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General server-side data flow templates with Swim

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// Copyright 2015-2024 Nstream, inc.
//
// Licensed under the Redis Source Available License 2.0 (RSALv2) Agreement;
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     https://redis.com/legal/rsalv2-agreement/
//
// 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 nstream.adapter.common.relay;

import swim.api.agent.AgentContext;
import swim.structure.Expression;
import swim.structure.Item;
import swim.structure.Record;
import swim.structure.Value;
import swim.util.Log;

/**
 * {@code Directive} to keep only a subset of the input.
 *
 * This {@code Directive} offers an alternative syntax that does not utilize
 * a declarative {@code Attr}:
 * 
 {@code
 * @command($data) { # implicit Take here
 *   ...
 * }}
 */
public final class Take extends Directive {

  private final Directive child;

  public Take(Directive parent, AgentContext context, Log log, Item scope,
              Expression expression, Value childStructure) {
    super(parent, context, log, scope);
    final Item childScope = expression.evaluate(scope);
    final Record trimmed = exciseAttrValue(childStructure);
    this.child = DslInterpret.createDirective(this.context, log, trimmed, childScope, this);
  }

  @SuppressWarnings("unchecked")
  @Override
  public T evaluate() throws InterpretException {
    return (T) this.child.evaluate();
  }

  private static Record exciseAttrValue(Value childStructure) {
    final Record shallowClone = Record.create(childStructure.length());
    shallowClone.attr(childStructure.head().key().stringValue());
    shallowClone.addAll(childStructure.tail());
    return shallowClone;
  }

  @Override
  public String toString() {
    return "Take{"
        + "scope=" + this.scope + ", "
        + "child=" + this.child
        + '}';
  }
}