javac/code/Scope.java

/*
 * Copyright (c) 1999, 2021, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */

package com.sun.tools.javac.code;

import com.sun.tools.javac.code.Kinds.Kind;
import java.lang.ref.WeakReference;
import java.util.*;
import java.util.function.BiConsumer;
import java.util.function.Predicate;

import com.sun.tools.javac.code.Symbol.CompletionFailure;
import com.sun.tools.javac.code.Symbol.TypeSymbol;
import com.sun.tools.javac.tree.JCTree.JCImport;
import com.sun.tools.javac.util.*;
import com.sun.tools.javac.util.List;

import static com.sun.tools.javac.code.Scope.LookupKind.NON_RECURSIVE;
import static com.sun.tools.javac.code.Scope.LookupKind.RECURSIVE;
import static com.sun.tools.javac.util.Iterators.createCompoundIterator;
import static com.sun.tools.javac.util.Iterators.createFilterIterator;

/** A scope represents an area of visibility in a Java program. The
 *  Scope class is a container for symbols which provides
 *  efficient access to symbols given their names. Scopes are implemented
 *  as hash tables with "open addressing" and "double hashing".
 *  Scopes can be nested. Nested scopes can share their hash tables.
 *
 *  <p><b>This is NOT part of any supported API.
 *  If you write code that depends on this, you do so at your own risk.
 *  This code and its internal interfaces are subject to change or
 *  deletion without notice.</b>
 */
public abstract class Scope {

    /** The scope's owner.
     */
    public final Symbol owner;

    protected Scope(Symbol owner) {
        this.owner = owner;
    }

    /**Returns all Symbols in this Scope. Symbols from outward Scopes are included.
     */
    public final Iterable<Symbol> getSymbols() {
        return getSymbols(noFilter);
    }

    /**Returns Symbols that match the given filter. Symbols from outward Scopes are included.
     */
    public final Iterable<Symbol> getSymbols(Predicate<Symbol> sf) {
        return getSymbols(sf, RECURSIVE);
    }

    /**Returns all Symbols in this Scope. Symbols from outward Scopes are included
     * iff lookupKind == RECURSIVE.
     */
    public final Iterable<Symbol> getSymbols(LookupKind lookupKind) {
        return getSymbols(noFilter, lookupKind);
    }

    /**Returns Symbols that match the given filter. Symbols from outward Scopes are included
     * iff lookupKind == RECURSIVE.
     */
    public abstract Iterable<Symbol> getSymbols(Predicate<Symbol> sf, LookupKind lookupKind);

    /**Returns Symbols with the given name. Symbols from outward Scopes are included.
     */
    public final Iterable<Symbol> getSymbolsByName(Name name) {
        return getSymbolsByName(name, RECURSIVE);
    }

    /**Returns Symbols with the given name that match the given filter.
     * Symbols from outward Scopes are included.
     */
    public final Iterable<Symbol> getSymbolsByName(final Name name, final Predicate<Symbol> sf) {
        return getSymbolsByName(name, sf, RECURSIVE);
    }

    /**Returns Symbols with the given name. Symbols from outward Scopes are included
     * iff lookupKind == RECURSIVE.
     */
    public final Iterable<Symbol> getSymbolsByName(Name name, LookupKind lookupKind) {
        return getSymbolsByName(name, noFilter, lookupKind);
    }

    /**Returns Symbols with the given name that match the given filter.
     * Symbols from outward Scopes are included iff lookupKind == RECURSIVE.
     */
    public abstract Iterable<Symbol> getSymbolsByName(final Name name, final Predicate<Symbol> sf,
            final LookupKind lookupKind);

    /** Return the first Symbol from this or outward scopes with the given name.
     * Returns null if none.
     */
    public final Symbol findFirst(Name name) {
        return findFirst(name, noFilter);
    }

    /** Return the first Symbol from this or outward scopes with the given name that matches the
     *  given filter. Returns null if none.
     */
    public Symbol findFirst(Name name, Predicate<Symbol> sf) {
        Iterator<Symbol> it = getSymbolsByName(name, sf).iterator();
        return it.hasNext() ? it.next() : null;
    }

    /** Returns true iff there are is at least one Symbol in this scope matching the given filter.
     *  Does not inspect outward scopes.
     */
    public boolean anyMatch(Predicate<Symbol> filter) {
        return getSymbols(filter, NON_RECURSIVE).iterator().hasNext();
    }

    /** Returns true iff the given Symbol is in this scope or any outward scope.
     */
    public boolean includes(final Symbol sym) {
        return includes(sym, RECURSIVE);
    }

    /** Returns true iff the given Symbol is in this scope, optionally checking outward scopes.
     */
    public boolean includes(final Symbol sym, LookupKind lookupKind) {
        return getSymbolsByName(sym.name, t -> t == sym, lookupKind).iterator().hasNext();
    }

    /** Returns true iff this scope does not contain any Symbol. Does not inspect outward scopes.
     */
    public boolean isEmpty() {
        return !getSymbols(NON_RECURSIVE).iterator().hasNext();
    }

    /** Returns the Scope from which the givins Symbol originates in this scope.
     */
    public abstract Scope getOrigin(Symbol byName);

    /** Returns true iff the given Symbol is part of this scope due to a static import.
     */
    public abstract boolean isStaticallyImported(Symbol byName);

    private static final Predicate<Symbol> noFilter = null;

    /** A list of scopes to be notified if items are to be removed from this scope.
     */
    ScopeListenerList listeners = new ScopeListenerList();

    public interface ScopeListener {
        void symbolAdded(Symbol sym, Scope s);
        void symbolRemoved(Symbol sym, Scope s);
    }

    /**
     * A list of scope listeners; listeners are stored in weak references, to avoid memory leaks.
     * When the listener list is scanned (upon notification), elements corresponding to GC-ed
     * listeners are removed so that the listener list size is kept in check.
     */
    public static class ScopeListenerList {

        List<WeakReference<ScopeListener>> listeners = List.nil();

        void add(ScopeListener sl) {
            listeners = listeners.prepend(new WeakReference<>(sl));
        }

        void symbolAdded(Symbol sym, Scope scope) {
            walkReferences(sym, scope, false);
        }

        void symbolRemoved(Symbol sym, Scope scope) {
            walkReferences(sym, scope, true);
        }

        private void walkReferences(Symbol sym, Scope scope, boolean isRemove) {
            ListBuffer<WeakReference<ScopeListener>> newListeners = new ListBuffer<>();
            for (WeakReference<ScopeListener> wsl : listeners) {
                ScopeListener sl = wsl.get();
                if (sl != null) {
                    if (isRemove) {
                        sl.symbolRemoved(sym, scope);
                    } else {
                        sl.symbolAdded(sym, scope);
                    }
                    newListeners.add(wsl);
                }
            }
            listeners = newListeners.toList();
        }
    }

    public enum LookupKind {
        RECURSIVE,
        NON_RECURSIVE;
    }

    /**A scope into which Symbols can be added.*/
    public abstract static class WriteableScope extends Scope {

        public WriteableScope(Symbol owner) {
            super(owner);
        }

        /** Enter the given Symbol into this scope.
         */
        public abstract void enter(Symbol c);
        /** Enter symbol sym in this scope if not already there.
         */
        public abstract void enterIfAbsent(Symbol c);

        public abstract void remove(Symbol c);

        /** Construct a fresh scope within this scope, with same owner. The new scope may
         *  shares internal structures with the this scope. Used in connection with
         *  method leave if scope access is stack-like in order to avoid allocation
         *  of fresh tables.
         */
        public final WriteableScope dup() {
            return dup(this.owner);
        }

        /** Construct a fresh scope within this scope, with new owner. The new scope may
         *  shares internal structures with the this scope. Used in connection with
         *  method leave if scope access is stack-like in order to avoid allocation
         *  of fresh tables.
         */
        public abstract WriteableScope dup(Symbol newOwner);

        /** Must be called on dup-ed scopes to be able to work with the outward scope again.
         */
        public abstract WriteableScope leave();

        /** Construct a fresh scope within this scope, with same owner. The new scope
         *  will not share internal structures with this scope.
         */
        public final WriteableScope dupUnshared() {
            return dupUnshared(owner);
        }

        /** Construct a fresh scope within this scope, with new owner. The new scope
         *  will not share internal structures with this scope.
         */
        public abstract WriteableScope dupUnshared(Symbol newOwner);

        /** Create a new WriteableScope.
         */
        public static WriteableScope create(Symbol owner) {
            return new ScopeImpl(owner);
        }

    }

    private static class ScopeImpl extends WriteableScope {
        /** The number of scopes that share this scope's hash table.
         */
        private int shared;

        /** Next enclosing scope (with whom this scope may share a hashtable)
         */
        public ScopeImpl next;

        /** A hash table for the scope's entries.
         */
        Entry[] table;

        /** Mask for hash codes, always equal to (table.length - 1).
         */
        int hashMask;

        /** A linear list that also contains all entries in
         *  reverse order of appearance (i.e later entries are pushed on top).
         */
        public Entry elems;

        /** The number of elements in this scope.
         * This includes deleted elements, whose value is the sentinel.
         */
        int nelems = 0;

        int removeCount = 0;

        /** Use as a "not-found" result for lookup.
         * Also used to mark deleted entries in the table.
         */
        private static final Entry sentinel = new Entry(null, null, null, null);

        /** The hash table's initial size.
         */
        private static final int INITIAL_SIZE = 0x10;

        /** Construct a new scope, within scope next, with given owner, using
         *  given table. The table's length must be an exponent of 2.
         */
        private ScopeImpl(ScopeImpl next, Symbol owner, Entry[] table) {
            super(owner);
            this.next = next;
            Assert.check(owner != null);
            this.table = table;
            this.hashMask = table.length - 1;
        }

        /** Convenience constructor used for dup and dupUnshared. */
        private ScopeImpl(ScopeImpl next, Symbol owner, Entry[] table, int nelems) {
            this(next, owner, table);
            this.nelems = nelems;
        }

        /** Construct a new scope, within scope next, with given owner,
         *  using a fresh table of length INITIAL_SIZE.
         */
        public ScopeImpl(Symbol owner) {
            this(null, owner, new Entry[INITIAL_SIZE]);
        }

        /** Construct a fresh scope within this scope, with new owner,
         *  which shares its table with the outer scope. Used in connection with
         *  method leave if scope access is stack-like in order to avoid allocation
         *  of fresh tables.
         */
        public WriteableScope dup(Symbol newOwner) {
            ScopeImpl result = new ScopeImpl(this, newOwner, this.table, this.nelems);
            shared++;
            // System.out.println("====> duping scope " + this.hashCode() + " owned by " + newOwner + " to " + result.hashCode());
            // new Error().printStackTrace(System.out);
            return result;
        }

        /** Construct a fresh scope within this scope, with new owner,
         *  with a new hash table, whose contents initially are those of
         *  the table of its outer scope.
         */
        public WriteableScope dupUnshared(Symbol newOwner) {
            if (shared > 0) {
                //The nested Scopes might have already added something to the table, so all items
                //that don't originate in this Scope or any of its outer Scopes need to be cleared:
                Set<Scope> acceptScopes = Collections.newSetFromMap(new IdentityHashMap<>());
                ScopeImpl c = this;
                while (c != null) {
                    acceptScopes.add(c);
                    c = c.next;
                }
                int n = 0;
                Entry[] oldTable = this.table;
                Entry[] newTable = new Entry[this.table.length];
                for (int i = 0; i < oldTable.length; i++) {
                    Entry e = oldTable[i];
                    while (e != null && e != sentinel && !acceptScopes.contains(e.scope)) {
                        e = e.shadowed;
                    }
                    if (e != null) {
                        n++;
                        newTable[i] = e;
                    }
                }
                return new ScopeImpl(this, newOwner, newTable, n);
            } else {
                return new ScopeImpl(this, newOwner, this.table.clone(), this.nelems);
            }
        }

        /** Remove all entries of this scope from its table, if shared
         *  with next.
         */
        public WriteableScope leave() {
            Assert.check(shared == 0);
            if (table != next.table) return next;
            while (elems != null) {
                int hash = getIndex(elems.sym.name);
                Entry e = table[hash];
                Assert.check(e == elems, elems.sym);
                table[hash] = elems.shadowed;
                elems = elems.nextSibling;
            }
            Assert.check(next.shared > 0);
            next.shared--;
            next.nelems = nelems;
            // System.out.println("====> leaving scope " + this.hashCode() + " owned by " + this.owner + " to " + next.hashCode());
            // new Error().printStackTrace(System.out);
            return next;
        }

        /** Double size of hash table.
         */
        private void dble() {
            Assert.check(shared == 0);
            Entry[] oldtable = table;
            Entry[] newtable = new Entry[oldtable.length * 2];
            for (ScopeImpl s = this; s != null; s = s.next) {
                if (s.table == oldtable) {
                    Assert.check(s == this || s.shared != 0);
                    s.table = newtable;
                    s.hashMask = newtable.length - 1;
                }
            }
            int n = 0;
            for (int i = oldtable.length; --i >= 0; ) {
                Entry e = oldtable[i];
                if (e != null && e != sentinel) {
                    table[getIndex(e.sym.name)] = e;
                    n++;
                }
            }
            // We don't need to update nelems for shared inherited scopes,
            // since that gets handled by leave().
            nelems = n;
        }

        /** Enter symbol sym in this scope.
         */
        public void enter(Symbol sym) {
            Assert.check(shared == 0);
            if (nelems * 3 >= hashMask * 2)
                dble();
            int hash = getIndex(sym.name);
            Entry old = table[hash];
            if (old == null) {
                old = sentinel;
                nelems++;
            }
            Entry e = new Entry(sym, old, elems, this);
            table[hash] = e;
            elems = e;

            //notify listeners
            listeners.symbolAdded(sym, this);
        }

        /** Remove symbol from this scope.
         */
        public void remove(Symbol sym) {
            Assert.check(shared == 0);
            Entry e = lookup(sym.name, candidate -> candidate == sym);
            if (e.scope == null) return;

            // remove e from table and shadowed list;
            int i = getIndex(sym.name);
            Entry te = table[i];
            if (te == e)
                table[i] = e.shadowed;
            else while (true) {
                if (te.shadowed == e) {
                    te.shadowed = e.shadowed;
                    break;
                }
                te = te.shadowed;
            }

            // remove e from elems and sibling list
            if (elems == e) {
                elems = e.nextSibling;
                if (elems != null)
                    elems.prevSibling = null;
            } else {
                Assert.check(e.prevSibling != null, e.sym);
                e.prevSibling.nextSibling = e.nextSibling;
                if (e.nextSibling != null)
                    e.nextSibling.prevSibling = e.prevSibling;
            }

            removeCount++;

            //notify listeners
            listeners.symbolRemoved(sym, this);
        }

        /** Enter symbol sym in this scope if not already there.
         */
        public void enterIfAbsent(Symbol sym) {
            Assert.check(shared == 0);
            Entry e = lookup(sym.name);
            while (e.scope == this && e.sym.kind != sym.kind) e = e.next();
            if (e.scope != this) enter(sym);
        }

        /** Given a class, is there already a class with same fully
         *  qualified name in this (import) scope?
         */
        public boolean includes(Symbol c) {
            for (Scope.Entry e = lookup(c.name);
                 e.scope == this;
                 e = e.next()) {
                if (e.sym == c) return true;
            }
            return false;
        }

        /** Return the entry associated with given name, starting in
         *  this scope and proceeding outwards. If no entry was found,
         *  return the sentinel, which is characterized by having a null in
         *  both its scope and sym fields, whereas both fields are non-null
         *  for regular entries.
         */
        protected Entry lookup(Name name) {
            return lookup(name, noFilter);
        }

        protected Entry lookup(Name name, Predicate<Symbol> sf) {
            Entry e = table[getIndex(name)];
            if (e == null || e == sentinel)
                return sentinel;
            while (e.scope != null && (e.sym.name != name || (sf != null && !sf.test(e.sym))))
                e = e.shadowed;
            return e;
        }

        public Symbol findFirst(Name name, Predicate<Symbol> sf) {
            return lookup(name, sf).sym;
        }

        /*void dump (java.io.PrintStream out) {
            out.println(this);
            for (int l=0; l < table.length; l++) {
                Entry le = table[l];
                out.print("#"+l+": ");
                if (le==sentinel) out.println("sentinel");
                else if(le == null) out.println("null");
                else out.println(""+le+" s:"+le.sym);
            }
        }*/

        /** Look for slot in the table.
         *  We use open addressing with double hashing.
         */
        int getIndex (Name name) {
            int h = name.hashCode();
            int i = h & hashMask;
            // The expression below is always odd, so it is guaranteed
            // to be mutually prime with table.length, a power of 2.
            int x = hashMask - ((h + (h >> 16)) << 1);
            int d = -1; // Index of a deleted item.
            for (;;) {
                Entry e = table[i];
                if (e == null)
                    return d >= 0 ? d : i;
                if (e == sentinel) {
                    // We have to keep searching even if we see a deleted item.
                    // However, remember the index in case we fail to find the name.
                    if (d < 0)
                        d = i;
                } else if (e.sym.name == name)
                    return i;
                i = (i + x) & hashMask;
            }
        }

        public boolean anyMatch(Predicate<Symbol> sf) {
            return getSymbols(sf, NON_RECURSIVE).iterator().hasNext();
        }

        public Iterable<Symbol> getSymbols(final Predicate<Symbol> sf,
                                           final LookupKind lookupKind) {
            return () -> new Iterator<Symbol>() {
                private ScopeImpl currScope = ScopeImpl.this;
                private Entry currEntry = elems;
                private int seenRemoveCount = currScope.removeCount;
                {
                    update();
                }

                public boolean hasNext() {
                    if (seenRemoveCount != currScope.removeCount &&
                        currEntry != null &&
                        !currEntry.scope.includes(currEntry.sym)) {
                        doNext(); //skip entry that is no longer in the Scope
                        seenRemoveCount = currScope.removeCount;
                    }
                    return currEntry != null;
                }

                public Symbol next() {
                    if (!hasNext()) {
                        throw new NoSuchElementException();
                    }

                    return doNext();
                }
                private Symbol doNext() {
                    Symbol sym = (currEntry == null ? null : currEntry.sym);
                    if (currEntry != null) {
                        currEntry = currEntry.nextSibling;
                    }
                    update();
                    return sym;
                }

                private void update() {
                    skipToNextMatchingEntry();
                    if (lookupKind == RECURSIVE) {
                        while (currEntry == null && currScope.next != null) {
                            currScope = currScope.next;
                            currEntry = currScope.elems;
                            seenRemoveCount = currScope.removeCount;
                            skipToNextMatchingEntry();
                        }
                    }
                }

                void skipToNextMatchingEntry() {
                    while (currEntry != null && sf != null && !sf.test(currEntry.sym)) {
                        currEntry = currEntry.nextSibling;
                    }
                }
            };
        }

        public Iterable<Symbol> getSymbolsByName(final Name name,
                                                 final Predicate<Symbol> sf,
                                                 final LookupKind lookupKind) {
            return () -> new Iterator<Symbol>() {
               Entry currentEntry = lookup(name, sf);
               int seenRemoveCount = currentEntry.scope != null ?
                       currentEntry.scope.removeCount : -1;

               public boolean hasNext() {
                   if (currentEntry.scope != null &&
                       seenRemoveCount != currentEntry.scope.removeCount &&
                       !currentEntry.scope.includes(currentEntry.sym)) {
                       doNext(); //skip entry that is no longer in the Scope
                   }
                   return currentEntry.scope != null &&
                           (lookupKind == RECURSIVE ||
                            currentEntry.scope == ScopeImpl.this);
               }
               public Symbol next() {
                   if (!hasNext()) {
                       throw new NoSuchElementException();
                   }
                   return doNext();
               }
               private Symbol doNext() {
                   Entry prevEntry = currentEntry;
                   currentEntry = currentEntry.next(sf);
                   return prevEntry.sym;
               }
               public void remove() {
                   throw new UnsupportedOperationException();
               }
           };
        }

        public Scope getOrigin(Symbol s) {
            for (Scope.Entry e = lookup(s.name); e.scope != null ; e = e.next()) {
                if (e.sym == s) {
                    return this;
                }
            }
            return null;
        }

        @Override
        public boolean isStaticallyImported(Symbol s) {
            return false;
        }

        public String toString() {
            StringBuilder result = new StringBuilder();
            result.append("Scope[");
            for (ScopeImpl s = this; s != null ; s = s.next) {
                if (s != this) result.append(" | ");
                for (Entry e = s.elems; e != null; e = e.nextSibling) {
                    if (e != s.elems) result.append(", ");
                    result.append(e.sym);
                }
            }
            result.append("]");
            return result.toString();
        }
    }

    /** A class for scope entries.
     */
    private static class Entry {

        /** The referenced symbol.
         *  sym == null   iff   this == sentinel
         */
        public Symbol sym;

        /** An entry with the same hash code, or sentinel.
         */
        private Entry shadowed;

        /** Next entry in same scope.
         */
        public Entry nextSibling;

        /** Prev entry in same scope.
         */
        public Entry prevSibling;

        /** The entry's scope.
         *  scope == null   iff   this == sentinel
         */
        public ScopeImpl scope;

        public Entry(Symbol sym, Entry shadowed, Entry nextSibling, ScopeImpl scope) {
            this.sym = sym;
            this.shadowed = shadowed;
            this.nextSibling = nextSibling;
            this.scope = scope;
            if (nextSibling != null)
                nextSibling.prevSibling = this;
        }

        /** Return next entry with the same name as this entry, proceeding
         *  outwards if not found in this scope.
         */
        public Entry next() {
            return shadowed;
        }

        public Entry next(Predicate<Symbol> sf) {
            if (shadowed.sym == null || sf == null || sf.test(shadowed.sym)) return shadowed;
            else return shadowed.next(sf);
        }

    }

    public static class ImportScope extends CompoundScope {

        public ImportScope(Symbol owner) {
            super(owner);
        }

        /**Finalize the content of the ImportScope to speed-up future lookups.
         * No further changes to class hierarchy or class content will be reflected.
         */
        public void finalizeScope() {
            for (List<Scope> scopes = this.subScopes.toList(); scopes.nonEmpty(); scopes = scopes.tail) {
                scopes.head = finalizeSingleScope(scopes.head);
            }
        }

        protected Scope finalizeSingleScope(Scope impScope) {
            if (impScope instanceof FilterImportScope filterImportScope
                    && impScope.owner.kind == Kind.TYP
                    && filterImportScope.isStaticallyImported()) {
                WriteableScope finalized = WriteableScope.create(impScope.owner);

                for (Symbol sym : impScope.getSymbols()) {
                    finalized.enter(sym);
                }

                finalized.listeners.add(new ScopeListener() {
                    @Override
                    public void symbolAdded(Symbol sym, Scope s) {
                        Assert.error("The scope is sealed.");
                    }

                    @Override
                    public void symbolRemoved(Symbol sym, Scope s) {
                        Assert.error("The scope is sealed.");
                    }
                });

                return finalized;
            }

            return impScope;
        }

    }

    public static class NamedImportScope extends ImportScope {

        /*A cache for quick lookup of Scopes that may contain the given name.
          ScopeImpl and Entry is not used, as it is maps names to Symbols,
          but it is necessary to map names to Scopes at this place (so that any
          changes to the content of the Scopes is reflected when looking up the
          Symbols.
         */
        private final Map<Name, Scope[]> name2Scopes = new HashMap<>();

        public NamedImportScope(Symbol owner) {
            super(owner);
        }

        public Scope importByName(Types types, Scope origin, Name name, ImportFilter filter, JCImport imp, BiConsumer<JCImport, CompletionFailure> cfHandler) {
            return appendScope(new FilterImportScope(types, origin, name, filter, imp, cfHandler), name);
        }

        public Scope importType(Scope delegate, Scope origin, Symbol sym) {
            return appendScope(new SingleEntryScope(delegate.owner, sym, origin), sym.name);
        }

        private Scope appendScope(Scope newScope, Name name) {
            appendSubScope(newScope);
            Scope[] existing = name2Scopes.get(name);
            if (existing != null)
                existing = Arrays.copyOf(existing, existing.length + 1);
            else
                existing = new Scope[1];
            existing[existing.length - 1] = newScope;
            name2Scopes.put(name, existing);
            return newScope;
        }

        @Override
        public Iterable<Symbol> getSymbolsByName(Name name, Predicate<Symbol> sf, LookupKind lookupKind) {
            Scope[] scopes = name2Scopes.get(name);
            if (scopes == null)
                return Collections.emptyList();
            return () -> Iterators.createCompoundIterator(Arrays.asList(scopes),
                                                          scope -> scope.getSymbolsByName(name,
                                                                                          sf,
                                                                                          lookupKind)
                                                                        .iterator());
        }
        public void finalizeScope() {
            super.finalizeScope();
            for (Scope[] scopes : name2Scopes.values()) {
                for (int i = 0; i < scopes.length; i++) {
                    scopes[i] = finalizeSingleScope(scopes[i]);
                }
            }
        }

        private static class SingleEntryScope extends Scope {

            private final Symbol sym;
            private final List<Symbol> content;
            private final Scope origin;

            public SingleEntryScope(Symbol owner, Symbol sym, Scope origin) {
                super(owner);
                this.sym = sym;
                this.content = List.of(sym);
                this.origin = origin;
            }

            @Override
            public Iterable<Symbol> getSymbols(Predicate<Symbol> sf, LookupKind lookupKind) {
                return sf == null || sf.test(sym) ? content : Collections.emptyList();
            }

            @Override
            public Iterable<Symbol> getSymbolsByName(Name name,
                                                     Predicate<Symbol> sf,
                                                     LookupKind lookupKind) {
                return sym.name == name &&
                       (sf == null || sf.test(sym)) ? content : Collections.emptyList();
            }

            @Override
            public Scope getOrigin(Symbol byName) {
                return sym == byName ? origin : null;
            }

            @Override
            public boolean isStaticallyImported(Symbol byName) {
                return false;
            }

        }
    }

    public static class StarImportScope extends ImportScope {

        public StarImportScope(Symbol owner) {
            super(owner);
        }

        public void importAll(Types types, Scope origin,
                              ImportFilter filter,
                              JCImport imp,
                              BiConsumer<JCImport, CompletionFailure> cfHandler) {
            for (Scope existing : subScopes) {
                Assert.check(existing instanceof FilterImportScope);
                FilterImportScope fis = (FilterImportScope) existing;
                if (fis.origin == origin && fis.filter == filter &&
                    fis.imp.staticImport == imp.staticImport)
                    return ; //avoid entering the same scope twice
            }
            prependSubScope(new FilterImportScope(types, origin, null, filter, imp, cfHandler));
        }

        public boolean isFilled() {
            return subScopes.nonEmpty();
        }

    }

    public interface ImportFilter {
        public boolean accepts(Scope origin, Symbol sym);
    }

    private static class FilterImportScope extends Scope {

        private final Types types;
        private final Scope origin;
        private final Name  filterName;
        private final ImportFilter filter;
        private final JCImport imp;
        private final BiConsumer<JCImport, CompletionFailure> cfHandler;

        public FilterImportScope(Types types,
                                 Scope origin,
                                 Name  filterName,
                                 ImportFilter filter,
                                 JCImport imp,
                                 BiConsumer<JCImport, CompletionFailure> cfHandler) {
            super(origin.owner);
            this.types = types;
            this.origin = origin;
            this.filterName = filterName;
            this.filter = filter;
            this.imp = imp;
            this.cfHandler = cfHandler;
        }

        @Override
        public Iterable<Symbol> getSymbols(final Predicate<Symbol> sf, final LookupKind lookupKind) {
            if (filterName != null)
                return getSymbolsByName(filterName, sf, lookupKind);
            try {
                SymbolImporter si = new SymbolImporter(imp.staticImport) {
                    @Override
                    Iterable<Symbol> doLookup(TypeSymbol tsym) {
                        return tsym.members().getSymbols(sf, lookupKind);
                    }
                };
                List<Iterable<Symbol>> results =
                        si.importFrom((TypeSymbol) origin.owner, List.nil());
                return () -> createFilterIterator(createCompoundIterator(results,
                                                                         Iterable::iterator),
                                                  s -> filter.accepts(origin, s));
            } catch (CompletionFailure cf) {
                cfHandler.accept(imp, cf);
                return Collections.emptyList();
            }
        }

        @Override
        public Iterable<Symbol> getSymbolsByName(final Name name,
                                                 final Predicate<Symbol> sf,
                                                 final LookupKind lookupKind) {
            if (filterName != null && filterName != name)
                return Collections.emptyList();
            try {
                SymbolImporter si = new SymbolImporter(imp.staticImport) {
                    @Override
                    Iterable<Symbol> doLookup(TypeSymbol tsym) {
                        return tsym.members().getSymbolsByName(name, sf, lookupKind);
                    }
                };
                List<Iterable<Symbol>> results =
                        si.importFrom((TypeSymbol) origin.owner, List.nil());
                return () -> createFilterIterator(createCompoundIterator(results,
                                                                         Iterable::iterator),
                                                  s -> filter.accepts(origin, s));
            } catch (CompletionFailure cf) {
                cfHandler.accept(imp, cf);
                return Collections.emptyList();
            }
        }

        @Override
        public Scope getOrigin(Symbol byName) {
            return origin;
        }

        @Override
        public boolean isStaticallyImported(Symbol byName) {
            return isStaticallyImported();
        }

        public boolean isStaticallyImported() {
            return imp.staticImport;
        }

        abstract class SymbolImporter {
            Set<Symbol> processed = new HashSet<>();
            List<Iterable<Symbol>> delegates = List.nil();
            final boolean inspectSuperTypes;
            public SymbolImporter(boolean inspectSuperTypes) {
                this.inspectSuperTypes = inspectSuperTypes;
            }
            List<Iterable<Symbol>> importFrom(TypeSymbol tsym, List<Iterable<Symbol>> results) {
                if (tsym == null || !processed.add(tsym))
                    return results;


                if (inspectSuperTypes) {
                    // also import inherited names
                    results = importFrom(types.supertype(tsym.type).tsym, results);
                    for (Type t : types.interfaces(tsym.type))
                        results = importFrom(t.tsym, results);
                }

                return results.prepend(doLookup(tsym));
            }
            abstract Iterable<Symbol> doLookup(TypeSymbol tsym);
        }

    }

    /** A class scope adds capabilities to keep track of changes in related
     *  class scopes - this allows client to realize whether a class scope
     *  has changed, either directly (because a new member has been added/removed
     *  to this scope) or indirectly (i.e. because a new member has been
     *  added/removed into a supertype scope)
     */
    public static class CompoundScope extends Scope implements ScopeListener {

        ListBuffer<Scope> subScopes = new ListBuffer<>();
        private int mark = 0;

        public CompoundScope(Symbol owner) {
            super(owner);
        }

        public void prependSubScope(Scope that) {
           if (that != null) {
                subScopes.prepend(that);
                that.listeners.add(this);
                mark++;
                listeners.symbolAdded(null, this);
           }
        }

        public void appendSubScope(Scope that) {
           if (that != null) {
                subScopes.append(that);
                that.listeners.add(this);
                mark++;
                listeners.symbolAdded(null, this);
           }
        }

        public void symbolAdded(Symbol sym, Scope s) {
            mark++;
            listeners.symbolAdded(sym, s);
        }

        public void symbolRemoved(Symbol sym, Scope s) {
            mark++;
            listeners.symbolRemoved(sym, s);
        }

        public int getMark() {
            return mark;
        }

        @Override
        public String toString() {
            StringBuilder buf = new StringBuilder();
            buf.append("CompoundScope{");
            String sep = "";
            for (Scope s : subScopes) {
                buf.append(sep);
                buf.append(s);
                sep = ",";
            }
            buf.append("}");
            return buf.toString();
        }

        @Override
        public Iterable<Symbol> getSymbols(final Predicate<Symbol> sf,
                                           final LookupKind lookupKind) {
            return () -> Iterators.createCompoundIterator(subScopes,
                                                          scope -> scope.getSymbols(sf,
                                                                                    lookupKind)
                                                                        .iterator());
        }

        @Override
        public Iterable<Symbol> getSymbolsByName(final Name name,
                                                 final Predicate<Symbol> sf,
                                                 final LookupKind lookupKind) {
            return () -> Iterators.createCompoundIterator(subScopes,
                                                          scope -> scope.getSymbolsByName(name,
                                                                                          sf,
                                                                                          lookupKind)
                                                                        .iterator());
        }

        @Override
        public Scope getOrigin(Symbol sym) {
            for (Scope delegate : subScopes) {
                if (delegate.includes(sym))
                    return delegate.getOrigin(sym);
            }

            return null;
        }

        @Override
        public boolean isStaticallyImported(Symbol sym) {
            for (Scope delegate : subScopes) {
                if (delegate.includes(sym))
                    return delegate.isStaticallyImported(sym);
            }

            return false;
        }

    }

    /** An error scope, for which the owner should be an error symbol. */
    public static class ErrorScope extends ScopeImpl {
        ErrorScope(ScopeImpl next, Symbol errSymbol, Entry[] table) {
            super(next, /*owner=*/errSymbol, table);
        }
        public ErrorScope(Symbol errSymbol) {
            super(errSymbol);
        }
        public WriteableScope dup(Symbol newOwner) {
            return new ErrorScope(this, newOwner, table);
        }
        public WriteableScope dupUnshared(Symbol newOwner) {
            return new ErrorScope(this, newOwner, table.clone());
        }
        public Entry lookup(Name name) {
            Entry e = super.lookup(name);
            if (e.scope == null)
                return new Entry(owner, null, null, null);
            else
                return e;
        }
    }
}