Java tutorial
/* * Copyright (c) 2012-2017 The ANTLR Project. All rights reserved. * Use of this file is governed by the BSD 3-clause license that * can be found in the LICENSE.txt file in the project root. */ package org.antlr.v4.runtime.atn; import org.antlr.v4.runtime.misc.AbstractEqualityComparator; import org.antlr.v4.runtime.misc.FlexibleHashMap; import org.antlr.v4.runtime.misc.MurmurHash; import java.util.BitSet; import java.util.Collection; import java.util.HashMap; import java.util.Iterator; import java.util.Map; /** * This enumeration defines the prediction modes available in ANTLR 4 along with * utility methods for analyzing configuration sets for conflicts and/or * ambiguities. */ public enum PredictionMode { /** * The SLL(*) prediction mode. This prediction mode ignores the current * parser context when making predictions. This is the fastest prediction * mode, and provides correct results for many grammars. This prediction * mode is more powerful than the prediction mode provided by ANTLR 3, but * may result in syntax errors for grammar and input combinations which are * not SLL. * * <p> * When using this prediction mode, the parser will either return a correct * parse tree (i.e. the same parse tree that would be returned with the * {@link #LL} prediction mode), or it will report a syntax error. If a * syntax error is encountered when using the {@link #SLL} prediction mode, * it may be due to either an actual syntax error in the input or indicate * that the particular combination of grammar and input requires the more * powerful {@link #LL} prediction abilities to complete successfully.</p> * * <p> * This prediction mode does not provide any guarantees for prediction * behavior for syntactically-incorrect inputs.</p> */ SLL, /** * The LL(*) prediction mode. This prediction mode allows the current parser * context to be used for resolving SLL conflicts that occur during * prediction. This is the fastest prediction mode that guarantees correct * parse results for all combinations of grammars with syntactically correct * inputs. * * <p> * When using this prediction mode, the parser will make correct decisions * for all syntactically-correct grammar and input combinations. However, in * cases where the grammar is truly ambiguous this prediction mode might not * report a precise answer for <em>exactly which</em> alternatives are * ambiguous.</p> * * <p> * This prediction mode does not provide any guarantees for prediction * behavior for syntactically-incorrect inputs.</p> */ LL, /** * The LL(*) prediction mode with exact ambiguity detection. In addition to * the correctness guarantees provided by the {@link #LL} prediction mode, * this prediction mode instructs the prediction algorithm to determine the * complete and exact set of ambiguous alternatives for every ambiguous * decision encountered while parsing. * * <p> * This prediction mode may be used for diagnosing ambiguities during * grammar development. Due to the performance overhead of calculating sets * of ambiguous alternatives, this prediction mode should be avoided when * the exact results are not necessary.</p> * * <p> * This prediction mode does not provide any guarantees for prediction * behavior for syntactically-incorrect inputs.</p> */ LL_EXACT_AMBIG_DETECTION; /** A Map that uses just the state and the stack context as the key. */ static class AltAndContextMap extends FlexibleHashMap<ATNConfig, BitSet> { public AltAndContextMap() { super(AltAndContextConfigEqualityComparator.INSTANCE); } } private static final class AltAndContextConfigEqualityComparator extends AbstractEqualityComparator<ATNConfig> { public static final AltAndContextConfigEqualityComparator INSTANCE = new AltAndContextConfigEqualityComparator(); private AltAndContextConfigEqualityComparator() { } /** * The hash code is only a function of the {@link ATNState#stateNumber} * and {@link ATNConfig#context}. */ @Override public int hashCode(ATNConfig o) { int hashCode = MurmurHash.initialize(7); hashCode = MurmurHash.update(hashCode, o.state.stateNumber); hashCode = MurmurHash.update(hashCode, o.context); hashCode = MurmurHash.finish(hashCode, 2); return hashCode; } @Override public boolean equals(ATNConfig a, ATNConfig b) { if (a == b) return true; if (a == null || b == null) return false; return a.state.stateNumber == b.state.stateNumber && a.context.equals(b.context); } } /** * Computes the SLL prediction termination condition. * * <p> * This method computes the SLL prediction termination condition for both of * the following cases.</p> * * <ul> * <li>The usual SLL+LL fallback upon SLL conflict</li> * <li>Pure SLL without LL fallback</li> * </ul> * * <p><strong>COMBINED SLL+LL PARSING</strong></p> * * <p>When LL-fallback is enabled upon SLL conflict, correct predictions are * ensured regardless of how the termination condition is computed by this * method. Due to the substantially higher cost of LL prediction, the * prediction should only fall back to LL when the additional lookahead * cannot lead to a unique SLL prediction.</p> * * <p>Assuming combined SLL+LL parsing, an SLL configuration set with only * conflicting subsets should fall back to full LL, even if the * configuration sets don't resolve to the same alternative (e.g. * {@code {1,2}} and {@code {3,4}}. If there is at least one non-conflicting * configuration, SLL could continue with the hopes that more lookahead will * resolve via one of those non-conflicting configurations.</p> * * <p>Here's the prediction termination rule them: SLL (for SLL+LL parsing) * stops when it sees only conflicting configuration subsets. In contrast, * full LL keeps going when there is uncertainty.</p> * * <p><strong>HEURISTIC</strong></p> * * <p>As a heuristic, we stop prediction when we see any conflicting subset * unless we see a state that only has one alternative associated with it. * The single-alt-state thing lets prediction continue upon rules like * (otherwise, it would admit defeat too soon):</p> * * <p>{@code [12|1|[], 6|2|[], 12|2|[]]. s : (ID | ID ID?) ';' ;}</p> * * <p>When the ATN simulation reaches the state before {@code ';'}, it has a * DFA state that looks like: {@code [12|1|[], 6|2|[], 12|2|[]]}. Naturally * {@code 12|1|[]} and {@code 12|2|[]} conflict, but we cannot stop * processing this node because alternative to has another way to continue, * via {@code [6|2|[]]}.</p> * * <p>It also let's us continue for this rule:</p> * * <p>{@code [1|1|[], 1|2|[], 8|3|[]] a : A | A | A B ;}</p> * * <p>After matching input A, we reach the stop state for rule A, state 1. * State 8 is the state right before B. Clearly alternatives 1 and 2 * conflict and no amount of further lookahead will separate the two. * However, alternative 3 will be able to continue and so we do not stop * working on this state. In the previous example, we're concerned with * states associated with the conflicting alternatives. Here alt 3 is not * associated with the conflicting configs, but since we can continue * looking for input reasonably, don't declare the state done.</p> * * <p><strong>PURE SLL PARSING</strong></p> * * <p>To handle pure SLL parsing, all we have to do is make sure that we * combine stack contexts for configurations that differ only by semantic * predicate. From there, we can do the usual SLL termination heuristic.</p> * * <p><strong>PREDICATES IN SLL+LL PARSING</strong></p> * * <p>SLL decisions don't evaluate predicates until after they reach DFA stop * states because they need to create the DFA cache that works in all * semantic situations. In contrast, full LL evaluates predicates collected * during start state computation so it can ignore predicates thereafter. * This means that SLL termination detection can totally ignore semantic * predicates.</p> * * <p>Implementation-wise, {@link ATNConfigSet} combines stack contexts but not * semantic predicate contexts so we might see two configurations like the * following.</p> * * <p>{@code (s, 1, x, {}), (s, 1, x', {p})}</p> * * <p>Before testing these configurations against others, we have to merge * {@code x} and {@code x'} (without modifying the existing configurations). * For example, we test {@code (x+x')==x''} when looking for conflicts in * the following configurations.</p> * * <p>{@code (s, 1, x, {}), (s, 1, x', {p}), (s, 2, x'', {})}</p> * * <p>If the configuration set has predicates (as indicated by * {@link ATNConfigSet#hasSemanticContext}), this algorithm makes a copy of * the configurations to strip out all of the predicates so that a standard * {@link ATNConfigSet} will merge everything ignoring predicates.</p> */ public static boolean hasSLLConflictTerminatingPrediction(PredictionMode mode, ATNConfigSet configs) { /* Configs in rule stop states indicate reaching the end of the decision * rule (local context) or end of start rule (full context). If all * configs meet this condition, then none of the configurations is able * to match additional input so we terminate prediction. */ if (allConfigsInRuleStopStates(configs)) { return true; } // pure SLL mode parsing if (mode == PredictionMode.SLL) { // Don't bother with combining configs from different semantic // contexts if we can fail over to full LL; costs more time // since we'll often fail over anyway. if (configs.hasSemanticContext) { // dup configs, tossing out semantic predicates ATNConfigSet dup = new ATNConfigSet(); for (ATNConfig c : configs) { c = new ATNConfig(c, SemanticContext.NONE); dup.add(c); } configs = dup; } // now we have combined contexts for configs with dissimilar preds } // pure SLL or combined SLL+LL mode parsing Collection<BitSet> altsets = getConflictingAltSubsets(configs); boolean heuristic = hasConflictingAltSet(altsets) && !hasStateAssociatedWithOneAlt(configs); return heuristic; } /** * Checks if any configuration in {@code configs} is in a * {@link RuleStopState}. Configurations meeting this condition have reached * the end of the decision rule (local context) or end of start rule (full * context). * * @param configs the configuration set to test * @return {@code true} if any configuration in {@code configs} is in a * {@link RuleStopState}, otherwise {@code false} */ public static boolean hasConfigInRuleStopState(ATNConfigSet configs) { for (ATNConfig c : configs) { if (c.state instanceof RuleStopState) { return true; } } return false; } /** * Checks if all configurations in {@code configs} are in a * {@link RuleStopState}. Configurations meeting this condition have reached * the end of the decision rule (local context) or end of start rule (full * context). * * @param configs the configuration set to test * @return {@code true} if all configurations in {@code configs} are in a * {@link RuleStopState}, otherwise {@code false} */ public static boolean allConfigsInRuleStopStates(ATNConfigSet configs) { for (ATNConfig config : configs) { if (!(config.state instanceof RuleStopState)) { return false; } } return true; } /** * Full LL prediction termination. * * <p>Can we stop looking ahead during ATN simulation or is there some * uncertainty as to which alternative we will ultimately pick, after * consuming more input? Even if there are partial conflicts, we might know * that everything is going to resolve to the same minimum alternative. That * means we can stop since no more lookahead will change that fact. On the * other hand, there might be multiple conflicts that resolve to different * minimums. That means we need more look ahead to decide which of those * alternatives we should predict.</p> * * <p>The basic idea is to split the set of configurations {@code C}, into * conflicting subsets {@code (s, _, ctx, _)} and singleton subsets with * non-conflicting configurations. Two configurations conflict if they have * identical {@link ATNConfig#state} and {@link ATNConfig#context} values * but different {@link ATNConfig#alt} value, e.g. {@code (s, i, ctx, _)} * and {@code (s, j, ctx, _)} for {@code i!=j}.</p> * * <p>Reduce these configuration subsets to the set of possible alternatives. * You can compute the alternative subsets in one pass as follows:</p> * * <p>{@code A_s,ctx = {i | (s, i, ctx, _)}} for each configuration in * {@code C} holding {@code s} and {@code ctx} fixed.</p> * * <p>Or in pseudo-code, for each configuration {@code c} in {@code C}:</p> * * <pre> * map[c] U= c.{@link ATNConfig#alt alt} # map hash/equals uses s and x, not * alt and not pred * </pre> * * <p>The values in {@code map} are the set of {@code A_s,ctx} sets.</p> * * <p>If {@code |A_s,ctx|=1} then there is no conflict associated with * {@code s} and {@code ctx}.</p> * * <p>Reduce the subsets to singletons by choosing a minimum of each subset. If * the union of these alternative subsets is a singleton, then no amount of * more lookahead will help us. We will always pick that alternative. If, * however, there is more than one alternative, then we are uncertain which * alternative to predict and must continue looking for resolution. We may * or may not discover an ambiguity in the future, even if there are no * conflicting subsets this round.</p> * * <p>The biggest sin is to terminate early because it means we've made a * decision but were uncertain as to the eventual outcome. We haven't used * enough lookahead. On the other hand, announcing a conflict too late is no * big deal; you will still have the conflict. It's just inefficient. It * might even look until the end of file.</p> * * <p>No special consideration for semantic predicates is required because * predicates are evaluated on-the-fly for full LL prediction, ensuring that * no configuration contains a semantic context during the termination * check.</p> * * <p><strong>CONFLICTING CONFIGS</strong></p> * * <p>Two configurations {@code (s, i, x)} and {@code (s, j, x')}, conflict * when {@code i!=j} but {@code x=x'}. Because we merge all * {@code (s, i, _)} configurations together, that means that there are at * most {@code n} configurations associated with state {@code s} for * {@code n} possible alternatives in the decision. The merged stacks * complicate the comparison of configuration contexts {@code x} and * {@code x'}. Sam checks to see if one is a subset of the other by calling * merge and checking to see if the merged result is either {@code x} or * {@code x'}. If the {@code x} associated with lowest alternative {@code i} * is the superset, then {@code i} is the only possible prediction since the * others resolve to {@code min(i)} as well. However, if {@code x} is * associated with {@code j>i} then at least one stack configuration for * {@code j} is not in conflict with alternative {@code i}. The algorithm * should keep going, looking for more lookahead due to the uncertainty.</p> * * <p>For simplicity, I'm doing a equality check between {@code x} and * {@code x'} that lets the algorithm continue to consume lookahead longer * than necessary. The reason I like the equality is of course the * simplicity but also because that is the test you need to detect the * alternatives that are actually in conflict.</p> * * <p><strong>CONTINUE/STOP RULE</strong></p> * * <p>Continue if union of resolved alternative sets from non-conflicting and * conflicting alternative subsets has more than one alternative. We are * uncertain about which alternative to predict.</p> * * <p>The complete set of alternatives, {@code [i for (_,i,_)]}, tells us which * alternatives are still in the running for the amount of input we've * consumed at this point. The conflicting sets let us to strip away * configurations that won't lead to more states because we resolve * conflicts to the configuration with a minimum alternate for the * conflicting set.</p> * * <p><strong>CASES</strong></p> * * <ul> * * <li>no conflicts and more than 1 alternative in set => continue</li> * * <li> {@code (s, 1, x)}, {@code (s, 2, x)}, {@code (s, 3, z)}, * {@code (s', 1, y)}, {@code (s', 2, y)} yields non-conflicting set * {@code {3}} U conflicting sets {@code min({1,2})} U {@code min({1,2})} = * {@code {1,3}} => continue * </li> * * <li>{@code (s, 1, x)}, {@code (s, 2, x)}, {@code (s', 1, y)}, * {@code (s', 2, y)}, {@code (s'', 1, z)} yields non-conflicting set * {@code {1}} U conflicting sets {@code min({1,2})} U {@code min({1,2})} = * {@code {1}} => stop and predict 1</li> * * <li>{@code (s, 1, x)}, {@code (s, 2, x)}, {@code (s', 1, y)}, * {@code (s', 2, y)} yields conflicting, reduced sets {@code {1}} U * {@code {1}} = {@code {1}} => stop and predict 1, can announce * ambiguity {@code {1,2}}</li> * * <li>{@code (s, 1, x)}, {@code (s, 2, x)}, {@code (s', 2, y)}, * {@code (s', 3, y)} yields conflicting, reduced sets {@code {1}} U * {@code {2}} = {@code {1,2}} => continue</li> * * <li>{@code (s, 1, x)}, {@code (s, 2, x)}, {@code (s', 3, y)}, * {@code (s', 4, y)} yields conflicting, reduced sets {@code {1}} U * {@code {3}} = {@code {1,3}} => continue</li> * * </ul> * * <p><strong>EXACT AMBIGUITY DETECTION</strong></p> * * <p>If all states report the same conflicting set of alternatives, then we * know we have the exact ambiguity set.</p> * * <p><code>|A_<em>i</em>|>1</code> and * <code>A_<em>i</em> = A_<em>j</em></code> for all <em>i</em>, <em>j</em>.</p> * * <p>In other words, we continue examining lookahead until all {@code A_i} * have more than one alternative and all {@code A_i} are the same. If * {@code A={{1,2}, {1,3}}}, then regular LL prediction would terminate * because the resolved set is {@code {1}}. To determine what the real * ambiguity is, we have to know whether the ambiguity is between one and * two or one and three so we keep going. We can only stop prediction when * we need exact ambiguity detection when the sets look like * {@code A={{1,2}}} or {@code {{1,2},{1,2}}}, etc...</p> */ public static int resolvesToJustOneViableAlt(Collection<BitSet> altsets) { return getSingleViableAlt(altsets); } /** * Determines if every alternative subset in {@code altsets} contains more * than one alternative. * * @param altsets a collection of alternative subsets * @return {@code true} if every {@link BitSet} in {@code altsets} has * {@link BitSet#cardinality cardinality} > 1, otherwise {@code false} */ public static boolean allSubsetsConflict(Collection<BitSet> altsets) { return !hasNonConflictingAltSet(altsets); } /** * Determines if any single alternative subset in {@code altsets} contains * exactly one alternative. * * @param altsets a collection of alternative subsets * @return {@code true} if {@code altsets} contains a {@link BitSet} with * {@link BitSet#cardinality cardinality} 1, otherwise {@code false} */ public static boolean hasNonConflictingAltSet(Collection<BitSet> altsets) { for (BitSet alts : altsets) { if (alts.cardinality() == 1) { return true; } } return false; } /** * Determines if any single alternative subset in {@code altsets} contains * more than one alternative. * * @param altsets a collection of alternative subsets * @return {@code true} if {@code altsets} contains a {@link BitSet} with * {@link BitSet#cardinality cardinality} > 1, otherwise {@code false} */ public static boolean hasConflictingAltSet(Collection<BitSet> altsets) { for (BitSet alts : altsets) { if (alts.cardinality() > 1) { return true; } } return false; } /** * Determines if every alternative subset in {@code altsets} is equivalent. * * @param altsets a collection of alternative subsets * @return {@code true} if every member of {@code altsets} is equal to the * others, otherwise {@code false} */ public static boolean allSubsetsEqual(Collection<BitSet> altsets) { Iterator<BitSet> it = altsets.iterator(); BitSet first = it.next(); while (it.hasNext()) { BitSet next = it.next(); if (!next.equals(first)) return false; } return true; } /** * Returns the unique alternative predicted by all alternative subsets in * {@code altsets}. If no such alternative exists, this method returns * {@link ATN#INVALID_ALT_NUMBER}. * * @param altsets a collection of alternative subsets */ public static int getUniqueAlt(Collection<BitSet> altsets) { BitSet all = getAlts(altsets); if (all.cardinality() == 1) return all.nextSetBit(0); return ATN.INVALID_ALT_NUMBER; } /** * Gets the complete set of represented alternatives for a collection of * alternative subsets. This method returns the union of each {@link BitSet} * in {@code altsets}. * * @param altsets a collection of alternative subsets * @return the set of represented alternatives in {@code altsets} */ public static BitSet getAlts(Collection<BitSet> altsets) { BitSet all = new BitSet(); for (BitSet alts : altsets) { all.or(alts); } return all; } /** * Get union of all alts from configs. * * @since 4.5.1 */ public static BitSet getAlts(ATNConfigSet configs) { BitSet alts = new BitSet(); for (ATNConfig config : configs) { alts.set(config.alt); } return alts; } /** * This function gets the conflicting alt subsets from a configuration set. * For each configuration {@code c} in {@code configs}: * * <pre> * map[c] U= c.{@link ATNConfig#alt alt} # map hash/equals uses s and x, not * alt and not pred * </pre> */ public static Collection<BitSet> getConflictingAltSubsets(ATNConfigSet configs) { AltAndContextMap configToAlts = new AltAndContextMap(); for (ATNConfig c : configs) { BitSet alts = configToAlts.get(c); if (alts == null) { alts = new BitSet(); configToAlts.put(c, alts); } alts.set(c.alt); } return configToAlts.values(); } /** * Get a map from state to alt subset from a configuration set. For each * configuration {@code c} in {@code configs}: * * <pre> * map[c.{@link ATNConfig#state state}] U= c.{@link ATNConfig#alt alt} * </pre> */ public static Map<ATNState, BitSet> getStateToAltMap(ATNConfigSet configs) { Map<ATNState, BitSet> m = new HashMap<ATNState, BitSet>(); for (ATNConfig c : configs) { BitSet alts = m.get(c.state); if (alts == null) { alts = new BitSet(); m.put(c.state, alts); } alts.set(c.alt); } return m; } public static boolean hasStateAssociatedWithOneAlt(ATNConfigSet configs) { Map<ATNState, BitSet> x = getStateToAltMap(configs); for (BitSet alts : x.values()) { if (alts.cardinality() == 1) return true; } return false; } public static int getSingleViableAlt(Collection<BitSet> altsets) { BitSet viableAlts = new BitSet(); for (BitSet alts : altsets) { int minAlt = alts.nextSetBit(0); viableAlts.set(minAlt); if (viableAlts.cardinality() > 1) { // more than 1 viable alt return ATN.INVALID_ALT_NUMBER; } } return viableAlts.nextSetBit(0); } }