#include <pic16f84a.h>

typedef unsigned char uint8_t;

__sfr __at(0x70) UART_TX;
__sfr __at(0x71) UART_RX;
__sfr __at(0x72) UART_STATUS;

#define GRID_W       20
#define GRID_H       10
#define MAX_LEN      20
#define OVF_PER_FRAME 60

#define DIR_UP    0
#define DIR_DOWN  1
#define DIR_LEFT  2
#define DIR_RIGHT 3

static uint8_t snake_x[MAX_LEN];
static uint8_t snake_y[MAX_LEN];
static uint8_t snake_len;
static uint8_t direction;
static uint8_t food_x;
static uint8_t food_y;
static uint8_t score;
static volatile uint8_t ovf_count;

void isr(void) __interrupt(0) {
    if (INTCON & 0x04) {
        ovf_count++;
        INTCON &= ~(uint8_t)0x04;
    }
}

static void uart_send(uint8_t b) {
    while (UART_STATUS & 0x01);
    UART_TX = b;
}

static void uart_puts(const char *s) {
    while (*s) uart_send((uint8_t)*s++);
}

static uint8_t uart_recv(void) {
    if (UART_STATUS & 0x02)
        return 0;
    return UART_RX;
}

static uint8_t uart_getc(void) {
    while (UART_STATUS & 0x02);
    return UART_RX;
}

static void handle_input(void) {
    uint8_t c = uart_recv();
    if (c != 0x1B) return;
    if (uart_getc() != '[') return;
    switch (uart_getc()) {
        case 'A': if (direction != DIR_DOWN)  direction = DIR_UP;    break;
        case 'B': if (direction != DIR_UP)    direction = DIR_DOWN;  break;
        case 'C': if (direction != DIR_LEFT)  direction = DIR_RIGHT; break;
        case 'D': if (direction != DIR_RIGHT) direction = DIR_LEFT;  break;
    }
}

static void place_food(void) {
    food_x = (ovf_count * 7 + 3) % GRID_W;
    food_y = (ovf_count * 5 + 1) % GRID_H;
}

static void print_uint8(uint8_t v) {
    uint8_t started = 0;
    if (v >= 200) { uart_send('2'); v -= 200; started = 1; }
    else if (v >= 100) { uart_send('1'); v -= 100; started = 1; }
    {
        uint8_t t = '0';
        while (v >= 10) { v -= 10; t++; }
        if (started || t != '0') uart_send(t);
    }
    uart_send(v + '0');
}

static void render(void) {
    uint8_t x, y, i;

    uart_puts("\x1B[H");

    /* top border */
    uart_send('+');
    for (x = 0; x < GRID_W; x++) uart_send('-');
    uart_puts("+\r\n");

    for (y = 0; y < GRID_H; y++) {
        uart_send('|');
        for (x = 0; x < GRID_W; x++) {
            uint8_t ch = ' ';
            if (x == food_x && y == food_y) {
                ch = '*';
            } else {
                for (i = 0; i < snake_len; i++) {
                    if (snake_x[i] == x && snake_y[i] == y) {
                        ch = (i == 0) ? 'O' : 'o';
                        break;
                    }
                }
            }
            uart_send(ch);
        }
        uart_puts("|\r\n");
    }

    /* bottom border */
    uart_send('+');
    for (x = 0; x < GRID_W; x++) uart_send('-');
    uart_puts("+\r\n");

    uart_puts("Score: ");
    print_uint8(score);
    uart_puts("   \r\n");
}

/* returns 1 if alive, 0 if dead */
static uint8_t update(void) {
    uint8_t nx, ny, i;

    nx = snake_x[0];
    ny = snake_y[0];
    switch (direction) {
        case DIR_UP:    if (ny == 0)         return 0; ny--; break;
        case DIR_DOWN:  if (ny == GRID_H-1)  return 0; ny++; break;
        case DIR_LEFT:  if (nx == 0)         return 0; nx--; break;
        case DIR_RIGHT: if (nx == GRID_W-1)  return 0; nx++; break;
    }

    /* self-collision (exclude tail: it moves away this step) */
    for (i = 0; i < snake_len - 1; i++) {
        if (snake_x[i] == nx && snake_y[i] == ny) return 0;
    }

    if (nx == food_x && ny == food_y) {
        if (snake_len < MAX_LEN) snake_len++;
        score++;
        place_food();
    }

    /* shift body back, add new head */
    for (i = snake_len - 1; i > 0; i--) {
        snake_x[i] = snake_x[i-1];
        snake_y[i] = snake_y[i-1];
    }
    snake_x[0] = nx;
    snake_y[0] = ny;

    return 1;
}

void main(void) {
    uint8_t alive;

    OPTION_REG = 0x07;
    TMR0 = 0;
    INTCON = 0xa0;

    while (1) {
        /* init */
        direction  = DIR_RIGHT;
        snake_len  = 3;
        score      = 0;
        snake_x[0] = 5; snake_y[0] = 5;
        snake_x[1] = 4; snake_y[1] = 5;
        snake_x[2] = 3; snake_y[2] = 5;
        place_food();

        uart_puts("\x1B[2J\x1B[H");

        alive = 1;
        while (alive) {
            ovf_count = 0;
            while (ovf_count < OVF_PER_FRAME);

            handle_input();
            alive = update();
            render();
            PORTA ^= 0x01;
        }

        uart_puts("\x1B[2J\x1B[H");
        uart_puts("GAME OVER  score: ");
        print_uint8(score);
        uart_puts("\r\npress any key\r\n");
        uart_getc();
    }
}