+/* returns 0 if IN is not an integer. OUT must already be
+ initialized. */
+static int
+coerce_to_big (SCM in, mpz_t out)
+{
+ if (SCM_BIGP (in))
+ mpz_set (out, SCM_I_BIG_MPZ (in));
+ else if (SCM_I_INUMP (in))
+ mpz_set_si (out, SCM_I_INUM (in));
+ else
+ return 0;
+
+ return 1;
+}
+
+SCM_DEFINE (scm_modulo_expt, "modulo-expt", 3, 0, 0,
+ (SCM n, SCM k, SCM m),
+ "Return @var{n} raised to the integer exponent\n"
+ "@var{k}, modulo @var{m}.\n"
+ "\n"
+ "@lisp\n"
+ "(modulo-expt 2 3 5)\n"
+ " @result{} 3\n"
+ "@end lisp")
+#define FUNC_NAME s_scm_modulo_expt
+{
+ mpz_t n_tmp;
+ mpz_t k_tmp;
+ mpz_t m_tmp;
+
+ /* There are two classes of error we might encounter --
+ 1) Math errors, which we'll report by calling scm_num_overflow,
+ and
+ 2) wrong-type errors, which of course we'll report by calling
+ SCM_WRONG_TYPE_ARG.
+ We don't report those errors immediately, however; instead we do
+ some cleanup first. These variables tell us which error (if
+ any) we should report after cleaning up.
+ */
+ int report_overflow = 0;
+
+ int position_of_wrong_type = 0;
+ SCM value_of_wrong_type = SCM_INUM0;
+
+ SCM result = SCM_UNDEFINED;
+
+ mpz_init (n_tmp);
+ mpz_init (k_tmp);
+ mpz_init (m_tmp);
+
+ if (scm_is_eq (m, SCM_INUM0))
+ {
+ report_overflow = 1;
+ goto cleanup;
+ }
+
+ if (!coerce_to_big (n, n_tmp))
+ {
+ value_of_wrong_type = n;
+ position_of_wrong_type = 1;
+ goto cleanup;
+ }
+
+ if (!coerce_to_big (k, k_tmp))
+ {
+ value_of_wrong_type = k;
+ position_of_wrong_type = 2;
+ goto cleanup;
+ }
+
+ if (!coerce_to_big (m, m_tmp))
+ {
+ value_of_wrong_type = m;
+ position_of_wrong_type = 3;
+ goto cleanup;
+ }
+
+ /* if the exponent K is negative, and we simply call mpz_powm, we
+ will get a divide-by-zero exception when an inverse 1/n mod m
+ doesn't exist (or is not unique). Since exceptions are hard to
+ handle, we'll attempt the inversion "by hand" -- that way, we get
+ a simple failure code, which is easy to handle. */
+
+ if (-1 == mpz_sgn (k_tmp))
+ {
+ if (!mpz_invert (n_tmp, n_tmp, m_tmp))
+ {
+ report_overflow = 1;
+ goto cleanup;
+ }
+ mpz_neg (k_tmp, k_tmp);
+ }
+
+ result = scm_i_mkbig ();
+ mpz_powm (SCM_I_BIG_MPZ (result),
+ n_tmp,
+ k_tmp,
+ m_tmp);
+
+ if (mpz_sgn (m_tmp) < 0 && mpz_sgn (SCM_I_BIG_MPZ (result)) != 0)
+ mpz_add (SCM_I_BIG_MPZ (result), SCM_I_BIG_MPZ (result), m_tmp);
+
+ cleanup:
+ mpz_clear (m_tmp);
+ mpz_clear (k_tmp);
+ mpz_clear (n_tmp);
+
+ if (report_overflow)
+ scm_num_overflow (FUNC_NAME);
+
+ if (position_of_wrong_type)
+ SCM_WRONG_TYPE_ARG (position_of_wrong_type,
+ value_of_wrong_type);
+
+ return scm_i_normbig (result);
+}
+#undef FUNC_NAME
+