public enum PredictionMode extends Enum<PredictionMode>
Enum Constant and Description 

LL
The LL(*) prediction mode.

LL_EXACT_AMBIG_DETECTION
The LL(*) prediction mode with exact ambiguity detection.

SLL
The SLL(*) prediction mode.

Modifier and Type  Method and Description 

static boolean 
allConfigsInRuleStopStates(ATNConfigSet configs)
Checks if all configurations in
configs are in a
RuleStopState . 
static boolean 
allSubsetsConflict(Collection<BitSet> altsets)
Determines if every alternative subset in
altsets contains more
than one alternative. 
static boolean 
allSubsetsEqual(Collection<BitSet> altsets)
Determines if every alternative subset in
altsets is equivalent. 
static BitSet 
getAlts(ATNConfigSet configs)
Get union of all alts from configs.

static BitSet 
getAlts(Collection<BitSet> altsets)
Gets the complete set of represented alternatives for a collection of
alternative subsets.

static Collection<BitSet> 
getConflictingAltSubsets(ATNConfigSet configs)
This function gets the conflicting alt subsets from a configuration set.

static int 
getSingleViableAlt(Collection<BitSet> altsets) 
static Map<ATNState,BitSet> 
getStateToAltMap(ATNConfigSet configs)
Get a map from state to alt subset from a configuration set.

static int 
getUniqueAlt(Collection<BitSet> altsets)
Returns the unique alternative predicted by all alternative subsets in
altsets . 
static boolean 
hasConfigInRuleStopState(ATNConfigSet configs)
Checks if any configuration in
configs is in a
RuleStopState . 
static boolean 
hasConflictingAltSet(Collection<BitSet> altsets)
Determines if any single alternative subset in
altsets contains
more than one alternative. 
static boolean 
hasNonConflictingAltSet(Collection<BitSet> altsets)
Determines if any single alternative subset in
altsets contains
exactly one alternative. 
static boolean 
hasSLLConflictTerminatingPrediction(PredictionMode mode,
ATNConfigSet configs)
Computes the SLL prediction termination condition.

static boolean 
hasStateAssociatedWithOneAlt(ATNConfigSet configs) 
static int 
resolvesToJustOneViableAlt(Collection<BitSet> altsets)
Full LL prediction termination.

static PredictionMode 
valueOf(String name)
Returns the enum constant of this type with the specified name.

static PredictionMode[] 
values()
Returns an array containing the constants of this enum type, in
the order they are declared.

public static final PredictionMode 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
LL
prediction mode), or it will report a syntax error. If a
syntax error is encountered when using the 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 LL
prediction abilities to complete successfully.
This prediction mode does not provide any guarantees for prediction behavior for syntacticallyincorrect inputs.
public static final PredictionMode LL
When using this prediction mode, the parser will make correct decisions for all syntacticallycorrect 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 syntacticallyincorrect inputs.
public static final PredictionMode LL_EXACT_AMBIG_DETECTION
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 syntacticallyincorrect inputs.
public static PredictionMode[] values()
for (PredictionMode c : PredictionMode.values()) System.out.println(c);
public static PredictionMode valueOf(String name)
name
 the name of the enum constant to be returned.IllegalArgumentException
 if this enum type has no constant with the specified nameNullPointerException
 if the argument is nullpublic static boolean hasSLLConflictTerminatingPrediction(PredictionMode mode, ATNConfigSet configs)
This method computes the SLL prediction termination condition for both of the following cases.
COMBINED SLL+LL PARSING
When LLfallback 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 nonconflicting
configuration, SLL could continue with the hopes that more lookahead will
resolve via one of those nonconflicting 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 singlealtstate thing lets prediction continue upon rules like (otherwise, it would admit defeat too soon):
[121[], 62[], 122[]]. s : (ID  ID ID?) ';' ;
When the ATN simulation reaches the state before ';'
, it has a
DFA state that looks like: [121[], 62[], 122[]]
. Naturally
121[]
and 122[]
conflict, but we cannot stop
processing this node because alternative to has another way to continue,
via [62[]]
.
It also let's us continue for this rule:
[11[], 12[], 83[]] 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.
Implementationwise, 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
ATNConfigSet.hasSemanticContext
), this algorithm makes a copy of
the configurations to strip out all of the predicates so that a standard
ATNConfigSet
will merge everything ignoring predicates.
public static boolean hasConfigInRuleStopState(ATNConfigSet configs)
configs
is in a
RuleStopState
. Configurations meeting this condition have reached
the end of the decision rule (local context) or end of start rule (full
context).configs
 the configuration set to testtrue
if any configuration in configs
is in a
RuleStopState
, otherwise false
public static boolean allConfigsInRuleStopStates(ATNConfigSet configs)
configs
are in a
RuleStopState
. Configurations meeting this condition have reached
the end of the decision rule (local context) or end of start rule (full
context).configs
 the configuration set to testtrue
if all configurations in configs
are in a
RuleStopState
, otherwise false
public static int resolvesToJustOneViableAlt(Collection<BitSet> altsets)
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
nonconflicting configurations. Two configurations conflict if they have
identical ATNConfig.state
and ATNConfig.context
values
but different 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 pseudocode, for each configuration c
in C
:
map[c] U= c.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 onthefly 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
or
x'
. If the x
associated with lowest alternative i
is 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 nonconflicting 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
(s, 1, x)
, (s, 2, x)
, (s, 3, z)
,
(s', 1, y)
, (s', 2, y)
yields nonconflicting 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 nonconflicting 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}
=> continueEXACT 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...
public static boolean allSubsetsConflict(Collection<BitSet> altsets)
altsets
contains more
than one alternative.altsets
 a collection of alternative subsetstrue
if every BitSet
in altsets
has
cardinality
> 1, otherwise false
public static boolean hasNonConflictingAltSet(Collection<BitSet> altsets)
altsets
contains
exactly one alternative.altsets
 a collection of alternative subsetstrue
if altsets
contains a BitSet
with
cardinality
1, otherwise false
public static boolean hasConflictingAltSet(Collection<BitSet> altsets)
altsets
contains
more than one alternative.altsets
 a collection of alternative subsetstrue
if altsets
contains a BitSet
with
cardinality
> 1, otherwise false
public static boolean allSubsetsEqual(Collection<BitSet> altsets)
altsets
is equivalent.altsets
 a collection of alternative subsetstrue
if every member of altsets
is equal to the
others, otherwise false
public static int getUniqueAlt(Collection<BitSet> altsets)
altsets
. If no such alternative exists, this method returns
ATN.INVALID_ALT_NUMBER
.altsets
 a collection of alternative subsetspublic static BitSet getAlts(Collection<BitSet> altsets)
BitSet
in altsets
.altsets
 a collection of alternative subsetsaltsets
public static BitSet getAlts(ATNConfigSet configs)
public static Collection<BitSet> getConflictingAltSubsets(ATNConfigSet configs)
c
in configs
:
map[c] U= c.alt
# map hash/equals uses s and x, not
alt and not pred
public static Map<ATNState,BitSet> getStateToAltMap(ATNConfigSet configs)
public static boolean hasStateAssociatedWithOneAlt(ATNConfigSet configs)
public static int getSingleViableAlt(Collection<BitSet> altsets)
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