/* This code comes from MAME 0.53 and according to changes log was written
   by Nicola Salmoria.  The MAME license says:

   VI. Reuse of Source Code
   --------------------------
   This chapter might not apply to specific portions of MAME (e.g. CPU
   emulators) which bear different copyright notices.
   The source code cannot be used in a commercial product without the written
   authorization of the authors. Use in non-commercial products is allowed, and
   indeed encouraged.  If you use portions of the MAME source code in your
   program, however, you must make the full source code freely available as
   well.
   Usage of the _information_ contained in the source code is free for any use.
   However, given the amount of time and energy it took to collect this
   information, if you find new information we would appreciate if you made it
   freely available as well.

*/

/***************************************************************************

  sn76496.c

  Routines to emulate the Texas Instruments SN76489 / SN76496 programmable
  tone /noise generator. Also known as (or at least compatible with) TMS9919.

  Noise emulation is not accurate due to lack of documentation. The noise
  generator uses a shift register with a XOR-feedback network, but the exact
  layout is unknown. It can be set for either period or white noise; again,
  the details are unknown.

***************************************************************************/

/* get uint16 definition */
#include "generator.h"

#include "sn76496.h"

#define MAX_OUTPUT 0x7fff
#define STEP 0x10000

/* Formulas for noise generator */
/* bit0 = output */

/* noise feedback for white noise mode */
#define FB_WNOISE 0x12000       /* bit15.d(16bits) = bit0(out) ^ bit2 */
//#define FB_WNOISE 0x14000     /* bit15.d(16bits) = bit0(out) ^ bit1 */
//#define FB_WNOISE 0x28000     /* bit16.d(17bits) = bit0(out) ^ bit2 (same to AY-3-8910) */
//#define FB_WNOISE 0x50000     /* bit17.d(18bits) = bit0(out) ^ bit2 */

/* noise feedback for periodic noise mode */
/* it is correct maybe (it was in the Megadrive sound manual) */
//#define FB_PNOISE 0x10000     /* 16bit rorate */
#define FB_PNOISE 0x08000       /* JH 981127 - fixes Do Run Run */

/* noise generator start preset (for periodic noise) */
#define NG_PRESET 0x0f35

struct SN76496 sn[MAX_76496];

void SN76496Write(int chip, int data)
{
  struct SN76496 *R = &sn[chip];

  /* should update buffer before getting here */

  if (data & 0x80) {
    int r = (data & 0x70) >> 4;
    int c = r / 2;

    R->LastRegister = r;
    R->Register[r] = (R->Register[r] & 0x3f0) | (data & 0x0f);
    switch (r) {
    case 0:                    /* tone 0 : frequency */
    case 2:                    /* tone 1 : frequency */
    case 4:                    /* tone 2 : frequency */
      R->Period[c] = R->UpdateStep * R->Register[r];
      if (R->Period[c] == 0)
        R->Period[c] = R->UpdateStep;
      if (r == 4) {
        /* update noise shift frequency */
        if ((R->Register[6] & 0x03) == 0x03)
          R->Period[3] = 2 * R->Period[2];
      }
      break;
    case 1:                    /* tone 0 : volume */
    case 3:                    /* tone 1 : volume */
    case 5:                    /* tone 2 : volume */
    case 7:                    /* noise  : volume */
      R->Volume[c] = R->VolTable[data & 0x0f];
      break;
    case 6:                    /* noise  : frequency, mode */
      {
        int n = R->Register[6];
        R->NoiseFB = (n & 4) ? FB_WNOISE : FB_PNOISE;
        n &= 3;
        /* N/512,N/1024,N/2048,Tone #3 output */
        R->Period[3] =
          (n == 3) ? 2 * R->Period[2] : (R->UpdateStep << (5 + n));

        /* reset noise shifter */
        R->RNG = NG_PRESET;
        R->Output[3] = R->RNG & 1;
      }
      break;
    }
  } else {
    int r = R->LastRegister;
    int c = r / 2;

    switch (r) {
    case 0:                    /* tone 0 : frequency */
    case 2:                    /* tone 1 : frequency */
    case 4:                    /* tone 2 : frequency */
      R->Register[r] = (R->Register[r] & 0x0f) | ((data & 0x3f) << 4);
      R->Period[c] = R->UpdateStep * R->Register[r];
      if (R->Period[c] == 0)
        R->Period[c] = R->UpdateStep;
      if (r == 4) {
        /* update noise shift frequency */
        if ((R->Register[6] & 0x03) == 0x03)
          R->Period[3] = 2 * R->Period[2];
      }
      break;
    }
  }
}

void SN76496Update(int chip, uint16 *buffer, int length)
{
  int i;
  struct SN76496 *R = &sn[chip];

  /* If the volume is 0, increase the counter */
  for (i = 0; i < 4; i++) {
    if (R->Volume[i] == 0) {
      /* note that I do count += length, NOT count = length + 1. You might
         think it's the same since the volume is 0, but doing the latter
         could cause interferencies when the program is rapidly modulating
         the volume. */
      if (R->Count[i] <= length * STEP)
        R->Count[i] += length * STEP;
    }
  }

  while (length > 0) {
    int vol[4];
    unsigned int out;
    int left;

    /* vol[] keeps track of how long each square wave stays */
    /* in the 1 position during the sample period. */
    vol[0] = vol[1] = vol[2] = vol[3] = 0;

    for (i = 0; i < 3; i++) {
      if (R->Output[i])
        vol[i] += R->Count[i];
      R->Count[i] -= STEP;
      /* Period[i] is the half period of the square wave. Here, in each */
      /* loop I add Period[i] twice, so that at the end of the loop the */
      /* square wave is in the same status (0 or 1) it was at the start. */
      /* vol[i] is also incremented by Period[i], since the wave has been 1 */
      /* exactly half of the time, regardless of the initial position. */
      /* If we exit the loop in the middle, Output[i] has to be inverted */
      /* and vol[i] incremented only if the exit status of the square */
      /* wave is 1. */
      while (R->Count[i] <= 0) {
        R->Count[i] += R->Period[i];
        if (R->Count[i] > 0) {
          R->Output[i] ^= 1;
          if (R->Output[i])
            vol[i] += R->Period[i];
          break;
        }
        R->Count[i] += R->Period[i];
        vol[i] += R->Period[i];
      }
      if (R->Output[i])
        vol[i] -= R->Count[i];
    }

    left = STEP;
    do {
      int nextevent;

      if (R->Count[3] < left)
        nextevent = R->Count[3];
      else
        nextevent = left;
      if (R->Output[3])
        vol[3] += R->Count[3];
      R->Count[3] -= nextevent;
      if (R->Count[3] <= 0) {
        if (R->RNG & 1)
          R->RNG ^= R->NoiseFB;
        R->RNG >>= 1;
        R->Output[3] = R->RNG & 1;
        R->Count[3] += R->Period[3];
        if (R->Output[3])
          vol[3] += R->Period[3];
      }
      if (R->Output[3])
        vol[3] -= R->Count[3];
      left -= nextevent;
    } while (left > 0);

    out = vol[0] * R->Volume[0] + vol[1] * R->Volume[1] +
      vol[2] * R->Volume[2] + vol[3] * R->Volume[3];
    if (out > MAX_OUTPUT * STEP)
      out = MAX_OUTPUT * STEP;
    *(buffer++) = out / STEP;
    length--;
  }
}

static void SN76496_set_clock(int chip, int clock)
{
  struct SN76496 *R = &sn[chip];

  /* the base clock for the tone generators is the chip clock divided by 16; */
  /* for the noise generator, it is clock / 256. */
  /* Here we calculate the number of steps which happen during one sample */
  /* at the given sample rate. No. of events = sample rate / (clock/16). */
  /* STEP is a multiplier used to turn the fraction into a fixed point */
  /* number. */
  R->UpdateStep = ((double)STEP * R->SampleRate * 16) / clock;
}

static void SN76496_set_gain(int chip, int gain)
{
  struct SN76496 *R = &sn[chip];
  int i;
  double out;

  gain &= 0xff;
  /* increase max output basing on gain (0.2 dB per step) */
  out = MAX_OUTPUT / 3;
  while (gain-- > 0)
    out *= 1.023292992;         /* = (10 ^ (0.2/20)) */
  /* build volume table (2dB per step) */
  for (i = 0; i < 15; i++) {
    /* limit volume to avoid clipping */
    if (out > MAX_OUTPUT / 3)
      R->VolTable[i] = MAX_OUTPUT / 3;
    else
      R->VolTable[i] = out;
    out /= 1.258925412;         /* = 10 ^ (2/20) = 2dB */
  }
  R->VolTable[15] = 0;
}

int SN76496Init(int chip, int clock, int gain, int sample_rate)
{
  int i;
  struct SN76496 *R = &sn[chip];
  char name[40];

  R->SampleRate = sample_rate;
  SN76496_set_clock(chip, clock);

  for (i = 0; i < 4; i++)
    R->Volume[i] = 0;

  R->LastRegister = 0;
  for (i = 0; i < 8; i += 2) {
    R->Register[i] = 0;
    R->Register[i + 1] = 0x0f;  /* volume = 0 */
  }

  for (i = 0; i < 4; i++) {
    R->Output[i] = 0;
    R->Period[i] = R->Count[i] = R->UpdateStep;
  }
  R->RNG = NG_PRESET;
  R->Output[3] = R->RNG & 1;
  SN76496_set_gain(chip, gain & 0xff);

  return 0;
}