content: /*
   * Licensed to the Apache Software Foundation (ASF) under one
   * or more contributor license agreements.  See the NOTICE file
   * distributed with this work for additional information
   * regarding copyright ownership.  The ASF licenses this file
   * to you under the Apache License, Version 2.0 (the
   * "License"); you may not use this file except in compliance
   * with the License.  You may obtain a copy of the License at
   *
   *     https://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS,
   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   * See the License for the specific language governing permissions and
   * limitations under the License.
   */
  package org.apache.avro.util;
  
  import org.apache.avro.Schema;
  import org.apache.avro.Schema.Field;
  
  import java.util.ArrayDeque;
  import java.util.Collections;
  import java.util.Deque;
  import java.util.IdentityHashMap;
  import java.util.function.Supplier;
  import java.util.stream.Collectors;
  
  /**
   * Avro Schema utilities, to traverse...
   */
  public final class Schemas {
  
    private Schemas() {
    }
  
    /**
     * Depth first visit.
     */
    public static <T> T visit(final Schema start, final SchemaVisitor<T> visitor) {
      // Set of Visited Schemas
      IdentityHashMap<Schema, Schema> visited = new IdentityHashMap<>();
      // Stack that contains the Schemas to process and afterVisitNonTerminal
      // functions.
      // Deque<Either<Schema, Supplier<SchemaVisitorAction>>>
      // Using Either<...> has a cost we want to avoid...
      Deque<Object> dq = new ArrayDeque<>();
      dq.push(start);
      Object current;
      while ((current = dq.poll()) != null) {
        if (current instanceof Supplier) {
          // We are executing a non-terminal post visit.
          SchemaVisitor.SchemaVisitorAction action = ((Supplier<SchemaVisitor.SchemaVisitorAction>) current).get();
          switch (action) {
          case CONTINUE:
            break;
          case SKIP_SIBLINGS:
            while (dq.peek() instanceof Schema) {
              dq.remove();
            }
            break;
          case TERMINATE:
            return visitor.get();
          case SKIP_SUBTREE:
          default:
            throw new UnsupportedOperationException("Invalid action " + action);
          }
        } else {
          Schema schema = (Schema) current;
          boolean terminate;
          if (visited.containsKey(schema)) {
            terminate = visitTerminal(visitor, schema, dq);
          } else {
            Schema.Type type = schema.getType();
            switch (type) {
            case ARRAY:
              terminate = visitNonTerminal(visitor, schema, dq, Collections.singleton(schema.getElementType()));
              visited.put(schema, schema);
              break;
            case RECORD:
              terminate = visitNonTerminal(visitor, schema, dq, () -> schema.getFields().stream().map(Field::schema)
                  .collect(Collectors.toCollection(ArrayDeque::new)).descendingIterator());
              visited.put(schema, schema);
              break;
            case UNION:
              terminate = visitNonTerminal(visitor, schema, dq, schema.getTypes());
              visited.put(schema, schema);
              break;
            case MAP:
              terminate = visitNonTerminal(visitor, schema, dq, Collections.singleton(schema.getValueType()));
              visited.put(schema, schema);
              break;
            default:
              terminate = visitTerminal(visitor, schema, dq);
              break;
            }
          }
          if (terminate) {
            return visitor.get();
          }
        }
      }
      return visitor.get();
    }
  
    private static boolean visitNonTerminal(final SchemaVisitor<?> visitor, final Schema schema, final Deque<Object> dq,
        final Iterable<Schema> itSupp) {
      SchemaVisitor.SchemaVisitorAction action = visitor.visitNonTerminal(schema);
      switch (action) {
      case CONTINUE:
        dq.push((Supplier<SchemaVisitor.SchemaVisitorAction>) () -> visitor.afterVisitNonTerminal(schema));
        itSupp.forEach(dq::push);
        break;
      case SKIP_SUBTREE:
        dq.push((Supplier<SchemaVisitor.SchemaVisitorAction>) () -> visitor.afterVisitNonTerminal(schema));
        break;
      case SKIP_SIBLINGS:
        while (dq.peek() instanceof Schema) {
          dq.remove();
        }
        break;
      case TERMINATE:
        return true;
      default:
        throw new UnsupportedOperationException("Invalid action " + action + " for " + schema);
      }
      return false;
    }
  
    private static boolean visitTerminal(final SchemaVisitor<?> visitor, final Schema schema, final Deque<Object> dq) {
      SchemaVisitor.SchemaVisitorAction action = visitor.visitTerminal(schema);
      switch (action) {
      case CONTINUE:
        break;
      case SKIP_SIBLINGS:
        while (dq.peek() instanceof Schema) {
          dq.remove();
        }
        break;
      case TERMINATE:
        return true;
      case SKIP_SUBTREE:
      default:
        throw new UnsupportedOperationException("Invalid action " + action + " for " + schema);
      }
      return false;
    }
  }
  