program.ts

/**
 * AssemblyScript's intermediate representation describing a program's elements.
 * @module program
 *//***/

import {
  CommonFlags,
  PATH_DELIMITER,
  STATIC_DELIMITER,
  INSTANCE_DELIMITER,
  LIBRARY_PREFIX,
  GETTER_PREFIX,
  SETTER_PREFIX,
  FILESPACE_PREFIX
} from "./common";

import {
  Options,
  Feature
} from "./compiler";

import {
  DiagnosticCode,
  DiagnosticMessage,
  DiagnosticEmitter
} from "./diagnostics";

import {
  Type,
  TypeKind,
  TypeFlags,
  Signature
} from "./types";

import {
  Node,
  NodeKind,
  Source,
  Range,
  CommonTypeNode,
  TypeParameterNode,
  DecoratorNode,
  DecoratorKind,

  Expression,
  IdentifierExpression,
  LiteralExpression,
  LiteralKind,
  StringLiteralExpression,

  ClassDeclaration,
  DeclarationStatement,
  EnumDeclaration,
  EnumValueDeclaration,
  ExportMember,
  ExportStatement,
  FieldDeclaration,
  FunctionDeclaration,
  ImportDeclaration,
  ImportStatement,
  InterfaceDeclaration,
  MethodDeclaration,
  NamespaceDeclaration,
  TypeDeclaration,
  VariableDeclaration,
  VariableLikeDeclarationStatement,
  VariableStatement,

  decoratorNameToKind,
  findDecorator
} from "./ast";

import {
  Module,
  NativeType,
  FunctionRef,
  ExpressionRef,
  ExpressionId,
  BinaryOp,
  UnaryOp,

  getExpressionId,
  getGetLocalIndex,
  isTeeLocal,
  getSetLocalValue,
  getBinaryOp,
  getConstValueI32,
  getBinaryLeft,
  getBinaryRight,
  getUnaryOp,
  getExpressionType,
  getLoadBytes,
  isLoadSigned,
  getIfTrue,
  getIfFalse,
  getSelectThen,
  getSelectElse,
  getCallTarget,
  getBlockChildCount,
  getBlockChild,
  getBlockName,
  getConstValueF32,
  getConstValueF64,
  getConstValueI64Low,
  getGetGlobalName
} from "./module";

import {
  CharCode
} from "./util";

import {
  Resolver
} from "./resolver";

/** Represents a yet unresolved import. */
class QueuedImport {
  localName: string;
  externalName: string;
  externalNameAlt: string;
  declaration: ImportDeclaration | null; // not set if a filespace
}

/** Represents a yet unresolved export. */
class QueuedExport {
  externalName: string;
  isReExport: bool;
  member: ExportMember;
}

/** Represents a type alias. */
class TypeAlias {
  typeParameters: TypeParameterNode[] | null;
  type: CommonTypeNode;
}

/** Represents a module-level export. */
class ModuleExport {
  element: Element;
  identifier: IdentifierExpression;
}

/** Represents the kind of an operator overload. */
export enum OperatorKind {
  INVALID,

  // indexed access
  INDEXED_GET,            // a[]
  INDEXED_SET,            // a[]=b
  UNCHECKED_INDEXED_GET,  // unchecked(a[])
  UNCHECKED_INDEXED_SET,  // unchecked(a[]=b)

  // binary
  ADD,                    // a + b
  SUB,                    // a - b
  MUL,                    // a * b
  DIV,                    // a / b
  REM,                    // a % b
  POW,                    // a ** b
  BITWISE_AND,            // a & b
  BITWISE_OR,             // a | b
  BITWISE_XOR,            // a ^ b
  BITWISE_SHL,            // a << b
  BITWISE_SHR,            // a >> b
  BITWISE_SHR_U,          // a >>> b
  EQ,                     // a == b
  NE,                     // a != b
  GT,                     // a > b
  GE,                     // a >= b
  LT,                     // a < b
  LE,                     // a <= b

  // unary prefix
  PLUS,                   // +a
  MINUS,                  // -a
  NOT,                    // !a
  BITWISE_NOT,            // ~a
  PREFIX_INC,             // ++a
  PREFIX_DEC,             // --a

  // unary postfix
  POSTFIX_INC,            // a++
  POSTFIX_DEC             // a--

  // not overridable:
  // IDENTITY             // a === b
  // LOGICAL_AND          // a && b
  // LOGICAL_OR           // a || b
}

/** Returns the operator kind represented by the specified decorator and string argument. */
function operatorKindFromDecorator(decoratorKind: DecoratorKind, arg: string): OperatorKind {
  assert(arg.length);
  switch (decoratorKind) {
    case DecoratorKind.OPERATOR:
    case DecoratorKind.OPERATOR_BINARY: {
      switch (arg.charCodeAt(0)) {
        case CharCode.OPENBRACKET: {
          if (arg == "[]") return OperatorKind.INDEXED_GET;
          if (arg == "[]=") return OperatorKind.INDEXED_SET;
          break;
        }
        case CharCode.OPENBRACE: {
          if (arg == "{}") return OperatorKind.UNCHECKED_INDEXED_GET;
          if (arg == "{}=") return OperatorKind.UNCHECKED_INDEXED_SET;
          break;
        }
        case CharCode.PLUS: {
          if (arg == "+") return OperatorKind.ADD;
          break;
        }
        case CharCode.MINUS: {
          if (arg == "-") return OperatorKind.SUB;
          break;
        }
        case CharCode.ASTERISK: {
          if (arg == "*") return OperatorKind.MUL;
          if (arg == "**") return OperatorKind.POW;
          break;
        }
        case CharCode.SLASH: {
          if (arg == "/") return OperatorKind.DIV;
          break;
        }
        case CharCode.PERCENT: {
          if (arg == "%") return OperatorKind.REM;
          break;
        }
        case CharCode.AMPERSAND: {
          if (arg == "&") return OperatorKind.BITWISE_AND;
          break;
        }
        case CharCode.BAR: {
          if (arg == "|") return OperatorKind.BITWISE_OR;
          break;
        }
        case CharCode.CARET: {
          if (arg == "^") return OperatorKind.BITWISE_XOR;
          break;
        }
        case CharCode.EQUALS: {
          if (arg == "==") return OperatorKind.EQ;
          break;
        }
        case CharCode.EXCLAMATION: {
          if (arg == "!=") return OperatorKind.NE;
          break;
        }
        case CharCode.GREATERTHAN: {
          if (arg == ">") return OperatorKind.GT;
          if (arg == ">=") return OperatorKind.GE;
          if (arg == ">>") return OperatorKind.BITWISE_SHR;
          if (arg == ">>>") return OperatorKind.BITWISE_SHR_U;
          break;
        }
        case CharCode.LESSTHAN: {
          if (arg == "<") return OperatorKind.LT;
          if (arg == "<=") return OperatorKind.LE;
          if (arg == "<<") return OperatorKind.BITWISE_SHL;
          break;
        }
      }
      break;
    }
    case DecoratorKind.OPERATOR_PREFIX: {
      switch (arg.charCodeAt(0)) {
        case CharCode.PLUS: {
          if (arg == "+") return OperatorKind.PLUS;
          if (arg == "++") return OperatorKind.PREFIX_INC;
          break;
        }
        case CharCode.MINUS: {
          if (arg == "-") return OperatorKind.MINUS;
          if (arg == "--") return OperatorKind.PREFIX_DEC;
          break;
        }
        case CharCode.EXCLAMATION: {
          if (arg == "!") return OperatorKind.NOT;
          break;
        }
        case CharCode.TILDE: {
          if (arg == "~") return OperatorKind.BITWISE_NOT;
          break;
        }
      }
      break;
    }
    case DecoratorKind.OPERATOR_POSTFIX: {
      switch (arg.charCodeAt(0)) {
        case CharCode.PLUS: {
          if (arg == "++") return OperatorKind.POSTFIX_INC;
          break;
        }
        case CharCode.MINUS: {
          if (arg == "--") return OperatorKind.POSTFIX_DEC;
          break;
        }
      }
      break;
    }
  }
  return OperatorKind.INVALID;
}

const noTypesYet = new Map<string,Type>();

/** Represents an AssemblyScript program. */
export class Program extends DiagnosticEmitter {

  /** Array of source files. */
  sources: Source[];
  /** Resolver instance. */
  resolver: Resolver;
  /** Diagnostic offset used where successively obtaining the next diagnostic. */
  diagnosticsOffset: i32 = 0;
  /** Compiler options. */
  options: Options;

  /** Elements by internal name. */
  elementsLookup: Map<string,Element> = new Map();
  /** Class and function instances by internal name. */
  instancesLookup: Map<string,Element> = new Map();
  /** Types by internal name. */
  typesLookup: Map<string,Type> = noTypesYet;
  /** Declared type aliases. */
  typeAliases: Map<string,TypeAlias> = new Map();
  /** File-level exports by exported name. */
  fileLevelExports: Map<string,Element> = new Map();
  /** Module-level exports by exported name. */
  moduleLevelExports: Map<string,ModuleExport> = new Map();

  /** ArrayBuffer instance reference. */
  arrayBufferInstance: Class | null = null;
  /** Array prototype reference. */
  arrayPrototype: ClassPrototype | null = null;
  /** String instance reference. */
  stringInstance: Class | null = null;
  /** Start function reference. */
  startFunction: FunctionPrototype;
  /** Main function reference, if present. */
  mainFunction: FunctionPrototype | null = null;
  /** Abort function reference, if present. */
  abortInstance: Function | null = null;
  /** Memory allocation function. */
  memoryAllocateInstance: Function | null = null;

  /** Whether a garbage collector is present or not. */
  hasGC: bool = false;
  /** Garbage collector allocation function. */
  gcAllocateInstance: Function | null = null;
  /** Garbage collector link function called when a managed object is referenced from a parent. */
  gcLinkInstance: Function | null = null;
  /** Garbage collector mark function called to on reachable managed objects. */
  gcMarkInstance: Function | null = null;
  /** Size of a managed object header. */
  gcHeaderSize: u32 = 0;
  /** Offset of the GC hook. */
  gcHookOffset: u32 = 0;

  /** Currently processing filespace. */
  currentFilespace: Filespace;

  /** Constructs a new program, optionally inheriting parser diagnostics. */
  constructor(diagnostics: DiagnosticMessage[] | null = null) {
    super(diagnostics);
    this.resolver = new Resolver(this);
    this.sources = [];
  }

  /** Gets a source by its exact path. */
  getSource(normalizedPath: string): Source | null {
    var sources = this.sources;
    for (let i = 0, k = sources.length; i < k; ++i) {
      let source = sources[i];
      if (source.normalizedPath == normalizedPath) return source;
    }
    return null;
  }

  /** Looks up the source for the specified possibly ambiguous path. */
  lookupSourceByPath(normalizedPathWithoutExtension: string): Source | null {
    var tmp: string;
    return (
      this.getSource(normalizedPathWithoutExtension + ".ts") ||
      this.getSource(normalizedPathWithoutExtension + "/index.ts") ||
      this.getSource((tmp = LIBRARY_PREFIX + normalizedPathWithoutExtension) + ".ts") ||
      this.getSource( tmp                                                    + "/index.ts")
    );
  }

  /** Initializes the program and its elements prior to compilation. */
  initialize(options: Options): void {
    this.options = options;

    // add built-in types
    this.typesLookup = new Map([
      ["i8", Type.i8],
      ["i16", Type.i16],
      ["i32", Type.i32],
      ["i64", Type.i64],
      ["isize", options.isizeType],
      ["u8", Type.u8],
      ["u16", Type.u16],
      ["u32", Type.u32],
      ["u64", Type.u64],
      ["usize", options.usizeType],
      ["bool", Type.bool],
      ["f32", Type.f32],
      ["f64", Type.f64],
      ["void", Type.void],
      ["number", Type.f64],
      ["boolean", Type.bool]
    ]);

    // add compiler hints
    this.setConstantInteger("ASC_TARGET", Type.i32,
      i64_new(options.isWasm64 ? 2 : 1));
    this.setConstantInteger("ASC_NO_TREESHAKING", Type.bool,
      i64_new(options.noTreeShaking ? 1 : 0, 0));
    this.setConstantInteger("ASC_NO_ASSERT", Type.bool,
      i64_new(options.noAssert ? 1 : 0, 0));
    this.setConstantInteger("ASC_MEMORY_BASE", Type.i32,
      i64_new(options.memoryBase, 0));
    this.setConstantInteger("ASC_OPTIMIZE_LEVEL", Type.i32,
      i64_new(options.optimizeLevelHint, 0));
    this.setConstantInteger("ASC_SHRINK_LEVEL", Type.i32,
      i64_new(options.shrinkLevelHint, 0));
    this.setConstantInteger("ASC_FEATURE_MUTABLE_GLOBAL", Type.bool,
      i64_new(options.hasFeature(Feature.MUTABLE_GLOBAL) ? 1 : 0, 0));
    this.setConstantInteger("ASC_FEATURE_SIGN_EXTENSION", Type.bool,
      i64_new(options.hasFeature(Feature.SIGN_EXTENSION) ? 1 : 0, 0));

    // remember deferred elements
    var queuedImports = new Array<QueuedImport>();
    var queuedExports = new Map<string,QueuedExport>();
    var queuedExtends = new Array<ClassPrototype>();
    var queuedImplements = new Array<ClassPrototype>();

    // build initial lookup maps of internal names to declarations
    for (let i = 0, k = this.sources.length; i < k; ++i) {
      let source = this.sources[i];

      // create one filespace per source
      let filespace = new Filespace(this, source);
      this.elementsLookup.set(filespace.internalName, filespace);
      this.currentFilespace = filespace;

      // process this source's statements
      let statements = source.statements;
      for (let j = 0, l = statements.length; j < l; ++j) {
        let statement = statements[j];
        switch (statement.kind) {
          case NodeKind.CLASSDECLARATION: {
            this.initializeClass(<ClassDeclaration>statement, queuedExtends, queuedImplements);
            break;
          }
          case NodeKind.ENUMDECLARATION: {
            this.initializeEnum(<EnumDeclaration>statement);
            break;
          }
          case NodeKind.EXPORT: {
            this.initializeExports(<ExportStatement>statement, queuedExports);
            break;
          }
          case NodeKind.FUNCTIONDECLARATION: {
            this.initializeFunction(<FunctionDeclaration>statement);
            break;
          }
          case NodeKind.IMPORT: {
            this.initializeImports(<ImportStatement>statement, queuedExports, queuedImports);
            break;
          }
          case NodeKind.INTERFACEDECLARATION: {
            this.initializeInterface(<InterfaceDeclaration>statement);
            break;
          }
          case NodeKind.NAMESPACEDECLARATION: {
            this.initializeNamespace(<NamespaceDeclaration>statement, queuedExtends, queuedImplements);
            break;
          }
          case NodeKind.TYPEDECLARATION: {
            this.initializeTypeAlias(<TypeDeclaration>statement);
            break;
          }
          case NodeKind.VARIABLE: {
            this.initializeVariables(<VariableStatement>statement);
            break;
          }
        }
      }
    }

    // queued imports should be resolvable now through traversing exports and queued exports
    for (let i = 0; i < queuedImports.length;) {
      let queuedImport = queuedImports[i];
      let declaration = queuedImport.declaration;
      if (declaration) { // named
        let element = this.tryLocateImport(queuedImport.externalName, queuedExports);
        if (element) {
          this.elementsLookup.set(queuedImport.localName, element);
          queuedImports.splice(i, 1);
        } else {
          if (element = this.tryLocateImport(queuedImport.externalNameAlt, queuedExports)) {
            this.elementsLookup.set(queuedImport.localName, element);
            queuedImports.splice(i, 1);
          } else {
            this.error(
              DiagnosticCode.Module_0_has_no_exported_member_1,
              declaration.range,
              (<ImportStatement>declaration.parent).path.value,
              declaration.externalName.text
            );
            ++i;
          }
        }
      } else { // filespace
        let element = this.elementsLookup.get(queuedImport.externalName);
        if (element) {
          this.elementsLookup.set(queuedImport.localName, element);
          queuedImports.splice(i, 1);
        } else {
          if (element = this.elementsLookup.get(queuedImport.externalNameAlt)) {
            this.elementsLookup.set(queuedImport.localName, element);
            queuedImports.splice(i, 1);
          } else {
            assert(false); // already reported by the parser not finding the file
            ++i;
          }
        }
      }
    }

    // queued exports should be resolvable now that imports are finalized
    for (let [exportName, queuedExport] of queuedExports) {
      let currentExport: QueuedExport | null = queuedExport; // nullable below
      let element: Element | null;
      do {
        if (currentExport.isReExport) {
          if (element = this.fileLevelExports.get(currentExport.externalName)) {
            this.setExportAndCheckLibrary(
              exportName,
              element,
              queuedExport.member.externalName
            );
            break;
          }
          currentExport = queuedExports.get(currentExport.externalName);
          if (!currentExport) {
            this.error(
              DiagnosticCode.Module_0_has_no_exported_member_1,
              queuedExport.member.externalName.range,
              (<StringLiteralExpression>(<ExportStatement>queuedExport.member.parent).path).value,
              queuedExport.member.externalName.text
            );
          }
        } else {
          if (
            // normal export
            (element = this.elementsLookup.get(currentExport.externalName)) ||
            // library re-export
            (element = this.elementsLookup.get(currentExport.member.name.text))
          ) {
            this.setExportAndCheckLibrary(
              exportName,
              element,
              queuedExport.member.externalName
            );
          } else {
            this.error(
              DiagnosticCode.Cannot_find_name_0,
              queuedExport.member.range, queuedExport.member.name.text
            );
          }
          break;
        }
      } while (currentExport);
    }

    // resolve base prototypes of derived classes
    var resolver = this.resolver;
    for (let i = 0, k = queuedExtends.length; i < k; ++i) {
      let derivedPrototype = queuedExtends[i];
      let derivedDeclaration = derivedPrototype.declaration;
      let derivedType = assert(derivedDeclaration.extendsType);
      let baseElement = resolver.resolveIdentifier(derivedType.name, null); // reports
      if (!baseElement) continue;
      if (baseElement.kind == ElementKind.CLASS_PROTOTYPE) {
        let basePrototype = <ClassPrototype>baseElement;
        derivedPrototype.basePrototype = basePrototype;
      } else {
        this.error(
          DiagnosticCode.A_class_may_only_extend_another_class,
          derivedType.range
        );
      }
    }

    // set up global aliases
    {
      let globalAliases = options.globalAliases;
      if (globalAliases) {
        for (let [alias, name] of globalAliases) {
          if (!name.length) continue; // explicitly disabled
          let element = this.elementsLookup.get(name);
          if (element) this.elementsLookup.set(alias, element);
          else throw new Error("element not found: " + name);
        }
      }
    }

    // register 'ArrayBuffer'
    if (this.elementsLookup.has("ArrayBuffer")) {
      let element = assert(this.elementsLookup.get("ArrayBuffer"));
      assert(element.kind == ElementKind.CLASS_PROTOTYPE);
      this.arrayBufferInstance = resolver.resolveClass(<ClassPrototype>element, null);
    }

    // register 'Array'
    if (this.elementsLookup.has("Array")) {
      let element = assert(this.elementsLookup.get("Array"));
      assert(element.kind == ElementKind.CLASS_PROTOTYPE);
      this.arrayPrototype = <ClassPrototype>element;
    }

    // register 'String'
    if (this.elementsLookup.has("String")) {
      let element = assert(this.elementsLookup.get("String"));
      assert(element.kind == ElementKind.CLASS_PROTOTYPE);
      let instance = resolver.resolveClass(<ClassPrototype>element, null);
      if (instance) {
        if (this.typesLookup.has("string")) {
          let declaration = (<ClassPrototype>element).declaration;
          this.error(
            DiagnosticCode.Duplicate_identifier_0,
            declaration.name.range, declaration.programLevelInternalName
          );
        } else {
          this.stringInstance = instance;
          this.typesLookup.set("string", instance.type);
        }
      }
    }

    // register 'start'
    {
      let element = assert(this.elementsLookup.get("start"));
      assert(element.kind == ElementKind.FUNCTION_PROTOTYPE);
      this.startFunction = <FunctionPrototype>element;
    }

    // register 'main' if present
    if (this.moduleLevelExports.has("main")) {
      let element = (<ModuleExport>this.moduleLevelExports.get("main")).element;
      if (
        element.kind == ElementKind.FUNCTION_PROTOTYPE &&
        !(<FunctionPrototype>element).isAny(CommonFlags.GENERIC | CommonFlags.AMBIENT)
      ) {
        (<FunctionPrototype>element).set(CommonFlags.MAIN);
        this.mainFunction = <FunctionPrototype>element;
      }
    }

    // register 'abort' if present
    if (this.elementsLookup.has("abort")) {
      let element = <Element>this.elementsLookup.get("abort");
      assert(element.kind == ElementKind.FUNCTION_PROTOTYPE);
      let instance = this.resolver.resolveFunction(<FunctionPrototype>element, null);
      if (instance) this.abortInstance = instance;
    }

    // register 'memory.allocate' if present
    if (this.elementsLookup.has("memory")) {
      let element = <Element>this.elementsLookup.get("memory");
      let members = element.members;
      if (members) {
        if (members.has("allocate")) {
          element = assert(members.get("allocate"));
          assert(element.kind == ElementKind.FUNCTION_PROTOTYPE);
          let instance = this.resolver.resolveFunction(<FunctionPrototype>element, null);
          if (instance) this.memoryAllocateInstance = instance;
        }
      }
    }

    // register GC hooks if present
    if (
      this.elementsLookup.has("__gc_allocate") &&
      this.elementsLookup.has("__gc_link") &&
      this.elementsLookup.has("__gc_mark")
    ) {
      // __gc_allocate(usize, (ref: usize) => void): usize
      let element = <Element>this.elementsLookup.get("__gc_allocate");
      assert(element.kind == ElementKind.FUNCTION_PROTOTYPE);
      let gcAllocateInstance = assert(this.resolver.resolveFunction(<FunctionPrototype>element, null));
      let signature = gcAllocateInstance.signature;
      assert(signature.parameterTypes.length == 2);
      assert(signature.parameterTypes[0] == this.options.usizeType);
      assert(signature.parameterTypes[1].signatureReference);
      assert(signature.returnType == this.options.usizeType);

      // __gc_link(usize, usize): void
      element = <Element>this.elementsLookup.get("__gc_link");
      assert(element.kind == ElementKind.FUNCTION_PROTOTYPE);
      let gcLinkInstance = assert(this.resolver.resolveFunction(<FunctionPrototype>element, null));
      signature = gcLinkInstance.signature;
      assert(signature.parameterTypes.length == 2);
      assert(signature.parameterTypes[0] == this.options.usizeType);
      assert(signature.parameterTypes[1] == this.options.usizeType);
      assert(signature.returnType == Type.void);

      // __gc_mark(usize): void
      element = <Element>this.elementsLookup.get("__gc_mark");
      assert(element.kind == ElementKind.FUNCTION_PROTOTYPE);
      let gcMarkInstance = assert(this.resolver.resolveFunction(<FunctionPrototype>element, null));
      signature = gcMarkInstance.signature;
      assert(signature.parameterTypes.length == 1);
      assert(signature.parameterTypes[0] == this.options.usizeType);
      assert(signature.returnType == Type.void);

      this.gcAllocateInstance = gcAllocateInstance;
      this.gcLinkInstance = gcLinkInstance;
      this.gcMarkInstance = gcMarkInstance;
      let gcHookOffset = 2 * options.usizeType.byteSize; // .next + .prev
      this.gcHookOffset =  gcHookOffset;
      this.gcHeaderSize = (gcHookOffset + 4 + 7) & ~7;   // + .hook index + alignment
      this.hasGC = true;
    }
  }

  /** Sets a constant integer value. */
  setConstantInteger(globalName: string, type: Type, value: I64): void {
    assert(type.is(TypeFlags.INTEGER));
    var global = new Global(this, globalName, globalName, type, null, DecoratorFlags.NONE)
      .withConstantIntegerValue(value);
    global.set(CommonFlags.RESOLVED);
    this.elementsLookup.set(globalName, global);
  }

  /** Sets a constant float value. */
  setConstantFloat(globalName: string, type: Type, value: f64): void {
    assert(type.is(TypeFlags.FLOAT));
    var global = new Global(this, globalName, globalName, type, null, DecoratorFlags.NONE)
      .withConstantFloatValue(value);
    global.set(CommonFlags.RESOLVED);
    this.elementsLookup.set(globalName, global);
  }

  /** Tries to locate an import by traversing exports and queued exports. */
  private tryLocateImport(
    externalName: string,
    queuedNamedExports: Map<string,QueuedExport>
  ): Element | null {
    var element: Element | null;
    var fileLevelExports = this.fileLevelExports;
    do {
      if (element = fileLevelExports.get(externalName)) return element;
      let queuedExport = queuedNamedExports.get(externalName);
      if (!queuedExport) break;
      if (queuedExport.isReExport) {
        externalName = queuedExport.externalName;
        continue;
      }
      return this.elementsLookup.get(queuedExport.externalName);
    } while (true);
    return null;
  }

  /** Checks that only supported decorators are present. */
  private checkDecorators(
    decorators: DecoratorNode[],
    acceptedFlags: DecoratorFlags
  ): DecoratorFlags {
    var presentFlags = DecoratorFlags.NONE;
    for (let i = 0, k = decorators.length; i < k; ++i) {
      let decorator = decorators[i];
      let kind = decoratorNameToKind(decorator.name);
      let flag = decoratorKindToFlag(kind);
      if (flag) {
        if (flag == DecoratorFlags.BUILTIN) {
          if (decorator.range.source.isLibrary) {
            presentFlags |= flag;
          } else {
            this.error(
              DiagnosticCode.Decorator_0_is_not_valid_here,
              decorator.range, decorator.name.range.toString()
            );
          }
        } else if (!(acceptedFlags & flag)) {
          this.error(
            DiagnosticCode.Decorator_0_is_not_valid_here,
            decorator.range, decorator.name.range.toString()
          );
        } else if (presentFlags & flag) {
          this.error(
            DiagnosticCode.Duplicate_decorator,
            decorator.range, decorator.name.range.toString()
          );
        } else {
          presentFlags |= flag;
        }
      }
    }
    return presentFlags;
  }

  /** Checks and sets up global options of an element. */
  private checkGlobal(
    element: Element,
    declaration: DeclarationStatement
  ): void {
    var parentNode = declaration.parent;
    // alias globally if explicitly annotated @global or exported from a top-level library file
    if (
      (element.hasDecorator(DecoratorFlags.GLOBAL)) ||
      (
        declaration.range.source.isLibrary &&
        element.is(CommonFlags.EXPORT) &&
        (
          assert(parentNode).kind == NodeKind.SOURCE ||
          (
            <Node>parentNode).kind == NodeKind.VARIABLE &&
            assert((<Node>parentNode).parent).kind == NodeKind.SOURCE
          )
        )
    ) {
      let globalName = declaration.programLevelInternalName;
      if (this.elementsLookup.has(globalName)) {
        this.error(
          DiagnosticCode.Duplicate_identifier_0,
          declaration.name.range, element.internalName
        );
      } else {
        this.elementsLookup.set(globalName, element);
      }
    }
    // builtins use the global name directly
    if (element.hasDecorator(DecoratorFlags.BUILTIN)) {
      element.internalName = declaration.programLevelInternalName;
    }
  }

  /** Initializes a class declaration. */
  private initializeClass(
    declaration: ClassDeclaration,
    queuedExtends: ClassPrototype[],
    queuedImplements: ClassPrototype[],
    namespace: Element | null = null
  ): void {
    var internalName = declaration.fileLevelInternalName;
    if (this.elementsLookup.has(internalName)) {
      this.error(
        DiagnosticCode.Duplicate_identifier_0,
        declaration.name.range, internalName
      );
      return;
    }

    var decorators = declaration.decorators;
    var simpleName = declaration.name.text;
    var prototype = new ClassPrototype(
      this,
      simpleName,
      internalName,
      declaration,
      decorators
        ? this.checkDecorators(decorators,
            DecoratorFlags.GLOBAL |
            DecoratorFlags.SEALED |
            DecoratorFlags.UNMANAGED
          )
        : DecoratorFlags.NONE
    );
    prototype.parent = namespace;
    this.elementsLookup.set(internalName, prototype);

    var implementsTypes = declaration.implementsTypes;
    if (implementsTypes) {
      let numImplementsTypes = implementsTypes.length;
      if (prototype.hasDecorator(DecoratorFlags.UNMANAGED)) {
        if (numImplementsTypes) {
          this.error(
            DiagnosticCode.Unmanaged_classes_cannot_implement_interfaces,
            Range.join(
              declaration.name.range,
              implementsTypes[numImplementsTypes - 1].range
            )
          );
        }

      // remember classes that implement interfaces
      } else if (numImplementsTypes) {
        for (let i = 0; i < numImplementsTypes; ++i) {
          this.warning( // TODO
            DiagnosticCode.Operation_not_supported,
            implementsTypes[i].range
          );
        }
        queuedImplements.push(prototype);
      }
    }

    // remember classes that extend another one
    if (declaration.extendsType) queuedExtends.push(prototype);

    // add as namespace member if applicable
    if (namespace) {
      if (namespace.members) {
        if (namespace.members.has(simpleName)) {
          this.error(
            DiagnosticCode.Duplicate_identifier_0,
            declaration.name.range, internalName
          );
          return;
        }
      } else {
        namespace.members = new Map();
      }
      namespace.members.set(simpleName, prototype);
      if (namespace.is(CommonFlags.MODULE_EXPORT) && prototype.is(CommonFlags.EXPORT)) {
        prototype.set(CommonFlags.MODULE_EXPORT);
      }

    // otherwise add to file-level exports if exported
    } else if (prototype.is(CommonFlags.EXPORT)) {
      if (this.fileLevelExports.has(internalName)) {
        this.error(
          DiagnosticCode.Export_declaration_conflicts_with_exported_declaration_of_0,
          declaration.name.range, internalName
        );
        return;
      }
      this.fileLevelExports.set(internalName, prototype);
      this.currentFilespace.members.set(simpleName, prototype);
      if (prototype.is(CommonFlags.EXPORT) && declaration.range.source.isEntry) {
        if (this.moduleLevelExports.has(simpleName)) {
          let existingExport = <ModuleExport>this.moduleLevelExports.get(simpleName);
          this.error(
            DiagnosticCode.Export_declaration_conflicts_with_exported_declaration_of_0,
            declaration.name.range, existingExport.element.internalName
          );
          return;
        }
        prototype.set(CommonFlags.MODULE_EXPORT);
        this.moduleLevelExports.set(simpleName, <ModuleExport>{
          element: prototype,
          identifier: declaration.name
        });
      }
    }

    // initialize members
    var memberDeclarations = declaration.members;
    for (let i = 0, k = memberDeclarations.length; i < k; ++i) {
      let memberDeclaration = memberDeclarations[i];
      switch (memberDeclaration.kind) {
        case NodeKind.FIELDDECLARATION: {
          this.initializeField(<FieldDeclaration>memberDeclaration, prototype);
          break;
        }
        case NodeKind.METHODDECLARATION: {
          if (memberDeclaration.isAny(CommonFlags.GET | CommonFlags.SET)) {
            this.initializeAccessor(<MethodDeclaration>memberDeclaration, prototype);
          } else {
            this.initializeMethod(<MethodDeclaration>memberDeclaration, prototype);
          }
          break;
        }
        case NodeKind.INDEXSIGNATUREDECLARATION: break; // ignored for now
        default: {
          assert(false); // should have been reported while parsing
          return;
        }
      }
    }

    this.checkGlobal(prototype, declaration);
  }

  /** Initializes a field of a class or interface. */
  private initializeField(
    declaration: FieldDeclaration,
    classPrototype: ClassPrototype
  ): void {
    var name = declaration.name.text;
    var internalName = declaration.fileLevelInternalName;
    var decorators = declaration.decorators;
    var isInterface = classPrototype.kind == ElementKind.INTERFACE_PROTOTYPE;

    // static fields become global variables
    if (declaration.is(CommonFlags.STATIC)) {
      if (isInterface) {
        // should have been reported while parsing
        assert(false);
      }
      if (this.elementsLookup.has(internalName)) {
        this.error(
          DiagnosticCode.Duplicate_identifier_0,
          declaration.name.range, internalName
        );
        return;