#include <string.h>

typedef unsigned char byte;
typedef unsigned short word;

// PLATFORM DEFINITION

__sfr __at (0x0) input0;
__sfr __at (0x1) input1;
__sfr __at (0x2) input2;
__sfr __at (0x3) input3;

__sfr __at (0x1) ay8910_reg;
__sfr __at (0x2) ay8910_data;
__sfr __at (0x40) palette;

byte __at (0xe000) cellram[28][32];
byte __at (0xe800) tileram[256][8];

#define LEFT1 !(input1 & 0x10)
#define RIGHT1 !(input1 & 0x20)
#define UP1 !(input1 & 0x40)
#define DOWN1 !(input1 & 0x80)
#define FIRE1 !(input2 & 0x20)
#define COIN1 (input3 & 0x8)
#define START1 !(input2 & 0x10)
#define START2 !(input3 & 0x20)
#define VSYNC (input1 & 0x8)

// GAME DATA

typedef struct {
  byte x;
  byte y;
  byte dir;
  word score;
  char head_attr;
  char tail_attr;
  char collided:1;
  char human:1;
} Player;

Player players[2];

byte attract;
byte credits = 0;
byte frames_per_move;

#define START_SPEED 12
#define MAX_SPEED 5
#define MAX_SCORE 7

// GAME CODE

void main();
void gsinit();

// start routine @ 0x0
void start() {
__asm
  	LD    SP,#0xE800 ; set up stack pointer
        DI		 ; disable interrupts
__endasm;
  	gsinit();
	main();
}

#define INIT_MAGIC 0xdeadbeef
static long is_initialized = INIT_MAGIC;

// set initialized portion of global memory
// by copying INITIALIZER area -> INITIALIZED area
void gsinit() {
  // already initialized? skip it
  if (is_initialized == INIT_MAGIC)
    return;
__asm
; copy initialized data to RAM
	LD    BC, #l__INITIALIZER
	LD    A, B
	LD    DE, #s__INITIALIZED
	LD    HL, #s__INITIALIZER
      	LDIR
__endasm;
}

// SOUND CODE

inline void set8910(byte reg, byte data) {
  if (attract) return; // no sound in attract mode
  ay8910_reg = reg;
  ay8910_data = data;
}

////////

// https://en.wikipedia.org/wiki/Linear-feedback_shift_register#Galois_LFSRs
static word lfsr = 1;
word rand() {
  unsigned lsb = lfsr & 1;   /* Get LSB (i.e., the output bit). */
  lfsr >>= 1;                /* Shift register */
  if (lsb) {                 /* If the output bit is 1, apply toggle mask. */
    lfsr ^= 0xB400u;
  }
  return lfsr;
}

void wait_for_vsync() {
  while (VSYNC != 0) lfsr++; // wait for VSYNC end
  while (VSYNC == 0) lfsr++; // wait for VSYNC start
}

#define LOCHAR 0x0
#define HICHAR 0xff

#define CHAR(ch) (ch-LOCHAR)

void clrscr() {
  memset(cellram, CHAR(' '), sizeof(cellram));
}

byte getchar(byte x, byte y) {
  return cellram[x][y];
}

void putchar(byte x, byte y, byte attr) {
  cellram[x][y] = attr;
}

void putstring(byte x, byte y, const char* string) {
  while (*string) {
    putchar(x++, y, CHAR(*string++));
  }
}

// PC font (code page 437)
const byte font8x8[0x100][8] = {
{ 0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00 }, { 0x7e,0x81,0x95,0xb1,0xb1,0x95,0x81,0x7e }, { 0x7e,0xff,0xeb,0xcf,0xcf,0xeb,0xff,0x7e }, { 0x0e,0x1f,0x3f,0x7e,0x3f,0x1f,0x0e,0x00 }, { 0x08,0x1c,0x3e,0x7f,0x3e,0x1c,0x08,0x00 }, { 0x38,0x3a,0x9f,0xff,0x9f,0x3a,0x38,0x00 }, { 0x10,0x38,0xbc,0xff,0xbc,0x38,0x10,0x00 }, { 0x00,0x00,0x18,0x3c,0x3c,0x18,0x00,0x00 }, { 0xff,0xff,0xe7,0xc3,0xc3,0xe7,0xff,0xff }, { 0x00,0x3c,0x66,0x42,0x42,0x66,0x3c,0x00 }, { 0xff,0xc3,0x99,0xbd,0xbd,0x99,0xc3,0xff }, { 0x70,0xf8,0x88,0x88,0xfd,0x7f,0x07,0x0f }, { 0x00,0x4e,0x5f,0xf1,0xf1,0x5f,0x4e,0x00 }, { 0xc0,0xe0,0xff,0x7f,0x05,0x05,0x07,0x07 }, { 0xc0,0xff,0x7f,0x05,0x05,0x65,0x7f,0x3f }, { 0x99,0x5a,0x3c,0xe7,0xe7,0x3c,0x5a,0x99 }, { 0x7f,0x3e,0x3e,0x1c,0x1c,0x08,0x08,0x00 }, { 0x08,0x08,0x1c,0x1c,0x3e,0x3e,0x7f,0x00 }, { 0x00,0x24,0x66,0xff,0xff,0x66,0x24,0x00 }, { 0x00,0x5f,0x5f,0x00,0x00,0x5f,0x5f,0x00 }, { 0x06,0x0f,0x09,0x7f,0x7f,0x01,0x7f,0x7f }, { 0xda,0xbf,0xa5,0xa5,0xfd,0x59,0x03,0x02 }, { 0x00,0x70,0x70,0x70,0x70,0x70,0x70,0x00 }, { 0x80,0x94,0xb6,0xff,0xff,0xb6,0x94,0x80 }, { 0x00,0x04,0x06,0x7f,0x7f,0x06,0x04,0x00 }, { 0x00,0x10,0x30,0x7f,0x7f,0x30,0x10,0x00 }, { 0x08,0x08,0x08,0x2a,0x3e,0x1c,0x08,0x00 }, { 0x08,0x1c,0x3e,0x2a,0x08,0x08,0x08,0x00 }, { 0x3c,0x3c,0x20,0x20,0x20,0x20,0x20,0x00 }, { 0x08,0x1c,0x3e,0x08,0x08,0x3e,0x1c,0x08 }, { 0x30,0x38,0x3c,0x3e,0x3e,0x3c,0x38,0x30 }, { 0x06,0x0e,0x1e,0x3e,0x3e,0x1e,0x0e,0x06 }, { 0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00 }, { 0x00,0x06,0x5f,0x5f,0x06,0x00,0x00,0x00 }, { 0x00,0x07,0x07,0x00,0x07,0x07,0x00,0x00 }, { 0x14,0x7f,0x7f,0x14,0x7f,0x7f,0x14,0x00 }, { 0x24,0x2e,0x6b,0x6b,0x3a,0x12,0x00,0x00 }, { 0x46,0x66,0x30,0x18,0x0c,0x66,0x62,0x00 }, { 0x30,0x7a,0x4f,0x5d,0x37,0x7a,0x48,0x00 }, { 0x04,0x07,0x03,0x00,0x00,0x00,0x00,0x00 }, { 0x00,0x1c,0x3e,0x63,0x41,0x00,0x00,0x00 }, { 0x00,0x41,0x63,0x3e,0x1c,0x00,0x00,0x00 }, { 0x08,0x2a,0x3e,0x1c,0x1c,0x3e,0x2a,0x08 }, { 0x08,0x08,0x3e,0x3e,0x08,0x08,0x00,0x00 }, { 0x00,0xa0,0xe0,0x60,0x00,0x00,0x00,0x00 }, { 0x08,0x08,0x08,0x08,0x08,0x08,0x00,0x00 }, { 0x00,0x00,0x60,0x60,0x00,0x00,0x00,0x00 }, { 0x60,0x30,0x18,0x0c,0x06,0x03,0x01,0x00 }, { 0x3e,0x7f,0x59,0x4d,0x7f,0x3e,0x00,0x00 }, { 0x42,0x42,0x7f,0x7f,0x40,0x40,0x00,0x00 }, { 0x62,0x73,0x59,0x49,0x6f,0x66,0x00,0x00 }, { 0x22,0x63,0x49,0x49,0x7f,0x36,0x00,0x00 }, { 0x18,0x1c,0x16,0x13,0x7f,0x7f,0x10,0x00 }, { 0x27,0x67,0x45,0x45,0x7d,0x39,0x00,0x00 }, { 0x3c,0x7e,0x4b,0x49,0x79,0x30,0x00,0x00 }, { 0x03,0x63,0x71,0x19,0x0f,0x07,0x00,0x00 }, { 0x36,0x7f,0x49,0x49,0x7f,0x36,0x00,0x00 }, { 0x06,0x4f,0x49,0x69,0x3f,0x1e,0x00,0x00 }, { 0x00,0x00,0x6c,0x6c,0x00,0x00,0x00,0x00 }, { 0x00,0xa0,0xec,0x6c,0x00,0x00,0x00,0x00 }, { 0x08,0x1c,0x36,0x63,0x41,0x00,0x00,0x00 }, { 0x14,0x14,0x14,0x14,0x14,0x14,0x00,0x00 }, { 0x00,0x41,0x63,0x36,0x1c,0x08,0x00,0x00 }, { 0x02,0x03,0x51,0x59,0x0f,0x06,0x00,0x00 }, { 0x3e,0x7f,0x41,0x5d,0x5d,0x1f,0x1e,0x00 }, { 0x7c,0x7e,0x13,0x13,0x7e,0x7c,0x00,0x00 }, { 0x41,0x7f,0x7f,0x49,0x49,0x7f,0x36,0x00 }, { 0x1c,0x3e,0x63,0x41,0x41,0x63,0x22,0x00 }, { 0x41,0x7f,0x7f,0x41,0x63,0x7f,0x1c,0x00 }, { 0x41,0x7f,0x7f,0x49,0x5d,0x41,0x63,0x00 }, { 0x41,0x7f,0x7f,0x49,0x1d,0x01,0x03,0x00 }, { 0x1c,0x3e,0x63,0x41,0x51,0x73,0x72,0x00 }, { 0x7f,0x7f,0x08,0x08,0x7f,0x7f,0x00,0x00 }, { 0x00,0x41,0x7f,0x7f,0x41,0x00,0x00,0x00 }, { 0x30,0x70,0x40,0x41,0x7f,0x3f,0x01,0x00 }, { 0x41,0x7f,0x7f,0x08,0x1c,0x77,0x63,0x00 }, { 0x41,0x7f,0x7f,0x41,0x40,0x60,0x70,0x00 }, { 0x7f,0x7f,0x06,0x0c,0x06,0x7f,0x7f,0x00 }, { 0x7f,0x7f,0x06,0x0c,0x18,0x7f,0x7f,0x00 }, { 0x1c,0x3e,0x63,0x41,0x63,0x3e,0x1c,0x00 }, { 0x41,0x7f,0x7f,0x49,0x09,0x0f,0x06,0x00 }, { 0x1e,0x3f,0x21,0x71,0x7f,0x5e,0x00,0x00 }, { 0x41,0x7f,0x7f,0x19,0x39,0x6f,0x46,0x00 }, { 0x26,0x67,0x4d,0x59,0x7b,0x32,0x00,0x00 }, { 0x03,0x41,0x7f,0x7f,0x41,0x03,0x00,0x00 }, { 0x7f,0x7f,0x40,0x40,0x7f,0x7f,0x00,0x00 }, { 0x1f,0x3f,0x60,0x60,0x3f,0x1f,0x00,0x00 }, { 0x7f,0x7f,0x30,0x18,0x30,0x7f,0x7f,0x00 }, { 0x63,0x77,0x1c,0x08,0x1c,0x77,0x63,0x00 }, { 0x07,0x4f,0x78,0x78,0x4f,0x07,0x00,0x00 }, { 0x67,0x73,0x59,0x4d,0x47,0x63,0x71,0x00 }, { 0x00,0x7f,0x7f,0x41,0x41,0x00,0x00,0x00 }, { 0x01,0x03,0x06,0x0c,0x18,0x30,0x60,0x00 }, { 0x00,0x41,0x41,0x7f,0x7f,0x00,0x00,0x00 }, { 0x08,0x0c,0x06,0x03,0x06,0x0c,0x08,0x00 }, { 0x80,0x80,0x80,0x80,0x80,0x80,0x80,0x80 }, { 0x00,0x00,0x03,0x07,0x04,0x00,0x00,0x00 }, { 0x20,0x74,0x54,0x54,0x3c,0x78,0x40,0x00 }, { 0x41,0x3f,0x7f,0x44,0x44,0x7c,0x38,0x00 }, { 0x38,0x7c,0x44,0x44,0x6c,0x28,0x00,0x00 }, { 0x30,0x78,0x48,0x49,0x3f,0x7f,0x40,0x00 }, { 0x38,0x7c,0x54,0x54,0x5c,0x18,0x00,0x00 }, { 0x48,0x7e,0x7f,0x49,0x03,0x02,0x00,0x00 }, { 0x98,0xbc,0xa4,0xa4,0xf8,0x7c,0x04,0x00 }, { 0x41,0x7f,0x7f,0x08,0x04,0x7c,0x78,0x00 }, { 0x00,0x44,0x7d,0x7d,0x40,0x00,0x00,0x00 }, { 0x40,0xc4,0x84,0xfd,0x7d,0x00,0x00,0x00 }, { 0x41,0x7f,0x7f,0x10,0x38,0x6c,0x44,0x00 }, { 0x00,0x41,0x7f,0x7f,0x40,0x00,0x00,0x00 }, { 0x7c,0x7c,0x0c,0x18,0x0c,0x7c,0x78,0x00 }, { 0x7c,0x7c,0x04,0x04,0x7c,0x78,0x00,0x00 }, { 0x38,0x7c,0x44,0x44,0x7c,0x38,0x00,0x00 }, { 0x84,0xfc,0xf8,0xa4,0x24,0x3c,0x18,0x00 }, { 0x18,0x3c,0x24,0xa4,0xf8,0xfc,0x84,0x00 }, { 0x44,0x7c,0x78,0x44,0x1c,0x18,0x00,0x00 }, { 0x48,0x5c,0x54,0x54,0x74,0x24,0x00,0x00 }, { 0x00,0x04,0x3e,0x7f,0x44,0x24,0x00,0x00 }, { 0x3c,0x7c,0x40,0x40,0x3c,0x7c,0x40,0x00 }, { 0x1c,0x3c,0x60,0x60,0x3c,0x1c,0x00,0x00 }, { 0x3c,0x7c,0x60,0x30,0x60,0x7c,0x3c,0x00 }, { 0x44,0x6c,0x38,0x10,0x38,0x6c,0x44,0x00 }, { 0x9c,0xbc,0xa0,0xa0,0xfc,0x7c,0x00,0x00 }, { 0x4c,0x64,0x74,0x5c,0x4c,0x64,0x00,0x00 }, { 0x08,0x08,0x3e,0x77,0x41,0x41,0x00,0x00 }, { 0x00,0x00,0x00,0x77,0x77,0x00,0x00,0x00 }, { 0x41,0x41,0x77,0x3e,0x08,0x08,0x00,0x00 }, { 0x02,0x03,0x01,0x03,0x02,0x03,0x01,0x00 }, { 0x78,0x7c,0x46,0x43,0x46,0x7c,0x78,0x00 }, { 0x0e,0x9f,0x91,0xb1,0xfb,0x4a,0x00,0x00 }, { 0x3a,0x7a,0x40,0x40,0x7a,0x7a,0x40,0x00 }, { 0x38,0x7c,0x54,0x55,0x5d,0x19,0x00,0x00 }, { 0x02,0x23,0x75,0x55,0x55,0x7d,0x7b,0x42 }, { 0x21,0x75,0x54,0x54,0x7d,0x79,0x40,0x00 }, { 0x21,0x75,0x55,0x54,0x7c,0x78,0x40,0x00 }, { 0x20,0x74,0x57,0x57,0x7c,0x78,0x40,0x00 }, { 0x18,0x3c,0xa4,0xa4,0xa4,0xe4,0x40,0x00 }, { 0x02,0x3b,0x7d,0x55,0x55,0x5d,0x1b,0x02 }, { 0x39,0x7d,0x54,0x54,0x5d,0x19,0x00,0x00 }, { 0x39,0x7d,0x55,0x54,0x5c,0x18,0x00,0x00 }, { 0x01,0x45,0x7c,0x7c,0x41,0x01,0x00,0x00 }, { 0x02,0x03,0x45,0x7d,0x7d,0x43,0x02,0x00 }, { 0x01,0x45,0x7d,0x7c,0x40,0x00,0x00,0x00 }, { 0x79,0x7d,0x26,0x26,0x7d,0x79,0x00,0x00 }, { 0x70,0x78,0x2b,0x2b,0x78,0x70,0x00,0x00 }, { 0x44,0x7c,0x7c,0x55,0x55,0x45,0x00,0x00 }, { 0x20,0x74,0x54,0x54,0x7c,0x7c,0x54,0x54 }, { 0x7c,0x7e,0x0b,0x09,0x7f,0x7f,0x49,0x00 }, { 0x32,0x7b,0x49,0x49,0x7b,0x32,0x00,0x00 }, { 0x32,0x7a,0x48,0x48,0x7a,0x32,0x00,0x00 }, { 0x32,0x7a,0x4a,0x48,0x78,0x30,0x00,0x00 }, { 0x3a,0x7b,0x41,0x41,0x7b,0x7a,0x40,0x00 }, { 0x3a,0x7a,0x42,0x40,0x78,0x78,0x40,0x00 }, { 0xba,0xba,0xa0,0xa0,0xfa,0x7a,0x00,0x00 }, { 0x19,0x3d,0x66,0x66,0x66,0x3d,0x19,0x00 }, { 0x3d,0x7d,0x40,0x40,0x7d,0x3d,0x00,0x00 }, { 0x18,0x3c,0x24,0xe7,0xe7,0x24,0x24,0x00 }, { 0x68,0x7e,0x7f,0x49,0x43,0x66,0x20,0x00 }, { 0x2b,0x2f,0x7c,0x7c,0x2f,0x2b,0x00,0x00 }, { 0x7f,0x7f,0x09,0x2f,0xf6,0xf8,0xa0,0x00 }, { 0x40,0xc8,0x88,0xfe,0x7f,0x09,0x0b,0x02 }, { 0x20,0x74,0x54,0x55,0x7d,0x79,0x40,0x00 }, { 0x00,0x44,0x7d,0x7d,0x41,0x00,0x00,0x00 }, { 0x30,0x78,0x48,0x4a,0x7a,0x32,0x00,0x00 }, { 0x38,0x78,0x40,0x42,0x7a,0x7a,0x40,0x00 }, { 0x7a,0x7a,0x0a,0x0a,0x7a,0x70,0x00,0x00 }, { 0x7d,0x7d,0x19,0x31,0x7d,0x7d,0x00,0x00 }, { 0x00,0x26,0x2f,0x29,0x2f,0x2f,0x28,0x00 }, { 0x00,0x26,0x2f,0x29,0x29,0x2f,0x26,0x00 }, { 0x30,0x78,0x4d,0x45,0x60,0x20,0x00,0x00 }, { 0x38,0x38,0x08,0x08,0x08,0x08,0x00,0x00 }, { 0x08,0x08,0x08,0x08,0x38,0x38,0x00,0x00 }, { 0x67,0x37,0x18,0xcc,0xee,0xab,0xb9,0x90 }, { 0x6f,0x3f,0x18,0x4c,0x66,0x73,0xf9,0xf8 }, { 0x00,0x00,0x60,0xfa,0xfa,0x60,0x00,0x00 }, { 0x08,0x1c,0x36,0x22,0x08,0x1c,0x36,0x22 }, { 0x22,0x36,0x1c,0x08,0x22,0x36,0x1c,0x08 }, { 0xaa,0x00,0x55,0x00,0xaa,0x00,0x55,0x00 }, { 0xaa,0x55,0xaa,0x55,0xaa,0x55,0xaa,0x55 }, { 0x55,0xff,0xaa,0xff,0x55,0xff,0xaa,0xff }, { 0x00,0x00,0x00,0xff,0xff,0x00,0x00,0x00 }, { 0x10,0x10,0x10,0xff,0xff,0x00,0x00,0x00 }, { 0x14,0x14,0x14,0xff,0xff,0x00,0x00,0x00 }, { 0x10,0x10,0xff,0xff,0x00,0xff,0xff,0x00 }, { 0x10,0x10,0xf0,0xf0,0x10,0xf0,0xf0,0x00 }, { 0x14,0x14,0x14,0xfc,0xfc,0x00,0x00,0x00 }, { 0x14,0x14,0xf7,0xf7,0x00,0xff,0xff,0x00 }, { 0x00,0x00,0xff,0xff,0x00,0xff,0xff,0x00 }, { 0x14,0x14,0xf4,0xf4,0x04,0xfc,0xfc,0x00 }, { 0x14,0x14,0x17,0x17,0x10,0x1f,0x1f,0x00 }, { 0x10,0x10,0x1f,0x1f,0x10,0x1f,0x1f,0x00 }, { 0x14,0x14,0x14,0x1f,0x1f,0x00,0x00,0x00 }, { 0x10,0x10,0x10,0xf0,0xf0,0x00,0x00,0x00 }, { 0x00,0x00,0x00,0x1f,0x1f,0x10,0x10,0x10 }, { 0x10,0x10,0x10,0x1f,0x1f,0x10,0x10,0x10 }, { 0x10,0x10,0x10,0xf0,0xf0,0x10,0x10,0x10 }, { 0x00,0x00,0x00,0xff,0xff,0x10,0x10,0x10 }, { 0x10,0x10,0x10,0x10,0x10,0x10,0x10,0x10 }, { 0x10,0x10,0x10,0xff,0xff,0x10,0x10,0x10 }, { 0x00,0x00,0x00,0xff,0xff,0x14,0x14,0x14 }, { 0x00,0x00,0xff,0xff,0x00,0xff,0xff,0x10 }, { 0x00,0x00,0x1f,0x1f,0x10,0x17,0x17,0x14 }, { 0x00,0x00,0xfc,0xfc,0x04,0xf4,0xf4,0x14 }, { 0x14,0x14,0x17,0x17,0x10,0x17,0x17,0x14 }, { 0x14,0x14,0xf4,0xf4,0x04,0xf4,0xf4,0x14 }, { 0x00,0x00,0xff,0xff,0x00,0xf7,0xf7,0x14 }, { 0x14,0x14,0x14,0x14,0x14,0x14,0x14,0x14 }, { 0x14,0x14,0xf7,0xf7,0x00,0xf7,0xf7,0x14 }, { 0x14,0x14,0x14,0x17,0x17,0x14,0x14,0x14 }, { 0x10,0x10,0x1f,0x1f,0x10,0x1f,0x1f,0x10 }, { 0x14,0x14,0x14,0xf4,0xf4,0x14,0x14,0x14 }, { 0x10,0x10,0xf0,0xf0,0x10,0xf0,0xf0,0x10 }, { 0x00,0x00,0x1f,0x1f,0x10,0x1f,0x1f,0x10 }, { 0x00,0x00,0x00,0x1f,0x1f,0x14,0x14,0x14 }, { 0x00,0x00,0x00,0xfc,0xfc,0x14,0x14,0x14 }, { 0x00,0x00,0xf0,0xf0,0x10,0xf0,0xf0,0x10 }, { 0x10,0x10,0xff,0xff,0x00,0xff,0xff,0x10 }, { 0x14,0x14,0x14,0xf7,0xf7,0x14,0x14,0x14 }, { 0x10,0x10,0x10,0x1f,0x1f,0x00,0x00,0x00 }, { 0x00,0x00,0x00,0xf0,0xf0,0x10,0x10,0x10 }, { 0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff }, { 0xf0,0xf0,0xf0,0xf0,0xf0,0xf0,0xf0,0xf0 }, { 0xff,0xff,0xff,0xff,0x00,0x00,0x00,0x00 }, { 0x00,0x00,0x00,0x00,0xff,0xff,0xff,0xff }, { 0x0f,0x0f,0x0f,0x0f,0x0f,0x0f,0x0f,0x0f }, { 0x38,0x7c,0x44,0x6c,0x38,0x6c,0x44,0x00 }, { 0xfc,0xfe,0x2a,0x2a,0x3e,0x14,0x00,0x00 }, { 0x7e,0x7e,0x02,0x02,0x02,0x06,0x06,0x00 }, { 0x02,0x7e,0x7e,0x02,0x7e,0x7e,0x02,0x00 }, { 0x41,0x63,0x77,0x5d,0x49,0x63,0x63,0x00 }, { 0x38,0x7c,0x44,0x44,0x7c,0x3c,0x04,0x00 }, { 0x80,0xfe,0x7e,0x20,0x20,0x3e,0x1e,0x00 }, { 0x04,0x06,0x02,0x7e,0x7c,0x06,0x02,0x00 }, { 0x99,0xbd,0xe7,0xe7,0xbd,0x99,0x00,0x00 }, { 0x1c,0x3e,0x6b,0x49,0x6b,0x3e,0x1c,0x00 }, { 0x4c,0x7e,0x73,0x01,0x73,0x7e,0x4c,0x00 }, { 0x30,0x78,0x4a,0x4f,0x7d,0x39,0x00,0x00 }, { 0x18,0x3c,0x24,0x3c,0x3c,0x24,0x3c,0x18 }, { 0x98,0xfc,0x64,0x3c,0x3e,0x27,0x3d,0x18 }, { 0x1c,0x3e,0x6b,0x49,0x49,0x49,0x00,0x00 }, { 0x7e,0x7f,0x01,0x01,0x7f,0x7e,0x00,0x00 }, { 0x2a,0x2a,0x2a,0x2a,0x2a,0x2a,0x00,0x00 }, { 0x44,0x44,0x5f,0x5f,0x44,0x44,0x00,0x00 }, { 0x40,0x51,0x5b,0x4e,0x44,0x40,0x00,0x00 }, { 0x40,0x44,0x4e,0x5b,0x51,0x40,0x00,0x00 }, { 0x00,0x00,0x00,0xfe,0xff,0x01,0x07,0x06 }, { 0x60,0xe0,0x80,0xff,0x7f,0x00,0x00,0x00 }, { 0x08,0x08,0x6b,0x6b,0x08,0x08,0x00,0x00 }, { 0x24,0x12,0x12,0x36,0x24,0x24,0x12,0x00 }, { 0x00,0x06,0x0f,0x09,0x0f,0x06,0x00,0x00 }, { 0x00,0x00,0x00,0x18,0x18,0x00,0x00,0x00 }, { 0x00,0x00,0x00,0x10,0x10,0x00,0x00,0x00 }, { 0x10,0x30,0x70,0xc0,0xff,0xff,0x01,0x01 }, { 0x00,0x1f,0x1f,0x01,0x1f,0x1e,0x00,0x00 }, { 0x00,0x19,0x1d,0x15,0x17,0x12,0x00,0x00 }, { 0x00,0x00,0x3c,0x3c,0x3c,0x3c,0x00,0x00 }, { 0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00 }
};

const char BOX_CHARS[8] = { 218, 191, 192, 217, 196, 196, 179, 179 };

void draw_box(byte x, byte y, byte x2, byte y2, const char* chars) {
  byte x1 = x;
  putchar(x, y, chars[2]);
  putchar(x2, y, chars[3]);
  putchar(x, y2, chars[0]);
  putchar(x2, y2, chars[1]);
  while (++x < x2) {
    putchar(x, y, chars[5]);
    putchar(x, y2, chars[4]);
  }
  while (++y < y2) {
    putchar(x1, y, chars[6]);
    putchar(x2, y, chars[7]);
  }
}

void draw_bcd_word(byte x, byte y, word bcd) {
  byte j;
  x += 3;
  for (j=0; j<4; j++) {
    putchar(x, y, CHAR('0'+(bcd&0xf)));
    x--;
    bcd >>= 4;
  }
}

/*
void draw_bcd_byte(byte x, byte y, byte bcd) {
  putchar(CHAR('0'+(bcd&0xf)), x+1, y);
  putchar(CHAR('0'+((bcd>>4)&0xf)), x, y);
}

void draw_bcd_word2(byte x, byte y, word bcd) {
  draw_bcd_byte(x+2, y, bcd);
  draw_bcd_byte(x, y, bcd>>8);
}
*/

void draw_playfield() {
  draw_box(0,0,27,29,BOX_CHARS);
  putstring(0,31,"PLAYER 1");
  putstring(20,31,"PLAYER 2");
  draw_bcd_word(0,30,players[0].score);
  draw_bcd_word(24,30,players[1].score);
  if (attract) {
    if (credits) {
      putstring(8,29,"PRESS START");
      putstring(9,0,"CREDITS ");
      putchar(9+8, 0, (credits>9?9:credits)+CHAR('0'));
    } else {
      putstring(9,29,"GAME OVER");
      putstring(8,0,"INSERT COIN");
    }
  }
}

typedef enum { D_RIGHT, D_DOWN, D_LEFT, D_UP } dir_t;
const char DIR_X[4] = { 1, 0, -1, 0 };
const char DIR_Y[4] = { 0, -1, 0, 1 };

void init_game() {
  memset(players, 0, sizeof(players));
  players[0].head_attr = CHAR('1');
  players[1].head_attr = CHAR('2');
  players[0].tail_attr = 254;
  players[1].tail_attr = 254;
  frames_per_move = START_SPEED;
}

void reset_players() {
  players[0].x = players[0].y = 6;
  players[0].dir = D_RIGHT;
  players[1].x = players[1].y = 21;
  players[1].dir = D_LEFT;
  players[0].collided = players[1].collided = 0;
}

void draw_player(Player* p) {
  putchar(p->x, p->y, p->head_attr);
}

void move_player(Player* p) {
  putchar(p->x, p->y, p->tail_attr);
  p->x += DIR_X[p->dir];
  p->y += DIR_Y[p->dir];
  if (getchar(p->x, p->y) != CHAR(' '))
    p->collided = 1;
  draw_player(p);
}

void human_control(Player* p) {
  byte dir = 0xff;
  if (!p->human) return;
  if (LEFT1) dir = D_LEFT;
  if (RIGHT1) dir = D_RIGHT;
  if (UP1) dir = D_UP;
  if (DOWN1) dir = D_DOWN;
  // don't let the player reverse
  if (dir < 0x80 && dir != (p->dir ^ 2)) {
    p->dir = dir;
  }
}

byte ai_try_dir(Player* p, dir_t dir, byte shift) {
  byte x,y;
  dir &= 3;
  x = p->x + (DIR_X[dir] << shift);
  y = p->y + (DIR_Y[dir] << shift);
  if (x < 29 && y < 27 && getchar(x, y) == CHAR(' ')) {
    p->dir = dir;
    return 1;
  } else {
    return 0;
  }
}

void ai_control(Player* p) {
  dir_t dir;
  if (p->human) return;
  dir = p->dir;
  if (!ai_try_dir(p, dir, 0)) {
    ai_try_dir(p, dir+1, 0);
    ai_try_dir(p, dir-1, 0);
  } else {
    ai_try_dir(p, dir-1, 0) && ai_try_dir(p, dir-1, 1+(rand() & 3));
    ai_try_dir(p, dir+1, 0) && ai_try_dir(p, dir+1, 1+(rand() & 3));
    ai_try_dir(p, dir, rand() & 3);
  }
}

// add two 16-bit BCD values
word bcd_add(word a, word b) {
  a; b; // to avoid warning
__asm
	ld	hl,#4
	add	hl,sp
	ld	iy,#2
	add	iy,sp
	ld	a,0 (iy)
	add	a, (hl)
	daa
	ld	c,a
	ld	a,1 (iy)
	inc	hl
	adc	a, (hl)
  	daa
	ld	b,a
	ld	l, c
	ld	h, b
__endasm;
}

void slide_right() {
  byte j;
  for (j=0; j<32; j++) {
    memmove(&cellram[1], &cellram[0], sizeof(cellram)-sizeof(cellram[0]));
    memset(&cellram[0], 0, sizeof(cellram[0]));
  }
}

void flash_colliders() {
  byte i;
  // flash players that collided
  for (i=0; i<60; i++) {
    if (players[0].collided) players[0].head_attr ^= 0x80;
    if (players[1].collided) players[1].head_attr ^= 0x80;
    wait_for_vsync();
    wait_for_vsync();
    draw_player(&players[0]);
    draw_player(&players[1]);
    palette = i;
    set8910(7, 0xff ^ 0x8);
    set8910(6, i>>1);
    set8910(8, 10);
  }
  set8910(8, 0);
  palette = 0;
}

void make_move() {
  byte i;
  for (i=0; i<frames_per_move; i++) {
    human_control(&players[0]);
    wait_for_vsync();
  }
  ai_control(&players[0]);
  ai_control(&players[1]);
  // if players collide, 2nd player gets the point
  move_player(&players[1]);
  move_player(&players[0]);
}

void play_game();

char start_pressed() {
  if (attract) {
    if (credits > 0 && START1) {
      credits--;
      return 1;
    }
  }
  return 0;
}

void declare_winner(byte winner) {
  byte i;
  for (i=0; i<10; i++) {
    draw_box(i,i,27-i,29-i,BOX_CHARS);
    wait_for_vsync();
  }
  putstring(10,16,"WINNER:");
  putstring(10,13,"PLAYER ");
  putchar(10+7, 13, CHAR('1')+winner);
  for (i=0; i<250; i++)
    wait_for_vsync();
  slide_right();
  attract = 1;
}

void play_round() {
  reset_players();
  clrscr();
  draw_playfield();
  while (1) {
    make_move();
    if (players[0].collided || players[1].collided) break;
    if (start_pressed()) {
      play_game();
      return;
    }
  }
  flash_colliders();
  // don't keep score in attract mode
  if (attract) return;
  // add scores to players that didn't collide
  if (players[0].collided)
    players[1].score = bcd_add(players[1].score, 1);
  if (players[1].collided)
    players[0].score = bcd_add(players[0].score, 1);
  // increase speed
  if (frames_per_move > MAX_SPEED) frames_per_move--;
  // game over?
  if (players[0].score != players[1].score) {
    if (players[0].score >= MAX_SCORE)
      declare_winner(0);
    else if (players[1].score >= MAX_SCORE)
      declare_winner(1);
  }
}

void play_game() {
  attract = 0;
  init_game();
  players[0].human = 1;
  while (!attract) {
    play_round();
  }
}

void attract_mode() {
  attract = 1;
  init_game();
  frames_per_move = 9;
  players[0].human = 0;
  while (1) {
    play_round();
  }
}

void test_ram() {
  word i;
  for (i=0; i<0x800; i++) {
    cellram[0][i & 0x3ff] = rand();
  }
}

void main() {
  if (COIN1) {
    credits++;
  } else {
    test_ram();
  }
  palette = 0;
  memcpy(tileram, font8x8, sizeof(font8x8));
  draw_playfield();
  attract_mode();
}