cpal 0.12.1

Low-level cross-platform audio I/O library in pure Rust.
Documentation 
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//! Feeds back the input stream directly into the output stream.
//!
//! Assumes that the input and output devices can use the same stream configuration and that they
//! support the f32 sample format.
//!
//! Uses a delay of `LATENCY_MS` milliseconds in case the default input and output streams are not
//! precisely synchronised.

extern crate anyhow;
extern crate cpal;
extern crate ringbuf;

use cpal::traits::{DeviceTrait, HostTrait, StreamTrait};
use ringbuf::RingBuffer;

const LATENCY_MS: f32 = 150.0;

fn main() -> Result<(), anyhow::Error> {
    let host = cpal::default_host();

    // Default devices.
    let input_device = host
        .default_input_device()
        .expect("failed to get default input device");
    let output_device = host
        .default_output_device()
        .expect("failed to get default output device");
    println!("Using default input device: \"{}\"", input_device.name()?);
    println!("Using default output device: \"{}\"", output_device.name()?);

    // We'll try and use the same configuration between streams to keep it simple.
    let config: cpal::StreamConfig = input_device.default_input_config()?.into();

    // Create a delay in case the input and output devices aren't synced.
    let latency_frames = (LATENCY_MS / 1_000.0) * config.sample_rate.0 as f32;
    let latency_samples = latency_frames as usize * config.channels as usize;

    // The buffer to share samples
    let ring = RingBuffer::new(latency_samples * 2);
    let (mut producer, mut consumer) = ring.split();

    // Fill the samples with 0.0 equal to the length of the delay.
    for _ in 0..latency_samples {
        // The ring buffer has twice as much space as necessary to add latency here,
        // so this should never fail
        producer.push(0.0).unwrap();
    }

    let input_data_fn = move |data: &[f32], _: &cpal::InputCallbackInfo| {
        let mut output_fell_behind = false;
        for &sample in data {
            if producer.push(sample).is_err() {
                output_fell_behind = true;
            }
        }
        if output_fell_behind {
            eprintln!("output stream fell behind: try increasing latency");
        }
    };

    let output_data_fn = move |data: &mut [f32], _: &cpal::OutputCallbackInfo| {
        let mut input_fell_behind = None;
        for sample in data {
            *sample = match consumer.pop() {
                Ok(s) => s,
                Err(err) => {
                    input_fell_behind = Some(err);
                    0.0
                }
            };
        }
        if let Some(err) = input_fell_behind {
            eprintln!(
                "input stream fell behind: {:?}: try increasing latency",
                err
            );
        }
    };

    // Build streams.
    println!(
        "Attempting to build both streams with f32 samples and `{:?}`.",
        config
    );
    let input_stream = input_device.build_input_stream(&config, input_data_fn, err_fn)?;
    let output_stream = output_device.build_output_stream(&config, output_data_fn, err_fn)?;
    println!("Successfully built streams.");

    // Play the streams.
    println!(
        "Starting the input and output streams with `{}` milliseconds of latency.",
        LATENCY_MS
    );
    input_stream.play()?;
    output_stream.play()?;

    // Run for 3 seconds before closing.
    println!("Playing for 3 seconds... ");
    std::thread::sleep(std::time::Duration::from_secs(3));
    drop(input_stream);
    drop(output_stream);
    println!("Done!");
    Ok(())
}

fn err_fn(err: cpal::StreamError) {
    eprintln!("an error occurred on stream: {}", err);
}