/* 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. */ #pragma once #include "support/BitSet.h" namespace antlr4 { namespace atn { /** * This enumeration defines the prediction modes available in ANTLR 4 along with * utility methods for analyzing configuration sets for conflicts and/or * ambiguities. */ enum class 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. * *

* 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.

* *

* This prediction mode does not provide any guarantees for prediction * behavior for syntactically-incorrect inputs.

*/ 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. * *

* 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 exactly which alternatives are * ambiguous.

* *

* This prediction mode does not provide any guarantees for prediction * behavior for syntactically-incorrect inputs.

*/ 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. * *

* 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.

* *

* This prediction mode does not provide any guarantees for prediction * behavior for syntactically-incorrect inputs.

*/ LL_EXACT_AMBIG_DETECTION }; class ANTLR4CPP_PUBLIC PredictionModeClass { public: /** * Computes the SLL prediction termination condition. * *

* This method computes the SLL prediction termination condition for both of * the following cases.

* * * *

COMBINED SLL+LL PARSING

* *

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.

* *

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.

* *

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.

* *

HEURISTIC

* *

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):

* *

{@code [12|1|[], 6|2|[], 12|2|[]]. s : (ID | ID ID?) ';' ;}

* *

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|[]]}.

* *

It also let's us continue for this rule:

* *

{@code [1|1|[], 1|2|[], 8|3|[]] a : A | A | A B ;}

* *

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.

* *

PURE SLL PARSING

* *

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.

* *

PREDICATES IN SLL+LL PARSING

* *

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.

* *

Implementation-wise, {@link ATNConfigSet} combines stack contexts but not * semantic predicate contexts so we might see two configurations like the * following.

* *

{@code (s, 1, x, {}), (s, 1, x', {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.

* *

{@code (s, 1, x, {}), (s, 1, x', {p}), (s, 2, x'', {})}

* *

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.

*/ static bool hasSLLConflictTerminatingPrediction(PredictionMode mode, ATNConfigSet *configs); /// /// Checks if any configuration in {@code configs} is in a /// . Configurations meeting this condition have /// reached /// the end of the decision rule (local context) or end of start rule (full /// context). /// /// the configuration set to test /// {@code true} if any configuration in {@code configs} is in a /// , otherwise {@code false} static bool hasConfigInRuleStopState(ATNConfigSet *configs); /// /// Checks if all configurations in {@code configs} are in a /// . Configurations meeting this condition have /// reached /// the end of the decision rule (local context) or end of start rule (full /// context). /// /// the configuration set to test /// {@code true} if all configurations in {@code configs} are in a /// , otherwise {@code false} static bool allConfigsInRuleStopStates(ATNConfigSet *configs); /** * Full LL prediction termination. * *

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.

* *

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}.

* *

Reduce these configuration subsets to the set of possible alternatives. * You can compute the alternative subsets in one pass as follows:

* *

{@code A_s,ctx = {i | (s, i, ctx, _)}} for each configuration in * {@code C} holding {@code s} and {@code ctx} fixed.

* *

Or in pseudo-code, for each configuration {@code c} in {@code C}:

* * 
     * map[c] U= c.{@link ATNConfig#alt alt} # map hash/equals uses s and x, not
     * alt and not pred
     *
* *

The values in {@code map} are the set of {@code A_s,ctx} sets.

* *

If {@code |A_s,ctx|=1} then there is no conflict associated with * {@code s} and {@code ctx}.

* *

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.

* *

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.

* *

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.

* *

CONFLICTING CONFIGS

* *

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.

* *

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.

* *

CONTINUE/STOP RULE

* *

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.

* *

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.

* *

CASES

* * * *

EXACT AMBIGUITY DETECTION

* *

If all states report the same conflicting set of alternatives, then we * know we have the exact ambiguity set.

* *

|A_i|>1 and * A_i = A_j for all i, j.

* *

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...

*/ static size_t resolvesToJustOneViableAlt(const std::vector &altsets); /// /// Determines if every alternative subset in {@code altsets} contains more /// than one alternative. /// /// a collection of alternative subsets /// {@code true} if every in {@code altsets} /// has /// > 1, otherwise {@code /// false} static bool allSubsetsConflict(const std::vector &altsets); /// /// Determines if any single alternative subset in {@code altsets} contains /// exactly one alternative. /// /// a collection of alternative subsets /// {@code true} if {@code altsets} contains a with /// 1, otherwise {@code false} /// static bool hasNonConflictingAltSet(const std::vector &altsets); /// /// Determines if any single alternative subset in {@code altsets} contains /// more than one alternative. /// /// a collection of alternative subsets /// {@code true} if {@code altsets} contains a with /// > 1, otherwise {@code /// false} static bool hasConflictingAltSet(const std::vector &altsets); /// /// Determines if every alternative subset in {@code altsets} is equivalent. /// /// a collection of alternative subsets /// {@code true} if every member of {@code altsets} is equal to the /// others, otherwise {@code false} static bool allSubsetsEqual(const std::vector &altsets); /// /// Returns the unique alternative predicted by all alternative subsets in /// {@code altsets}. If no such alternative exists, this method returns /// . /// /// a collection of alternative subsets static size_t getUniqueAlt(const std::vector &altsets); /// /// Gets the complete set of represented alternatives for a collection of /// alternative subsets. This method returns the union of each /// in {@code altsets}. /// /// a collection of alternative subsets /// the set of represented alternatives in {@code altsets} static antlrcpp::BitSet getAlts(const std::vector &altsets); /** Get union of all alts from configs. @since 4.5.1 */ static antlrcpp::BitSet getAlts(ATNConfigSet *configs); /// /// This function gets the conflicting alt subsets from a configuration set. /// For each configuration {@code c} in {@code configs}: /// /// 
    /// map[c] U= c. # map hash/equals uses s and
    /// x, not
    /// alt and not pred
    ///
/// static std::vector getConflictingAltSubsets(ATNConfigSet *configs); /// /// Get a map from state to alt subset from a configuration set. For each /// configuration {@code c} in {@code configs}: /// /// 
    /// map[c.] U= c.
    ///
/// static std::map getStateToAltMap(ATNConfigSet *configs); static bool hasStateAssociatedWithOneAlt(ATNConfigSet *configs); static size_t getSingleViableAlt(const std::vector &altsets); }; } // namespace atn } // namespace antlr4