C Program that keeps count of the no. of steps of Kaprekar routine's execution

#include <stdio.h>
#include <stdbool.h>

void sort(int arr[], int n, bool reverse){
    int i, j, key;

    if(reverse==true){
        for(i=1;i<n;i++){
            key=arr[i];
            j=i-1;

            while(j>=0&&arr[j]<key){
                arr[j+1] = arr[j];
                j--;
            }
            arr[j+1]=key;
        }
    } else {
        for(i=1;i<n;i++){
            key=arr[i];
            j=i-1;

            while(j>=0&&arr[j]>key){
                arr[j+1] = arr[j];
                j--;
            }
            arr[j+1]=key;
        }
    }
}

int kaprekar_routine(int n, int diff){
    if((1000>=n||n>=9999)&&(n!=0)){printf("Number should be only 4 digits"); return 0;}
    int arr[4], arr_res[2], i=0, largest=0, smallest=0, ldiff;
    static int res, steps=0;

    while(n>0){
        arr[i] = n%10;
        i++;
        n /= 10;
    }
    sort(arr, 4, false);
    for(i=0; i<4; i++){
        smallest *= 10;
        smallest += arr[i];
    }
    sort(arr, 4, true);
    for(i=0; i<4; i++){
        largest *= 10;
        largest += arr[i];
    }
    ldiff = largest-smallest;
    steps++;

    if((ldiff==diff)&&(ldiff!=0)){
        return steps;
    } else if(ldiff==0){
        return 0;
    } else {
        return kaprekar_routine(ldiff, ldiff);
    }
}

int main(){
    int overall_steps = kaprekar_routine(3456, 0);
    printf("The Kaprekar Routine happened for %d steps", overall_steps);
    return 0;
}