chains.c

#include <stdio.h>
#include <memory.h>

#define MAX 1000 /* max input line length + 1 */

static char line0[MAX]; /* the 2nd to last line of input */
static char line1[MAX]; /* the last line of input */

static int mark0[MAX]; /* markings for line0 */
static int mark1[MAX]; /* markings for line1 */

static int chain_count = 0; /* number of chains so far */
static int high_chain = 0; /* highest chain mark so far */

static void merge_chains(int a, int b)
{
        for( int i = 0; i < MAX; i++ )
        {
                if( mark0[i] == a )
                        mark0[i] = b;
                if( mark1[i] == a )
                        mark1[i] = b;
        }
}

static void set_mark(int i)
{
        int mark_up = mark0[i];
        int mark_left = i ? mark1[i-1] : 0;

        if( mark_up && mark_left && mark_up != mark_left )
        {
                merge_chains(mark_left, mark_up);
                mark1[i] = mark_up;
                chain_count--;
        }
        else if( mark_up || mark_left )
        {
                mark1[i] = mark_up ? mark_up : mark_left;
        }
        else
        {
                mark1[i] = ++high_chain;
                chain_count++;
        }
}

int main()
{
        while( !feof(stdin) )
        {
                memcpy(line0, line1, MAX * sizeof *line1);
                memcpy(mark0, mark1, MAX * sizeof *mark1);
                memset(line1, 0, MAX * sizeof *line0);
                memset(mark1, 0, MAX * sizeof *mark1);
                fgets(line1, MAX, stdin);
                for( int i = 0; i < MAX; i++ )
                        if( line1[i] == 'x' )
                                set_mark(i);
        }

        printf("%d chain(s) found\n", chain_count);
}