/// <summary>Computes the SLL prediction termination condition.</summary>
/// <remarks>
/// 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.
/// <c/>
///
/// 1,2}} and
/// <c/>
///
/// 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>
/// <c>[12|1|[], 6|2|[], 12|2|[]]. s : (ID | ID ID?) ';' ;</c>
/// </p>
/// <p>When the ATN simulation reaches the state before
/// <c>';'</c>
/// , it has a
/// DFA state that looks like:
/// <c>[12|1|[], 6|2|[], 12|2|[]]</c>
/// . Naturally
/// <c>12|1|[]</c>
/// and
/// <c>12|2|[]</c>
/// conflict, but we cannot stop
/// processing this node because alternative to has another way to continue,
/// via
/// <c>[6|2|[]]</c>
/// .</p>
/// <p>It also let's us continue for this rule:</p>
/// <p>
/// <c>[1|1|[], 1|2|[], 8|3|[]] a : A | A | A B ;</c>
/// </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,
/// <see cref="ATNConfigSet"/>
/// combines stack contexts but not
/// semantic predicate contexts so we might see two configurations like the
/// following.</p>
/// <p>
/// <c/>
/// (s, 1, x,
/// ), (s, 1, x', {p})}</p>
/// <p>Before testing these configurations against others, we have to merge
/// <c>x</c>
/// and
/// <c>x'</c>
/// (without modifying the existing configurations).
/// For example, we test
/// <c>(x+x')==x''</c>
/// when looking for conflicts in
/// the following configurations.</p>
/// <p>
/// <c/>
/// (s, 1, x,
/// ), (s, 1, x', {p}), (s, 2, x'', {})}</p>
/// <p>If the configuration set has predicates (as indicated by
/// <see cref="ATNConfigSet.hasSemanticContext"/>
/// ), this algorithm makes a copy of
/// the configurations to strip out all of the predicates so that a standard
/// <see cref="ATNConfigSet"/>
/// will merge everything ignoring predicates.</p>
/// </remarks>
public static bool HasSLLConflictTerminatingPrediction(PredictionMode mode, ATNConfigSet configSet)
{
if (AllConfigsInRuleStopStates(configSet.configs))
{
return(true);
}
// pure SLL mode parsing
if (mode == PredictionMode.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 (configSet.hasSemanticContext)
{
// dup configs, tossing out semantic predicates
ATNConfigSet dup = new ATNConfigSet();
foreach (ATNConfig c in configSet.configs)
{
dup.Add(new ATNConfig(c, SemanticContext.NONE));
}
configSet = dup;
}
}
// now we have combined contexts for configs with dissimilar preds
// pure SLL or combined SLL+LL mode parsing
ICollection <BitSet> altsets = GetConflictingAltSubsets(configSet.configs);
bool heuristic = HasConflictingAltSet(altsets) && !HasStateAssociatedWithOneAlt(configSet.configs);
return(heuristic);
}
/// <summary>Computes the SLL prediction termination condition.</summary>
/// <remarks>
/// 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.
/// <c/>
///
/// 1,2}} and
/// <c/>
///
/// 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>
/// <c>[12|1|[], 6|2|[], 12|2|[]]. s : (ID | ID ID?) ';' ;</c>
/// </p>
/// <p>When the ATN simulation reaches the state before
/// <c>';'</c>
/// , it has a
/// DFA state that looks like:
/// <c>[12|1|[], 6|2|[], 12|2|[]]</c>
/// . Naturally
/// <c>12|1|[]</c>
/// and
/// <c>12|2|[]</c>
/// conflict, but we cannot stop
/// processing this node because alternative to has another way to continue,
/// via
/// <c>[6|2|[]]</c>
/// .</p>
/// <p>It also let's us continue for this rule:</p>
/// <p>
/// <c>[1|1|[], 1|2|[], 8|3|[]] a : A | A | A B ;</c>
/// </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,
/// <see cref="ATNConfigSet"/>
/// combines stack contexts but not
/// semantic predicate contexts so we might see two configurations like the
/// following.</p>
/// <p>
/// <c/>
/// (s, 1, x,
/// ), (s, 1, x', {p})}</p>
/// <p>Before testing these configurations against others, we have to merge
/// <c>x</c>
/// and
/// <c>x'</c>
/// (without modifying the existing configurations).
/// For example, we test
/// <c>(x+x')==x''</c>
/// when looking for conflicts in
/// the following configurations.</p>
/// <p>
/// <c/>
/// (s, 1, x,
/// ), (s, 1, x', {p}), (s, 2, x'', {})}</p>
/// <p>If the configuration set has predicates (as indicated by
/// <see cref="ATNConfigSet.HasSemanticContext()"/>
/// ), this algorithm makes a copy of
/// the configurations to strip out all of the predicates so that a standard
/// <see cref="ATNConfigSet"/>
/// will merge everything ignoring predicates.</p>
/// </remarks>
public static bool HasSLLConflictTerminatingPrediction(PredictionMode mode, [NotNull] ATNConfigSet configs)
{
/* 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 (AllConfigsInRuleStopStates(configs))
{
return(true);
}
// pure SLL mode parsing
if (mode == PredictionMode.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
ATNConfigSet dup = new ATNConfigSet();
foreach (ATNConfig c in configs)
{
ATNConfig config = c.Transform(c.State, SemanticContext.None, false);
dup.Add(c);
}
configs = dup;
}
}
// now we have combined contexts for configs with dissimilar preds
// pure SLL or combined SLL+LL mode parsing
ICollection <BitSet> altsets = GetConflictingAltSubsets(configs);
bool heuristic = HasConflictingAltSet(altsets) && !HasStateAssociatedWithOneAlt(configs);
return(heuristic);
}
