Initial version of a serialization framework

pkg/serialization/lib/src/reader_writer.dart

+// Copyright (c) 2012, 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.
+
+part of serialization;
+
+/**
+ * This writes out the state of the objects to an external format. It holds
+ * all of the intermediate state needed. The primary API for it is the
+ * [write] method.
+ */
+// TODO(alanknight): For simple serialization formats this does a lot of work
+// that isn't necessary, e.g. detecting cycles and maintaining references.
+// Consider having an abstract superclass with the basic functionality and
+// simple serialization subclasses where we know there aren't cycles.
+class Writer {
+  /**
+   * The [serialization] holds onto the rules that define how objects
+   * are serialized.
+   */
+  final Serialization serialization;
+
+  /** The [trace] object keeps track of the objects to be visited while finding
+   * the full set of objects to be written.*/
+  Trace trace;
+
+  /**
+   * When we write out objects, should we also write out a description
+   * of the rules for the serialization. This defaults to the corresponding
+   * value on the Serialization.
+   */
+  bool selfDescribing;
+
+  /**
+   * Objects that cannot be represented in-place in the serialized form need
+   * to have references to them stored. The [Reference] objects are computed
+   * once and stored here for each object. This provides some space-saving,
+   * but also serves to record which objects we have already seen.
+   */
+  Map<Object, Reference> references = new Map<Object, Reference>();
+
+  /**
+   * The state of objects that need to be serialized is stored here.
+   * Each rule has a number, and rules keep track of the objects that they
+   * serialize, in order. So the state of any object can be found by indexing
+   * from the rule number and the object number within the rule.
+   * The actual representation of the state is determined by the rule. Lists
+   * and Maps are common, but it is arbitrary.
+   */
+  List<List> states = new List<List>();
+
+  /** Return the list of rules we use. */
+  List<SerializationRule> get rules => serialization.rules;
+
+  /**
+   * Creates a new [Writer] that uses the rules from its parent
+   * [Serialization]. Serializations are do not keep any state
+   * related to a particular read/write, so the same one can be used
+   * for multiple different Readers/Writers.
+   */
+  Writer(this.serialization) {
+    trace = new Trace(this);
+    selfDescribing = serialization.selfDescribing;
+  }
+
+  /**
+   * This is the main API for a [Writer]. It writes the objects and returns
+   * the serialized representation, currently a JSON format of a map
+   * whose data is either lists indexed by field position or maps indexed
+   * by field name, and holding either primitives or references. See [toMaps]
+   */
+  // TODO(alanknight): Generalize the output representation. Probably requires
+  // introducing some sort of OutputFormat object.
+  String write(anObject) {
+    trace.addRoot(anObject);
+    trace.traceAll();
+    flatten();
+    return toStringFormat();
+  }
+
+  /**
+   * This is an alternate writing API that writes the objects and returns
+   * the serialized representation as a List of simple objects.
+   * See [toFlatFormat].
+   */
+  List writeFlat(anObject) {
+    shouldUseReferencesForPrimitives = true;
+    trace.addRoot(anObject);
+    trace.traceAll();
+    flatten();
+    return toFlatFormat();
+  }
+
+  /**
+   * Write an incredibly cheesy flat format, just to verify that we can handle
+   * formats of that general type. Very rough right now. Ignores low-level
+   * issues like representing strings, and just produces a List that contains
+   * nulls, ints, and strings.
+   */
+  List toFlatFormat() {
+    var result = new List(3);
+    // TODO(alanknight): Don't make it call toMaps in order to make non-maps.
+    var stuff = toMaps();
+    result[0] = stuff["rules"];
+    var roots = new List();
+    stuff["roots"].forEach((x) => x.writeToList(roots));
+    result[2] = roots;
+
+    // TODO(alanknight): This needs serious generalization. Do we introduce
+    // an output format object that the rules talk to? Do we make use of the
+    // fact that rules talk to something that looks to them like a List. Do
+    // we then mandate that instead of saying they have complete charge of
+    // their own storage?
+    var flatData = new List();
+    for (var eachRule in rules) {
+      var ruleData = stuff["data"][eachRule.number];
+      flatData.add(ruleData.length);
+      eachRule.dumpStateInto(ruleData, flatData);
+    }
+    result[1] = flatData;
+    return result;
+  }
+
+  /**
+   * Given that we have fully populated the list of [states], and more
+   * importantly, the list of [references], go through each state and turn
+   * anything that requires a [Reference] into one. Since only the rules
+   * know the representation they use for state, delegate to them.
+   */
+  void flatten() {
+    for (var eachRule in rules) {
+      _growStates(eachRule);
+      var index = eachRule.number;
+      for (var eachState in states[index]) {
+        eachRule.flatten(eachState, this);
+      }
+    }
+  }
+
+  /**
+   * As the [trace] processes each object, it will call this method on us.
+   * We find the rules for this object, and record the state of the object
+   * as determined by each rule.
+   */
+  void _process(object, Trace trace) {
+    var real = (object is DesignatedRuleForObject) ? object.target : object;
+    for (var eachRule in serialization.rulesFor(object)) {
+      _record(real, eachRule);
+    }
+  }
+
+  /**
+   * Record the state of [object] as determined by [rule] and keep
+   * track of it. Generate a [Reference] for this object if required.
+   * When it's required is up to the particular rule, but generally everything
+   * gets a reference except a primitive.
+   * Note that at this point the states are just the same as the fields of the
+   * object, and haven't been flattened.
+   */
+  void _record(Object object, SerializationRule rule) {
+    if (rule.shouldUseReferenceFor(object, this)) {
+      references.putIfAbsent(object, () =>
+        new Reference(this, rule.number, _nextObjectNumberFor(rule)));
+      var state = rule.extractState(object, trace.note);
+      _addStateForRule(rule, state);
+    }
+  }
+
+  /**
+   * Should we store primitive objects directly or create references for them.
+   * That depends on which format we're using, so a flat format will want
+   * references, but the Map format can store them directly.
+   */
+  bool shouldUseReferencesForPrimitives = false;
+
+  /** Record a [state] entry for a particular rule. */
+  void _addStateForRule(eachRule, Object state) {
+    _growStates(eachRule);
+    states[eachRule.number].add(state);
+  }
+
+  /** Find what the object number for the thing we're about to add will be.*/
+  int _nextObjectNumberFor(SerializationRule rule) {
+    _growStates(rule);
+    return states[rule.number].length;
+  }
+
+  /**
+   * We store the states in a List, indexed by rule number. But rules can be
+   * dynamically added, so we may have to grow the list.
+   */
+  void _growStates(eachRule) {
+    while (states.length <= eachRule.number) states.add(new List());
+  }
+
+  /**
+   * Return true if we have an object number for this object. This is used to
+   * tell if we have processed the object or not. This relies on checking if we
+   * have a reference or not. That saves some space by not having to keep track
+   * of simple objects, but means that if someone refers to the identical string
+   * from several places, we will process it several times, and store it
+   * several times. That seems an acceptable tradeoff, and in cases where it
+   * isn't, it's possible to apply a rule for String, or even for Strings larger
+   * than x, which gives them references.
+   */
+  bool _hasIndexFor(Object object) {
+    return _objectNumberFor(object) != -1;
+  }
+
+  /**
+   * Given an object, find what number it has. The number is valid only in
+   * the context of a particular rule, and if the rule has more than one,
+   * this will return the one for the primary rule, defined as the one that
+   * is listed in its canonical reference.
+   */
+  int _objectNumberFor(Object object) {
+    //TODO(alanknight): http://dartbug.com/6642  A bool can't be a map key.
+    if (object == true || object == false) {
+      return -1;
+    }
+    var reference = references[object];
+    return (reference == null) ? -1 : reference.objectNumber;
+  }
+
+  /** Return the serialized data in string format. Currently hard-coded to
+   * our custom JSON format.
+   */
+  String toStringFormat() {
+    return toJSON(toMaps());
+  }
+
+  /** The implementation of our custom JSON format. This is trivial, since
+   * that format is essentially the same as the internal representation.
+   */
+  String toJSON(data) {
+    return JSON.stringify(data);
+  }
+
+  /**
+   * Returns the full serialized structure as nested maps. The top-level
+   * has 3 fields, "rules" which may hold a definition of the rules used,
+   * "data" which holds the serialized data, and "roots", which holds
+   * [Reference] objects indicating the root objects. Note that roots are
+   * necessary because the data is organized in the same way as the object
+   * structure, it's a list of lists holding self-contained maps which only
+   * refer to other parts via [Reference] objects.
+   * This effectively defines a custom JSON serialization format, although
+   * the details of the format vary depending which rules were used.
+   */
+  Map toMaps() {
+    var result = new Map();
+    var savedRules;
+    if (selfDescribing) {
+      var meta = serialization._ruleSerialization();
+      var writer = new Writer(meta);
+      writer.selfDescribing = false;
+      savedRules = writer.write(serialization.rules);
+    }
+    result["rules"] = savedRules;
+    result["data"] = states;
+    result["roots"] = _rootReferences(trace.roots);
+    return result;
+  }
+
+  /**
+   * Return a list of [Reference] objects pointing to our roots. This will be
+   * stored in the output under "roots" in the default format.
+   */
+  _rootReferences(roots) =>
+    roots.map((x) => _referenceFor(x));
+
+  /**
+   * Given an object, return a reference for it if one exists. If there's
+   * no reference, return null. Once we have finished the tracing step, all
+   * objects that should have a reference (roughly speaking, non-primitives)
+   * can be relied on to have a reference.
+   */
+  _referenceFor(Object o) {
+    if (o == null) return null;
+    //TODO(alanknight): http://dartbug.com/6642  A bool can't be a map key.
+    if (o == true || o == false) return null;
+    return references[o];
+  }
+
+  // For debugging/testing purposes. Find what state a reference points to.
+  stateForReference(Reference r) =>
+      states[r.ruleNumber][r.objectNumber];
+}
+
+/**
+ * The main class responsible for reading. It holds
+ * onto the necessary state and to the objects that have been inflated.
+ */
+class Reader {
+
+  /**
+   * The serialization that specifies how we read. Note that in contrast
+   * to the Writer, this is not final. This is because we may be created
+   * with an empty [Serialization] and then read the rules from the data,
+   * if [selfDescribing] is true.
+   */
+  Serialization serialization;
+
+  /**
+   * When we read objects, should we read a description of the rules if
+   * present. This defaults to the corresponding value on the Serialization.
+   */
+  bool selfDescribing;
+
+  /**
+   * The state of objects that have been serialized is stored here.
+   * Each rule has a number, and rules keep track of the objects that they
+   * serialize, in order. So the state of any object can be found by indexing
+   * from the rule number and the object number within the rule.
+   * The actual representation of the state is determined by the rule. Lists
+   * and Maps are common, but it is arbitrary. See [Writer.states].
+   */
+  List<List> _data;
+
+  /**
+   * The resulting objects, indexed according to the same scheme as
+   * [data], where each rule has a number, and rules keep track of the objects
+   * that they serialize, in order.
+   */
+  List<List> objects;
+
+  /**
+   * Creates a new [Reader] that uses the rules from its parent
+   * [Serialization]. Serializations do not keep any state related to 
+   * a particular read or write operation, so the same one can be used
+   * for multiple different Writers/Readers.
+   */
+  Reader(this.serialization) {
+    selfDescribing = serialization.selfDescribing;
+  }
+
+  /**
+   * When we read, we may need to look up objects by name in order to link to
+   * them. This is particularly true if we have references to classes,
+   * functions, mirrors, or other non-portable entities. The map in which we
+   * look things up can be provided as an argument to read, but we can also
+   * provide a map here, and objects will be looked up in both places.
+   */
+  Map externalObjects;
+
+  /**
+   * Look up the reference to an external object. This can be held either in
+   * the reader-specific list of externals or in the serializer's
+   */
+  externalObjectNamed(key) {
+    var map = (externalObjects.containsKey(key))
+        ? externalObjects : serialization.externalObjects;
+    if (!map.containsKey(key)) {
+      throw 'Cannot find named object to link to: $key';
+    }
+    return map[key];
+  }
+
+  get rules => serialization.rules;
+
+  get data => _data;
+
+  // When we set the data, initialize the object storage to a matching size.
+  set data(x) {
+    _data = x;
+    objects = keysAndValues(serialization.rules).map(
+        (index, rule) => new List(data[index].length));
+  }
+
+  /**
+   * This is the primary method for a [Reader]. It takes the input data,
+   * currently hard-coded to expect our custom JSON format, and returns
+   * the root objects.
+   */
+  read(String input, [Map externals = const {}]) {
+    externalObjects = externals;
+    var topLevel = JSON.parse(input);
+    var ruleString = topLevel["rules"];
+    readRules(ruleString, externals);
+    data = topLevel["data"];
+    rules.forEach(inflateForRule);
+    var roots = topLevel["roots"];
+    return roots.map((x) => inflateReference(x));
+  }
+
+  /**
+   * If the data we are reading from has rules written to it, read them back
+   * and set them as the rules we will use.
+   */
+  void readRules(String newRules, Map externals) {
+    // TODO(alanknight): Replacing the serialization is kind of confusing.
+    List rulesWeRead = (newRules == null) ?
+        null : serialization._ruleSerialization().readOne(newRules, externals);
+    if (rulesWeRead != null && !rulesWeRead.isEmpty) {
+      serialization = new Serialization.noDefaultRules();
+      rulesWeRead.forEach(serialization.addRule);
+    }
+  }
+
+  /**
+   * This is a hard-coded read method for a vaguely flat format. It's just a
+   * proof of concept of handling more flat formats right now, and needs a lot
+   * of fixing and generalization.
+   */
+  readFlat(List input, [Map externals = const {}]) {
+    // TODO(alanknight): Way too much code duplication with read. Numerous
+    // code smells.
+    externalObjects = externals;
+    var topLevel = input;
+    var ruleString = topLevel[0];
+    readRules(ruleString, externals);
+    var flatData = topLevel[1];
+    var stream = flatData.iterator();
+    var tempData = new List(rules.length);
+     for (var eachRule in rules) {
+       tempData[eachRule.number] = eachRule.pullStateFrom(stream);
+    }
+    data = tempData;
+    for (var eachRule in rules) {
+      inflateForRule(eachRule);
+    }
+    var rootsAsInts = topLevel[2];
+    var rootStream = rootsAsInts.iterator();
+    var roots = new List();
+    while (rootStream.hasNext) {
+      roots.add(new Reference(this, rootStream.next(), rootStream.next()));
+    }
+    var x = inflateReference(roots[0]);
+    return roots.map((x) => inflateReference(x));
+  }
+
+
+  /**
+   * A convenient alternative to [read] when you know there is only
+   * one object.
+   */
+  readOne(String input, [Map externals = const {}])
+    => read(input, externals)[0];
+
+  /**
+   * A convenient alternative to [readFlat] when you know there is only
+   * one object.
+   */
+  readOneFlat(List input, [Map externals = const {}]) =>
+      readFlat(input, externals)[0];
+
+  /**
+   * Inflate all of the objects for [rule]. Does the essential state for all
+   * objects first, then the non-essential state. This avoids cycles in
+   * non-essential state, because all the objects will have already been
+   * created.
+   */
+  inflateForRule(rule) {
+    var dataForThisRule = data[rule.number];
+    keysAndValues(dataForThisRule).forEach((position, state) {
+      inflateOne(rule, position, state);
+    });
+    keysAndValues(dataForThisRule).forEach((position, state) {
+      rule.inflateNonEssential(state, allObjectsForRule(rule)[position], this);
+    });
+  }
+
+  /**
+   * Create a new object, based on [rule] and [state], which will
+   * be stored in [position] in the storage for [rule]. This will
+   * follow references and recursively inflate them, leaving Sentinel objects
+   * to detect cycles.
+   */
+  Object inflateOne(SerializationRule rule, position, state) {
+    var existing = allObjectsForRule(rule)[position];
+    // We may already be in progress and hitting this in a cycle.
+    if (existing is Sentinel) {
+      throw new SerializationException('Cycle in essential state');
+    }
+    // We may have already inflated this object, at least its essential state.
+    if (existing != null) return existing;
+
+    // Put a sentinel there to mark this in case of recursion.
+    allObjectsForRule(rule)[position] = const Sentinel();
+    var newObject = rule.inflateEssential(state, this);
+    allObjectsForRule(rule)[position] = newObject;
+    return newObject;
+  }
+
+  /**
+   * The parameter [possibleReference] might be a reference. If it isn't, just
+   * return it. If it is, then inflate the target of the reference and return
+   * the resulting object.
+   */
+  Object inflateReference(possibleReference) {
+    // If this is a primitive, return it directly.
+    // TODO This seems too complicated.
+    return asReference(possibleReference,
+        ifReference: (reference) {
+          var rule = ruleFor(reference);
+          var state = _stateFor(reference);
+          inflateOne(rule, reference.objectNumber, state);
+          return _objectFor(reference);
+        });
+  }
+
+  /**
+   * Given [reference], return what we have stored as an object for it. Note
+   * that, depending on the current state, this might be null or a Sentinel.
+   */
+  Object _objectFor(Reference reference) =>
+      objects[reference.ruleNumber][reference.objectNumber];
+
+  /** Given [rule], return the storage for its objects. */
+  allObjectsForRule(SerializationRule rule) => objects[rule.number];
+
+  /** Given [reference], return the the state we have stored for it. */
+  Object _stateFor(Reference reference) =>
+      data[reference.ruleNumber][reference.objectNumber];
+
+  /** Given a reference, return the rule it references. */
+  SerializationRule ruleFor(Reference reference) =>
+      serialization.rules[reference.ruleNumber];
+
+  /**
+   * Given a possible reference [anObject], call either [ifReference] or
+   * [ifNotReference], depending if it's a reference or not. This is the
+   * primary place that knows about the serialized representation of a
+   * reference.
+   */
+  asReference(anObject, {Function ifReference: doNothing,
+      Function ifNotReference : doNothing}) {
+    if (anObject is Reference) return ifReference(anObject);
+    if (anObject is Map && (anObject["__Ref"] == true)) {
+      var ref =
+          new Reference(this, anObject["rule"], anObject["object"]);
+      return ifReference(ref);
+    } else {
+      return ifNotReference(anObject);
+    }
+  }
+}
+
+/**
+ * This serves as a marker to indicate a object that is in the process of
+ * being de-serialized. So if we look for an object slot and find one of these,
+ * we know we've hit a cycle.
+ */
+class Sentinel {
+  const Sentinel();
+}
+
+/**
+ * This represents the transitive closure of the referenced objects to be
+ * used for serialization. It works closely in conjunction with the Writer,
+ * and is kept as a separate object primarily for the possibility of wanting
+ * to plug in different sorts of tracing rules.
+ */
+class Trace {
+  // TODO(alanknight): It seems likely that the mechanism for cutting off
+  // tracings is by specifying rules. So is there any reason any more to have
+  // this as a separate class?
+  Writer writer;
+
+  /**
+   * This class works by doing a breadth-first traversal of the objects,
+   * with the traversal order maintained in [queue].
+   */
+  Queue queue = new Queue();
+
+  /** The root objects from which we will be tracing. */
+  List roots = [];
+
+  Trace(this.writer);
+
+  addRoot(object) {
+    roots.add(object);
+  }
+
+  /** A convenience method to add a single root and trace it in one step. */
+  trace(Object o) {
+    addRoot(o);
+    traceAll();
+  }
+
+  /**
+   * Process all of the objects reachable from our roots via state that the
+   * serialization rules access.
+   */
+  traceAll() {
+    queue.addAll(roots);
+    while (!queue.isEmpty) {
+      var next = queue.removeFirst();
+      if (!hasProcessed(next)) writer._process(next, this);
+    }
+  }
+
+  /**
+   * Has this object been seen yet? We test for this by checking if the
+   * writer has a reference for it. See comment for _hasIndexFor.
+   */
+  bool hasProcessed(object) {
+   return writer._hasIndexFor(object);
+  }
+
+  /** Note that we've seen [value], and add it to the queue to be processed. */
+  note(Object value) {
+    if (value != null) {
+      queue.add(value);
+    }
+    return value;
+  }
+}
+
+/**
+ * Any pointers to objects that can't be represented directly in the
+ * serialization format has to be stored as a reference. A reference encodes
+ * the rule number of the rule that saved it in the Serialization that was used
+ * for writing, and the object number within that rule.
+ */
+class Reference {
+  /** The [Reader] or [Writer] that owns this reference. */
+  final parent;
+  /** The position of the rule that controls this reference in [parent]. */
+  final int ruleNumber;
+  /** The index of the referred-to object in the storage of [parent] */
+  final int objectNumber;
+
+  const Reference(this.parent, this.ruleNumber, this.objectNumber);
+
+  /**
+   * Convert the reference to a map in JSON format. This is specific to the
+   * custom JSON format we define, and must be consistent with the
+   * [asReference] method.
+   */
+  // TODO(alanknight): This is a hack both in defining a toJson specific to a
+  // particular representation, and the use of a bogus sentinel "__Ref"
+  toJson() => { "__Ref" : true, "rule" : ruleNumber,
+    "object" : objectNumber};
+
+  /** Write our information to [list]. Useful in writing to flat formats.*/
+  writeToList(List list) {
+    list.add(ruleNumber);
+    list.add(objectNumber);
+  }
+}
+
+/**
+ * This is used during tracing to indicate that an object should be processed
+ * using a particular rule, rather than the one that might ordinarily be
+ * found for it. This normally only makes sense if the object is uniquely
+ * referenced, and is a more or less internal collection. See ListRuleEssential
+ * for an example. It knows how to return its object and how to filter.
+ */
+class DesignatedRuleForObject {
+  Function rulePredicate;
+  final target;
+
+  DesignatedRuleForObject(this.target, this.rulePredicate);
+
+  possibleRules(List rules) => rules.filter(rulePredicate);
+}
+