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#include <dirent.h>
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>

struct LList {
    dev_t dev;
    ino_t inode;
    nlink_t nlink;
    char **names;
    unsigned int nnames;
    unsigned int max_nnames;
    struct LList *next;
};

static struct LList *list = NULL;

int dirsreglnkfilter(const struct dirent *dir) {
    switch (dir->d_type) {
        case DT_DIR:
            if (!strcmp(dir->d_name, ".") || !strcmp(dir->d_name, ".."))
                return 0;
        case DT_REG:
        case DT_LNK:
            return 1;
        default:
            return 0;
    }
}

char *dircat(char *a, char *b) {
    char *res = malloc((strlen(a) + strlen(b) + 2) * sizeof(char));
    strcpy(res, a);
    res[strlen(a)] = '/';
    strcpy(res + strlen(a) + 1, b);

    return res;
}

int dirslast(const struct dirent **a, const struct dirent **b) {
    if ((*a)->d_type == (*b)->d_type) {
        return 0;
    } else if ((*a)->d_type == DT_DIR) {
        return 1;
    } else if ((*b)->d_type == DT_DIR) {
        return -1;
    } else {
        return 0;
    }
}

void append_link(char *name, struct stat *sstat) {
    struct LList *found = list;
    struct LList *prev = NULL;

    while (found != NULL) {
        if (found->dev == sstat->st_dev &&
                found->inode == sstat->st_ino) {
            break;
        }
        prev = found;
        found = found->next;
    }

    if (found == NULL) {
        found = malloc(sizeof(struct LList));
        if (prev != NULL) {
            prev->next = found;
        } else {
            list = found;
        }
        found->max_nnames = 10;
        found->nnames = 0;
        found->names = malloc(10 * sizeof(char *));
        found->next = NULL;
        found->dev = sstat->st_dev;
        found->inode = sstat->st_ino;
    }

    if (found->nnames == found->max_nnames) {
        found->max_nnames *= 2;
        found->names = realloc(found->names, found->max_nnames * sizeof(char *));
    }

    found->names[found->nnames++] = strdup(name);
}

int handle_dir(char *name) {
    struct dirent **dirent;
    struct stat buf;
    char *cat;
    int count;

    count = scandir(name, &dirent, dirsreglnkfilter, dirslast);
    if (count == -1) {
        fprintf(stderr, "An error occurred while scanning directory '%s': %s\n", name, strerror(errno));
        return -1;
    } else {
        while (count--) {
            switch (dirent[count]->d_type) {
                case DT_LNK:
                case DT_REG:
                    cat = dircat(name, dirent[count]->d_name);
                    if (lstat(cat, &buf) == -1) {
                        fprintf(stderr, "lstat '%s': %s\n", cat, strerror(errno));
                    } else if (buf.st_nlink > 1) {
                        append_link(cat, &buf);
                    }
                    free(dirent[count]);
                    free(cat);
                    break;
                case DT_DIR:
                    cat = dircat(name, dirent[count]->d_name);
                    handle_dir(cat);
                    free(dirent[count]);
                    free(cat);
                    break;
            }
        }

        free(dirent);
    }

    return 0;
}

void print_hlinks() {
    struct LList *current = list;
    int i;

    while (current != NULL) {
        for (i = 0; i < current->nnames; i++) {
            printf("%s\n", current->names[i]);
        }
        printf("\n");
        current = current->next;
    }
}

void free_hlinks() {
    struct LList *prev, *current = list;
    int i;

    while (current != NULL) {
        prev = current;
        current = prev->next;
        for (i = 0; i < prev->nnames; i++) {
            free(prev->names[i]);
        }
        free(prev->names);
        free(prev);
    }

    list = NULL;
}

int main(int argc, char **argv) {
    if (argc != 2) {
        printf("Usage: %s <DIR>\n", argv[0]);
        return 0;
    }

    handle_dir(argv[1]);

    printf("Here are all the hard links, grouped together:\n");
    print_hlinks();
    free_hlinks();

    return 0;
}