// Copyright 2018 The Fuchsia Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

library fuchsia.media;

//
// AudioRenderers can be in one of two states at any point in time, either
// the configurable state or the operational state. A renderer is considered
// to be operational any time it has packets queued and waiting to be
// rendered; otherwise it is considered to be in the configurable state. When an
// AudioRenderer has entered the operational state of its life, any attempt to
// call a config method in the interface is considered to be illegal and will
// result in termination of the interface's connection to the audio service.
//
// If an AudioRenderer must be reconfigured, it is best practice to always call
// `DiscardAllPackets` on the AudioRenderer, before starting to reconfigure it.
//

[FragileBase]
interface AudioRenderer : StreamBufferSet, StreamSink {
    // Sets the type of the stream to be delivered by the client. Using this
    // method implies that the stream encoding is AUDIO_ENCODING_LPCM.
    SetPcmStreamType(AudioStreamType type);

    // Sets the stream type to be delivered by the client. This method is used for
    // compressed pass-through. The media_specific field must be of type audio.
    // NOTE: Not currently implemented.
    SetStreamType(StreamType type);

    // Sets the units used by the presentation (media) timeline. By default, PTS
    // units are nanoseconds (as if this were called with values of 1e9 and 1).
    SetPtsUnits(uint32 tick_per_second_numerator,
                uint32 tick_per_second_denominator);

    // Sets the maximum threshold (in frames) between an explicit PTS (user-
    // provided) and an expected PTS (determined using interpolation). Beyond this
    // threshold, a stream is no longer considered 'continuous' by the renderer.
    //
    // Defaults to RoundUp((AudioFPS/PTSTicksPerSec) / 2.0) / AudioFPS
    // Most users should not need to change this value from its default.
    //
    // Example:
    // A user is playing back 48KHz audio from a container, which also contains
    // video and needs to be synchronized with the audio. The timestamps are
    // provided explicitly per packet by the container, and expressed in mSec
    // units. This means that a single tick of the media timeline (1 mSec)
    // represents exactly 48 frames of audio. The application in this scenario
    // delivers packets of audio to the AudioRenderer, each with exactly 470
    // frames of audio, and each with an explicit timestamp set to the best
    // possible representation of the presentation time (given this media clock's
    // resolution). So, starting from zero, the timestamps would be..
    //
    // [ 0, 10, 20, 29, 39, 49, 59, 69, 78, 88, ... ]
    //
    // In this example, attempting to use the presentation time to compute the
    // starting frame number of the audio in the packet would be wrong the
    // majority of the time. The first timestamp is correct (by definition), but
    // it will be 24 packets before the timestamps and frame numbers come back
    // into alignment (the 24th packet would start with the 11280th audio frame
    // and have a PTS of exactly 235).
    //
    // One way to fix this situation is to set the PTS continuity threshold
    // (henceforth, CT) for the stream to be equal to 1/2 of the time taken by the
    // number of frames contained within a single tick of the media clock, rounded
    // up. In this scenario, that would be 24.0 frames of audio, or 500 uSec.
    // Any packets whose expected PTS was within +/-CT frames of the explicitly
    // provided PTS would be considered to be a continuation of the previous frame
    // of audio.
    //
    // Other possible uses:
    // Users who are scheduling audio explicitly, relative to a clock which has
    // not been configured as the reference clock, can use this value to control
    // the maximum acceptable synchronization error before a discontinuity is
    // introduced. E.g., if a user is scheduling audio based on a recovered common
    // media clock, and has not published that clock as the reference clock, and
    // they set the CT to 20mSec, then up to 20mSec of drift error can accumulate
    // before the AudioRenderer deliberately inserts a presentation discontinuity
    // to account for the error.
    //
    // Users whose need to deal with a container where their timestamps may be
    // even less correct than +/- 1/2 of a PTS tick may set this value to
    // something larger. This should be the maximum level of inaccuracy present
    // in the container timestamps, if known. Failing that, it could be set to
    // the maximum tolerable level of drift error before absolute timestamps are
    // explicitly obeyed. Finally, a user could set this number to a very large
    // value (86400.0 seconds, for example) to effectively cause *all* timestamps
    // to be ignored after the first, thus treating all audio as continuous with
    // previously delivered packets. Conversely, users who wish to *always*
    // explicitly schedule their audio packets exactly may specify a CT of 0.
    //
    SetPtsContinuityThreshold(float32 threshold_seconds);

    // Set the reference clock used to control playback rate.
    //
    // TODO(mpuryear): refine this type when we solidly define what a clock handle
    // is/looks like. Also should we allow users to lock their rates to CLOCK_MONO
    // instead of following the default (perhaps dynamic) system rate?
    SetReferenceClock(handle reference_clock);

    // Immediately put the AudioRenderer into a playing state. Start the advance
    // of the media timeline, using specific values provided by the caller (or
    // default values if not specified). In an optional callback, return the
    // timestamp values ultimately used -- these set the ongoing relationship
    // between the media and reference timelines (i.e., how to translate between
    // the domain of presentation timestamps, and the realm of local system time).
    //
    // Local system time is specified in units of nanoseconds; media_time is
    // specified in the units defined by the user in the `SetPtsUnits` function,
    // or nanoseconds if `SetPtsUnits` is not called.
    //
    // The act of placing an AudioRenderer into the playback state establishes a
    // relationship between 1) the user-defined media (or presentation) timeline
    // for this particular AudioRenderer, and 2) the real-world system reference
    // timeline. To communicate how to translate between timelines, the Play()
    // callback provides an equivalent timestamp in each time domain. The first
    // value ('reference_time') is given in terms of the local system clock; the
    // second value ('media_time') is what media instant exactly corresponds to
    // that local time. Restated, the frame at 'media_time' in the audio stream
    // should be presented at system local time 'reference_time'.
    //
    // Note: on calling this API, media_time immediately starts advancing. It is
    // possible, although uncommon, for a caller to specify a system time that is
    // far in the past, or far into the future. This, along with the specified
    // media time, is simply used to determine what media time corresponds to
    // 'now', and THAT media time is then intersected with presentation
    // timestamps of packets already submitted, to determine which media frames
    // should be presented next.
    //
    // With the corresponding reference_time and media_time values, a user can
    // translate arbitrary time values from one timeline into the other. After
    // calling `SetPtsUnits(pts_per_sec_numerator, pts_per_sec_denominator)` and
    // given the 'ref_start' and 'media_start' values from `Play()`, then for any
    // 'ref_time':
    //
    // media_time = ( (ref_time - ref_start) / 1e9
    //                * (pts_per_sec_numerator / pts_per_sec_denominator) )
    //              + media_start
    //
    // Conversely, for any presentation timestamp 'media_time':
    //
    // ref_time = ( (media_time - media_start)
    //              * (pts_per_sec_denominator / pts_per_sec_numerator)
    //              * 1e9 )
    //            + ref_start
    //
    // Users, depending on their use case, may optionally choose not to specify
    // one or both of these timestamps. A timestamp may be omitted by supplying
    // the special value 'kNoTimestamp'. The AudioRenderer automatically deduces
    // any omitted timestamp value using the following rules:
    //
    // Reference Time
    // If 'reference_time' is omitted, the AudioRenderer will select a "safe"
    // reference time to begin presentation, based on the minimum lead times for
    // the output devices that are currently bound to this AudioRenderer. For
    // example, if an AudioRenderer is bound to an internal audio output
    // requiring at least 3 mSec of lead time, and an HDMI output requiring at
    // least 75 mSec of lead time, the AudioRenderer might (if 'reference_time'
    // is omitted) select a reference time 80 mSec from now.
    //
    // Media Time
    // If media_time is omitted, the AudioRenderer will select one of two values.
    // - If the AudioRenderer is resuming from the paused state, and packets have
    // not been discarded since being paused, then the AudioRenderer will use a
    // media_time corresponding to the instant at which the presentation became
    // paused.
    // - If the AudioRenderer is being placed into a playing state for the first
    // time following startup or a 'discard packets' operation, the initial
    // media_time will be set to the PTS of the first payload in the pending
    // packet queue. If the pending queue is empty, initial media_time will be
    // set to zero.
    //
    // Return Value
    // When requested, the AudioRenderer will return the 'reference_time' and
    // 'media_time' which were selected and used (whether they were explicitly
    // specified or not) in the return value of the play call.
    //
    // Examples
    // 1. A user has queued some audio using `SendPacket` and simply wishes them
    // to start playing as soon as possible. The user may call Play without
    // providing explicit timestamps -- `Play(kNoTimestamp, kNoTimestamp)`.
    //
    // 2. A user has queued some audio using `SendPacket`, and wishes to start
    // playback at a specified 'reference_time', in sync with some other media
    // stream, either initially or after discarding packets. The user would call
    // `Play(reference_time, kNoTimestamp)`.
    //
    // 3. A user has queued some audio using `SendPacket`. The first of these
    // packets has a PTS of zero, and the user wishes playback to begin as soon as
    // possible, but wishes to skip all of the audio content between PTS 0 and PTS
    // 'media_time'. The user would call `Play(kNoTimestamp, media_time)`.
    //
    // 4. A user has queued some audio using `SendPacket` and want to present this
    // media in synch with another player in a different device. The coordinator
    // of the group of distributed players sends an explicit message to each
    // player telling them to begin presentation of audio at PTS 'media_time', at
    // the time (based on the group's shared reference clock) 'reference_time'.
    // Here the user would call `Play(reference_time, media_time)`.
    //
    // TODO(mpuryear): Define behavior in the case that a user calls `Play` while
    // the system is already playing. We should probably do nothing but return a
    // valid correspondence pair in response -- unless both reference and media
    // times are provided (and do not equate to the current timeline
    // relationship), in which case we should introduce a discontinuity.
    //
    // TODO(mpuryear): Collapse these if we ever have optional retvals in FIDL
    Play(int64 reference_time, int64 media_time)
        -> (int64 reference_time, int64 media_time);
    PlayNoReply(int64 reference_time, int64 media_time);

    // Immediately put the AudioRenderer into the paused state and then report the
    // relationship between the media and reference timelines which was
    // established (if requested).
    //
    // TODO(mpuryear): Define behavior in the case that a user calls `Pause` while
    // the system is already in the paused state. We should probably do nothing
    // but provide a valid correspondence pair in response.
    //
    // TODO(mpuryear): Collapse these if we ever have optional retvals in FIDL
    Pause() -> (int64 reference_time, int64 media_time);
    PauseNoReply();

    // Enable or disable notifications about changes to the minimum clock lead
    // time (in nanoseconds) for this AudioRenderer. Calling this method with
    // 'enabled' set to true will trigger an immediate `OnMinLeadTimeChanged`
    // event with the current minimum lead time for the AudioRenderer. If the
    // value changes, an `OnMinLeadTimeChanged` event will be raised with the new
    // value. This behavior will continue until the user calls
    // `EnableMinLeadTimeEvents(false)`.
    //
    // The minimum clock lead time is the amount of time ahead of the reference
    // clock's understanding of "now" that packets needs to arrive (relative to
    // the playback clock transformation) in order for the mixer to be able to mix
    // packet. For example...
    //
    // ++ Let the PTS of packet X be P(X)
    // ++ Let the function which transforms PTS -> RefClock be R(p) (this function
    //    is determined by the call to Play(...)
    // ++ Let the minimum lead time be MLT
    //
    // If R(P(X)) < RefClock.Now() + MLT
    // Then the packet is late, and some (or all) of the packet's payload will
    // need to be skipped in order to present the packet at the scheduled time.
    //
    // TODO(mpuryear): What should the units be here?  Options include...
    //
    // 1) Normalized to nanoseconds (this is the current API)
    // 2) Reference clock units (what happens if the reference clock changes?)
    // 3) PTS units (what happens when the user changes the PTS units?)
    //
    // TODO(mpuryear): Should `EnableMinLeadTimeEvents` have an optional -> ()
    // return value for synchronization purposes?  Probably not; users should be
    // able to send a disable request and clear their event handler if they no
    // longer want notifications. Their in-process dispatcher framework can handle
    // draining and dropping any lead time changed events that were already in
    // flight when the disable message was sent.
    //
    EnableMinLeadTimeEvents(bool enabled);
    -> OnMinLeadTimeChanged(int64 min_lead_time_nsec);

    // TODO(mpuryear): Eliminate this method when possible. Right now, it is used
    // by code requiring synchronous FIDL interfaces to talk to AudioRenderers.
    GetMinLeadTime() -> (int64 min_lead_time_nsec);

    // Binds to the gain control for this AudioRenderer.
    BindGainControl(request<GainControl> gain_control_request);

    /////////////////////////////////////////////////////////////////////////////
    // StreamBufferSet methods
    // See stream.fidl.

    /////////////////////////////////////////////////////////////////////////////
    // StreamSink methods
    // See stream.fidl.

    // TODO(mpuryear): Spec methods/events which can be used for unintentional
    // discontinuity/underflow detection.
    //
    // TODO(mpuryear): Spec methods/events which can be used to report routing
    // changes. (Presuming that they belong at this level at all; they may belong
    // on some sort of policy object).
    //
    // TODO(mpuryear): Spec methods/events which can be used to report policy
    // induced gain/ducking changes. (Presuming that they belong at this level at
    // all; they may belong on some sort of policy object).
};