fib.c

#include <stdio.h>
#include <string.h>
#define aSize 1001

// 8 bit data type
typedef char byte;

int thousandDigitFib();

int addTo(byte* , byte*, int, int);

//print debugger
//void printNum(byte*, int);

int main(void) {
   printf("%d\n", thousandDigitFib());
}


int thousandDigitFib() {

  byte a[aSize] = {0};
  byte b[aSize]= {0};
  byte c[aSize]= {0};

  a[aSize - 1] = 0;
  b[aSize-1] = 1;
  c[aSize-1] = 1;

  int digits = 1;
  int count = 1;
  while(1) {
    count += 1;
    memcpy(c, b, aSize);
    int digits = addTo(&b, &a, aSize, digits+1);
    memcpy(a, c, aSize);
    if (digits >= 1000) return count;
  }
  return count;
}

int addTo(byte* a, byte* b, int size, int end) {
  int carry = 0;
  int temp = 0;
  int digits = 0;
  for (int i = (size - 1); i >= size-end; i--) {
    temp = a[i] + b[i] + carry;
    if (temp) digits = (size-i);
    carry = (temp >=10);
    a[i] = (temp % 10);
  }
  return digits;
}


/*
  void printNum(byte* a, int size) {
  byte printZ = 0;
  for (int i = 0; i < size; i++) {
    if(!printZ) {
      if(a[i]){
  printZ = 1;
  printf("%d",a[i]);
      }
    } else {
      printf("%d",a[i]);
    }

  }
  puts("");
}
*/