#include <pic16f84a.h>

typedef unsigned char uint8_t;

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

/*
 * how many OVF per refresh of the screen
 */
#define OVF_PER_REFRESH 80

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

uint8_t direction;
uint8_t position[2];

static volatile uint8_t ovf_count = 0;

void isr(void) __interrupt(0) {
    if (INTCON & 0x04) {        /* T0IF = bit 2 */
        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;           /* rx_empty: nothing to read */
    return UART_RX;         /* reading pops the FIFO */
}

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': direction = DIR_UP;    break;  /* ESC [ A */
        case 'B': direction = DIR_DOWN;  break;  /* ESC [ B */
        case 'C': direction = DIR_RIGHT; break;  /* ESC [ C */
        case 'D': direction = DIR_LEFT;  break;  /* ESC [ D */
    }
}

static void update_position(void) {
    switch (direction) {
        case DIR_UP:    position[1]--; break;
        case DIR_DOWN:  position[1]++; break;
        case DIR_LEFT:  position[0]--; break;
        case DIR_RIGHT: position[0]++; break;
    }
}

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) {
    uart_puts("\x1B[H");   /* cursor home */

    uart_puts("pos: ");
    print_uint8(position[0]);
    uart_puts(", ");
    print_uint8(position[1]);
    uart_puts("\r\n");
}


void main(void) {

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

    direction = DIR_RIGHT;
    position[0] = 10;
    position[1] = 5;

    uart_puts("\x1B[2J\x1B[H");
    uart_puts("snake\r\n");
    uart_puts("-----------\r\n");

    while (1) {
        ovf_count = 0;
        while (ovf_count < OVF_PER_REFRESH);

        handle_input();
        update_position();
        render();

        PORTA ^= 0x01;
    }
}