Comments on hash_combine.

Author: evoskuil

include/bitcoin/system/impl/hash/functions.ipp

@@ -237,30 +237,10 @@ INLINE constexpr size_t djb2_hash(const data_slice& data) NOEXCEPT
 }
 
 // Combine hash values, such as djb2_hash or unique_hash outputs.
+// Right should always be the lower entropy value (e.g. point index).
 INLINE constexpr size_t hash_combine(size_t left, size_t right) NOEXCEPT
 {
-    //// This leads to mixing null points (850k identical) with other buckets.
-    ////constexpr auto prime1 = possible_narrow_cast<size_t>(0x9e3779b97f4a7c15_u64);
-    ////constexpr auto prime2 = possible_narrow_cast<size_t>(0x517cc1b727220a95_u64);
-    ////
-    ////auto first = left;
-    ////first ^= shift_right(first, 23);
-    ////first *= prime1;
-    ////first ^= shift_right(first, 19);
-    ////
-    ////auto second = right;
-    ////second ^= shift_right(second, 13);
-    ////second *= prime2;
-    ////second ^= shift_right(second, 31);
-    ////
-    ////// seed parameter, defaults to zero.
-    ////first ^= second;
-    ////first += seed;
-    ////first ^= shift_right(first, 17);
-    ////first *= prime1;
-    ////first ^= shift_right(first, 29);
-
-    ////return first;
+    // Shift ensures left == right does not always return zero.
     return bit_xor(left, shift_left(right));
 }