/* Dumbest possible run-time code generation for ARM Thumb? */
#include <sys/mman.h>
#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>

typedef int (*func_type)();

void assemble_halfword(char **buf, unsigned short h)
{
  *(*buf)++ = h & 0xff;
  *(*buf)++ = h >> 8;
}

void assemble_pushn(char **buf, int regs)
{
  assemble_halfword(buf, 0xb400 + regs);
}

void assemble_popn(char **buf, int regs)
{
  assemble_halfword(buf, 0xbc00 + regs);
}

void assemble_movs8(char **buf, int reg, int imm8)
{
  assemble_halfword(buf, 0x2000 + (reg << 8) + imm8);
}


int main()
{
  char *codebuf = mmap(0,       /* no requested address */
                       4096,    /* requested size */
                       PROT_READ | PROT_WRITE | PROT_EXEC,
                       MAP_ANONYMOUS | MAP_PRIVATE,
                       -1,      /* MAP_ANONYMOUS portability requires fd -1 */
                       0        /* offset is meaningless with no fd */
                       );
  printf("codebuf = %lx\n", (long)codebuf);
  if (!codebuf) return 1;

  char *p = codebuf;
  assemble_pushn(&p, 1 << 8 | 1 << 3); /* only push lr and r3 */
  assemble_movs8(&p, 0, 53);           /* set r0 to #53 */
  assemble_movs8(&p, 1, 53);           /* set r1 to #53 */
  assemble_popn(&p, 1 << 8 | 1 << 3);  /* only pop pc and r3 */

  int n = p - codebuf;
  for (int i = 0; i < n; i++) {
    printf("%02x ", codebuf[i]);
  }
  printf("\n");

  /* Write it to a file so I can run a disassembler on it.

     As in:

     arm-linux-gnueabi-objdump -b binary -D -Mforce-thumb -mcortex-m4 nullcomplr.out.bin

  */
  char *outfile = "nullcomplr.out.bin";
  int fd = open(outfile, O_WRONLY | O_CREAT | O_TRUNC, 0666);
  if (fd < 0) {
    perror(outfile);
    return 2;
  }

  if (n != write(fd, codebuf, n)) {
    perror("write");
    return 3;
  }

  if (0 != close(fd)) {
    perror("close");
    return 4;
  }

  /* +1 for Thumb! */
  printf("result = %d\n", ((func_type)(codebuf+1))());
  return 0;
}