A tiny Turing machine for 2-symbols Busy Beavers. No variable for state. Program is kept on input string.

ttm.c

// Tiny Turing machine in C
// for 2-symbols Busy Beavers
// Nicolas Seriot, 2023-05-07

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

#define TAPE_LEN 50000
#define MAX_COUNT 100000000

int runMachine(_Bool *t, char *p) {
    int h = TAPE_LEN/2;
    int i = 0; // instruction pointer
    int c = 0; // counter

    while (++c) {
        t[h] = p[i]-'0';      // write tape
        h+=p[i+1]=='R'?1:-1;  // move head
        if(p[i+2]=='H')break; // halt if needed
        if(h==0||h==TAPE_LEN-1){return -1;} // out of tape
        if(c==MAX_COUNT){return -2;} // max count reached
        i = 8*(p[i+2]-'A')+4*t[h];   // set ip for next state
    }
    return c;
}

int main(int argc, const char * argv[]) {    
    char *program = "1LB 1RC 1LC 1LB 1LD 0RE 1RA 1RD 1LH 0RA";
    int64_t sigma = 0;
    _Bool tape[TAPE_LEN] = {0};
    printf("* Tiny Turing Machine *\n");
    printf("Program: %s\n", program);
    int c = runMachine(tape, program);
    for (int i = 0; i < TAPE_LEN; i++) {sigma += tape[i];}
    printf("S = %d, Sigma = %llu\n", c, sigma);
    return 0;
}