Method: PredictionContextFunctions::Methods#mergeRoot

Defined in:
lib/antlr4/PredictionContext.rb

#mergeRoot(a, b, rootIsWildcard) ⇒ Object

Handle case where at least one of a or b is #EMPTY. In the following diagrams, the symbol $ is used to represent #EMPTY.

<h2>Local-Context Merges</h2>

<p>These local-context merge operations are used when rootIsWildcard is true.</p>

<p>#EMPTY is superset of any graph; return #EMPTY.
<embed src=“images/LocalMerge_EmptyRoot.svg” type=“image/svg+xml”/></p>

<p>#EMPTY and anything is #EMPTY, so merged parent is #EMPTY; return left graph.
<embed src=“images/LocalMerge_EmptyParent.svg” type=“image/svg+xml”/></p>

<p>Special case of last merge if local context.
<embed src=“images/LocalMerge_DiffRoots.svg” type=“image/svg+xml”/></p>

<h2>Full-Context Merges</h2>

<p>These full-context merge operations are used when rootIsWildcard is false.</p>

<p><embed src=“images/FullMerge_EmptyRoots.svg” type=“image/svg+xml”/></p>

<p>Must keep all contexts; #EMPTY in array is a special value (and null parent).
<embed src=“images/FullMerge_EmptyRoot.svg” type=“image/svg+xml”/></p>

<p><embed src=“images/FullMerge_SameRoot.svg” type=“image/svg+xml”/></p>

otherwise false to indicate a full-context merge / def mergeRoot(a:SingletonPredictionContext, b:SingletonPredictionContext, rootIsWildcard:bool):

Parameters:

  • a —

    the first SingletonPredictionContext

  • b —

    the second SingletonPredictionContext

  • rootIsWildcard (@code true) —

    if this is a local-context merge,



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# File 'lib/antlr4/PredictionContext.rb', line 456

def mergeRoot(a, b, rootIsWildcard)
    if rootIsWildcard
        return PredictionContext.EMPTY if PredictionContext.EMPTY == a ## + b =#
        return PredictionContext.EMPTY if PredictionContext.EMPTY == b # a +# =#
    else
        if PredictionContext.EMPTY == a and PredictionContext.EMPTY == b then
            return PredictionContext.EMPTY # $ + $ = $
        elsif PredictionContext.EMPTY == a # $ + x = [$,x]
            payloads = [ b.returnState, PredictionContext::EMPTY_RETURN_STATE ]
            parents = [ b.parentCtx, nil ]
            return ArrayPredictionContext.new(parents, payloads)
        elsif PredictionContext.EMPTY == b # x + $ = [$,x] ($ is always first if present)
            payloads = [ a.returnState, PredictionContext::EMPTY_RETURN_STATE ]
            parents = [ a.parentCtx, nil ]
            return ArrayPredictionContext.new(parents, payloads)
        end
    end
    return nil
end