presto-main/src/main/java/io/prestosql/operator/window/WindowPartition.java/WindowPartition/updatePeerGroup
class WindowPartition:    private void updatePeerGroup()
    {
        peerGroupStart = currentPosition;
        // find end of peer group
        peerGroupEnd = peerGroupStart + 1;
        while ((peerGroupEnd < partitionEnd) && pagesIndex.positionEqualsPosition(peerGroupHashStrategy, peerGroupStart, peerGroupEnd)) {
            peerGroupEnd++;
        }
    }