using System;
using System.Linq;

namespace NAudio.Dsp
{
    // C# ADSR based on work by Nigel Redmon, EarLevel Engineering: earlevel.com
    // http://www.earlevel.com/main/2013/06/03/envelope-generators-adsr-code/
    /// <summary>
    /// Envelope generator (ADSR)
    /// </summary>
    public class EnvelopeGenerator
    {
        private EnvelopeState state;
        private float output;
        private float attackRate;
        private float decayRate;
        private float releaseRate;
        private float attackCoef;
        private float decayCoef;
        private float releaseCoef;
        private float sustainLevel;
        private float targetRatioAttack;
        private float targetRatioDecayRelease;
        private float attackBase;
        private float decayBase;
        private float releaseBase;

        /// <summary>
        /// Envelope State
        /// </summary>
        public enum EnvelopeState
        {
            /// <summary>
            /// Idle
            /// </summary>
            Idle = 0,
            /// <summary>
            /// Attack
            /// </summary>
            Attack,
            /// <summary>
            /// Decay
            /// </summary>
            Decay,
            /// <summary>
            /// Sustain
            /// </summary>
            Sustain,
            /// <summary>
            /// Release
            /// </summary>
            Release
        };

        /// <summary>
        /// Creates and Initializes an Envelope Generator
        /// </summary>
        public EnvelopeGenerator()
        {
            Reset();
            AttackRate = 0;
            DecayRate = 0;
            ReleaseRate = 0;
            SustainLevel = 1.0f;
            SetTargetRatioAttack(0.3f);
            SetTargetRatioDecayRelease(0.0001f);
        }

        /// <summary>
        /// Attack Rate (seconds * SamplesPerSecond)
        /// </summary>
        public float AttackRate
        {
            get
            {
                return attackRate;
            }
            set
            {
                attackRate = value;
                attackCoef = CalcCoef(value, targetRatioAttack);
                attackBase = (1.0f + targetRatioAttack) * (1.0f - attackCoef);
            }
        }

        /// <summary>
        /// Decay Rate (seconds * SamplesPerSecond)
        /// </summary>
        public float DecayRate
        {
            get
            {
                return decayRate;
            }
            set
            {
                decayRate = value;
                decayCoef = CalcCoef(value, targetRatioDecayRelease);
                decayBase = (sustainLevel - targetRatioDecayRelease) * (1.0f - decayCoef);
            }
        }

        /// <summary>
        /// Release Rate (seconds * SamplesPerSecond)
        /// </summary>
        public float ReleaseRate
        {
            get
            {
                return releaseRate;
            }
            set
            {
                releaseRate = value;
                releaseCoef = CalcCoef(value, targetRatioDecayRelease);
                releaseBase = -targetRatioDecayRelease * (1.0f - releaseCoef);
            }
        }

        private static float CalcCoef(float rate, float targetRatio)
        {
            return (float)Math.Exp(-Math.Log((1.0f + targetRatio) / targetRatio) / rate);
        }

        /// <summary>
        /// Sustain Level (1 = 100%)
        /// </summary>
        public float SustainLevel
        {
            get
            {
                return sustainLevel;
            }
            set
            {
                sustainLevel = value;
                decayBase = (sustainLevel - targetRatioDecayRelease) * (1.0f - decayCoef);
            }
        }

        /// <summary>
        /// Sets the attack curve
        /// </summary>
        void SetTargetRatioAttack(float targetRatio)
        {
            if (targetRatio < 0.000000001f)
                targetRatio = 0.000000001f;  // -180 dB
            targetRatioAttack = targetRatio;
            attackBase = (1.0f + targetRatioAttack) * (1.0f - attackCoef);
        }

        /// <summary>
        /// Sets the decay release curve
        /// </summary>
        void SetTargetRatioDecayRelease(float targetRatio)
        {
            if (targetRatio < 0.000000001f)
                targetRatio = 0.000000001f;  // -180 dB
            targetRatioDecayRelease = targetRatio;
            decayBase = (sustainLevel - targetRatioDecayRelease) * (1.0f - decayCoef);
            releaseBase = -targetRatioDecayRelease * (1.0f - releaseCoef);
        }

        /// <summary>
        /// Read the next volume multiplier from the envelope generator
        /// </summary>
        /// <returns>A volume multiplier</returns>
        public float Process()
        {
            switch (state)
            {
                case EnvelopeState.Idle:
                    break;
                case EnvelopeState.Attack:
                    output = attackBase + output * attackCoef;
                    if (output >= 1.0f)
                    {
                        output = 1.0f;
                        state = EnvelopeState.Decay;
                    }
                    break;
                case EnvelopeState.Decay:
                    output = decayBase + output * decayCoef;
                    if (output <= sustainLevel)
                    {
                        output = sustainLevel;
                        state = EnvelopeState.Sustain;
                    }
                    break;
                case EnvelopeState.Sustain:
                    break;
                case EnvelopeState.Release:
                    output = releaseBase + output * releaseCoef;
                    if (output <= 0.0)
                    {
                        output = 0.0f;
                        state = EnvelopeState.Idle;
                    }
                    break;
            }
            return output;
        }

        /// <summary>
        /// Trigger the gate
        /// </summary>
        /// <param name="gate">If true, enter attack phase, if false enter release phase (unless already idle)</param>
        public void Gate(bool gate)
        {
            if (gate)
                state = EnvelopeState.Attack;
            else if (state != EnvelopeState.Idle)
                state = EnvelopeState.Release;
        }

        /// <summary>
        /// Current envelope state
        /// </summary>
        public EnvelopeState State
        {
            get
            {
                return state;
            }
        }

        /// <summary>
        /// Reset to idle state
        /// </summary>
        public void Reset()
        {
            state = EnvelopeState.Idle;
            output = 0.0f;
        }

        /// <summary>
        /// Get the current output level
        /// </summary>
        public float GetOutput()
        {
            return output;
        }
    }
}