bfc/lib/antlr4/include/atn/PredictionMode.h

/* 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.
     *
     * <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
  };

  class ANTLR4CPP_PUBLIC PredictionModeClass {
  public:
    /**
     * 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>
     */
    static bool hasSLLConflictTerminatingPrediction(PredictionMode mode, ATNConfigSet *configs);

    /// <summary>
    /// Checks if any configuration in {@code configs} is in a
    /// <seealso cref="RuleStopState"/>. Configurations meeting this condition have
    /// reached
    /// the end of the decision rule (local context) or end of start rule (full
    /// context).
    /// </summary>
    /// <param name="configs"> the configuration set to test </param>
    /// <returns> {@code true} if any configuration in {@code configs} is in a
    /// <seealso cref="RuleStopState"/>, otherwise {@code false} </returns>
    static bool hasConfigInRuleStopState(ATNConfigSet *configs);

    /// <summary>
    /// Checks if all configurations in {@code configs} are in a
    /// <seealso cref="RuleStopState"/>. Configurations meeting this condition have
    /// reached
    /// the end of the decision rule (local context) or end of start rule (full
    /// context).
    /// </summary>
    /// <param name="configs"> the configuration set to test </param>
    /// <returns> {@code true} if all configurations in {@code configs} are in a
    /// <seealso cref="RuleStopState"/>, otherwise {@code false} </returns>
    static bool allConfigsInRuleStopStates(ATNConfigSet *configs);

    /**
     * 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 =&gt; 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}} =&gt; 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}} =&gt; 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}} =&gt; 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}} =&gt; 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}} =&gt; 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>|&gt;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>
     */
    static size_t resolvesToJustOneViableAlt(const std::vector<antlrcpp::BitSet> &altsets);

    /// <summary>
    /// Determines if every alternative subset in {@code altsets} contains more
    /// than one alternative.
    /// </summary>
    /// <param name="altsets"> a collection of alternative subsets </param>
    /// <returns> {@code true} if every <seealso cref="BitSet"/> in {@code altsets}
    /// has
    /// <seealso cref="BitSet#cardinality cardinality"/> &gt; 1, otherwise {@code
    /// false} </returns>
    static bool allSubsetsConflict(const std::vector<antlrcpp::BitSet> &altsets);

    /// <summary>
    /// Determines if any single alternative subset in {@code altsets} contains
    /// exactly one alternative.
    /// </summary>
    /// <param name="altsets"> a collection of alternative subsets </param>
    /// <returns> {@code true} if {@code altsets} contains a <seealso
    /// cref="BitSet"/> with
    /// <seealso cref="BitSet#cardinality cardinality"/> 1, otherwise {@code false}
    /// </returns>
    static bool hasNonConflictingAltSet(const std::vector<antlrcpp::BitSet> &altsets);

    /// <summary>
    /// Determines if any single alternative subset in {@code altsets} contains
    /// more than one alternative.
    /// </summary>
    /// <param name="altsets"> a collection of alternative subsets </param>
    /// <returns> {@code true} if {@code altsets} contains a <seealso
    /// cref="BitSet"/> with
    /// <seealso cref="BitSet#cardinality cardinality"/> &gt; 1, otherwise {@code
    /// false} </returns>
    static bool hasConflictingAltSet(const std::vector<antlrcpp::BitSet> &altsets);

    /// <summary>
    /// Determines if every alternative subset in {@code altsets} is equivalent.
    /// </summary>
    /// <param name="altsets"> a collection of alternative subsets </param>
    /// <returns> {@code true} if every member of {@code altsets} is equal to the
    /// others, otherwise {@code false} </returns>
    static bool allSubsetsEqual(const std::vector<antlrcpp::BitSet> &altsets);

    /// <summary>
    /// Returns the unique alternative predicted by all alternative subsets in
    /// {@code altsets}. If no such alternative exists, this method returns
    /// <seealso cref="ATN#INVALID_ALT_NUMBER"/>.
    /// </summary>
    /// <param name="altsets"> a collection of alternative subsets </param>
    static size_t getUniqueAlt(const std::vector<antlrcpp::BitSet> &altsets);

    /// <summary>
    /// Gets the complete set of represented alternatives for a collection of
    /// alternative subsets. This method returns the union of each <seealso
    /// cref="BitSet"/>
    /// in {@code altsets}.
    /// </summary>
    /// <param name="altsets"> a collection of alternative subsets </param>
    /// <returns> the set of represented alternatives in {@code altsets} </returns>
    static antlrcpp::BitSet getAlts(const std::vector<antlrcpp::BitSet> &altsets);

    /** Get union of all alts from configs. @since 4.5.1 */
    static antlrcpp::BitSet getAlts(ATNConfigSet *configs);

    /// <summary>
    /// This function gets the conflicting alt subsets from a configuration set.
    /// For each configuration {@code c} in {@code configs}:
    ///
    /// <pre>
    /// map[c] U= c.<seealso cref="ATNConfig#alt alt"/> # map hash/equals uses s and
    /// x, not
    /// alt and not pred
    /// </pre>
    /// </summary>
    static std::vector<antlrcpp::BitSet> getConflictingAltSubsets(ATNConfigSet *configs);

    /// <summary>
    /// Get a map from state to alt subset from a configuration set. For each
    /// configuration {@code c} in {@code configs}:
    ///
    /// <pre>
    /// map[c.<seealso cref="ATNConfig#state state"/>] U= c.<seealso
    /// cref="ATNConfig#alt alt"/>
    /// </pre>
    /// </summary>
    static std::map<ATNState*, antlrcpp::BitSet> getStateToAltMap(ATNConfigSet *configs);

    static bool hasStateAssociatedWithOneAlt(ATNConfigSet *configs);

    static size_t getSingleViableAlt(const std::vector<antlrcpp::BitSet> &altsets);
  };

} // namespace atn
} // namespace antlr4