private Map<ServerName, List<RegionInfo>>
    getRegionAssignmentsByServer(Collection<RegionInfo> regions) {
    return provider != null
      ? new HashMap<>(provider.getSnapShotOfAssignment(regions))
      : new HashMap<>();
  }
Round-robin a list of regions to a list of servers
hbase-balancer/src/main/java/org/apache/hadoop/hbase/master/balancer/BaseLoadBalancer.java