using System;

namespace Susurrus.XAudio2
{
    public static class OutputMatrix
    {
        private static float Magnitude(float toLeft, float toRight)
        {
            return (float)Math.Sqrt(toLeft * toLeft + toRight * toRight);
        }

        /// <summary>
        /// Fills <paramref name="levels"/> with the levels that send a pan to a device, laid out the way XAudio2 reads
        /// them, with a row for every device channel and a column for every source channel.
        ///
        /// A device with two or more channels plays left and right on its first two, which is where every standard
        /// speaker layout puts its front pair.  A mono device gets each side's power folded together.
        /// </summary>
        public static void Fill(float[] levels, in PanMatrix pan, int sourceChannels, int deviceChannels)
        {
            if (levels == null)
            {
                throw new ArgumentNullException(nameof(levels));
            }

            if (sourceChannels < 1 || sourceChannels > 2)
            {
                throw new ArgumentOutOfRangeException(nameof(sourceChannels), sourceChannels,
                    "Only mono and stereo sounds can be panned.");
            }

            if (deviceChannels < 1)
            {
                throw new ArgumentOutOfRangeException(nameof(deviceChannels), deviceChannels,
                    "A device plays at least one channel.");
            }

            int count = sourceChannels * deviceChannels;

            if (levels.Length < count)
            {
                throw new ArgumentException("The levels are too short for the channels.", nameof(levels));
            }

            for (int i = 0; i < count; ++i)
            {
                levels[i] = 0f;
            }

            if (deviceChannels == 1)
            {
                // A stereo source's two channels add, so each gives up half its power to keep the sum from clipping.
                float share = sourceChannels == 2 ? (float)Math.Sqrt(0.5) : 1f;
                levels[0] = share * Magnitude(pan.LeftToLeft, pan.LeftToRight);

                if (sourceChannels == 2)
                {
                    levels[1] = share * Magnitude(pan.RightToLeft, pan.RightToRight);
                }

                return;
            }

            levels[0] = pan.LeftToLeft;
            levels[sourceChannels] = pan.LeftToRight;

            if (sourceChannels == 2)
            {
                levels[1] = pan.RightToLeft;
                levels[sourceChannels + 1] = pan.RightToRight;
            }
        }
    }
}