/* 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
