/*
  lang.c - simple bytecode language, written in 20 mins
  Written by Harry "Piru" Sintonen <sintonen@iki.fi>

  features:
   - 64k memory
   - input from stdin
   - output to stdout
   - add immediate (8-bit)
   - sub immediate (8-bit)
   - absolute jump (16-bit address)
   - absolute conditional jump (16-bit address)
   - return
*/

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

typedef enum
{
  ST_INIT,
  ST_RUN

} state_t;

typedef unsigned char u8;
#define INC(x) (((x) + 1) & 65535)
#define DEC(x) (((x) - 1) & 65535)

int main(int argc, char **argv)
{
  state_t state = ST_INIT;
  u8 mem[65536];
  int pc;
  int ac;
  int c;

  if (argc != 2)
  {
    fprintf(stderr, "usage: %s PROGRAM\n", argv[0]);
    exit(1);
  }

  for (;;) switch (state)
  {
    case ST_INIT:
      strncpy((char *) mem, argv[1], sizeof(mem));
      pc = 0;
      ac = 0x8000;
      state = ST_RUN;
      break;

    case ST_RUN:
      switch (mem[pc])
      {
        case 'I':
          c = getchar();
          if (c == -1)
            exit(1);
          mem[ac] = (u8) c;
          ac = INC(ac);
          break;

        case 'O':
          ac = DEC(ac);
          putchar(mem[ac]);
          break;

        case 'A':
          mem[ac] = mem[ac] + mem[++pc];
          break;

        case 'S':
          mem[ac] = mem[ac] - mem[++pc];
          break;

        case 'C':
          if (mem[ac])
            break;

        case 'J':
          pc = DEC((mem[pc + 1] << 8) | mem[pc + 2]);
          break;

        case 'R':
          exit(0);

      }
      pc = INC(pc);
      break;
  }

  return 0;
}