3 files changed, 31 insertions, 0 deletions
diff --git a/src/util/bits.c b/src/util/bits.c
index e5f28c1..6311ba8 100644
--- a/src/util/bits.c
+++ b/src/util/bits.c
@@ -49,6 +49,24 @@ bits_t *bits_from_i(GEN i) {
 	return result;
 }
 
+bits_t *bits_from_i_len(GEN i, size_t bit_len) {
+	pari_sp ltop = avma;
+	GEN bitvec = binary_zv(i);
+	size_t i_len = (size_t)glength(bitvec);
+	bits_t *result = bits_new(bit_len);
+	size_t offset = 0;
+	if (i_len < bit_len) {
+		offset = bit_len - i_len;
+	}
+	for (size_t j = 0; j < bit_len; ++j) {
+		if (j < i_len && gel(bitvec, j + 1) == (GEN)1) {
+			result->bits[(j + offset) / 8] |= 1 << (7 - ((j + offset) % 8));
+		}
+	}
+	avma = ltop;
+	return result;
+}
+
 bits_t *bits_from_hex(const char *hex_str) {
 	size_t nibble_len = strlen(hex_str);
 	bits_t *result = bits_new(nibble_len * 4);
diff --git a/src/util/bits.h b/src/util/bits.h
index 2766cc3..9b37539 100644
--- a/src/util/bits.h
+++ b/src/util/bits.h
@@ -35,6 +35,8 @@ void bits_free(bits_t **bits);
 
 bits_t *bits_from_i(GEN i);
 
+bits_t *bits_from_i_len(GEN i, size_t bitlen);
+
 bits_t *bits_from_hex(const char *hex_str);
 
 bits_t *bits_from_bin(const char *bin_str);
diff --git a/test/src/util/test_bits.c b/test/src/util/test_bits.c
index 70060c9..a7b38e3 100644
--- a/test/src/util/test_bits.c
+++ b/test/src/util/test_bits.c
@@ -48,6 +48,17 @@ Test(bits, test_bits_from_i) {
 	bits_free(&bits);
 }
 
+Test(bits, test_bits_from_i_len) {
+	GEN i = int2n(5);
+
+	bits_t *bits = bits_from_i_len(i, 7);
+	cr_assert_not_null(bits, );
+	cr_assert_eq(bits->bitlen, 7, );
+	cr_assert_eq(bits->allocated, 1, );
+	cr_assert_eq(bits->bits[0], 0b01000000, );
+	bits_free(&bits);
+}
+
 Test(bits, test_bits_from_hex) {
 	char *hex = "0ab";