src/source/skip.rs

use std::time::Duration;

use crate::{Sample, Source};

const NS_PER_SECOND: u128 = 1_000_000_000;

/// Internal function that builds a `SkipDuration` object.
pub fn skip_duration<I>(mut input: I, duration: Duration) -> SkipDuration<I>
where
    I: Source,
    I::Item: Sample,
{
    do_skip_duration(&mut input, duration);
    SkipDuration {
        input,
        skipped_duration: duration,
    }
}

/// Skips specified `duration` of the given `input` source from it's current position.
fn do_skip_duration<I>(input: &mut I, mut duration: Duration)
where
    I: Source,
    I::Item: Sample,
{
    while duration > Duration::new(0, 0) {
        if input.current_frame_len().is_none() {
            // Sample rate and the amount of channels will be the same till the end.
            do_skip_duration_unchecked(input, duration);
            return;
        }

        // .unwrap() safety: if `current_frame_len()` is None, the body of the `if` statement
        // above returns before we get here.
        let frame_len: usize = input.current_frame_len().unwrap();
        let ns_per_sample: u128 =
            NS_PER_SECOND / input.sample_rate() as u128 / input.channels() as u128;

        // Check if we need to skip only part of the current frame.
        if frame_len as u128 * ns_per_sample > duration.as_nanos() {
            skip_samples(input, (duration.as_nanos() / ns_per_sample) as usize);
            return;
        }

        skip_samples(input, frame_len as usize);

        duration -= Duration::from_nanos((frame_len * ns_per_sample as usize) as u64);
    }
}

/// Skips specified `duration` from the `input` source assuming that sample rate
/// and amount of channels are not changing.
fn do_skip_duration_unchecked<I>(input: &mut I, duration: Duration)
where
    I: Source,
    I::Item: Sample,
{
    let samples_per_channel: u128 =
        duration.as_nanos() * input.sample_rate() as u128 / NS_PER_SECOND;
    let samples_to_skip: u128 = samples_per_channel * input.channels() as u128;

    skip_samples(input, samples_to_skip as usize);
}

/// Skips `n` samples from the given `input` source.
fn skip_samples<I>(input: &mut I, n: usize)
where
    I: Source,
    I::Item: Sample,
{
    for _ in 0..n {
        if input.next().is_none() {
            break;
        }
    }
}

/// A source that skips specified duration of the given source from it's current position.
#[derive(Clone, Debug)]
pub struct SkipDuration<I> {
    input: I,
    skipped_duration: Duration,
}

impl<I> SkipDuration<I>
where
    I: Source,
    I::Item: Sample,
{
    /// Returns a reference to the inner source.
    #[inline]
    pub fn inner(&self) -> &I {
        &self.input
    }

    /// Returns a mutable reference to the inner source.
    #[inline]
    pub fn inner_mut(&mut self) -> &mut I {
        &mut self.input
    }

    /// Returns the inner source.
    #[inline]
    pub fn into_inner(self) -> I {
        self.input
    }
}

impl<I> Iterator for SkipDuration<I>
where
    I: Source,
    I::Item: Sample,
{
    type Item = <I as Iterator>::Item;

    #[inline]
    fn next(&mut self) -> Option<Self::Item> {
        self.input.next()
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        self.input.size_hint()
    }
}

impl<I> Source for SkipDuration<I>
where
    I: Source,
    I::Item: Sample,
{
    #[inline]
    fn current_frame_len(&self) -> Option<usize> {
        self.input.current_frame_len()
    }

    #[inline]
    fn channels(&self) -> u16 {
        self.input.channels()
    }

    #[inline]
    fn sample_rate(&self) -> u32 {
        self.input.sample_rate()
    }

    #[inline]
    fn total_duration(&self) -> Option<Duration> {
        self.input.total_duration().map(|val| {
            val.checked_sub(self.skipped_duration)
                .unwrap_or_else(|| Duration::from_secs(0))
        })
    }
}

#[cfg(test)]
mod tests {
    use std::time::Duration;

    use crate::buffer::SamplesBuffer;
    use crate::source::Source;

    fn test_skip_duration_samples_left(
        channels: u16,
        sample_rate: u32,
        seconds: u32,
        seconds_to_skip: u32,
    ) {
        let data: Vec<f32> = (1..=(sample_rate * channels as u32 * seconds))
            .map(|_| 0f32)
            .collect();
        let test_buffer = SamplesBuffer::new(channels, sample_rate, data);
        let seconds_left = seconds.saturating_sub(seconds_to_skip);

        let samples_left_expected = (sample_rate * channels as u32 * seconds_left) as usize;
        let samples_left = test_buffer
            .skip_duration(Duration::from_secs(seconds_to_skip as u64))
            .count();

        assert_eq!(samples_left, samples_left_expected);
    }

    macro_rules! skip_duration_test_block {
        ($(channels: $ch:expr, sample rate: $sr:expr, seconds: $sec:expr, seconds to skip: $sec_to_skip:expr;)+) => {
            $(
                test_skip_duration_samples_left($ch, $sr, $sec, $sec_to_skip);
            )+
        }
    }

    #[test]
    fn skip_duration_shorter_than_source() {
        skip_duration_test_block! {
            channels: 1, sample rate: 44100, seconds: 5, seconds to skip: 3;
            channels: 1, sample rate: 96000, seconds: 5, seconds to skip: 3;

            channels: 2, sample rate: 44100, seconds: 5, seconds to skip: 3;
            channels: 2, sample rate: 96000, seconds: 5, seconds to skip: 3;

            channels: 4, sample rate: 44100, seconds: 5, seconds to skip: 3;
            channels: 4, sample rate: 96000, seconds: 5, seconds to skip: 3;
        }
    }

    #[test]
    fn skip_duration_zero_duration() {
        skip_duration_test_block! {
            channels: 1, sample rate: 44100, seconds: 5, seconds to skip: 0;
            channels: 1, sample rate: 96000, seconds: 5, seconds to skip: 0;

            channels: 2, sample rate: 44100, seconds: 5, seconds to skip: 0;
            channels: 2, sample rate: 96000, seconds: 5, seconds to skip: 0;

            channels: 4, sample rate: 44100, seconds: 5, seconds to skip: 0;
            channels: 4, sample rate: 96000, seconds: 5, seconds to skip: 0;
        }
    }

    #[test]
    fn skip_duration_longer_than_source() {
        skip_duration_test_block! {
            channels: 1, sample rate: 44100, seconds: 1, seconds to skip: 5;
            channels: 1, sample rate: 96000, seconds: 10, seconds to skip: 11;

            channels: 2, sample rate: 44100, seconds: 1, seconds to skip: 5;
            channels: 2, sample rate: 96000, seconds: 10, seconds to skip: 11;

            channels: 4, sample rate: 44100, seconds: 1, seconds to skip: 5;
            channels: 4, sample rate: 96000, seconds: 10, seconds to skip: 11;
        }
    }

    #[test]
    fn skip_duration_equal_to_source_length() {
        skip_duration_test_block! {
            channels: 1, sample rate: 44100, seconds: 1, seconds to skip: 1;
            channels: 1, sample rate: 96000, seconds: 10, seconds to skip: 10;

            channels: 2, sample rate: 44100, seconds: 1, seconds to skip: 1;
            channels: 2, sample rate: 96000, seconds: 10, seconds to skip: 10;

            channels: 4, sample rate: 44100, seconds: 1, seconds to skip: 1;
            channels: 4, sample rate: 96000, seconds: 10, seconds to skip: 10;
        }
    }
}