com.google.javascript.jscomp.MinimizedCondition Maven / Gradle / Ivy
/*
* Copyright 2013 The Closure Compiler Authors.
*
* 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 com.google.javascript.jscomp;
import com.google.common.base.Preconditions;
import com.google.common.collect.ImmutableList;
import com.google.javascript.rhino.IR;
import com.google.javascript.rhino.Node;
import com.google.javascript.rhino.Token;
import java.util.Collections;
import java.util.Comparator;
/**
* A class that represents a minimized conditional expression.
* This is a conditional expression that has been massaged according to
* DeMorgan's laws in order to minimize the length of the source
* representation.
*
* Depending on the context, a leading NOT node in front of the conditional
* may or may not be counted as a cost, so this class provides ways to
* access minimized versions of both of those abstract syntax trees (ASTs).
*
* @author [email protected] (Ben Lickly)
*/
class MinimizedCondition {
/** Definitions of the style of minimization preferred. */
enum MinimizationStyle {
/** Compute the length of the minimized condition as including
* any leading NOT node, if present. */
PREFER_UNNEGATED,
/** Compute the length of the minimized condition without penalizing
* a leading NOT node, if present. */
ALLOW_LEADING_NOT
}
/** A representation equivalent to the original condition. */
private final MeasuredNode positive;
/** A representation equivalent to the negation of the original condition. */
private final MeasuredNode negative;
/** A placeholder at the same AST location as the original condition */
private Node placeholder;
private MinimizedCondition(MeasuredNode p, MeasuredNode n) {
Preconditions.checkArgument(p.node.getParent() == null);
Preconditions.checkArgument(n.node.getParent() == null);
positive = p;
negative = n.change();
}
Node getPlaceholder() {
return placeholder;
}
MinimizedCondition setPlaceholder(Node placeholder) {
this.placeholder = placeholder;
return this;
}
/**
* Remove the passed condition node from the AST, and then return a
* MinimizedCondition that represents the condition node after
* minimization.
*/
static MinimizedCondition fromConditionNode(Node n) {
switch (n.getType()) {
case Token.NOT:
case Token.AND:
case Token.OR:
case Token.HOOK:
case Token.COMMA:
Node placeholder = swapWithPlaceholderNode(n);
return computeMinimizedCondition(n).setPlaceholder(placeholder);
default:
return unoptimized(n);
}
}
/**
* Return the shorter representation of the original condition node.
*
* Depending on the context, this may require to either penalize or
* not the existence of a leading NOT node.
*
- When {@code style} is {@code PREFER_UNNEGATED}, simply try to
* minimize the total length of the conditional.
* - When {@code style} is {@code ALLOW_LEADING_NOT}, prefer the right side
* in cases such as:
*
* !x || !y || z ==> !(x && y && !z)
*
* This is useful in contexts such as IFs or HOOKs where subsequent
* optimizations can efficiently deal with leading NOTs.
*
*
* @return the minimized condition MeasuredNode, with equivalent semantics
* to that passed to {@link #fromConditionNode}.
*/
MeasuredNode getMinimized(MinimizationStyle style) {
if (style == MinimizationStyle.PREFER_UNNEGATED
|| positive.node.isNot()
|| positive.length <= negative.length) {
return positive;
} else {
return negative.addNot();
}
}
/**
* Return a MeasuredNode of the given condition node, without minimizing
* the result.
*
* Since a MinimizedCondition necessarily must contain two trees, this
* method sets the negative side to a {@link Token#SCRIPT} node (never valid
* inside an expression) with an unreasonably high length so that it will
* never be chosen by {@link #getMinimized}.
*
* @param n the conditional expression tree to minimize.
* This must be connected to the AST, and will be swapped
* with a placeholder node during minimization.
* @return a MinimizedCondition object representing that tree.
*/
static MinimizedCondition unoptimized(Node n) {
Preconditions.checkNotNull(n.getParent());
Node placeholder = swapWithPlaceholderNode(n);
MeasuredNode pos = new MeasuredNode(n, 0, false);
MeasuredNode neg = new MeasuredNode(IR.script(), Integer.MAX_VALUE, true);
return new MinimizedCondition(pos, neg).setPlaceholder(placeholder);
}
/**
* Remove the given node from the AST, and replace it with a placeholder
* SCRIPT node.
* @return the new placeholder node.
*/
private static Node swapWithPlaceholderNode(Node n) {
Preconditions.checkNotNull(n.getParent());
Node placeholder = IR.script();
n.getParent().replaceChild(n, placeholder);
return placeholder;
}
/**
* Minimize the condition at the given node.
*
* @param n the conditional expression tree to minimize.
* This must be connected to the AST, and will be swapped
* with a placeholder node during minimization.
* @return a MinimizedCondition object representing that tree.
*/
private static MinimizedCondition computeMinimizedCondition(Node n) {
Preconditions.checkArgument(n.getParent() == null);
switch (n.getType()) {
case Token.NOT: {
MinimizedCondition subtree =
computeMinimizedCondition(n.getFirstChild().detachFromParent());
ImmutableList positiveAsts = ImmutableList.of(
subtree.positive.cloneTree().addNot(),
subtree.negative.cloneTree());
ImmutableList negativeAsts = ImmutableList.of(
subtree.negative.negate(),
subtree.positive);
return new MinimizedCondition(
Collections.min(positiveAsts, AST_LENGTH_COMPARATOR),
Collections.min(negativeAsts, AST_LENGTH_COMPARATOR));
}
case Token.AND:
case Token.OR: {
int opType = n.getType();
int complementType = opType == Token.AND ? Token.OR : Token.AND;
MinimizedCondition leftSubtree =
computeMinimizedCondition(n.getFirstChild().detachFromParent());
MinimizedCondition rightSubtree =
computeMinimizedCondition(n.getLastChild().detachFromParent());
ImmutableList positiveAsts = ImmutableList.of(
MeasuredNode.addNode(new Node(opType).srcref(n),
leftSubtree.positive.cloneTree(),
rightSubtree.positive.cloneTree()),
MeasuredNode.addNode(new Node(complementType).srcref(n),
leftSubtree.negative.cloneTree(),
rightSubtree.negative.cloneTree()).negate());
ImmutableList negativeAsts = ImmutableList.of(
MeasuredNode.addNode(new Node(opType).srcref(n),
leftSubtree.positive,
rightSubtree.positive).negate(),
MeasuredNode.addNode(new Node(complementType).srcref(n),
leftSubtree.negative,
rightSubtree.negative));
return new MinimizedCondition(
Collections.min(positiveAsts, AST_LENGTH_COMPARATOR),
Collections.min(negativeAsts, AST_LENGTH_COMPARATOR));
}
case Token.HOOK: {
Node cond = n.getFirstChild();
Node thenNode = cond.getNext();
Node elseNode = thenNode.getNext();
MinimizedCondition thenSubtree =
computeMinimizedCondition(thenNode.detachFromParent());
MinimizedCondition elseSubtree =
computeMinimizedCondition(elseNode.detachFromParent());
MeasuredNode posTree = MeasuredNode.addNode(
new Node(Token.HOOK, cond.cloneTree()).srcref(n),
thenSubtree.positive,
elseSubtree.positive);
MeasuredNode negTree = MeasuredNode.addNode(
new Node(Token.HOOK, cond.cloneTree()).srcref(n),
thenSubtree.negative,
elseSubtree.negative);
return new MinimizedCondition(posTree, negTree);
}
case Token.COMMA: {
Node lhs = n.getFirstChild();
MinimizedCondition rhsSubtree =
computeMinimizedCondition(lhs.getNext().detachFromParent());
MeasuredNode posTree = MeasuredNode.addNode(
new Node(Token.COMMA, lhs.cloneTree()).srcref(n),
rhsSubtree.positive);
MeasuredNode negTree = MeasuredNode.addNode(
new Node(Token.COMMA, lhs.cloneTree()).srcref(n),
rhsSubtree.negative);
return new MinimizedCondition(posTree, negTree);
}
default: {
MeasuredNode pos = new MeasuredNode(n, 0, false);
MeasuredNode neg = pos.cloneTree().negate();
return new MinimizedCondition(pos, neg);
}
}
}
private static final Comparator AST_LENGTH_COMPARATOR =
new Comparator() {
@Override
public int compare(MeasuredNode o1, MeasuredNode o2) {
return o1.length - o2.length;
}
};
/** An AST-node along with some additional metadata. */
static class MeasuredNode {
private Node node;
private int length;
private boolean changed;
Node getNode() {
return node;
}
boolean isChanged() {
return changed;
}
MeasuredNode(Node n, int len, boolean ch) {
node = n;
length = len;
changed = ch;
}
private MeasuredNode negate() {
this.change();
switch (node.getType()) {
case Token.EQ:
node.setType(Token.NE);
return this;
case Token.NE:
node.setType(Token.EQ);
return this;
case Token.SHEQ:
node.setType(Token.SHNE);
return this;
case Token.SHNE:
node.setType(Token.SHEQ);
return this;
default:
return this.addNot();
}
}
private MeasuredNode change() {
this.changed = true;
return this;
}
private MeasuredNode addNot() {
node = new Node(Token.NOT, node).srcref(node);
length += estimateCostOneLevel(node);
return this;
}
/**
* Estimate the number of characters in the textual representation of
* the given node and that will be devoted to negation or parentheses.
* Since these are the only characters that flipping a condition
* according to De Morgan's rule can affect, these are the only ones
* we count.
* Not nodes are counted by the NOT node itself, whereas
* parentheses around an expression are counted by the parent node.
* @param n the node to be checked.
* @return the number of negations and parentheses in the node.
*/
private static int estimateCostOneLevel(Node n) {
int cost = 0;
if (n.isNot()) {
cost++; // A negation is needed.
}
int parentPrecedence = NodeUtil.precedence(n.getType());
for (Node child = n.getFirstChild();
child != null; child = child.getNext()) {
if (PeepholeMinimizeConditions.isLowerPrecedence(child, parentPrecedence)) {
cost += 2; // A pair of parenthesis is needed.
}
}
return cost;
}
private MeasuredNode cloneTree() {
return new MeasuredNode(node.cloneTree(), length, changed);
}
private static MeasuredNode addNode(Node parent, MeasuredNode... children) {
int cost = 0;
boolean changed = false;
for (MeasuredNode child : children) {
parent.addChildrenToBack(child.node);
cost += child.length;
changed = changed || child.changed;
}
cost += estimateCostOneLevel(parent);
return new MeasuredNode(parent, cost, changed);
}
}
}