LUCENE-8796: Use exponential search in IntArrayDocIdSetIterator#advance (#667)

lucene/CHANGES.txt

@@ -103,6 +103,11 @@ Improvements
 * LUCENE-8845: Allow Intervals.prefix() and Intervals.wildcard() to specify
   their maximum allowed expansions (Alan Woodward)
 
+Optimizations
+
+* LUCENE-8796: Use exponential search instead of binary search in
+  IntArrayDocIdSet#advance method (Luca Cavanna via Adrien Grand)
+
 Test Framework
 
 * LUCENE-8825: CheckHits now display the shard index in case of mismatch

lucene/core/src/java/org/apache/lucene/util/IntArrayDocIdSet.java

@@ -52,7 +52,7 @@ final class IntArrayDocIdSet extends DocIdSet {
 
     private final int[] docs;
     private final int length;
-    private int i = -1;
+    private int i = 0;
     private int doc = -1;
 
     IntArrayDocIdSetIterator(int[] docs, int length) {
@@ -67,16 +67,21 @@ final class IntArrayDocIdSet extends DocIdSet {
 
     @Override
     public int nextDoc() throws IOException {
-      return doc = docs[++i];
+      return doc = docs[i++];
     }
 
     @Override
     public int advance(int target) throws IOException {
-      i = Arrays.binarySearch(docs, i + 1, length, target);
+      int bound = 1;
+      //given that we use this for small arrays only, this is very unlikely to overflow
+      while(i + bound < length && docs[i + bound] < target) {
+        bound *= 2;
+      }
+      i = Arrays.binarySearch(docs, i + bound / 2, Math.min(i + bound + 1, length), target);
       if (i < 0) {
         i = -1 - i;
       }
-      return doc = docs[i];
+      return doc = docs[i++];
     }
 
     @Override