Add cache for first entry hash in HashMaps

Author: mikera

convex-core/src/main/java/convex/core/data/AHashMap.java

@@ -175,6 +175,12 @@ public AVector<K> getKeys() {
 		return Vectors.wrap(keys);
 	}
 
+	/**
+	 * Gets the Hash for the first entry. Useful for prefix comparisons etc.
+	 * @return
+	 */
+	protected abstract Hash getFirstHash();
+	
 	@Override
 	public HashSet<Entry<K, V>> entrySet() {
 		int len = size();

convex-core/src/main/java/convex/core/data/MapLeaf.java

@@ -758,4 +758,9 @@ public MapLeaf<K, V> slice(long start, long end) {
 		return new MapLeaf<K,V>(nrefs);
 	}
 
+	@Override
+	protected Hash getFirstHash() {
+		return entries[0].getKeyHash();
+	}
+
 }

convex-core/src/main/java/convex/core/data/MapTree.java

@@ -933,5 +933,14 @@ public boolean isCVMValue() {
 		return true;
 	}
 
+	// Cache of first hash, we don't want to descend tree repeatedly to find this
+	private Hash firstHash;
+	
+	@Override
+	protected Hash getFirstHash() {
+		if (firstHash==null) firstHash=children[0].getValue().getFirstHash();
+		return firstHash;
+	}
+
 
 }