//
// Simple filter to pass the first of every N input lines.
//

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

long Skip = 0;
long Take = 0;
long Window = 1;

void main (int argc, char *argv[])
{
    long In;
    long Index;
    char Line[1000];
    long Out;

    if (argc > 1 && strcmp(argv[1], "/?") == 0) {
        fprintf(stderr, "%s [Window] [Skip] [Take]\n", argv[0]);
        exit(0);
    }

    if (argc > 1) {
        sscanf(argv[1], "%ld", &Window);
        if (Window < 1) {
            Window = 1;
        }
        fprintf(stderr, "Window %ld\n", Window);
    }

    if (argc > 2) {
        sscanf(argv[2], "%ld", &Skip);
        if (Skip < 0) {
            Skip = 0;
        }
        fprintf(stderr, "Skip %ld\n", Skip);
    }

    if (argc > 3) {
        sscanf(argv[3], "%ld", &Take);
        if (Take < 1) {
            Take = 1;
        }
        fprintf(stderr, "Take %ld\n", Take);
    }

    In = Out = Index = 0;

    while (fgets(Line, sizeof(Line), stdin) != NULL) {
        In += 1;
        if (In > Skip) {
            if (Index == 0) {
                fputs(Line, stdout);
                Out += 1;
                if (Take != 0 && Out == Take) {
                    break;
                }
            }
            Index = (Index + 1) % Window;
        }
    }

    fprintf(stderr, "%ld lines in", In);
    if (Skip != 0) {
        fprintf(stderr, ", %ld lines skipped", Skip);
    }
    fprintf(stderr, ", %ld lines out", Out);
    if (Index != 0 && Take == 0) {
        fprintf(stderr, ", %ld lines left", Index);
    }
    fprintf(stderr, "\n");

    return;
}