From 558bc429ec308dd9cac314810d1b2219b4f85863 Mon Sep 17 00:00:00 2001
From: Saleem Rashid <trezor@saleemrashid.com>
Date: Wed, 26 Jul 2017 11:32:48 +0100
Subject: [PATCH] bignum: Add bn_digitcount

---
 bignum.h     | 11 +++++++++++
 test_check.c | 31 +++++++++++++++++++++++++++++++
 2 files changed, 42 insertions(+)

diff --git a/bignum.h b/bignum.h
index 8e23ab9760..7ba6204eed 100644
--- a/bignum.h
+++ b/bignum.h
@@ -81,6 +81,17 @@ static inline void bn_copy(const bignum256 *a, bignum256 *b) {
 
 int bn_bitcount(const bignum256 *a);
 
+static inline int bn_digitcount(const bignum256 *a)
+{
+	int bitcount = bn_bitcount(a);
+
+	if (bitcount == 256) {
+		return 78;
+	} else {
+		return bitcount * 78 / 256 + 1;
+	}
+}
+
 void bn_zero(bignum256 *a);
 
 int bn_is_zero(const bignum256 *a);
diff --git a/test_check.c b/test_check.c
index 9dde969263..e81ddba156 100644
--- a/test_check.c
+++ b/test_check.c
@@ -350,6 +350,36 @@ START_TEST(test_bignum_bitcount)
 }
 END_TEST
 
+START_TEST(test_bignum_digitcount)
+{
+	bignum256 a;
+
+	bn_zero(&a);
+	ck_assert_int_eq(bn_digitcount(&a), 1);
+
+	bn_one(&a);
+	ck_assert_int_eq(bn_digitcount(&a), 1);
+
+	bn_read_uint32(10, &a);
+	ck_assert_int_eq(bn_digitcount(&a), 2);
+
+	bn_read_uint32(11, &a);
+	ck_assert_int_eq(bn_digitcount(&a), 2);
+
+	bn_read_uint32(100, &a);
+	ck_assert_int_eq(bn_digitcount(&a), 3);
+
+	bn_read_uint32(0x3fffffff, &a);
+	ck_assert_int_eq(bn_digitcount(&a), 10);
+
+	bn_read_uint32(0xffffffff, &a);
+	ck_assert_int_eq(bn_digitcount(&a), 10);
+
+	bn_read_be(fromhex("ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff"), &a);
+	ck_assert_int_eq(bn_digitcount(&a), 78);
+}
+END_TEST
+
 START_TEST(test_bignum_is_less)
 {
 	bignum256 a;
@@ -3193,6 +3223,7 @@ Suite *test_suite(void)
 	tcase_add_test(tc, test_bignum_is_even);
 	tcase_add_test(tc, test_bignum_is_odd);
 	tcase_add_test(tc, test_bignum_bitcount);
+	tcase_add_test(tc, test_bignum_digitcount);
 	tcase_add_test(tc, test_bignum_is_less);
 	tcase_add_test(tc, test_bignum_format);
 	suite_add_tcase(s, tc);