optimise prefix sums in Lucene104PostingsReader (#15175)

Author: RamakrishnaChilaka

lucene/core/src/java/org/apache/lucene/codecs/lucene104/Lucene104PostingsReader.java

@@ -190,10 +190,11 @@ public void init(IndexInput termsIn, SegmentReadState state) throws IOException
     }
   }
 
-  static void prefixSum(int[] buffer, int count, long base) {
-    buffer[0] += base;
-    for (int i = 1; i < count; ++i) {
-      buffer[i] += buffer[i - 1];
+  static void prefixSum(int[] buffer, int count, int base) {
+    int sum = base;
+    for (int i = 0; i < count; ++i) {
+      sum += buffer[i];
+      buffer[i] = sum;
     }
   }
 
@@ -593,11 +594,7 @@ private void refillFullBlock() throws IOException {
       if (bitsPerValue > 0) {
         // block is encoded as 256 packed integers that record the delta between doc IDs
         forUtil.decode(bitsPerValue, docInUtil, docBuffer);
-        int sum = prevDocID;
-        for (int i = 0; i < BLOCK_SIZE; ++i) {
-          sum += docBuffer[i];
-          docBuffer[i] = sum;
-        }
+        prefixSum(docBuffer, BLOCK_SIZE, prevDocID);
         encoding = DeltaEncoding.PACKED;
       } else {
         // block is encoded as a bit set
@@ -619,9 +616,7 @@ private void refillFullBlock() throws IOException {
           for (int i = 0; i < numLongs - 1; ++i) {
             docCumulativeWordPopCounts[i] = Long.bitCount(docBitSet.getBits()[i]);
           }
-          for (int i = 1; i < numLongs - 1; ++i) {
-            docCumulativeWordPopCounts[i] += docCumulativeWordPopCounts[i - 1];
-          }
+          prefixSum(docCumulativeWordPopCounts, numLongs - 1, 0);
           docCumulativeWordPopCounts[numLongs - 1] = BLOCK_SIZE;
           assert docCumulativeWordPopCounts[numLongs - 2]
                   + Long.bitCount(docBitSet.getBits()[numLongs - 1])