src/main/java/net/starlark/java/eval/StarlarkList.java - bazel - Git at Google

 // Copyright 2014 The Bazel Authors. All rights reserved.
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
 // You may obtain a copy of the License at
 //
 //    http://www.apache.org/licenses/LICENSE-2.0
 //
 // Unless required by applicable law or agreed to in writing, software
 // distributed under the License is distributed on an "AS IS" BASIS,
 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 // See the License for the specific language governing permissions and
 // limitations under the License.

 package net.starlark.java.eval;

 import com.google.common.base.Preconditions;
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.Iterables;
 import java.util.AbstractList;
 import java.util.Arrays;
 import java.util.Collection;
 import javax.annotation.Nullable;
 import net.starlark.java.annot.Param;
 import net.starlark.java.annot.ParamType;
 import net.starlark.java.annot.StarlarkBuiltin;
 import net.starlark.java.annot.StarlarkMethod;

 /**
  * A StarlarkList is a mutable finite sequence of values.
  *
  * <p>Starlark operations on lists, including element update and the {@code append}, {@code insert},
  * and {@code extend} methods, may insert arbitrary Starlark values as list elements, regardless of
  * the type argument used to reference to the list from Java code. Therefore, as long as a list is
  * mutable, Java code should refer to it only through a type such as {@code StarlarkList<Object>} or
  * {@code StarlarkList<?>} to avoid undermining the type-safety of the Java application. Once the
  * list becomes frozen, it is safe to {@link #cast} it to a more specific type that accurately
  * reflects its elements, such as {@code StarlarkList<String>}.
  *
  * <p>The following List methods, by inheriting their implementations from AbstractList, are
  * effectively disabled. Use the corresponding methods with "element" in their name; they may report
  * mutation failure by throwing a checked exception.
  *
  * <pre>
  *   boolean add(E)                    -- use addElement
  *   boolean remove(Object)            -- use removeElement
  *   boolean addAll(Collection)        -- use addElements
  *   boolean addAll(int, Collection)
  *   boolean removeAll(Collection)     -- use removeElements
  *   boolean retainAll(Collection)
  *   void clear()                      -- use clearElements
  *   E set(int, E)                     -- use setElementAt
  *   void add(int, E)                  -- use addElementAt
  *   E remove(int)                     -- use removeElementAt
  * </pre>
  */
 @StarlarkBuiltin(
     name = "list",
     category = "core",
     doc =
         "The built-in list type. Example list expressions:<br>"
             + "<pre class=language-python>x = [1, 2, 3]</pre>"
             + "Accessing elements is possible using indexing (starts from <code>0</code>):<br>"
             + "<pre class=language-python>e = x[1]   # e == 2</pre>"
             + "Lists support the <code>+</code> operator to concatenate two lists. Example:<br>"
             + "<pre class=language-python>x = [1, 2] + [3, 4]   # x == [1, 2, 3, 4]\n"
             + "x = [\"a\", \"b\"]\n"
             + "x += [\"c\"]            # x == [\"a\", \"b\", \"c\"]</pre>"
             + "Similar to strings, lists support slice operations:"
             + "<pre class=language-python>['a', 'b', 'c', 'd'][1:3]   # ['b', 'c']\n"
             + "['a', 'b', 'c', 'd'][::2]  # ['a', 'c']\n"
             + "['a', 'b', 'c', 'd'][3:0:-1]  # ['d', 'c', 'b']</pre>"
             + "Lists are mutable, as in Python.")
 public final class StarlarkList<E> extends AbstractList<E>
     implements Sequence<E>, StarlarkValue, Mutability.Freezable, Comparable<StarlarkList<?>> {

   // The implementation strategy is similar to ArrayList,
   // but without the extra indirection of using ArrayList.

   // elems[0:size] holds the logical elements, and elems[size:] are not used.
   // elems.getClass() == Object[].class. This is necessary to avoid ArrayStoreException.
   private int size;
   private int iteratorCount; // number of active iterators (unused once frozen)
   private Object[] elems = EMPTY_ARRAY; // elems[i] == null  iff  i >= size

   /** Final except for {@link #unsafeShallowFreeze}; must not be modified any other way. */
   private Mutability mutability;

   private static final Object[] EMPTY_ARRAY = {};

   private StarlarkList(@Nullable Mutability mutability, Object[] elems, int size) {
     Preconditions.checkArgument(elems.getClass() == Object[].class);
     this.elems = elems;
     this.size = size;
     this.mutability = mutability == null ? Mutability.IMMUTABLE : mutability;
   }

   /**
    * Takes ownership of the supplied array of class Object[].class, and returns a new StarlarkList
    * instance that initially wraps the array. The caller must not subsequently modify the array, but
    * the StarlarkList instance may do so.
    */
   static <T> StarlarkList<T> wrap(@Nullable Mutability mutability, Object[] elems) {
     return new StarlarkList<>(mutability, elems, elems.length);
   }

   @Override
   public boolean isImmutable() {
     return mutability().isFrozen();
   }

   @Override
   public void checkHashable() throws EvalException {
     // Even a frozen list is unhashable.
     throw Starlark.errorf("unhashable type: 'list'");
   }

   @Override
   public boolean updateIteratorCount(int delta) {
     if (mutability().isFrozen()) {
       return false;
     }
     if (delta > 0) {
       iteratorCount++;
     } else if (delta < 0) {
       iteratorCount--;
     }
     return iteratorCount > 0;
   }

   /**
    * A shared instance for the empty list with immutable mutability.
    *
    * <p>Other immutable empty list objects can exist, e.g. lists that were once mutable but whose
    * environments were then frozen. This instance is for empty lists that were always frozen from
    * the beginning.
    */
   private static final StarlarkList<?> EMPTY = wrap(Mutability.IMMUTABLE, EMPTY_ARRAY);

   /** Returns an empty frozen list of the desired type. */
   @SuppressWarnings("unchecked")
   public static <T> StarlarkList<T> empty() {
     return (StarlarkList<T>) EMPTY;
   }

   /** Returns a new, empty list with the specified Mutability. */
   public static <T> StarlarkList<T> newList(Mutability mutability) {
     return wrap(mutability, EMPTY_ARRAY);
   }

   /**
    * Returns a {@code StarlarkList} whose items are given by an iterable and which has the given
    * {@link Mutability}. If {@code mutability} is null, the list is immutable.
    */
   public static <T> StarlarkList<T> copyOf(
       @Nullable Mutability mutability, Iterable<? extends T> elems) {
     if (mutability == null
         && elems instanceof StarlarkList
         && ((StarlarkList) elems).isImmutable()) {
       @SuppressWarnings("unchecked")
       StarlarkList<T> list = (StarlarkList<T>) elems; // safe
       return list;
     }

     Object[] array = Iterables.toArray(elems, Object.class);
     checkElemsValid(array);
     return wrap(mutability, array);
   }

   private static void checkElemsValid(Object[] elems) {
     for (Object elem : elems) {
       Starlark.checkValid(elem);
     }
   }

   /**
    * Returns an immutable list with the given elements. Equivalent to {@code copyOf(null, elems)}.
    */
   public static <T> StarlarkList<T> immutableCopyOf(Iterable<? extends T> elems) {
     return copyOf(null, elems);
   }

   /**
    * Returns a {@code StarlarkList} with the given items and the {@link Mutability}. If {@code
    * mutability} is null, the list is immutable.
    */
   public static <T> StarlarkList<T> of(@Nullable Mutability mutability, T... elems) {
     if (elems.length == 0) {
       return newList(mutability);
     }

     checkElemsValid(elems);
     return wrap(mutability, Arrays.copyOf(elems, elems.length, Object[].class));
   }

   /** Returns an immutable {@code StarlarkList} with the given items. */
   public static <T> StarlarkList<T> immutableOf(T... elems) {
     checkElemsValid(elems);
     return wrap(null, Arrays.copyOf(elems, elems.length, Object[].class));
   }

   @Override
   public Mutability mutability() {
     return mutability;
   }

   @Override
   public void unsafeShallowFreeze() {
     Mutability.Freezable.checkUnsafeShallowFreezePrecondition(this);
     this.mutability = Mutability.IMMUTABLE;
   }

   @Override
   public ImmutableList<E> getImmutableList() {
     // Optimization: a frozen array needn't be copied.
     // If the entire array is full, we can wrap it directly.
     if (elems.length == size && mutability().isFrozen()) {
       return Tuple.wrapImmutable(elems);
     }

     return ImmutableList.copyOf(this);
   }

   /**
    * Returns a new {@code StarlarkList} that is the concatenation of two {@code StarlarkList}s. The
    * new list will have the given {@link Mutability}.
    */
   public static <T> StarlarkList<T> concat(
       StarlarkList<? extends T> x, StarlarkList<? extends T> y, Mutability mutability) {
     Object[] res = new Object[x.size + y.size];
     System.arraycopy(x.elems, 0, res, 0, x.size);
     System.arraycopy(y.elems, 0, res, x.size, y.size);
     return wrap(mutability, res);
   }

   @Override
   public int compareTo(StarlarkList<?> that) {
     return Sequence.compare(this, that);
   }

   @Override
   public boolean equals(Object that) {
     // This slightly violates the java.util.List equivalence contract
     // because it considers the class, not just the elements.
     return this == that || (that instanceof StarlarkList && sameElems(this, ((StarlarkList) that)));
   }

   private static boolean sameElems(StarlarkList<?> x, StarlarkList<?> y) {
     if (x.size != y.size) {
       return false;
     }
     for (int i = 0; i < x.size; i++) {
       if (!x.elems[i].equals(y.elems[i])) {
         return false;
       }
     }
     return true;
   }

   @Override
   public int hashCode() {
     // Hash the elements elems[0:size].
     int result = 1;
     for (int i = 0; i < size; i++) {
       result = 31 * result + elems[i].hashCode();
     }
     return 6047 + 4673 * result;
   }

   @Override
   public void repr(Printer printer) {
     printer.printList(this, "[", ", ", "]");
   }

   // TODO(adonovan): StarlarkValue has 3 String methods yet still we need this fourth. Why?
   @Override
   public String toString() {
     return Starlark.repr(this);
   }

   /** Returns a new StarlarkList containing n consecutive repeats of this tuple. */
   public StarlarkList<E> repeat(StarlarkInt n, Mutability mutability) throws EvalException {
     if (n.signum() <= 0) {
       return wrap(mutability, EMPTY_ARRAY);
     }

     // TODO(adonovan): reject unreasonably large n.
     int ni = n.toInt("repeat");
     Object[] res = new Object[ni * size];
     for (int i = 0; i < ni; i++) {
       System.arraycopy(elems, 0, res, i * size, size);
     }
     return wrap(mutability, res);
   }

   @Override
   @SuppressWarnings("unchecked")
   public E get(int i) {
     if (i >= size) {
       throw new IndexOutOfBoundsException();
     }
     return (E) elems[i]; // unchecked
   }

   @Override
   public int size() {
     return size;
   }

   @Override
   public StarlarkList<E> getSlice(Mutability mu, int start, int stop, int step) {
     RangeList indices = new RangeList(start, stop, step);
     int n = indices.size();
     Object[] res = new Object[n];
     if (step == 1) { // common case
       System.arraycopy(elems, indices.at(0), res, 0, n);
     } else {
       for (int i = 0; i < n; ++i) {
         res[i] = elems[indices.at(i)];
       }
     }
     return wrap(mu, res);
   }

   // Postcondition: elems.length >= mincap.
   private void grow(int mincap) {
     int oldcap = elems.length;
     if (oldcap < mincap) {
       int newcap = oldcap + (oldcap >> 1); // grow by at least 50%
       if (newcap < mincap) {
         newcap = mincap;
       }
       elems = Arrays.copyOf(elems, newcap);
     }
   }

   /**
    * Appends an element to the end of the list, after validating that mutation is allowed.
    *
    * @param element the element to add
    */
   public void addElement(E element) throws EvalException {
     Starlark.checkMutable(this);
     grow(size + 1);
     elems[size++] = element;
   }

   /**
    * Inserts an element at a given position to the list.
    *
    * @param index the new element's index
    * @param element the element to add
    */
   public void addElementAt(int index, E element) throws EvalException {
     Starlark.checkMutable(this);
     grow(size + 1);
     System.arraycopy(elems, index, elems, index + 1, size - index);
     elems[index] = element;
     size++;
   }

   /**
    * Appends all the elements to the end of the list.
    *
    * @param elements the elements to add
    */
   public void addElements(Iterable<? extends E> elements) throws EvalException {
     Starlark.checkMutable(this);
     if (elements instanceof StarlarkList) {
       StarlarkList<?> that = (StarlarkList) elements;
       // (safe even if this == that)
       grow(this.size + that.size);
       System.arraycopy(that.elems, 0, this.elems, this.size, that.size);
       this.size += that.size;
     } else if (elements instanceof Collection) {
       // collection of known size
       Collection<?> that = (Collection) elements;
       grow(size + that.size());
       for (Object x : that) {
         elems[size++] = x;
       }
     } else {
       // iterable
       for (Object x : elements) {
         grow(size + 1);
         elems[size++] = x;
       }
     }
   }

   /**
    * Removes the element at a given index. The index must already have been validated to be in
    * range.
    *
    * @param index the index of the element to remove
    */
   public void removeElementAt(int index) throws EvalException {
     Starlark.checkMutable(this);
     int n = size - index - 1;
     if (n > 0) {
       System.arraycopy(elems, index + 1, elems, index, n);
     }
     elems[--size] = null; // aid GC
   }

   @StarlarkMethod(
       name = "remove",
       doc =
           "Removes the first item from the list whose value is x. "
               + "It is an error if there is no such item.",
       parameters = {@Param(name = "x", doc = "The object to remove.")})
   public void removeElement(Object x) throws EvalException {
     for (int i = 0; i < size; i++) {
       if (elems[i].equals(x)) {
         removeElementAt(i);
         return;
       }
     }
     throw Starlark.errorf("item %s not found in list", Starlark.repr(x));
   }

   /**
    * Sets the position at the given index to contain the given value. Precondition: {@code 0 <=
    * index < size()}.
    */
   public void setElementAt(int index, E value) throws EvalException {
     Starlark.checkMutable(this);
     Preconditions.checkArgument(index < size);
     elems[index] = value;
   }

   @StarlarkMethod(
       name = "append",
       doc = "Adds an item to the end of the list.",
       parameters = {@Param(name = "item", doc = "Item to add at the end.")})
   @SuppressWarnings("unchecked")
   public void append(Object item) throws EvalException {
     addElement((E) item); // unchecked
   }

   @StarlarkMethod(name = "clear", doc = "Removes all the elements of the list.")
   public void clearElements() throws EvalException {
     Starlark.checkMutable(this);
     for (int i = 0; i < size; i++) {
       elems[i] = null; // aid GC
     }
     size = 0;
   }

   @StarlarkMethod(
       name = "insert",
       doc = "Inserts an item at a given position.",
       parameters = {
         @Param(name = "index", doc = "The index of the given position."),
         @Param(name = "item", doc = "The item.")
       })
   @SuppressWarnings("unchecked")
   public void insert(StarlarkInt index, Object item) throws EvalException {
     addElementAt(EvalUtils.toIndex(index.toInt("index"), size), (E) item); // unchecked
   }

   @StarlarkMethod(
       name = "extend",
       doc = "Adds all items to the end of the list.",
       parameters = {@Param(name = "items", doc = "Items to add at the end.")})
   public void extend(Object items) throws EvalException {
     @SuppressWarnings("unchecked")
     Iterable<? extends E> src = (Iterable<? extends E>) Starlark.toIterable(items);
     addElements(src);
   }

   @StarlarkMethod(
       name = "index",
       doc =
           "Returns the index in the list of the first item whose value is x. "
               + "It is an error if there is no such item.",
       parameters = {
         @Param(name = "x", doc = "The object to search."),
         @Param(
             name = "start",
             allowedTypes = {
               @ParamType(type = StarlarkInt.class),
               @ParamType(type = NoneType.class), // TODO(adonovan): this is wrong
             },
             defaultValue = "None",
             named = true, // TODO(adonovan): this is wrong
             doc = "The start index of the list portion to inspect."),
         @Param(
             name = "end",
             allowedTypes = {
               @ParamType(type = StarlarkInt.class),
               @ParamType(type = NoneType.class), // TODO(adonovan): this is wrong
             },
             defaultValue = "None",
             named = true, // TODO(adonovan): this is wrong
             doc = "The end index of the list portion to inspect.")
       })
   public int index(Object x, Object start, Object end) throws EvalException {
     int i = start == Starlark.NONE ? 0 : EvalUtils.toIndex(Starlark.toInt(start, "start"), size);
     int j = end == Starlark.NONE ? size : EvalUtils.toIndex(Starlark.toInt(end, "end"), size);
     for (; i < j; i++) {
       if (elems[i].equals(x)) {
         return i;
       }
     }
     throw Starlark.errorf("item %s not found in list", Starlark.repr(x));
   }

   @StarlarkMethod(
       name = "pop",
       doc =
           "Removes the item at the given position in the list, and returns it. "
               + "If no <code>index</code> is specified, "
               + "it removes and returns the last item in the list.",
       parameters = {
         @Param(
             name = "i",
             allowedTypes = {
               @ParamType(type = StarlarkInt.class),
               @ParamType(type = NoneType.class), // TODO(adonovan): this is not what Python3 does
             },
             defaultValue = "-1",
             doc = "The index of the item.")
       })
   public Object pop(Object i) throws EvalException {
     int arg = i == Starlark.NONE ? -1 : Starlark.toInt(i, "i");
     int index = EvalUtils.getSequenceIndex(arg, size);
     Object result = elems[index];
     removeElementAt(index);
     return result;
   }

   /** Returns a new array of class Object[] containing the list elements. */
   @Override
   public Object[] toArray() {
     return size != 0 ? Arrays.copyOf(elems, size, Object[].class) : EMPTY_ARRAY;
   }

   @SuppressWarnings("unchecked")
   @Override
   public <T> T[] toArray(T[] a) {
     if (a.length < size) {
       return (T[]) Arrays.copyOf(elems, size, a.getClass());
     } else {
       System.arraycopy(elems, 0, a, 0, size);
       Arrays.fill(a, size, a.length, null);
       return a;
     }
   }
 }
	// Copyright 2014 The Bazel Authors. All rights reserved.
	//
	// Licensed under the Apache License, Version 2.0 (the "License");
	// you may not use this file except in compliance with the License.
	// You may obtain a copy of the License at
	//
	// http://www.apache.org/licenses/LICENSE-2.0
	//
	// Unless required by applicable law or agreed to in writing, software
	// distributed under the License is distributed on an "AS IS" BASIS,
	// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
	// See the License for the specific language governing permissions and
	// limitations under the License.

	package net.starlark.java.eval;

	import com.google.common.base.Preconditions;
	import com.google.common.collect.ImmutableList;
	import com.google.common.collect.Iterables;
	import java.util.AbstractList;
	import java.util.Arrays;
	import java.util.Collection;
	import javax.annotation.Nullable;
	import net.starlark.java.annot.Param;
	import net.starlark.java.annot.ParamType;
	import net.starlark.java.annot.StarlarkBuiltin;
	import net.starlark.java.annot.StarlarkMethod;

	/**
	* A StarlarkList is a mutable finite sequence of values.
	*
	* <p>Starlark operations on lists, including element update and the {@code append}, {@code insert},
	* and {@code extend} methods, may insert arbitrary Starlark values as list elements, regardless of
	* the type argument used to reference to the list from Java code. Therefore, as long as a list is
	* mutable, Java code should refer to it only through a type such as {@code StarlarkList<Object>} or
	* {@code StarlarkList<?>} to avoid undermining the type-safety of the Java application. Once the
	* list becomes frozen, it is safe to {@link #cast} it to a more specific type that accurately
	* reflects its elements, such as {@code StarlarkList<String>}.
	*
	* <p>The following List methods, by inheriting their implementations from AbstractList, are
	* effectively disabled. Use the corresponding methods with "element" in their name; they may report
	* mutation failure by throwing a checked exception.
	*
	* <pre>
	* boolean add(E) -- use addElement
	* boolean remove(Object) -- use removeElement
	* boolean addAll(Collection) -- use addElements
	* boolean addAll(int, Collection)
	* boolean removeAll(Collection) -- use removeElements
	* boolean retainAll(Collection)
	* void clear() -- use clearElements
	* E set(int, E) -- use setElementAt
	* void add(int, E) -- use addElementAt
	* E remove(int) -- use removeElementAt
	* </pre>
	*/
	@StarlarkBuiltin(
	name = "list",
	category = "core",
	doc =
	"The built-in list type. Example list expressions:<br>"
	+ "<pre class=language-python>x = [1, 2, 3]</pre>"
	+ "Accessing elements is possible using indexing (starts from <code>0</code>):<br>"
	+ "<pre class=language-python>e = x[1] # e == 2</pre>"
	+ "Lists support the <code>+</code> operator to concatenate two lists. Example:<br>"
	+ "<pre class=language-python>x = [1, 2] + [3, 4] # x == [1, 2, 3, 4]\n"
	+ "x = [\"a\", \"b\"]\n"
	+ "x += [\"c\"] # x == [\"a\", \"b\", \"c\"]</pre>"
	+ "Similar to strings, lists support slice operations:"
	+ "<pre class=language-python>['a', 'b', 'c', 'd'][1:3] # ['b', 'c']\n"
	+ "['a', 'b', 'c', 'd'][::2] # ['a', 'c']\n"
	+ "['a', 'b', 'c', 'd'][3:0:-1] # ['d', 'c', 'b']</pre>"
	+ "Lists are mutable, as in Python.")
	public final class StarlarkList<E> extends AbstractList<E>
	implements Sequence<E>, StarlarkValue, Mutability.Freezable, Comparable<StarlarkList<?>> {

	// The implementation strategy is similar to ArrayList,
	// but without the extra indirection of using ArrayList.

	// elems[0:size] holds the logical elements, and elems[size:] are not used.
	// elems.getClass() == Object[].class. This is necessary to avoid ArrayStoreException.
	private int size;
	private int iteratorCount; // number of active iterators (unused once frozen)
	private Object[] elems = EMPTY_ARRAY; // elems[i] == null iff i >= size

	/** Final except for {@link #unsafeShallowFreeze}; must not be modified any other way. */
	private Mutability mutability;

	private static final Object[] EMPTY_ARRAY = {};

	private StarlarkList(@Nullable Mutability mutability, Object[] elems, int size) {
	Preconditions.checkArgument(elems.getClass() == Object[].class);
	this.elems = elems;
	this.size = size;
	this.mutability = mutability == null ? Mutability.IMMUTABLE : mutability;
	}

	/**
	* Takes ownership of the supplied array of class Object[].class, and returns a new StarlarkList
	* instance that initially wraps the array. The caller must not subsequently modify the array, but
	* the StarlarkList instance may do so.
	*/
	static <T> StarlarkList<T> wrap(@Nullable Mutability mutability, Object[] elems) {
	return new StarlarkList<>(mutability, elems, elems.length);
	}

	@Override
	public boolean isImmutable() {
	return mutability().isFrozen();
	}

	@Override
	public void checkHashable() throws EvalException {
	// Even a frozen list is unhashable.
	throw Starlark.errorf("unhashable type: 'list'");
	}

	@Override
	public boolean updateIteratorCount(int delta) {
	if (mutability().isFrozen()) {
	return false;
	}
	if (delta > 0) {
	iteratorCount++;
	} else if (delta < 0) {
	iteratorCount--;
	}
	return iteratorCount > 0;
	}

	/**
	* A shared instance for the empty list with immutable mutability.
	*
	* <p>Other immutable empty list objects can exist, e.g. lists that were once mutable but whose
	* environments were then frozen. This instance is for empty lists that were always frozen from
	* the beginning.
	*/
	private static final StarlarkList<?> EMPTY = wrap(Mutability.IMMUTABLE, EMPTY_ARRAY);

	/** Returns an empty frozen list of the desired type. */
	@SuppressWarnings("unchecked")
	public static <T> StarlarkList<T> empty() {
	return (StarlarkList<T>) EMPTY;
	}

	/** Returns a new, empty list with the specified Mutability. */
	public static <T> StarlarkList<T> newList(Mutability mutability) {
	return wrap(mutability, EMPTY_ARRAY);
	}

	/**
	* Returns a {@code StarlarkList} whose items are given by an iterable and which has the given
	* {@link Mutability}. If {@code mutability} is null, the list is immutable.
	*/
	public static <T> StarlarkList<T> copyOf(
	@Nullable Mutability mutability, Iterable<? extends T> elems) {
	if (mutability == null
	&& elems instanceof StarlarkList
	&& ((StarlarkList) elems).isImmutable()) {
	@SuppressWarnings("unchecked")
	StarlarkList<T> list = (StarlarkList<T>) elems; // safe
	return list;
	}

	Object[] array = Iterables.toArray(elems, Object.class);
	checkElemsValid(array);
	return wrap(mutability, array);
	}

	private static void checkElemsValid(Object[] elems) {
	for (Object elem : elems) {
	Starlark.checkValid(elem);
	}
	}

	/**
	* Returns an immutable list with the given elements. Equivalent to {@code copyOf(null, elems)}.
	*/
	public static <T> StarlarkList<T> immutableCopyOf(Iterable<? extends T> elems) {
	return copyOf(null, elems);
	}

	/**
	* Returns a {@code StarlarkList} with the given items and the {@link Mutability}. If {@code
	* mutability} is null, the list is immutable.
	*/
	public static <T> StarlarkList<T> of(@Nullable Mutability mutability, T... elems) {
	if (elems.length == 0) {
	return newList(mutability);
	}

	checkElemsValid(elems);
	return wrap(mutability, Arrays.copyOf(elems, elems.length, Object[].class));
	}

	/** Returns an immutable {@code StarlarkList} with the given items. */
	public static <T> StarlarkList<T> immutableOf(T... elems) {
	checkElemsValid(elems);
	return wrap(null, Arrays.copyOf(elems, elems.length, Object[].class));
	}

	@Override
	public Mutability mutability() {
	return mutability;
	}

	@Override
	public void unsafeShallowFreeze() {
	Mutability.Freezable.checkUnsafeShallowFreezePrecondition(this);
	this.mutability = Mutability.IMMUTABLE;
	}

	@Override
	public ImmutableList<E> getImmutableList() {
	// Optimization: a frozen array needn't be copied.
	// If the entire array is full, we can wrap it directly.
	if (elems.length == size && mutability().isFrozen()) {
	return Tuple.wrapImmutable(elems);
	}

	return ImmutableList.copyOf(this);
	}

	/**
	* Returns a new {@code StarlarkList} that is the concatenation of two {@code StarlarkList}s. The
	* new list will have the given {@link Mutability}.
	*/
	public static <T> StarlarkList<T> concat(
	StarlarkList<? extends T> x, StarlarkList<? extends T> y, Mutability mutability) {
	Object[] res = new Object[x.size + y.size];
	System.arraycopy(x.elems, 0, res, 0, x.size);
	System.arraycopy(y.elems, 0, res, x.size, y.size);
	return wrap(mutability, res);
	}

	@Override
	public int compareTo(StarlarkList<?> that) {
	return Sequence.compare(this, that);
	}

	@Override
	public boolean equals(Object that) {
	// This slightly violates the java.util.List equivalence contract
	// because it considers the class, not just the elements.
	return this == that \|\| (that instanceof StarlarkList && sameElems(this, ((StarlarkList) that)));
	}

	private static boolean sameElems(StarlarkList<?> x, StarlarkList<?> y) {
	if (x.size != y.size) {
	return false;
	}
	for (int i = 0; i < x.size; i++) {
	if (!x.elems[i].equals(y.elems[i])) {
	return false;
	}
	}
	return true;
	}

	@Override
	public int hashCode() {
	// Hash the elements elems[0:size].
	int result = 1;
	for (int i = 0; i < size; i++) {
	result = 31 * result + elems[i].hashCode();
	}
	return 6047 + 4673 * result;
	}

	@Override
	public void repr(Printer printer) {
	printer.printList(this, "[", ", ", "]");
	}

	// TODO(adonovan): StarlarkValue has 3 String methods yet still we need this fourth. Why?
	@Override
	public String toString() {
	return Starlark.repr(this);
	}

	/** Returns a new StarlarkList containing n consecutive repeats of this tuple. */
	public StarlarkList<E> repeat(StarlarkInt n, Mutability mutability) throws EvalException {
	if (n.signum() <= 0) {
	return wrap(mutability, EMPTY_ARRAY);
	}

	// TODO(adonovan): reject unreasonably large n.
	int ni = n.toInt("repeat");
	Object[] res = new Object[ni * size];
	for (int i = 0; i < ni; i++) {
	System.arraycopy(elems, 0, res, i * size, size);
	}
	return wrap(mutability, res);
	}

	@Override
	@SuppressWarnings("unchecked")
	public E get(int i) {
	if (i >= size) {
	throw new IndexOutOfBoundsException();
	}
	return (E) elems[i]; // unchecked
	}

	@Override
	public int size() {
	return size;
	}

	@Override
	public StarlarkList<E> getSlice(Mutability mu, int start, int stop, int step) {
	RangeList indices = new RangeList(start, stop, step);
	int n = indices.size();
	Object[] res = new Object[n];
	if (step == 1) { // common case
	System.arraycopy(elems, indices.at(0), res, 0, n);
	} else {
	for (int i = 0; i < n; ++i) {
	res[i] = elems[indices.at(i)];
	}
	}
	return wrap(mu, res);
	}

	// Postcondition: elems.length >= mincap.
	private void grow(int mincap) {
	int oldcap = elems.length;
	if (oldcap < mincap) {
	int newcap = oldcap + (oldcap >> 1); // grow by at least 50%
	if (newcap < mincap) {
	newcap = mincap;
	}
	elems = Arrays.copyOf(elems, newcap);
	}
	}

	/**
	* Appends an element to the end of the list, after validating that mutation is allowed.
	*
	* @param element the element to add
	*/
	public void addElement(E element) throws EvalException {
	Starlark.checkMutable(this);
	grow(size + 1);
	elems[size++] = element;
	}

	/**
	* Inserts an element at a given position to the list.
	*
	* @param index the new element's index
	* @param element the element to add
	*/
	public void addElementAt(int index, E element) throws EvalException {
	Starlark.checkMutable(this);
	grow(size + 1);
	System.arraycopy(elems, index, elems, index + 1, size - index);
	elems[index] = element;
	size++;
	}

	/**
	* Appends all the elements to the end of the list.
	*
	* @param elements the elements to add
	*/
	public void addElements(Iterable<? extends E> elements) throws EvalException {
	Starlark.checkMutable(this);
	if (elements instanceof StarlarkList) {
	StarlarkList<?> that = (StarlarkList) elements;
	// (safe even if this == that)
	grow(this.size + that.size);
	System.arraycopy(that.elems, 0, this.elems, this.size, that.size);
	this.size += that.size;
	} else if (elements instanceof Collection) {
	// collection of known size
	Collection<?> that = (Collection) elements;
	grow(size + that.size());
	for (Object x : that) {
	elems[size++] = x;
	}
	} else {
	// iterable
	for (Object x : elements) {
	grow(size + 1);
	elems[size++] = x;
	}
	}
	}

	/**
	* Removes the element at a given index. The index must already have been validated to be in
	* range.
	*
	* @param index the index of the element to remove
	*/
	public void removeElementAt(int index) throws EvalException {
	Starlark.checkMutable(this);
	int n = size - index - 1;
	if (n > 0) {
	System.arraycopy(elems, index + 1, elems, index, n);
	}
	elems[--size] = null; // aid GC
	}

	@StarlarkMethod(
	name = "remove",
	doc =
	"Removes the first item from the list whose value is x. "
	+ "It is an error if there is no such item.",
	parameters = {@Param(name = "x", doc = "The object to remove.")})
	public void removeElement(Object x) throws EvalException {
	for (int i = 0; i < size; i++) {
	if (elems[i].equals(x)) {
	removeElementAt(i);
	return;
	}
	}
	throw Starlark.errorf("item %s not found in list", Starlark.repr(x));
	}

	/**
	* Sets the position at the given index to contain the given value. Precondition: {@code 0 <=
	* index < size()}.
	*/
	public void setElementAt(int index, E value) throws EvalException {
	Starlark.checkMutable(this);
	Preconditions.checkArgument(index < size);
	elems[index] = value;
	}

	@StarlarkMethod(
	name = "append",
	doc = "Adds an item to the end of the list.",
	parameters = {@Param(name = "item", doc = "Item to add at the end.")})
	@SuppressWarnings("unchecked")
	public void append(Object item) throws EvalException {
	addElement((E) item); // unchecked
	}

	@StarlarkMethod(name = "clear", doc = "Removes all the elements of the list.")
	public void clearElements() throws EvalException {
	Starlark.checkMutable(this);
	for (int i = 0; i < size; i++) {
	elems[i] = null; // aid GC
	}
	size = 0;
	}

	@StarlarkMethod(
	name = "insert",
	doc = "Inserts an item at a given position.",
	parameters = {
	@Param(name = "index", doc = "The index of the given position."),
	@Param(name = "item", doc = "The item.")
	})
	@SuppressWarnings("unchecked")
	public void insert(StarlarkInt index, Object item) throws EvalException {
	addElementAt(EvalUtils.toIndex(index.toInt("index"), size), (E) item); // unchecked
	}

	@StarlarkMethod(
	name = "extend",
	doc = "Adds all items to the end of the list.",
	parameters = {@Param(name = "items", doc = "Items to add at the end.")})
	public void extend(Object items) throws EvalException {
	@SuppressWarnings("unchecked")
	Iterable<? extends E> src = (Iterable<? extends E>) Starlark.toIterable(items);
	addElements(src);
	}

	@StarlarkMethod(
	name = "index",
	doc =
	"Returns the index in the list of the first item whose value is x. "
	+ "It is an error if there is no such item.",
	parameters = {
	@Param(name = "x", doc = "The object to search."),
	@Param(
	name = "start",
	allowedTypes = {
	@ParamType(type = StarlarkInt.class),
	@ParamType(type = NoneType.class), // TODO(adonovan): this is wrong
	},
	defaultValue = "None",
	named = true, // TODO(adonovan): this is wrong
	doc = "The start index of the list portion to inspect."),
	@Param(
	name = "end",
	allowedTypes = {
	@ParamType(type = StarlarkInt.class),
	@ParamType(type = NoneType.class), // TODO(adonovan): this is wrong
	},
	defaultValue = "None",
	named = true, // TODO(adonovan): this is wrong
	doc = "The end index of the list portion to inspect.")
	})
	public int index(Object x, Object start, Object end) throws EvalException {
	int i = start == Starlark.NONE ? 0 : EvalUtils.toIndex(Starlark.toInt(start, "start"), size);
	int j = end == Starlark.NONE ? size : EvalUtils.toIndex(Starlark.toInt(end, "end"), size);
	for (; i < j; i++) {
	if (elems[i].equals(x)) {
	return i;
	}
	}
	throw Starlark.errorf("item %s not found in list", Starlark.repr(x));
	}

	@StarlarkMethod(
	name = "pop",
	doc =
	"Removes the item at the given position in the list, and returns it. "
	+ "If no <code>index</code> is specified, "
	+ "it removes and returns the last item in the list.",
	parameters = {
	@Param(
	name = "i",
	allowedTypes = {
	@ParamType(type = StarlarkInt.class),
	@ParamType(type = NoneType.class), // TODO(adonovan): this is not what Python3 does
	},
	defaultValue = "-1",
	doc = "The index of the item.")
	})
	public Object pop(Object i) throws EvalException {
	int arg = i == Starlark.NONE ? -1 : Starlark.toInt(i, "i");
	int index = EvalUtils.getSequenceIndex(arg, size);
	Object result = elems[index];
	removeElementAt(index);
	return result;
	}

	/** Returns a new array of class Object[] containing the list elements. */
	@Override
	public Object[] toArray() {
	return size != 0 ? Arrays.copyOf(elems, size, Object[].class) : EMPTY_ARRAY;
	}

	@SuppressWarnings("unchecked")
	@Override
	public <T> T[] toArray(T[] a) {
	if (a.length < size) {
	return (T[]) Arrays.copyOf(elems, size, a.getClass());
	} else {
	System.arraycopy(elems, 0, a, 0, size);
	Arrays.fill(a, size, a.length, null);
	return a;
	}
	}
	}