pkg/dev_compiler/lib/src/compiler/module_containers.dart

// Copyright (c) 2020, the Dart project authors.  Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

// @dart = 2.9

import '../compiler/js_names.dart' as js_ast;
import '../js_ast/js_ast.dart' as js_ast;
import '../js_ast/js_ast.dart' show js;

/// Defines how to emit a value of a table
typedef _EmitValue<K> = js_ast.Expression Function(K, ModuleItemData);

/// Represents a top-level property hoisted to a top-level object.
class ModuleItemData {
  /// The container that holds this module item in the emitted JS.
  js_ast.Identifier id;

  /// This module item's key in the emitted JS.
  ///
  /// A LiteralString if this object is backed by a JS Object/Map.
  /// A LiteralNumber if this object is backed by a JS Array.
  js_ast.Literal jsKey;

  /// This module item's value in the emitted JS.
  js_ast.Expression jsValue;

  ModuleItemData(this.id, this.jsKey, this.jsValue);
}

/// Holds variables emitted during code gen.
///
/// Associates a [K] with a container-unique JS key and arbitrary JS value.
/// The container is emitted as a single object:
/// ```
/// var C = {
///   jsKey: jsValue,
///   ...
/// };
/// ```
abstract class ModuleItemContainer<K> {
  /// Name of the container in the emitted JS.
  String name;

  /// Refers to the latest container if this container is sharded.
  js_ast.Identifier containerId;

  final Map<K, ModuleItemData> moduleItems = {};

  /// Incremental mode used for expression compilation
  bool _incrementalMode = false;

  /// Items accessed during incremental mode
  final Set<K> incrementalModuleItems = {};

  /// Indicates if this table is being used in an incremental context.
  ///
  /// Used during expression evaluation.
  /// Set by `emitFunctionIncremental` in kernel/compiler.dart.
  bool get incrementalMode => _incrementalMode;

  /// Sets the container to incremental mode.
  ///
  /// Used during expression evaluating so only referenced items
  /// will be emitted in a generated function.
  ///
  /// Note: the container cannot revert to non-incremental mode.
  void setIncrementalMode() {
    incrementalModuleItems.clear();
    _incrementalMode = true;
  }

  /// Holds keys that will not be emitted when calling [emit].
  final Set<K> _noEmit = {};

  /// Creates a container with a name, ID
  ModuleItemContainer._(this.name, this.containerId);

  /// Creates an automatically sharding container backed by JS Objects.
  factory ModuleItemContainer.asObject(String name,
      {String Function(K) keyToString}) {
    return ModuleItemObjectContainer<K>(name, keyToString);
  }

  /// Creates a container backed by a JS Array.
  factory ModuleItemContainer.asArray(String name) {
    return ModuleItemArrayContainer<K>(name);
  }

  bool get isNotEmpty => moduleItems.isNotEmpty;

  Iterable<K> get keys => moduleItems.keys;

  int get length => moduleItems.keys.length;

  bool get isEmpty => moduleItems.isEmpty;

  js_ast.Expression operator [](K key) => moduleItems[key]?.jsValue;

  void operator []=(K key, js_ast.Expression value);

  /// Returns the expression that retrieves [key]'s corresponding JS value via
  /// a property access through its container.
  js_ast.Expression access(K key);

  bool contains(K key) => moduleItems.containsKey(key);

  bool canEmit(K key) => !_noEmit.contains(key);

  /// Indicates that [K] should be treated as if it weren't hoisted.
  ///
  /// Used when we are managing the variable declarations manually (such as
  /// unhoisting specific symbols for performance reasons).
  void setNoEmit(K key) {
    _noEmit.add(key);
  }

  void setEmitIfIncremental(K key) {
    if (incrementalMode) {
      incrementalModuleItems.add(key);
    }
  }

  /// Emit the container declaration/initializer, using multiple statements if
  /// necessary.
  ///
  /// Uses [emitValue] to emit the values in the table.
  List<js_ast.Statement> emit({_EmitValue<K> emitValue});
}

/// Associates a [K] with a container-unique JS key and arbitrary JS value.
///
/// Emitted as a series of JS Objects, splitting them into groups of 500 for
/// JS optimization purposes:
/// ```
/// var C = {
///   jsKey: jsValue,
///   ...
/// };
/// var C$1 = { ... };
/// ```
class ModuleItemObjectContainer<K> extends ModuleItemContainer<K> {
  /// Tracks how often JS emitted field names appear.
  ///
  /// [keyToString] may resolve multiple unique keys to the same JS string.
  /// When this occurs, the resolved JS string will automatically be renamed.
  final Map<String, int> _nameFrequencies = {};

  /// Transforms a [K] into a valid name for a JS object property key.
  ///
  /// Non-unique generated strings are automatically renamed.
  String Function(K) keyToString;

  ModuleItemObjectContainer(String name, this.keyToString)
      : super._(name, js_ast.TemporaryId(name));

  @override
  void operator []=(K key, js_ast.Expression value) {
    if (contains(key)) {
      moduleItems[key].jsValue = value;
      return;
    }
    // Create a unique name for K when emitted as a JS field.
    var fieldString = keyToString(key);
    _nameFrequencies.update(fieldString, (v) {
      fieldString += '\$${v + 1}';
      return v + 1;
    }, ifAbsent: () {
      // Avoid shadowing common JS properties.
      if (js_ast.objectProperties.contains(fieldString)) {
        fieldString += '\$';
      }
      return 0;
    });
    moduleItems[key] = ModuleItemData(
        containerId, js_ast.LiteralString("'$fieldString'"), value);
    if (length % 500 == 0) containerId = js_ast.TemporaryId(name);
  }

  @override
  js_ast.Expression access(K key) {
    var id = moduleItems[key].id;
    return js.call('#.#', [id, moduleItems[key].jsKey]);
  }

  @override
  List<js_ast.Statement> emit({_EmitValue<K> emitValue}) {
    var containersToProperties = <js_ast.Identifier, List<js_ast.Property>>{};
    moduleItems.forEach((k, v) {
      if (!incrementalMode && _noEmit.contains(k)) return;
      if (incrementalMode && !incrementalModuleItems.contains(k)) return;

      if (!containersToProperties.containsKey(v.id)) {
        containersToProperties[v.id] = <js_ast.Property>[];
      }
      containersToProperties[v.id].add(js_ast.Property(
          v.jsKey, emitValue == null ? v.jsValue : emitValue(k, v)));
    });

    if (containersToProperties.isEmpty) return [];

    var statements = <js_ast.Statement>[];
    containersToProperties.forEach((containerId, properties) {
      var containerObject = js_ast.ObjectInitializer(properties,
          multiline: properties.length > 1);
      statements.add(js.statement('var # = #', [containerId, containerObject]));
    });

    return statements;
  }
}

/// Associates a unique [K] with an arbitrary JS value.
///
/// Emitted as a JS Array:
/// ```
/// var C = [
///   jsValue,
///   ...
/// ];
/// ```
class ModuleItemArrayContainer<K> extends ModuleItemContainer<K> {
  ModuleItemArrayContainer(String name)
      : super._(name, js_ast.TemporaryId(name));

  @override
  void operator []=(K key, js_ast.Expression value) {
    if (moduleItems.containsKey(key)) {
      moduleItems[key].jsValue = value;
      return;
    }
    moduleItems[key] =
        ModuleItemData(containerId, js_ast.LiteralNumber('$length'), value);
  }

  @override
  js_ast.Expression access(K key) {
    var id = containerId;
    return js.call('#[#]', [id, moduleItems[key].jsKey]);
  }

  @override
  List<js_ast.Statement> emit({_EmitValue<K> emitValue}) {
    var properties = List<js_ast.Expression>.filled(length, null);

    // If the entire array holds just one value, generate a short initializer.
    var valueSet = <js_ast.Expression>{};
    moduleItems.forEach((k, v) {
      if (!incrementalMode && _noEmit.contains(k)) return;
      if (incrementalMode && !incrementalModuleItems.contains(k)) return;
      valueSet.add(v.jsValue);
      properties[int.parse((v.jsKey as js_ast.LiteralNumber).value)] =
          emitValue == null ? v.jsValue : emitValue(k, v);
    });

    if (valueSet.isEmpty) return [];

    if (valueSet.length == 1 && moduleItems.length > 1) {
      return [
        js.statement('var # = Array(#).fill(#)', [
          containerId,
          js_ast.LiteralNumber('${properties.length}'),
          valueSet.first
        ])
      ];
    }
    // Array containers are not sharded, as we do not expect to hit V8's
    // dictionary-mode limit of 99999 elements.
    return [
      js.statement('var # = #', [
        containerId,
        js_ast.ArrayInitializer(properties, multiline: properties.length > 1)
      ])
    ];
  }
}