src/Atn/PredictionMode.php

<?php

declare(strict_types=1);

namespace Antlr\Antlr4\Runtime\Atn;

use Antlr\Antlr4\Runtime\Atn\SemanticContexts\SemanticContext;
use Antlr\Antlr4\Runtime\Atn\States\RuleStopState;
use Antlr\Antlr4\Runtime\Comparison\Equality;
use Antlr\Antlr4\Runtime\Comparison\Equivalence;
use Antlr\Antlr4\Runtime\Comparison\Hashable;
use Antlr\Antlr4\Runtime\Comparison\Hasher;
use Antlr\Antlr4\Runtime\Utils\BitSet;
use Antlr\Antlr4\Runtime\Utils\Map;

/**
 * This enumeration defines the prediction modes available in ANTLR 4 along with
 * utility methods for analyzing configuration sets for conflicts and/or
 * ambiguities.
 */
final 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
     * {@see PredictionMode::LL()} prediction mode), or it will report a syntax
     * error. If a syntax error is encountered when using the
     * {@see PredictionMode::SLL()} prediction mode, it may be due to either
     * an actual syntax error in the input or indicate that the particular
     * ombination of grammar and input requires the more powerful
     * {@see PredictionMode::LL()} prediction abilities to complete successfully.
     *
     * This prediction mode does not provide any guarantees for prediction
     * behavior for syntactically-incorrect inputs.
     */
    public const SLL = 0;

    /**
     * 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.
     */
    public const LL = 1;

    /**
     * The LL(*) prediction mode with exact ambiguity detection. In addition to
     * the correctness guarantees provided by the {@see PredictionMode::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.
     */
    public const LL_EXACT_AMBIG_DETECTION = 2;

    /**
     * Computes the SLL prediction termination condition.
     *
     * This method computes the SLL prediction termination condition for both of
     * the following cases.
     *
     * - The usual SLL+LL fallback upon SLL conflict
     * - Pure SLL without LL fallback
     *
     * 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.
     * `{1,2}` and `{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):
     *
     * `[12|1|[], 6|2|[], 12|2|[]]. s : (ID | ID ID?) ';' ;`
     *
     * When the ATN simulation reaches the state before `';'`, it has a
     * DFA state that looks like: `[12|1|[], 6|2|[], 12|2|[]]`. Naturally
     * `12|1|[]` and `12|2|[]` conflict, but we cannot stop processing this
     * node because alternative to has another way to continue, via `[6|2|[]]`.
     *
     * It also let's us continue for this rule: `[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, {@see ATNConfigSet} combines stack contexts but not
     * semantic predicate contexts so we might see two configurations like the
     * following.
     *
     * `s, 1, x, {}), (s, 1, x', {p})`
     *
     * Before testing these configurations against others, we have to merge
     * `x` and `x'` (without modifying the existing configurations).
     * For example, we test `(x+x') === x''` when looking for conflicts in
     * the following configurations.
     *
     * `(s, 1, x, {}), (s, 1, x', {p}), (s, 2, x'', {})`
     *
     * If the configuration set has predicates (as indicated by
     * {@see ATNConfigSet::hasSemanticContext()}), this algorithm makes a copy of
     * the configurations to strip out all of the predicates so that a standard
     * {@see ATNConfigSet} will merge everything ignoring predicates.
     */
    public static function hasSLLConflictTerminatingPrediction(int $mode, ATNConfigSet $configs) : bool
    {
        /* Configs in rule stop states indicate reaching the end of the decision
         * rule (local context) or end of start rule (full context). If all
         * configs meet this condition, then none of the configurations is able
         * to match additional input so we terminate prediction.
         */
        if (self::allConfigsInRuleStopStates($configs)) {
            return true;
        }

        // pure SLL mode parsing
        if ($mode === self::SLL) {
            // Don't bother with combining configs from different semantic
            // contexts if we can fail over to full LL; costs more time
            // since we'll often fail over anyway.
            if ($configs->hasSemanticContext) {
                // dup configs, tossing out semantic predicates
                $dup = new ATNConfigSet();

                foreach ($configs->elements() as $c) {
                    $c = new ATNConfig($c, null, null, 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
        $altsets = self::getConflictingAltSubsets($configs);

        return self::hasConflictingAltSet($altsets) && !self::hasStateAssociatedWithOneAlt($configs);
    }

    /**
     * Checks if any configuration in `configs` is in a {@see RuleStopState}.
     * Configurations meeting this condition have reached the end of the decision
     * rule (local context) or end of start rule (full context).
     *
     * @param ATNConfigSet $configs The configuration set to test.
     *
     * @return bool If any configuration in  is in a if any configuration in
     *              `configs` is in a {@see RuleStopState}, otherwise `false`.
     */
    public static function hasConfigInRuleStopState(ATNConfigSet $configs) : bool
    {
        foreach ($configs->elements() as $c) {
            if ($c->state instanceof RuleStopState) {
                return true;
            }
        }

        return false;
    }

    /**
     * Checks if all configurations in `configs` are in a {@see RuleStopState}.
     * Configurations meeting this condition have reached the end of the decision
     * rule (local context) or end of start rule (full context).
     *
     * @param ATNConfigSet $configs the configuration set to test.
     *
     * @return bool If all configurations in  are in a if all configurations in
     *              `configs` are in a {@see RuleStopState}, otherwise `false`.
     */
    public static function allConfigsInRuleStopStates(ATNConfigSet $configs) : bool
    {
        foreach ($configs->elements() as $c) {
            if (!$c->state instanceof RuleStopState) {
                return false;
            }
        }

        return true;
    }

    /**
     * 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 `C`, into
     * conflicting subsets `(s, _, ctx, _)` and singleton subsets with
     * non-conflicting configurations. Two configurations conflict if they have
     * identical {@see ATNConfig::state()} and {@see ATNConfig::context()} values
     * but different {@see ATNConfig::alt()} value, e.g. `(s, i, ctx, _)` and
     * `(s, j, ctx, _)` for `i!=j`.
     *
     * Reduce these configuration subsets to the set of possible alternatives.
     * You can compute the alternative subsets in one pass as follows:
     *
     * `A_s,ctx = {i | (s, i, ctx, _)}` for each configuration in `C` holding
     * `s` and `ctx` fixed.
     *
     * Or in pseudo-code, for each configuration `c` in `C`:
     *
     *     map[c] U= c.{@see ATNConfig::alt alt} # map hash/equals uses s and x,
     *     not alt and not pred
     *
     * The values in `map` are the set of `A_s,ctx` sets.
     *
     * If `|A_s,ctx|=1` then there is no conflict associated with `s` and `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 `(s, i, x)` and `(s, j, x')`, conflict when `i!=j`
     * but `x=x'`. Because we merge all `(s, i, _)` configurations together,
     * that means that there are at most `n` configurations associated with
     * state `s` for `n` possible alternatives in the decision. The merged stacks
     * complicate the comparison of configuration contexts `x` and `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 `x` orv`x'`. If the `x`
     * associated with lowest alternative `i`vis the superset, then `i` is the
     * only possible prediction since the others resolve to `min(i)` as well.
     * However, if `x` is associated with `j>i` then at least one stack
     * configuration for `j` is not in conflict with alternative `i`. The algorithm
     * should keep going, looking for more lookahead due to the uncertainty.
     *
     * For simplicity, I'm doing a equality check between `x` and `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, `[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
     *
     * - no conflicts and more than 1 alternative in set => continue
     * - `(s, 1, x)}, `(s, 2, x)`, `(s, 3, z)`, `(s', 1, y)`, `(s', 2, y)`
     *   yields non-conflicting set `{3}} U conflicting sets `min({1,2})} U
     *   `min({1,2`)` = `{1,3}` => continue
     * - `(s, 1, x)}, `(s, 2, x)`, `(s', 1, y)`, `(s', 2, y)`, `(s'', 1, z)`
     *    yields non-conflicting set `{1}} U conflicting sets `min({1,2})} U
     *    `min({1,2`)` = `{1`` => stop and predict 1
     * - `(s, 1, x)}, `(s, 2, x)`, `(s', 1, y)`, `(s', 2, y)` yields conflicting,
     *    reduced sets `{1`` U `{1}} = `{1`` => stop and predict 1, can announce
     *    ambiguity `{1,2}`
     * - `(s, 1, x)}, `(s, 2, x)`, `(s', 2, y)`, `(s', 3, y)` yields conflicting,
     *    reduced sets `{1`` U `{2}} = `{1,2`` => continue
     * - `(s, 1, x)}, `(s, 2, x)`, `(s', 3, y)`, `(s', 4, y)` yields conflicting,
     *    reduced sets `{1`` U `{3}} = `{1,3`` => continue
     *
     * 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 `A_i`
     * have more than one alternative and all `A_i` are the same. If
     * `A={{1,2}, {1,3}}`, then regular LL prediction would terminate
     * because the resolved set is `{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
     * `A={{1,2}}} or `{{1,2},{1,2}``, etc...
     *
     * @param array<BitSet> $altsets
     */
    public static function resolvesToJustOneViableAlt(array $altsets) : int
    {
        return self::getSingleViableAlt($altsets);
    }

    /**
     * Determines if every alternative subset in `altsets` contains more
     * than one alternative.
     *
     * @param array<BitSet> $altsets a collection of alternative subsets
     *
     * @return bool If every >BitSet in `altsets` {@see BitSet::length()} > 1,
     *              otherwise `false`.
     */
    public static function allSubsetsConflict(array $altsets) : bool
    {
        return !self::hasNonConflictingAltSet($altsets);
    }

    /**
     * Determines if any single alternative subset in `altsets` contains
     * exactly one alternative.
     *
     * @param array<BitSet> $altsets a collection of alternative subsets
     *
     * @return bool `true` if `altsets` contains a {@see BitSet} with
     *              {@see BitSet::length()} 1, otherwise `false`.
     */
    public static function hasNonConflictingAltSet(array $altsets) : bool
    {
        foreach ($altsets as $alts) {
            if ($alts->length() === 1) {
                return true;
            }
        }

        return false;
    }

    /**
     * Determines if any single alternative subset in `altsets` contains
     * more than one alternative.
     *
     * @param array<BitSet> $altsets a collection of alternative subsets
     *
     * @return bool `true` if `altsets` contains a {@see BitSet} with
     *              {@see BitSet::length()} > 1, otherwise `false`.
     */
    public static function hasConflictingAltSet(array $altsets) : bool
    {
        foreach ($altsets as $alts) {
            if ($alts->length() > 1) {
                return true;
            }
        }

        return false;
    }

    /**
     * Determines if every alternative subset in `altsets` is equivalent.
     *
     * @param array<BitSet> $altsets a collection of alternative subsets
     *
     * @return bool `true` if every member of `altsets` is equal to
     *              the others, otherwise `false`.
     */
    public static function allSubsetsEqual(array $altsets) : bool
    {
        $first = null;

        foreach ($altsets as $alts) {
            if ($first === null) {
                $first = $alts;
            } elseif ($alts !== $first) {
                return false;
            }
        }

        return true;
    }

    /**
     * Returns the unique alternative predicted by all alternative subsets in
     * `altsets`. If no such alternative exists, this method returns
     * {@see ATN::INVALID_ALT_NUMBER}.
     *
     * @param array<BitSet> $altsets a collection of alternative subsets
     */
    public static function getUniqueAlt(array $altsets) : int
    {
        $all = self::getAlts($altsets);

        if ($all->length() === 1) {
            return $all->minValue();
        }

        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 {@see BitSet}
     * in `altsets`.
     *
     * @param array<BitSet> $altsets a collection of alternative subsets.
     *
     * @return BitSet the set of represented alternatives in `altsets`.
     */
    public static function getAlts(array $altsets) : BitSet
    {
        $all = new BitSet();

        foreach ($altsets as $alts) {
            $all->or($alts);
        }

        return $all;
    }

    /**
     * This function gets the conflicting alt subsets from a configuration set.
     * For each configuration `c` in `configs`:
     *
     *     map[c] U= c.{@see ATNConfig::$alt} # map hash/equals uses s and x,
     *     not alt and not pred
     *
     * @return array<BitSet>
     */
    public static function getConflictingAltSubsets(ATNConfigSet $configs) : array
    {
        $configToAlts = new Map(new class implements Equivalence {
            public function equals(object $other) : bool
            {
                return $this instanceof self;
            }

            public function equivalent(Hashable $left, Hashable $right) : bool
            {
                return $left instanceof ATNConfig
                    && $right instanceof ATNConfig
                    && $left->state->stateNumber === $right->state->stateNumber
                    && Equality::equals($left->context, $right->context);
            }

            public function hash(Hashable $value) : int
            {
                if (!$value instanceof ATNConfig) {
                    throw new \InvalidArgumentException('Unsupported value.');
                }

                return Hasher::hash($value->state->stateNumber, $value->context);
            }
        });

        foreach ($configs->elements() as $cfg) {
            $alts = $configToAlts->get($cfg);

            if ($alts === null) {
                $alts = new BitSet();
                $configToAlts->put($cfg, $alts);
            }

            $alts->add($cfg->alt);
        }

        return $configToAlts->getValues();
    }

    /**
     * Get a map from state to alt subset from a configuration set. For each
     * configuration `c` in `configs`:
     *
     *     map[c.{@see ATNConfig::$state}] U= c.{@see ATNConfig::$alt}
     */
    public static function getStateToAltMap(ATNConfigSet $configs) : Map
    {
        $m = new Map();

        foreach ($configs->elements() as $c) {
            $alts = $m->get($c->state);

            if ($alts === null) {
                $alts = new BitSet();
                $m->put($c->state, $alts);
            }

            $alts->add($c->alt);
        }

        return $m;
    }

    public static function hasStateAssociatedWithOneAlt(ATNConfigSet $configs) : bool
    {
        foreach (self::getStateToAltMap($configs)->getValues() as $value) {
            if ($value instanceof BitSet && $value->length() === 1) {
                return true;
            }
        }

        return false;
    }

    /**
     * @param array<BitSet> $altsets
     */
    public static function getSingleViableAlt(array $altsets) : int
    {
        $result = 0;

        foreach ($altsets as $alts) {
            $minAlt = $alts->minValue();

            if ($result === 0) {
                $result = (int) $minAlt;
            } elseif ($result !== $minAlt) {
                // more than 1 viable alt
                return ATN::INVALID_ALT_NUMBER;
            }
        }

        return $result;
    }
}