add bn_divmod1000 including unit test

bignum.c

@@ -804,6 +804,30 @@ void bn_divmod58(bignum256 *a, uint32_t *r)
 	*r = rem;
 }
 
+// a / 1000 = a (+r)
+void bn_divmod1000(bignum256 *a, uint32_t *r)
+{
+	int i;
+	uint32_t rem, tmp;
+	rem = a->val[8] % 1000;
+	a->val[8] /= 1000;
+	for (i = 7; i >= 0; i--) {
+		// invariants:
+		//   rem = old(a) >> 30(i+1) % 1000
+		//   a[i+1..8] = old(a[i+1..8])/1000
+		//   a[0..i]   = old(a[0..i])
+		// 2^30 == 1073741*1000 + 824
+		tmp = rem * 824 + a->val[i];
+		// set a[i] = (rem * 2^30 + a[i])/1000
+		//          = rem * 1073741 + (rem * 824 + a[i])/1000
+		a->val[i] = rem * 1073741 + (tmp / 1000);
+		// set rem = (rem * 2^30 + a[i]) mod 1000
+		//         = (rem * 824 + a[i]) mod 1000
+		rem = tmp % 1000;
+	}
+	*r = rem;
+}
+
 #if USE_BN_PRINT
 void bn_print(const bignum256 *a)
 {

bignum.h

@@ -87,6 +87,8 @@ void bn_subtract(const bignum256 *a, const bignum256 *b, bignum256 *res);
 
 void bn_divmod58(bignum256 *a, uint32_t *r);
 
+void bn_divmod1000(bignum256 *a, uint32_t *r);
+
 #if USE_BN_PRINT
 void bn_print(const bignum256 *a);
 void bn_print_raw(const bignum256 *a);

tests.c

@@ -161,6 +161,35 @@ START_TEST(test_base58)
 }
 END_TEST
 
+START_TEST(test_bignum_divmod)
+{
+	uint32_t r;
+	int i;
+
+	bignum256 a = { { 0x3fffffff, 0x3fffffff, 0x3fffffff, 0x3fffffff, 0x3fffffff, 0x3fffffff, 0x3fffffff, 0x3fffffff, 0xffff} };
+	uint32_t ar[] = { 15, 14, 55, 29, 44, 24, 53, 49, 18, 55, 2, 28, 5, 4, 12, 43, 18, 37, 28, 14, 30, 46, 12, 11, 17, 10, 10, 13, 24, 45, 4, 33, 44, 42, 2, 46, 34, 43, 45, 28, 21, 18, 13, 17 };
+
+	i = 0;
+	while (!bn_is_zero(&a) && i < 44) {
+		bn_divmod58(&a, &r);
+		ck_assert_int_eq(r, ar[i]);
+		i++;
+	}
+	ck_assert_int_eq(i, 44);
+
+	bignum256 b = { { 0x3fffffff, 0x3fffffff, 0x3fffffff, 0x3fffffff, 0x3fffffff, 0x3fffffff, 0x3fffffff, 0x3fffffff, 0xffff} };
+	uint32_t br[] = { 935, 639, 129, 913, 7, 584, 457, 39, 564, 640, 665, 984, 269, 853, 907, 687, 8, 985, 570, 423, 195, 316, 237, 89, 792, 115 };
+
+	i = 0;
+	while (!bn_is_zero(&b) && i < 26) {
+		bn_divmod1000(&b, &r);
+		ck_assert_int_eq(r, br[i]);
+		i++;
+	}
+	ck_assert_int_eq(i, 26);
+
+}
+END_TEST
 
 // test vector 1 from https://en.bitcoin.it/wiki/BIP_0032_TestVectors
 START_TEST(test_bip32_vector_1)
@@ -1981,6 +2010,10 @@ Suite *test_suite(void)
 	tcase_add_test(tc, test_base58);
 	suite_add_tcase(s, tc);
 
+	tc = tcase_create("bignum_divmod");
+	tcase_add_test(tc, test_bignum_divmod);
+	suite_add_tcase(s, tc);
+
 	tc = tcase_create("bip32");
 	tcase_add_test(tc, test_bip32_vector_1);
 	tcase_add_test(tc, test_bip32_vector_2);