/*
 * Copyright (C) 2004-2010 NXP Software
 * Copyright (C) 2010 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#include "LVM_Types.h"
#include "LVM_Macros.h"
#include "Mixer.h"
#include "LVM_Mixer_FilterCoeffs.h"

/************************************************************************/
/* FUNCTION:                                                            */
/*   LVM_Mix_GetTimeConstant                                            */
/*                                                                      */
/* DESCRIPTION:                                                         */
/*  This function calculates the filter coefficient using the following */
/*  equation:                                                           */
/*       Alpha = exp(ln(0.1)/ (tc * Update + 1.0))                      */
/*                                                                      */
/*  This is to be used with the follow first order filter, called at a  */
/*  rate of Update times a second. tc is the required time constant in  */
/*  units of 100us.                                                     */
/*                                                                      */
/*       Output(n) = Alpha * Output(n-1) + (1 - Alpha) * Target(n)      */
/*                                                                      */
/*  The function assumes the block size is large, i.e. the update rate  */
/*  is approximately a fixed, and correct factor of the value of Fs     */
/*  given in the call. This is especially not true when the block size  */
/*  is very small, see the mixer documentation for further details.     */
/*                                                                      */
/*  The function does not support all possible combinations of input    */
/*  values:                                                             */
/*                                                                      */
/*  1. NumChannels is limited to the values 1 (Mono) and 2 (Stereo)     */
/*  2. The product tc * Fs is limited approximately to the range        */
/*      8 < (tc * Fs) < 2^35                                            */
/*                                                                      */
/* PARAMETERS:                                                          */
/*  tc              - the time constant in 100us steps, i.e. 10 = 1ms   */
/*  Fs              - the filter update rate in Hz                      */
/*  NumChannels     - Number of channels 1=Mono, 2=Stereo               */
/*                                                                      */
/* RETURNS:                                                             */
/*  Alpha   - the filter coefficient Q31 format                         */
/*                                                                      */
/************************************************************************/
LVM_FLOAT LVM_Mixer_TimeConstant(LVM_UINT32 tc, LVM_UINT32 Fs, LVM_UINT16 NumChannels) {
    LVM_UINT32 Product;
    LVM_FLOAT ProductFloat;
    LVM_INT16 InterpolateShort;
    LVM_FLOAT Interpolate;
    LVM_UINT16 Shift;
    LVM_FLOAT Diff;
    LVM_FLOAT Table[] = {
            ALPHA_Float_0, /* Log spaced look-up table */
            ALPHA_Float_1,  ALPHA_Float_2,  ALPHA_Float_3,  ALPHA_Float_4,  ALPHA_Float_5,
            ALPHA_Float_6,  ALPHA_Float_7,  ALPHA_Float_8,  ALPHA_Float_9,  ALPHA_Float_10,
            ALPHA_Float_11, ALPHA_Float_12, ALPHA_Float_13, ALPHA_Float_14, ALPHA_Float_15,
            ALPHA_Float_16, ALPHA_Float_17, ALPHA_Float_18, ALPHA_Float_19, ALPHA_Float_20,
            ALPHA_Float_21, ALPHA_Float_22, ALPHA_Float_23, ALPHA_Float_24, ALPHA_Float_25,
            ALPHA_Float_26, ALPHA_Float_27, ALPHA_Float_28, ALPHA_Float_29, ALPHA_Float_30,
            ALPHA_Float_31, ALPHA_Float_32, ALPHA_Float_33, ALPHA_Float_34, ALPHA_Float_35,
            ALPHA_Float_36, ALPHA_Float_37, ALPHA_Float_38, ALPHA_Float_39, ALPHA_Float_40,
            ALPHA_Float_41, ALPHA_Float_42, ALPHA_Float_43, ALPHA_Float_44, ALPHA_Float_45,
            ALPHA_Float_46, ALPHA_Float_47, ALPHA_Float_48, ALPHA_Float_49, ALPHA_Float_50};

    /* Calculate the product of the time constant and the sample rate */
    Product = ((tc >> 16) * (LVM_UINT32)Fs) << 13; /* Stereo value */
    Product = Product + (((tc & 0x0000FFFF) * (LVM_UINT32)Fs) >> 3);

    if (NumChannels == 1) {
        Product = Product >> 1; /* Mono value */
    }

    /* Normalize to get the table index and interpolation factor */
    for (Shift = 0; Shift < ((Alpha_TableSize - 1) / 2); Shift++) {
        if ((Product & 0x80000000) != 0) {
            break;
        }

        Product = Product << 1;
    }
    Shift = (LVM_UINT16)((Shift << 1));

    if ((Product & 0x40000000) == 0) {
        Shift++;
    }

    InterpolateShort = (LVM_INT16)((Product >> 15) & 0x00007FFF);
    Interpolate = (LVM_FLOAT)InterpolateShort / 32768.0f;

    Diff = (Table[Shift] - Table[Shift + 1]);
    Diff = Diff * Interpolate;
    ProductFloat = Table[Shift + 1] + Diff;

    return ProductFloat;
}