Common tokens.

#include <glib.h>


GHashTable* read_word_file(const gchar *filename, GError **error)
{
    GIOChannel *word_file;
    GHashTable *result;
    gchar *word;

    if ((word_file = g_io_channel_new_file(filename, "r", error)) == NULL) {
        return NULL;
    }
    result = g_hash_table_new_full(g_str_hash, g_str_equal, g_free, NULL);
    while (
        g_io_channel_read_line(word_file, &word, NULL, NULL, NULL)
            == G_IO_STATUS_NORMAL
    ) {
        g_hash_table_add(result, g_strstrip(word));
    }
    if (*error != NULL) {
        g_hash_table_unref(result);
        result = NULL;
    }
    g_io_channel_unref(word_file);
    return result;
}

void intersection(GHashTable *set_a, GHashTable *set_b)
{
    GHashTableIter iterator;
    gchar *word;

    g_hash_table_iter_init(&iterator, set_a);
    while (g_hash_table_iter_next(&iterator, (gpointer*) &word, NULL)) {
        if (!g_hash_table_lookup_extended(set_b, word, NULL, NULL)) {
            g_hash_table_iter_remove(&iterator);
        }
    }
}

gint compare_caseless(const gchar** a, const gchar** b)
{
    gint result;
    gchar *tmp_a, *tmp_b;

    tmp_a = g_utf8_casefold(*a, -1);
    tmp_b = g_utf8_casefold(*b, -1);
    result = g_strcmp0(tmp_a, tmp_b);

    g_free(tmp_a);
    g_free(tmp_b);
    return result;
}

void print_result(GHashTable *set)
{
    GPtrArray *words;
    GHashTableIter iterator;
    gchar *word, *line;

    words = g_ptr_array_sized_new(g_hash_table_size(set) + 1);

    g_hash_table_iter_init(&iterator, set);
    while (g_hash_table_iter_next(&iterator, (gpointer*) &word, NULL)) {
        g_ptr_array_add(words, word);
    }
    g_ptr_array_sort(words, (GCompareFunc) compare_caseless);
    g_ptr_array_add(words, NULL);
    line = g_strjoinv(", ", (char**) words->pdata);
    g_print("%s\n", line);

    g_free(line);
    g_ptr_array_unref(words);
}

int main(gint argc, gchar **argv)
{
    gint i;
    GHashTable *result, *word_set;
    GError *error = NULL;

    if (argc == 1) return 0;

    if ((result = read_word_file(argv[1], &error)) == NULL) {
        g_print("Problem reading '%s': %s\n", argv[1], error->message);
        g_error_free(error);
        return 1;
    }

    for (i = 2; i < argc; i++) {
        if ((word_set = read_word_file(argv[i], &error)) == NULL) {
            g_print("Problem reading '%s': %s\n", argv[i], error->message);
            g_error_free(error);
            g_hash_table_unref(result);
            return 1;
        } else {
            intersection(result, word_set);
            g_hash_table_unref(word_set);
        }
    }

    print_result(result);

    g_hash_table_unref(result);
    return 0;
}