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This patch introduces nested let-abbreviations in the ExprSMTLIBPrinter
to reduce the size of the SMTLIBv2 queries and the corresponding processing
time (bugfix for #170).
The current implementation of the let abbreviation mode does not consider
expression intra-dependencies and prints all abbreviations in the same
let scope. For a (simplified) example, it prints
(assert (let ( (?B1 (A + B)) (?B2 (A + B + C)) ) (= ?B1 ?B2) ) ).
This is extremely inefficient if the expressions (and there many of these!)
extensively reuse their subexpressions. Therefore, it's better to print
the query with nested let-expressions by reusing existing expression bindings
in the new let scope:
(assert (let ( (?B1 (A + B)) ) (let ( (?B2 (?B1 + C)) ) (= ?B1 ?B2) ) ) ).
This patch adds a new function ExprSMTLIBPrinter::scanBindingExprDeps() that
scans bindings for expression dependencies. The result is a vector of
new bindings (orderedBindings) that represents the expression dependency tree.
When printing in the let-abbreviation mode, the new code starts with
abbreviating expressions that have no dependencies and then gradually makes
these new bindings available in the upcoming let-scopes where expressions
with dependencies reuse them.
The effect of nested let-abbreviations is comparable to :named abbreviations.
However, the latter mode is not supported by the majority of the solvers.
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single method with two different implementations.
There is one version of this method for human readability
(printHumanReadableQuery()) and a version for machine consumption
(printMachineReadableQuery()).
The reason for having two versions is because different behaviour is
needed in different scenarios
* In machine readable mode the entire query is printed inside a single
``(assert ...)``. This is done to allow ``(let ...)`` to abbreviate
as much as possible.
* In human readable mode each constraint and query expression is printed
inside its own ``(assert ...)`` unless the abbreviation mode is
ABBR_LET in which case all constraints and query expr are printed
inside a single ``(assert ...)`` much like in the machine readable mode
Whilst I was here I also fixed a bug handling identation when printing
``(let ...)`` expressions in printAssert()
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* Set the default abbreviation mode to let (ExprSMTLIBPrinter::ABBR_LET)
* Remove the now defunct ExprSMTLIBLetPrinter
* Improve performance of ExprSMTLIBPrinter::scan() by keeping
track of visited Expr to avoid visiting them again
* Rename ExprSMTLIBPrinter::printQuery() to ExprSMTLIBPrinter::printQueryExpr()
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