/* NRJ (NOR and Reference Jump) OISC ANSI C emulator
 * Created by Luxferre, 2022, public domain
 * Build with: cc -Os -std=c89 -o nrj nrj.c */
#include <stdlib.h>
#include <stdio.h>
#include <termios.h>
#include <unistd.h>
#include <sys/select.h>
#ifndef NRJWORD /* we emulate NRJ16 by default but this can be overridden */
#define NRJWORD unsigned short
#endif
#define NRJWSIZE (sizeof(NRJWORD) << 3)
#define NRJSIZE (1 << NRJWSIZE)
#define MAXADDR ((NRJWORD) (NRJSIZE - 1))

int kbhit() { /* our best attempt to detect a keypress on a POSIX system */
  struct timeval tv = { 0L, 0L };
  fd_set fds;
  FD_ZERO(&fds);
  FD_SET(0, &fds);
  return select(1, &fds, NULL, NULL, &tv) > 0;
}

void nrj_in(NRJWORD *ctxid, NRJWORD *val) { /* context-dependent input handler */
  unsigned char c;
  if(*ctxid == (NRJWORD) 0) /* for now, only emulate standard context id */
    *val = (NRJWORD) ((kbhit() && read(0, &c, 1) > -1) ? c : 0);
}

void nrj_out(NRJWORD *ctxid, NRJWORD *val) { /* context-dependent output handler */
  if(*ctxid == (NRJWORD) 0) /* for now, only emulate standard context id */
    putchar((unsigned char) *val);
}

void nrj_run(NRJWORD *mem, NRJWORD pc) { /* Main NRJ engine - just 12 lines of C within this function */
  while(pc != MAXADDR) { /* halt if jumped to the last word */
    if(mem[0]) { /* handle input if word 0 is set */
      nrj_in(&mem[2], &mem[mem[0]]); /* input a value to the location specified in word 0 */
      mem[0] = (NRJWORD) 0; /* clear word 0 */
    }
    mem[mem[pc]] = (~(mem[mem[pc]] | mem[mem[pc+1]])) & MAXADDR; /* then perform the NOR operation */
    pc = mem[mem[pc+2]]; /* then perform the reference jump operation */
    if(mem[1]) { /* then handle output if word 1 is set */
      nrj_out(&mem[2], &mem[1]); /* output the value from the word 1 */
      mem[1] = (NRJWORD) 0; /* clear word 1 */
    }
  }
}

struct termios tty_opts_backup, tty_opts_raw; /* terminal I/O helper definitions */
void restore_term() {tcsetattr(0, TCSANOW, &tty_opts_backup);}

int main(int argc, char* argv[]) { /* emulator entry point: nrj program.bin */
  if(argc > 1) {
    FILE *prog = fopen(argv[1], "rb");
    if(prog) { /* load the program and prepare the terminal */
      NRJWORD mem[NRJSIZE]; /* NRJ memory: word 0 - input, 1 - output, 2 - I/O context, 3 - program start */
      fseek(prog, 0, SEEK_END);
      int flen = ftell(prog);
      fseek(prog, 0, SEEK_SET);
      fread(mem, sizeof(NRJWORD), flen/sizeof(NRJWORD), prog);
      fclose(prog);
      tcgetattr(0, &tty_opts_backup);
      atexit(&restore_term);
      cfmakeraw(&tty_opts_raw);
      tcsetattr(0, TCSANOW, &tty_opts_raw);
      nrj_run(mem, 3); /* now, start the engine */
      return 0; /* return the successful status to the outer OS */
    }
  } /* otherwise bail out with an error */
  printf("NRJ%u: no valid program binary specified\r\n", (unsigned int) NRJWSIZE);
  return 1;
}