bignum: Fix bn_digitcount

bn_digitcount used to use bn_bitcount. This would give the maximum
digits, which would often be higher than the actual number. This would
result in leading zeroes in bn_format.

bignum.c

@@ -195,6 +195,31 @@ int bn_bitcount(const bignum256 *a)
 	return 0;
 }
 
+#define DIGITS 78 // log10(2 ^ 256)
+
+unsigned int bn_digitcount(const bignum256 *a)
+{
+	bignum256 val;
+	memcpy(&val, a, sizeof(bignum256));
+
+	unsigned int digits = 1;
+
+	for (unsigned int i = 0; i < DIGITS; i += 3) {
+		uint32_t limb;
+		bn_divmod1000(&val, &limb);
+
+		if (limb >= 100) {
+			digits = i + 3;
+		} else if (limb >= 10) {
+			digits = i + 2;
+		} else if (limb >= 1) {
+			digits = i + 1;
+		}
+	}
+
+	return digits;
+}
+
 // sets a bignum to zero.
 void bn_zero(bignum256 *a)
 {

bignum.h

@@ -83,16 +83,7 @@ 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;
-	}
-}
+unsigned int bn_digitcount(const bignum256 *a);
 
 void bn_zero(bignum256 *a);