using System;
using Chernobyl.Extensions;
using Chernobyl.Mathematics.Space;

namespace Chernobyl.Generation
{
    /// <summary>
    /// Methods to help with the creation of the audio.
    /// </summary>
    public static class Audio
    {
        /// <summary>
        /// Returns a methods that produces a sin wave with the specified properties.
        /// </summary>
        /// <param name="frequency">Method that returns the number of times the wave repeats in hertz.
        /// This value dictates the horizontal squashing and stretching of the wave.</param>
        /// <param name="sampleRate">The method that returns the number of samples carried per second.</param>
        /// <param name="amplitude">Method that returns the volume of the produced wave. This is
        /// applied to the wave as a vertical scalar. For example, setting this to .5 reduces the
        /// height wave by half, setting this to 2 doubles the height of the wave.</param>
        public static Func<double> CreateSinOscillator(Func<double> frequency, Func<double> sampleRate, Func<double> amplitude)
        {
            // One cycle of a wave includes both the peak and trough and starts at the vertical origin (0 for
            // example but can be anything) and ends at the same vertical origin. This can be described
            // as the 360 degree circle (2PI radians). The 'sample' decides which wave along the
            // horizontal axis we are sampling from. The 'Frequency' dictates how close together or
            // how far apart these waves are. Dividing by the sample rate converts the frequency to
            // "normalized frequency" or cycles per sample.
            var oneCycleInRadians = 2 * Math.PI; // Use radians because Math.Sin takes radians.

            var phase = new Func<double, double>(sample => (oneCycleInRadians * sample * frequency()) / sampleRate())
                .Bind(Wave.Step(1, reset: sample => sample >= sampleRate()));
            return Wave.Sin.Bind(phase).Select(value => value * amplitude());
        }
    }
}