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323 lines
12 KiB
Python
323 lines
12 KiB
Python
# This code supports verifying group implementations which have branches
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# or conditional statements (like cmovs), by allowing each execution path
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# to independently set assumptions on input or intermediary variables.
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#
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# The general approach is:
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# * A constraint is a tuple of two sets of symbolic expressions:
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# the first of which are required to evaluate to zero, the second of which
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# are required to evaluate to nonzero.
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# - A constraint is said to be conflicting if any of its nonzero expressions
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# is in the ideal with basis the zero expressions (in other words: when the
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# zero expressions imply that one of the nonzero expressions are zero).
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# * There is a list of laws that describe the intended behaviour, including
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# laws for addition and doubling. Each law is called with the symbolic point
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# coordinates as arguments, and returns:
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# - A constraint describing the assumptions under which it is applicable,
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# called "assumeLaw"
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# - A constraint describing the requirements of the law, called "require"
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# * Implementations are transliterated into functions that operate as well on
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# algebraic input points, and are called once per combination of branches
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# executed. Each execution returns:
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# - A constraint describing the assumptions this implementation requires
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# (such as Z1=1), called "assumeFormula"
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# - A constraint describing the assumptions this specific branch requires,
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# but which is by construction guaranteed to cover the entire space by
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# merging the results from all branches, called "assumeBranch"
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# - The result of the computation
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# * All combinations of laws with implementation branches are tried, and:
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# - If the combination of assumeLaw, assumeFormula, and assumeBranch results
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# in a conflict, it means this law does not apply to this branch, and it is
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# skipped.
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# - For others, we try to prove the require constraints hold, assuming the
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# information in assumeLaw + assumeFormula + assumeBranch, and if this does
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# not succeed, we fail.
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# + To prove an expression is zero, we check whether it belongs to the
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# ideal with the assumed zero expressions as basis. This test is exact.
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# + To prove an expression is nonzero, we check whether each of its
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# factors is contained in the set of nonzero assumptions' factors.
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# This test is not exact, so various combinations of original and
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# reduced expressions' factors are tried.
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# - If we succeed, we print out the assumptions from assumeFormula that
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# weren't implied by assumeLaw already. Those from assumeBranch are skipped,
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# as we assume that all constraints in it are complementary with each other.
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#
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# Based on the sage verification scripts used in the Explicit-Formulas Database
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# by Tanja Lange and others, see http://hyperelliptic.org/EFD
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class fastfrac:
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"""Fractions over rings."""
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def __init__(self,R,top,bot=1):
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"""Construct a fractional, given a ring, a numerator, and denominator."""
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self.R = R
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if parent(top) == ZZ or parent(top) == R:
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self.top = R(top)
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self.bot = R(bot)
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elif top.__class__ == fastfrac:
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self.top = top.top
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self.bot = top.bot * bot
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else:
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self.top = R(numerator(top))
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self.bot = R(denominator(top)) * bot
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def iszero(self,I):
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"""Return whether this fraction is zero given an ideal."""
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return self.top in I and self.bot not in I
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def reduce(self,assumeZero):
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zero = self.R.ideal(map(numerator, assumeZero))
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return fastfrac(self.R, zero.reduce(self.top)) / fastfrac(self.R, zero.reduce(self.bot))
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def __add__(self,other):
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"""Add two fractions."""
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if parent(other) == ZZ:
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return fastfrac(self.R,self.top + self.bot * other,self.bot)
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if other.__class__ == fastfrac:
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return fastfrac(self.R,self.top * other.bot + self.bot * other.top,self.bot * other.bot)
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return NotImplemented
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def __sub__(self,other):
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"""Subtract two fractions."""
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if parent(other) == ZZ:
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return fastfrac(self.R,self.top - self.bot * other,self.bot)
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if other.__class__ == fastfrac:
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return fastfrac(self.R,self.top * other.bot - self.bot * other.top,self.bot * other.bot)
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return NotImplemented
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def __neg__(self):
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"""Return the negation of a fraction."""
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return fastfrac(self.R,-self.top,self.bot)
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def __mul__(self,other):
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"""Multiply two fractions."""
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if parent(other) == ZZ:
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return fastfrac(self.R,self.top * other,self.bot)
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if other.__class__ == fastfrac:
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return fastfrac(self.R,self.top * other.top,self.bot * other.bot)
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return NotImplemented
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def __rmul__(self,other):
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"""Multiply something else with a fraction."""
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return self.__mul__(other)
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def __div__(self,other):
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"""Divide two fractions."""
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if parent(other) == ZZ:
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return fastfrac(self.R,self.top,self.bot * other)
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if other.__class__ == fastfrac:
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return fastfrac(self.R,self.top * other.bot,self.bot * other.top)
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return NotImplemented
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def __pow__(self,other):
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"""Compute a power of a fraction."""
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if parent(other) == ZZ:
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if other < 0:
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# Negative powers require flipping top and bottom
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return fastfrac(self.R,self.bot ^ (-other),self.top ^ (-other))
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else:
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return fastfrac(self.R,self.top ^ other,self.bot ^ other)
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return NotImplemented
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def __str__(self):
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return "fastfrac((" + str(self.top) + ") / (" + str(self.bot) + "))"
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def __repr__(self):
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return "%s" % self
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def numerator(self):
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return self.top
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class constraints:
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"""A set of constraints, consisting of zero and nonzero expressions.
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Constraints can either be used to express knowledge or a requirement.
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Both the fields zero and nonzero are maps from expressions to description
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strings. The expressions that are the keys in zero are required to be zero,
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and the expressions that are the keys in nonzero are required to be nonzero.
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Note that (a != 0) and (b != 0) is the same as (a*b != 0), so all keys in
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nonzero could be multiplied into a single key. This is often much less
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efficient to work with though, so we keep them separate inside the
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constraints. This allows higher-level code to do fast checks on the individual
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nonzero elements, or combine them if needed for stronger checks.
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We can't multiply the different zero elements, as it would suffice for one of
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the factors to be zero, instead of all of them. Instead, the zero elements are
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typically combined into an ideal first.
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"""
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def __init__(self, **kwargs):
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if 'zero' in kwargs:
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self.zero = dict(kwargs['zero'])
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else:
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self.zero = dict()
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if 'nonzero' in kwargs:
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self.nonzero = dict(kwargs['nonzero'])
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else:
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self.nonzero = dict()
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def negate(self):
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return constraints(zero=self.nonzero, nonzero=self.zero)
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def __add__(self, other):
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zero = self.zero.copy()
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zero.update(other.zero)
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nonzero = self.nonzero.copy()
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nonzero.update(other.nonzero)
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return constraints(zero=zero, nonzero=nonzero)
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def __str__(self):
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return "constraints(zero=%s,nonzero=%s)" % (self.zero, self.nonzero)
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def __repr__(self):
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return "%s" % self
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def conflicts(R, con):
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"""Check whether any of the passed non-zero assumptions is implied by the zero assumptions"""
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zero = R.ideal(map(numerator, con.zero))
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if 1 in zero:
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return True
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# First a cheap check whether any of the individual nonzero terms conflict on
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# their own.
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for nonzero in con.nonzero:
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if nonzero.iszero(zero):
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return True
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# It can be the case that entries in the nonzero set do not individually
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# conflict with the zero set, but their combination does. For example, knowing
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# that either x or y is zero is equivalent to having x*y in the zero set.
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# Having x or y individually in the nonzero set is not a conflict, but both
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# simultaneously is, so that is the right thing to check for.
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if reduce(lambda a,b: a * b, con.nonzero, fastfrac(R, 1)).iszero(zero):
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return True
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return False
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def get_nonzero_set(R, assume):
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"""Calculate a simple set of nonzero expressions"""
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zero = R.ideal(map(numerator, assume.zero))
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nonzero = set()
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for nz in map(numerator, assume.nonzero):
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for (f,n) in nz.factor():
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nonzero.add(f)
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rnz = zero.reduce(nz)
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for (f,n) in rnz.factor():
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nonzero.add(f)
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return nonzero
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def prove_nonzero(R, exprs, assume):
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"""Check whether an expression is provably nonzero, given assumptions"""
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zero = R.ideal(map(numerator, assume.zero))
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nonzero = get_nonzero_set(R, assume)
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expl = set()
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ok = True
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for expr in exprs:
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if numerator(expr) in zero:
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return (False, [exprs[expr]])
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allexprs = reduce(lambda a,b: numerator(a)*numerator(b), exprs, 1)
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for (f, n) in allexprs.factor():
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if f not in nonzero:
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ok = False
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if ok:
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return (True, None)
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ok = True
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for (f, n) in zero.reduce(numerator(allexprs)).factor():
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if f not in nonzero:
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ok = False
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if ok:
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return (True, None)
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ok = True
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for expr in exprs:
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for (f,n) in numerator(expr).factor():
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if f not in nonzero:
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ok = False
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if ok:
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return (True, None)
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ok = True
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for expr in exprs:
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for (f,n) in zero.reduce(numerator(expr)).factor():
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if f not in nonzero:
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expl.add(exprs[expr])
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if expl:
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return (False, list(expl))
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else:
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return (True, None)
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def prove_zero(R, exprs, assume):
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"""Check whether all of the passed expressions are provably zero, given assumptions"""
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r, e = prove_nonzero(R, dict(map(lambda x: (fastfrac(R, x.bot, 1), exprs[x]), exprs)), assume)
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if not r:
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return (False, map(lambda x: "Possibly zero denominator: %s" % x, e))
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zero = R.ideal(map(numerator, assume.zero))
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nonzero = prod(x for x in assume.nonzero)
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expl = []
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for expr in exprs:
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if not expr.iszero(zero):
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expl.append(exprs[expr])
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if not expl:
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return (True, None)
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return (False, expl)
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def describe_extra(R, assume, assumeExtra):
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"""Describe what assumptions are added, given existing assumptions"""
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zerox = assume.zero.copy()
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zerox.update(assumeExtra.zero)
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zero = R.ideal(map(numerator, assume.zero))
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zeroextra = R.ideal(map(numerator, zerox))
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nonzero = get_nonzero_set(R, assume)
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ret = set()
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# Iterate over the extra zero expressions
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for base in assumeExtra.zero:
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if base not in zero:
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add = []
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for (f, n) in numerator(base).factor():
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if f not in nonzero:
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add += ["%s" % f]
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if add:
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ret.add((" * ".join(add)) + " = 0 [%s]" % assumeExtra.zero[base])
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# Iterate over the extra nonzero expressions
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for nz in assumeExtra.nonzero:
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nzr = zeroextra.reduce(numerator(nz))
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if nzr not in zeroextra:
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for (f,n) in nzr.factor():
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if zeroextra.reduce(f) not in nonzero:
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ret.add("%s != 0" % zeroextra.reduce(f))
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return ", ".join(x for x in ret)
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def check_symbolic(R, assumeLaw, assumeAssert, assumeBranch, require):
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"""Check a set of zero and nonzero requirements, given a set of zero and nonzero assumptions"""
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assume = assumeLaw + assumeAssert + assumeBranch
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if conflicts(R, assume):
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# This formula does not apply
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return None
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describe = describe_extra(R, assumeLaw + assumeBranch, assumeAssert)
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ok, msg = prove_zero(R, require.zero, assume)
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if not ok:
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return "FAIL, %s fails (assuming %s)" % (str(msg), describe)
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res, expl = prove_nonzero(R, require.nonzero, assume)
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if not res:
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return "FAIL, %s fails (assuming %s)" % (str(expl), describe)
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if describe != "":
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return "OK (assuming %s)" % describe
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else:
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return "OK"
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def concrete_verify(c):
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for k in c.zero:
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if k != 0:
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return (False, c.zero[k])
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for k in c.nonzero:
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if k == 0:
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return (False, c.nonzero[k])
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return (True, None)
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