--- /dev/null
+/* 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 => 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>
+ */
+ 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"/> > 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"/> > 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
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