#include <stdio.h>
#include <limits.h>

#define M       10              /* Nbr of birds */

#define CSTAB  100              /* Criterium of stability */

#define DOVE    'D'             /* Dove */
#define HAWK    'H'             /* Hawk */
#define VULT    'V'             /* Vulture */
                                
main() {
    char birds[M+1];            /* Bird string e.g. "DDHHVHVHVD" */
    int ibird;                  /* Attacking bird index */
    int jbird;                  /* Defending bird index */
    int t;                      /* Time */
    int c;                      /* Stability criterium counter */
    char buf[32];               /* Input buffer */
    FILE *fp;                   /* Output file */

    srand(time(0));             /* Set random seed */
    fp = fopen("A.DAT","w");    /* Open output file */

    while (
        printf("Enter %d-char bird-string (use DHV): ",M),
        gets(buf)
    ) {                         /* Input loop, end with CTRL-Z */
        sscanf(buf,"%s",birds); /* Initial bird string */
        printf("%s",birds);
        fprintf(fp,"%s",birds);

        for (c = 0, t = 0; c < CSTAB; t++) {
            ibird = (int)((double)rand()/(double)INT_MAX*M);
            if (birds[ibird] != VULT) {
                c++;
                continue;       /* Only vultures attack */
            }
            jbird = (int)((double)rand()/(double)INT_MAX*M);
            switch (birds[jbird]) {
            case DOVE:          /* Vulture wins */
                birds[jbird] = VULT;
                c = 0;
                break;
            case HAWK:          /* 50% vulture, 50% hawk */
                if (rand() < INT_MAX/2)
                    birds[jbird] = VULT;
                else
                    birds[ibird] = HAWK;
                c = 0;
                break;
            case VULT:          /* Nothing changes */
                c++;
                break;
            }
        }
        printf(" ==> %s in %3d\n",birds,t-CSTAB);
        fprintf(fp," ==> %s in %3d\n",birds,t-CSTAB);
    }
    fclose(fp);
}