Swap bit-endianness of value in note commitment.

src/util.rs

@@ -0,0 +1,27 @@
+pub fn swap_bits_u64(x: u64) -> u64
+{
+    let mut tmp = 0;
+    for i in 0..64 {
+        tmp |= ((x >> i) & 1) << (63 - i);
+    }
+    tmp
+}
+
+#[test]
+fn test_swap_bits_u64() {
+    assert_eq!(swap_bits_u64(17182120934178543809), 0b1000001100011011110000011000111000101111111001001100111001110111);
+    assert_eq!(swap_bits_u64(15135675916470734665), 0b1001001011110010001101010010001110110000100111010011000001001011);
+    assert_eq!(swap_bits_u64(6724233301461108393),  0b1001010101100000100011100001010111110001011000101000101010111010);
+    assert_eq!(swap_bits_u64(206708183275952289),   0b1000010100011010001010100011101011111111111110100111101101000000);
+    assert_eq!(swap_bits_u64(12712751566144824320), 0b0000000000100110010110111000001110001100001000110011011000001101);
+
+    let mut a = 15863238721320035327u64;
+    for _ in 0..1000 {
+        a = a.wrapping_mul(a);
+
+        let swapped = swap_bits_u64(a);
+        let unswapped = swap_bits_u64(swapped);
+
+        assert_eq!(a, unswapped);
+    }
+}