src/main/java/com/google/devtools/build/lib/packages/SkylarkClassObject.java

// Copyright 2016 The Bazel Authors. All rights reserved.
//
// Licensed 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
//
//    http://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 com.google.devtools.build.lib.packages;

import com.google.common.base.Joiner;
import com.google.common.collect.ImmutableCollection;
import com.google.common.collect.ImmutableMap;
import com.google.common.collect.Ordering;
import com.google.common.collect.Sets;
import com.google.common.collect.Sets.SetView;
import com.google.devtools.build.lib.events.Location;
import com.google.devtools.build.lib.skylarkinterface.SkylarkModule;
import com.google.devtools.build.lib.skylarkinterface.SkylarkModuleCategory;
import com.google.devtools.build.lib.skylarkinterface.SkylarkValue;
import com.google.devtools.build.lib.syntax.ClassObject;
import com.google.devtools.build.lib.syntax.Concatable;
import com.google.devtools.build.lib.syntax.EvalException;
import com.google.devtools.build.lib.syntax.EvalUtils;
import com.google.devtools.build.lib.syntax.Printer;
import com.google.devtools.build.lib.syntax.SkylarkType;
import com.google.devtools.build.lib.util.Preconditions;
import java.io.Serializable;
import java.util.Map;

/** An implementation class of ClassObject for structs created in Skylark code. */
@SkylarkModule(
  name = "struct",
  category = SkylarkModuleCategory.BUILTIN,
  doc =
      "A special language element to support structs (i.e. simple value objects). "
          + "See the global <a href=\"globals.html#struct\">struct</a> function "
          + "for more details."
)
public class SkylarkClassObject implements ClassObject, SkylarkValue, Concatable, Serializable {
  /** Error message to use when errorMessage argument is null. */
  private static final String DEFAULT_ERROR_MESSAGE = "'struct' object has no attribute '%s'";

  private final SkylarkClassObjectConstructor constructor;
  private final ImmutableMap<String, Object> values;
  private final Location creationLoc;
  private final String errorMessage;

  /**
   * Primarily for testing purposes where no location is available and the default
   * errorMessage suffices.
   */
  public SkylarkClassObject(SkylarkClassObjectConstructor constructor,
      Map<String, Object> values) {
    this.constructor = constructor;
    this.values = copyValues(values);
    this.creationLoc = null;
    this.errorMessage = DEFAULT_ERROR_MESSAGE;
  }

  /**
   * Creates a built-in struct (i.e. without creation loc). The errorMessage has to have
   * exactly one '%s' parameter to substitute the struct field name.
   */
  public SkylarkClassObject(SkylarkClassObjectConstructor constructor,
      Map<String, Object> values, String errorMessage) {
    this.constructor = constructor;
    this.values = copyValues(values);
    this.creationLoc = null;
    this.errorMessage = Preconditions.checkNotNull(errorMessage);
  }

  public SkylarkClassObject(SkylarkClassObjectConstructor constructor,
      Map<String, Object> values, Location creationLoc) {
    this.constructor = constructor;
    this.values = copyValues(values);
    this.creationLoc = Preconditions.checkNotNull(creationLoc);
    this.errorMessage = DEFAULT_ERROR_MESSAGE;
  }

  // Ensure that values are all acceptable to Skylark before to stuff them in a ClassObject
  private ImmutableMap<String, Object> copyValues(Map<String, Object> values) {
    ImmutableMap.Builder<String, Object> builder = ImmutableMap.builder();
    for (Map.Entry<String, Object> e : values.entrySet()) {
      builder.put(e.getKey(), SkylarkType.convertToSkylark(e.getValue(), null));
    }
    return builder.build();
  }

  @Override
  public Object getValue(String name) {
    return values.get(name);
  }

  /**
   *  Returns a value and try to cast it into specified type
   */
  public <TYPE> TYPE getValue(String key, Class<TYPE> type) throws EvalException {
    Object obj = values.get(key);
    if (obj == null) {
      return null;
    }
    SkylarkType.checkType(obj, type, key);
    return type.cast(obj);
  }

  @Override
  public ImmutableCollection<String> getKeys() {
    return values.keySet();
  }

  public Location getCreationLoc() {
    return Preconditions.checkNotNull(creationLoc,
        "This struct was not created in a Skylark code");
  }

  @Override
  public Concatter getConcatter() {
    return StructConcatter.INSTANCE;
  }
  
  public SkylarkClassObjectConstructor getConstructor() {
    return constructor;
  }

  private static class StructConcatter implements Concatter {
    private static final StructConcatter INSTANCE = new StructConcatter();

    private StructConcatter() {}

    @Override
    public SkylarkClassObject concat(
        Concatable left, Concatable right, Location loc) throws EvalException {
      SkylarkClassObject lval = (SkylarkClassObject) left;
      SkylarkClassObject rval = (SkylarkClassObject) right;
      if (!lval.constructor.equals(rval.constructor)) {
        throw new EvalException(loc,
            String.format("Cannot concat %s with %s",
                lval.constructor.getPrintableName(),
                rval.constructor.getPrintableName()));
      }
      SetView<String> commonFields = Sets
          .intersection(lval.values.keySet(), rval.values.keySet());
      if (!commonFields.isEmpty()) {
        throw new EvalException(loc, "Cannot concat structs with common field(s): "
            + Joiner.on(",").join(commonFields));
      }
      return new SkylarkClassObject(lval.constructor,
          ImmutableMap.<String, Object>builder()
              .putAll(lval.values)
              .putAll(rval.values)
              .build(),
          loc);
    }
  }

  @Override
  public String errorMessage(String name) {
    String suffix =
        "Available attributes: "
            + Joiner.on(", ").join(Ordering.natural().sortedCopy(values.keySet()));
    return String.format(errorMessage, name) + "\n" + suffix;
  }

  @Override
  public boolean isImmutable() {
    for (Object item : values.values()) {
      if (!EvalUtils.isImmutable(item)) {
        return false;
      }
    }
    return true;
  }

  /**
   * Convert the object to string using Skylark syntax. The output tries to be
   * reversible (but there is no guarantee, it depends on the actual values).
   */
  @Override
  public void write(Appendable buffer, char quotationMark) {
    boolean first = true;
    Printer.append(buffer, constructor.getPrintableName());
    Printer.append(buffer, "(");
    // Sort by key to ensure deterministic output.
    for (String key : Ordering.natural().sortedCopy(values.keySet())) {
      if (!first) {
        Printer.append(buffer, ", ");
      }
      first = false;
      Printer.append(buffer, key);
      Printer.append(buffer, " = ");
      Printer.write(buffer, values.get(key), quotationMark);
    }
    Printer.append(buffer, ")");
  }

  @Override
  public String toString() {
    return Printer.repr(this);
  }
}