Back out "Always use first syncmer as base"

This backs out commit 9fa9d41.

Author: marcelm

src/randstrobes.cpp

@@ -49,7 +49,8 @@ static inline syncmer_hash_t syncmer_smer_hash(uint64_t packed) {
  * This function combines two individual syncmer hashes into a single hash
  * for the randstrobe.
  *
- * One syncmer is designated as the "main", the other is the "auxiliary".
+ * The syncmer with the smaller hash is designated as the "main", the other is
+ * the "auxiliary".
  * The combined hash is obtained by setting the top bits to the bits of
  * the main hash and the bottom bits to the bits of the auxiliary
  * hash. Since entries in the index are sorted by randstrobe hash, this allows
@@ -58,6 +59,10 @@ static inline syncmer_hash_t syncmer_smer_hash(uint64_t packed) {
 static inline randstrobe_hash_t randstrobe_hash(
     syncmer_hash_t hash1, syncmer_hash_t hash2, randstrobe_hash_t main_hash_mask
 ) {
+    // Make the function symmetric
+    if (hash1 > hash2) {
+        std::swap(hash1, hash2);
+    }
     return ((hash1 & main_hash_mask) | (hash2 & ~main_hash_mask)) & RANDSTROBE_HASH_MASK;
 }
 
@@ -156,11 +161,12 @@ std::ostream& operator<<(std::ostream& os, const QueryRandstrobe& randstrobe) {
 }
 
 Randstrobe make_randstrobe(Syncmer strobe1, Syncmer strobe2, randstrobe_hash_t main_hash_mask) {
+    bool first_strobe_is_main = strobe1.hash < strobe2.hash;
     return Randstrobe{
         randstrobe_hash(strobe1.hash, strobe2.hash, main_hash_mask),
         static_cast<uint32_t>(strobe1.position),
         static_cast<uint32_t>(strobe2.position),
-        true
+        first_strobe_is_main
     };
 }