned int> prev_choices;
    unsigned int choice;
 
    if(prev_choices.find(files) != prev_choices.end()) {
        choice = randomness::rng::default_instance().get_random_int(0, ids.size()-1 - 1);
        if(choice >= prev_choices[files]) {
            ++choice;
        }
 
        prev_choices[files] = choice;
    } else {
        choice = randomness::rng::default_instance().get_random_int(0, ids.size()-1);
        prev_choices.emplace(files, choice);
    }
 
    return ids[choice];
}
 
namespace
{
struct audio_lock
{
    audio_lock()
    {
        SDL_LockAudio();
    }
 
    ~audio_lock()
    {
        SDL_UnlockAudio();
    }
};
 
} // end of anonymous namespace
 
namespace sound
{
// Removes channel-chunk and channel-id mapping
static void channel_finished_hook(int channel)
{
    channel_chunks[channel] = nullptr;
    channel_ids[channel] = -1;
}
 
std::string current_driver()
{
    const char* const drvname = SDL_GetCurrentAudioDriver();
    return drvname ? drvname : "<not initialized>";
}
 
std::vector<std::string> enumerate_drivers()
{
    std::vector<std::string> res;
    int num_drivers = SDL_GetNumVideoDrivers();
 
    for(int n = 0; n < num_drivers; ++n) {
        const char* drvname = SDL_GetAudioDriver(n);
        res.emplace_back(drvname ? drvname : "<invalid driver>");
    }
 
    return res;
}
 
driver_status driver_status::query()
{
    driver_status res{mix_ok, 0, 0, 0, 0};
 
    if(mix_ok) {
        Mix_QuerySpec(&res.frequency, &res.format, &res.channels);
        res.chunk_size = preferences::sound_buffer_size();
    }
 
    return res;
}
 
bool init_sound()
{
    LOG_AUDIO << "Initializing audio...";
    if(SDL_WasInit(SDL_INIT_AUDIO) == 0) {
        if(SDL_InitSubSystem(SDL_INIT_AUDIO) == -1) {
            return false;
        }
    }
 
    if(!mix_ok) {
        if(Mix_OpenAudio(preferences::sample_rate(), MIX_DEFAULT_FORMAT, 2, preferences::sound_buffer_size()) == -1) {
            mix_ok = false;
            ERR_AUDIO << "Could not initialize audio: " << Mix_GetError();
            return false;
        }
 
        mix_ok = true;
        Mix_AllocateChannels(n_of_channels);
        Mix_ReserveChannels(n_reserved_channels);
 
        channel_chunks.clear();
        channel_chunks.resize(n_of_channels, nullptr);
        channel_ids.resize(n_of_channels, -1);
 
        Mix_GroupChannel(bell_channel, SOUND_BELL);
        Mix_GroupChannel(timer_channel, SOUND_TIMER);
        Mix_GroupChannels(source_channel_start, source_channel_last, SOUND_SOURCES);
        Mix_GroupChannels(UI_sound_channel_start, UI_sound_channel_last, SOUND_UI);
        Mix_GroupChannels(n_reserved_channels, n_of_channels - 1, SOUND_FX);
 
        set_sound_volume(preferences::sound_volume());
        set_UI_volume(preferences::UI_volume());
        set_music_volume(preferences::music_volume());
        set_bell_volume(preferences::bell_volume());
 
        Mix_ChannelFinished(channel_finished_hook);
 
        LOG_AUDIO << "Audio initialized.";
 
        DBG_AUDIO << "Channel layout: " << n_of_channels << " channels (" << n_reserved_channels << " reserved)\n"
                  << "    " << bell_channel << " - bell\n"
                  << "    " << timer_channel << " - timer\n"
                  << "    " << source_channel_start << ".." << source_channel_last << " - sound sources\n"
                  << "    " << UI_sound_channel_start << ".." << UI_sound_channel_last << " - UI\n"
                  << "    " << UI_sound_channel_last + 1 << ".." << n_of_channels - 1 << " - sound effects";
 
        play_music();
    }
 
    return true;
}
 
void close_sound()
{
    int frequency, channels;
    uint16_t format;
 
    if(mix_ok) {
        stop_bell();
        stop_UI_sound();
        stop_sound();
        sound_cache.clear();
        stop_music();
        mix_ok = false;
 
        int numtimesopened = Mix_QuerySpec(&frequency, &format, &channels);
        if(numtimesopened == 0) {
            ERR_AUDIO << "Error closing audio device: " << Mix_GetError();
        }
 
        while(numtimesopened) {
            Mix_CloseAudio();
            --numtimesopened;
        }
    }
 
    if(SDL_WasInit(SDL_INIT_AUDIO) != 0) {
        SDL_QuitSubSystem(SDL_INIT_AUDIO);
    }
 
    LOG_AUDIO << "Audio device released.";
}
 
void reset_sound()
{
    bool music = preferences::music_on();
    bool sound = preferences::sound_on();
    bool UI_sound = preferences::UI_sound_on();
    bool bell = preferences::turn_bell();
 
    if(music || sound || bell || UI_sound) {
        sound::close_sound();
        if(!sound::init_sound()) {
            ERR_AUDIO << "Error initializing audio device: " << Mix_GetError();
        }
 
        if(!music) {
            sound::stop_music();
        }
 
        if(!sound) {
            sound::stop_sound();
        }
 
        if(!UI_sound) {
            sound::stop_UI_sound();
        }
 
        if(!bell) {
            sound::stop_bell();
        }
    }
}
 
void stop_music()
{
    if(mix_ok) {
        Mix_FadeOutMusic(500);
        Mix_HookMusicFinished([]() { unload_music = true; });
    }
}
 
void stop_sound()
{
    if(mix_ok) {
        Mix_HaltGroup(SOUND_SOURCES);
        Mix_HaltGroup(SOUND_FX);
 
        sound_cache.remove_if([](const sound_cache_chunk& c) {
            return c.group == SOUND_SOURCES || c.group == SOUND_FX;
        });
    }
}
 
/*
 * For the purpose of channel manipulation, we treat turn timer the same as bell
 */
void stop_bell()
{
    if(mix_ok) {
        Mix_HaltGroup(SOUND_BELL);
        Mix_HaltGroup(SOUND_TIMER);
 
        sound_cache.remove_if([](const sound_cache_chunk& c) {
            return c.group == SOUND_BELL || c.group == SOUND_TIMER;
        });
    }
}
 
void stop_UI_sound()
{
    if(mix_ok) {
        Mix_HaltGroup(SOUND_UI);
 
        sound_cache.remove_if([](const sound_cache_chunk& c) {
            return c.group == SOUND_UI;
        });
    }
}
 
void play_music_once(const std::string& file)
{
    set_previous_track(current_track);
    current_track = std::make_shared<music_track>(file);
    current_track->set_play_once(true);
    current_track_index = current_track_list.size();
    play_music();
}
 
void empty_playlist()
{
    current_track_list.clear();
}
 
void play_music()
{
    if(!current_track) {
        return;
    }
 
    music_start_time = 1; // immediate (same as effect as SDL_GetTicks())
    want_new_music = true;
    no_fading = false;
    fadingout_time = previous_track != nullptr ? previous_track->ms_after() : 0;
}
 
void play_track(unsigned int i)
{
    set_previous_track(current_track);
    if(i >= current_track_list.size()) {
        current_track = choose_track();
    } else {
        current_track_index = i;
        current_track = current_track_list[i];
    }
    play_music();
}
 
static void play_new_music()
{
    music_start_time = 0; // reset status: no start time
    want_new_music = true;
 
    if(!preferences::music_on() || !mix_ok || !current_track || !current_track->valid()) {
        return;
    }
 
    const std::string localized = filesystem::get_localized_path(current_track->file_path());
    const std::string& filename = localized.empty() ? current_track->file_path() : localized;
 
    auto itor = music_cache.find(filename);
    if(itor == music_cache.end()) {
        LOG_AUDIO << "attempting to insert track '" << filename << "' into cache";
 
        filesystem::rwops_ptr rwops = filesystem::make_read_RWops(filename);
        // SDL takes ownership of rwops
        const std::shared_ptr<Mix_Music> music(Mix_LoadMUSType_RW(rwops.release(), MUS_NONE, true), &Mix_FreeMusic);
 
        if(music == nullptr) {
            ERR_AUDIO << "Could not load music file '" << filename << "': " << Mix_GetError();
            return;
        }
 
        itor = music_cache.emplace(filename, music).first;
    }
 
    LOG_AUDIO << "Playing track '" << filename << "'";
    int fading_time = current_track->ms_before();
    if(no_fading) {
        fading_time = 0;
    }
 
    // Halt any existing music.
    // If we don't do this SDL_Mixer blocks everything until fade out is complete.
    // Do not remove this without ensuring that it does not block.
    // If you don't want it to halt the music, ensure that fades are completed
    // before attempting to play new music.
    Mix_HaltMusic();
 
    // Fade in the new music
    const int res = Mix_FadeInMusic(itor->second.get(), 1, fading_time);
    if(res < 0) {
        ERR_AUDIO << "Could not play music: " << Mix_GetError() << " " << filename << " ";
    }
 
    want_new_music = false;
}
 
void play_music_repeatedly(const std::string& id)
{
    // Can happen if scenario doesn't specify.
    if(id.empty()) {
        return;
    }
 
    current_track_list.clear();
    current_track_list.emplace_back(new music_track(id));
 
    std::shared_ptr<music_track> last_track = current_track;
    current_track = current_track_list.back();
    current_track_index = 0;
 
    // If we're already playing it, don't interrupt.
    if(!last_track || !current_track || *last_track != *current_track) {
        play_music();
    }
 
    last_track.reset();
}
 
void play_music_config(const config& music_node, bool allow_interrupt_current_track, int i)
{
    //
    // FIXME: there is a memory leak somewhere in this function, seemingly related to the shared_ptrs
    // stored in current_track_list.
    //
    // vultraz 5/8/2017
    //
 
    music_track track(music_node);
 
    if(!track.valid() && !track.id().empty()) {
        ERR_AUDIO << "cannot open track '" << track.id() << "'; disabled in this playlist.";
    }
 
    // If they say play once, we don't alter playlist.
    if(track.play_once()) {
        set_previous_track(current_track);
        current_track = std::make_shared<music_track>(track);
        current_track_index = current_track_list.size();
        play_music();
        return;
    }
 
    // Clear play list unless they specify append.
    if(!track.append()) {
        current_track_list.clear();
    }
 
    if(!track.valid()) {
        return;
    }
 
    auto iter = find_track(track);
    // Avoid 2 tracks with the same name, since that can cause an infinite loop
    // in choose_track(), 2 tracks with the same name will always return the
    // current track and track_ok() doesn't allow that.
    if(iter == current_track_list.end()) {
        if(i < 0 || static_cast<std::size_t>(i) >= current_track_list.size()) {
            current_track_list.emplace_back(new music_track(track));
            iter = current_track_list.end() - 1;
        } else {
            iter = current_track_list.emplace(current_track_list.begin() + 1, new music_track(track));
            if(current_track_index >= static_cast<std::size_t>(i)) {
                current_track_index++;
            }
        }
    } else {
        ERR_AUDIO << "tried to add duplicate track '" << track.file_path() << "'";
    }
 
    // They can tell us to start playing this list immediately.
    if(track.immediate()) {
        set_previous_track(current_track);
        current_track = *iter;
        current_track_index = iter - current_track_list.begin();
        play_music();
    } else if(!track.append() && !allow_interrupt_current_track && current_track) {
        // Make sure the current track will finish first
        current_track->set_play_once(true);
    }
}
 
void music_thinker::process(events::pump_info& info)
{
    if(Mix_FadingMusic() != MIX_NO_FADING) {
        // Do not block everything while fading.
        return;
    }
 
    if(preferences::music_on()) {
        if(!music_start_time && !current_track_list.empty() && !Mix_PlayingMusic()) {
            // Pick next track, add ending time to its start time.
            set_previous_track(current_track);
            current_track = choose_track();
            music_start_time = info.ticks();
            no_fading = true;
            fadingout_time = 0;
        }
 
        if(music_start_time && info.ticks(&music_refresh, music_refresh_rate) >= music_start_time - fadingout_time) {
            want_new_music = true;
        }
 
        if(want_new_music) {
            if(Mix_PlayingMusic()) {
                Mix_FadeOutMusic(fadingout_time);
                return;
            }
 
            unload_music = false;
            play_new_music();
        }
    }
 
    if(unload_music) {
        // The custom shared_ptr deleter (Mix_FreeMusic) will handle the freeing of each track.
        music_cache.clear();
 
        Mix_HookMusicFinished(nullptr);
 
        unload_music = false;
    }
}
 
music_muter::music_muter()
    : events::sdl_handler(false)
{
    join_global();
}
 
void music_muter::handle_window_event(const SDL_Event& event)
{
    if(preferences::stop_music_in_background() && preferences::music_on()) {
        if(event.window.event == SDL_WINDOWEVENT_FOCUS_GAINED) {
            Mix_ResumeMusic();
        } else if(event.window.event == SDL_WINDOWEVENT_FOCUS_LOST) {
            if(Mix_PlayingMusic()) {
                Mix_PauseMusic();
            }
        }
    }
}
 
void commit_music_changes()
{
    played_before.clear();
 
    // Play-once is OK if still playing.
    if(current_track) {
        if(current_track->play_once()) {
            return;
        }
 
        // If current track no longer on playlist, change it.
        for(auto m : current_track_list) {
            if(*current_track == *m) {
                return;
            }
        }
    }
 
    // Victory empties playlist: if next scenario doesn't specify one...
    if(current_track_list.empty()) {
        return;
    }
 
    // FIXME: we don't pause ms_before on this first track.  Should we?
    set_previous_track(current_track);
    current_track = choose_track();
    play_music();
}
 
void write_music_play_list(config& snapshot)
{
    // First entry clears playlist, others append to it.
    bool append = false;
    for(auto m : current_track_list) {
        m->write(snapshot, append);
        append = true;
    }
}
 
void reposition_sound(int id, unsigned int distance)
{
    audio_lock lock;
    for(unsigned ch = 0; ch < channel_ids.size(); ++ch) {
        if(channel_ids[ch] != id) {
            continue;
        }
 
        if(distance >= DISTANCE_SILENT) {
            Mix_HaltChannel(ch);
        } else {
            Mix_SetDistance(ch, distance);
        }
    }
}
 
bool is_sound_playing(int id)
{
    audio_lock lock;
    return std::find(channel_ids.begin(), channel_ids.end(), id) != channel_ids.end();
}
 
void stop_sound(int id)
{
    reposition_sound(id, DISTANCE_SILENT);
}
 
struct chunk_load_exception
{
};
 
static Mix_Chunk* load_chunk(const std::string& file, channel_group group)
{
    sound_cache_iterator it_bgn, it_end;
    sound_cache_iterator it;
 
    sound_cache_chunk temp_chunk(file); // search the sound cache on this key
    it_bgn = sound_cache.begin();
    it_end = sound_cache.end();
    it = std::find(it_bgn, it_end, temp_chunk);
 
    if(it != it_end) {
        if(it->group != group) {
            // cached item has been used in multiple sound groups
            it->group = NULL_CHANNEL;
        }
 
        // splice the most recently used chunk to the front of the cache
        sound_cache.splice(it_bgn, sound_cache, it);
    } else {
        // remove the least recently used chunk from cache if it's full
        bool cache_full = (sound_cache.size() == max_cached_chunks);
        while(cache_full && it != it_bgn) {
            // make sure this chunk is not being played before freeing it
            std::vector<Mix_Chunk*>::iterator ch_end = channel_chunks.end();
            if(std::find(channel_chunks.begin(), ch_end, (--it)->get_data()) == ch_end) {
                sound_cache.erase(it);
                cache_full = false;
            }
        }
 
        if(cache_full) {
            LOG_AUDIO << "Maximum sound cache size reached and all are busy, skipping.";
            throw chunk_load_exception();
        }
 
        temp_chunk.group = group;
        const std::string& filename = filesystem::get_binary_file_location("sounds", file);
        const std::string localized = filesystem::get_localized_path(filename);
 
        if(!filename.empty()) {
            filesystem::rwops_ptr rwops = filesystem::make_read_RWops(localized.empty() ? filename : localized);
            temp_chunk.set_data(Mix_LoadWAV_RW(rwops.release(), true)); // SDL takes ownership of rwops
        } else {
            ERR_AUDIO << "Could not load sound file '" << file << "'.";
            throw chunk_load_exception();
        }
 
        if(temp_chunk.get_data() == nullptr) {
            ERR_AUDIO << "Could not load sound file '" << filename << "': " << Mix_GetError();
            throw chunk_load_exception();
        }
 
        sound_cache.push_front(temp_chunk);
    }
 
    return sound_cache.begin()->get_data();
}
 
static void play_sound_internal(const std::string& files,
        channel_group group,
        unsigned int repeats = 0,
        unsigned int distance = 0,
        int id = -1,
        int loop_ticks = 0,
        int fadein_ticks = 0)
{
    if(files.empty() || distance >= DISTANCE_SILENT || !mix_ok) {
        return;
    }
 
    audio_lock lock;
 
    // find a free channel in the desired group
    int channel = Mix_GroupAvailable(group);
    if(channel == -1) {
        LOG_AUDIO << "All channels dedicated to sound group(" << group << ") are busy, skipping.";
        return;
    }
 
    Mix_Chunk* chunk;
    std::string file = pick_one(files);
 
    try {
        chunk = load_chunk(file, group);
        assert(chunk);
    } catch(const chunk_load_exception&) {
        return;
    }
 
    /*
     * This check prevents SDL_Mixer from blowing up on Windows when UI sound is played
     * in response to toggling the checkbox which disables sound.
     */
    if(group != SOUND_UI) {
        Mix_SetDistance(channel, distance);
    }
 
    int res;
    if(loop_ticks > 0) {
        if(fadein_ticks > 0) {
            res = Mix_FadeInChannelTimed(channel, chunk, -1, fadein_ticks, loop_ticks);
        } else {
            res = Mix_PlayChannel(channel, chunk, -1);
        }
 
        if(res >= 0) {
            Mix_ExpireChannel(channel, loop_ticks);
        }
    } else {
        if(fadein_ticks > 0) {
            res = Mix_FadeInChannel(channel, chunk, repeats, fadein_ticks);
        } else {
            res = Mix_PlayChannel(channel, chunk, repeats);
        }
    }
 
    if(res < 0) {
        ERR_AUDIO << "error playing sound effect: " << Mix_GetError();
        // still keep it in the sound cache, in case we want to try again later
        return;
    }
 
    channel_ids[channel] = id;
 
    // reserve the channel's chunk from being freed, since it is playing
    channel_chunks[res] = chunk;
}
 
void play_sound(const std::string& files, channel_group group, unsigned int repeats)
{
    if(preferences::sound_on()) {
        play_sound_internal(files, group, repeats);
    }
}
 
void play_sound_positioned(const std::string& files, int id, int repeats, unsigned int distance)
{
    if(preferences::sound_on()) {
        play_sound_internal(files, SOUND_SOURCES, repeats, distance, id);
    }
}
 
// Play bell with separate volume setting
void play_bell(const std::string& files)
{
    if(preferences::turn_bell()) {
        play_sound_internal(files, SOUND_BELL);
    }
}
 
// Play timer with separate volume setting
void play_timer(const std::string& files, int loop_ticks, int fadein_ticks)
{
    if(preferences::sound_on()) {
        play_sound_internal(files, SOUND_TIMER, 0, 0, -1, loop_ticks, fadein_ticks);
    }
}
 
// Play UI sounds on separate volume than soundfx
void play_UI_sound(const std::string& files)
{
    if(preferences::UI_sound_on()) {
        play_sound_internal(files, SOUND_UI);
    }
}
 
int get_music_volume()
{
    if(mix_ok) {
        return Mix_VolumeMusic(-1);
    }
 
    return 0;
}
 
void set_music_volume(int vol)
{
    if(mix_ok && vol >= 0) {
        if(vol > MIX_MAX_VOLUME) {
            vol = MIX_MAX_VOLUME;
        }
 
        Mix_VolumeMusic(vol);
    }
}
 
int get_sound_volume()
{
    if(mix_ok) {
        // Since set_sound_volume sets all main channels to the same, just return the volume of any main channel
        return Mix_Volume(source_channel_start, -1);
    }
    return 0;
}
 
void set_sound_volume(int vol)
{
    if(mix_ok && vol >= 0) {
        if(vol > MIX_MAX_VOLUME) {
            vol = MIX_MAX_VOLUME;
        }
 
        // Bell, timer and UI have separate channels which we can't set up from this
        for(unsigned i = 0; i < n_of_channels; ++i) {
            if(!(i >= UI_sound_channel_start && i <= UI_sound_channel_last) && i != bell_channel
                    && i != timer_channel) {
                Mix_Volume(i, vol);
            }
        }
    }
}
 
/*
 * For the purpose of volume setting, we treat turn timer the same as bell
 */
void set_bell_volume(int vol)
{
    if(mix_ok && vol >= 0) {
        if(vol > MIX_MAX_VOLUME) {
            vol = MIX_MAX_VOLUME;
        }
 
        Mix_Volume(bell_channel, vol);
        Mix_Volume(timer_channel, vol);
    }
}
 
void set_UI_volume(int vol)
{
    if(mix_ok && vol >= 0) {
        if(vol > MIX_MAX_VOLUME) {
            vol = MIX_MAX_VOLUME;
        }
 
        for(unsigned i = UI_sound_channel_start; i <= UI_sound_channel_last; ++i) {
            Mix_Volume(i, vol);
        }
    }
}
 
} // end namespace sound