math.big: fix big.from_string #7313 (#7351)

vlib/math/big/big.v

@@ -93,7 +93,13 @@ pub fn from_u64(u u64) Number {
 }
 
 // Converts a hex string to big.Number.
-pub fn from_string(s string) Number {
+pub fn from_string(input string) Number {
+	mut s := input.trim_prefix('0x')
+	if s.len == 0 {
+		s = '0'
+	}
+	padding := '0'.repeat((8 - s.len % 8) % 8)
+	s = padding + s
 	n := Number{}
 	C.bignum_from_string(&n, s.str, s.len)
 	return n

vlib/math/big/big_test.v

@@ -77,6 +77,19 @@ fn test_mod() {
 	assert (big.from_u64(7) % big.from_u64(5)).int() == 2
 }
 
+fn test_from_str() {
+	assert big.from_string('').hexstr() == '0'
+	assert big.from_string('1').hexstr() == '1'
+	assert big.from_string('0').hexstr() == '0'
+	assert big.from_string('0x123').hexstr() == '123'
+	for i in 1 .. 33 {
+		input := 'e'.repeat(i)
+		out := big.from_string(input).hexstr()
+		assert input == out
+	}
+	assert big.from_string('0').hexstr() == '0'
+}
+
 fn test_str() {
 	assert big.from_u64(255).str() == '255'
 	assert big.from_u64(127).str() == '127'