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from sympy.core.add import Add
from sympy.core.containers import Tuple
from sympy.core.expr import Expr
from sympy.core.mul import Mul
from sympy.core.power import Pow
from sympy.core.sorting import default_sort_key
from sympy.core.sympify import sympify
from sympy.matrices import Matrix
def _is_scalar(e):
""" Helper method used in Tr"""
# sympify to set proper attributes
e = sympify(e)
if isinstance(e, Expr):
if (e.is_Integer or e.is_Float or
e.is_Rational or e.is_Number or
(e.is_Symbol and e.is_commutative)
):
return True
return False
def _cycle_permute(l):
""" Cyclic permutations based on canonical ordering
Explanation
===========
This method does the sort based ascii values while
a better approach would be to used lexicographic sort.
TODO: Handle condition such as symbols have subscripts/superscripts
in case of lexicographic sort
"""
if len(l) == 1:
return l
min_item = min(l, key=default_sort_key)
indices = [i for i, x in enumerate(l) if x == min_item]
le = list(l)
le.extend(l) # duplicate and extend string for easy processing
# adding the first min_item index back for easier looping
indices.append(len(l) + indices[0])
# create sublist of items with first item as min_item and last_item
# in each of the sublist is item just before the next occurrence of
# minitem in the cycle formed.
sublist = [[le[indices[i]:indices[i + 1]]] for i in
range(len(indices) - 1)]
# we do comparison of strings by comparing elements
# in each sublist
idx = sublist.index(min(sublist))
ordered_l = le[indices[idx]:indices[idx] + len(l)]
return ordered_l
def _rearrange_args(l):
""" this just moves the last arg to first position
to enable expansion of args
A,B,A ==> A**2,B
"""
if len(l) == 1:
return l
x = list(l[-1:])
x.extend(l[0:-1])
return Mul(*x).args
class Tr(Expr):
""" Generic Trace operation than can trace over:
a) SymPy matrix
b) operators
c) outer products
Parameters
==========
o : operator, matrix, expr
i : tuple/list indices (optional)
Examples
========
# TODO: Need to handle printing
a) Trace(A+B) = Tr(A) + Tr(B)
b) Trace(scalar*Operator) = scalar*Trace(Operator)
>>> from sympy.physics.quantum.trace import Tr
>>> from sympy import symbols, Matrix
>>> a, b = symbols('a b', commutative=True)
>>> A, B = symbols('A B', commutative=False)
>>> Tr(a*A,[2])
a*Tr(A)
>>> m = Matrix([[1,2],[1,1]])
>>> Tr(m)
2
"""
def __new__(cls, *args):
""" Construct a Trace object.
Parameters
==========
args = SymPy expression
indices = tuple/list if indices, optional
"""
# expect no indices,int or a tuple/list/Tuple
if (len(args) == 2):
if not isinstance(args[1], (list, Tuple, tuple)):
indices = Tuple(args[1])
else:
indices = Tuple(*args[1])
expr = args[0]
elif (len(args) == 1):
indices = Tuple()
expr = args[0]
else:
raise ValueError("Arguments to Tr should be of form "
"(expr[, [indices]])")
if isinstance(expr, Matrix):
return expr.trace()
elif hasattr(expr, 'trace') and callable(expr.trace):
#for any objects that have trace() defined e.g numpy
return expr.trace()
elif isinstance(expr, Add):
return Add(*[Tr(arg, indices) for arg in expr.args])
elif isinstance(expr, Mul):
c_part, nc_part = expr.args_cnc()
if len(nc_part) == 0:
return Mul(*c_part)
else:
obj = Expr.__new__(cls, Mul(*nc_part), indices )
#this check is needed to prevent cached instances
#being returned even if len(c_part)==0
return Mul(*c_part)*obj if len(c_part) > 0 else obj
elif isinstance(expr, Pow):
if (_is_scalar(expr.args[0]) and
_is_scalar(expr.args[1])):
return expr
else:
return Expr.__new__(cls, expr, indices)
else:
if (_is_scalar(expr)):
return expr
return Expr.__new__(cls, expr, indices)
@property
def kind(self):
expr = self.args[0]
expr_kind = expr.kind
return expr_kind.element_kind
def doit(self, **hints):
""" Perform the trace operation.
#TODO: Current version ignores the indices set for partial trace.
>>> from sympy.physics.quantum.trace import Tr
>>> from sympy.physics.quantum.operator import OuterProduct
>>> from sympy.physics.quantum.spin import JzKet, JzBra
>>> t = Tr(OuterProduct(JzKet(1,1), JzBra(1,1)))
>>> t.doit()
1
"""
if hasattr(self.args[0], '_eval_trace'):
return self.args[0]._eval_trace(indices=self.args[1])
return self
@property
def is_number(self):
# TODO : improve this implementation
return True
#TODO: Review if the permute method is needed
# and if it needs to return a new instance
def permute(self, pos):
""" Permute the arguments cyclically.
Parameters
==========
pos : integer, if positive, shift-right, else shift-left
Examples
========
>>> from sympy.physics.quantum.trace import Tr
>>> from sympy import symbols
>>> A, B, C, D = symbols('A B C D', commutative=False)
>>> t = Tr(A*B*C*D)
>>> t.permute(2)
Tr(C*D*A*B)
>>> t.permute(-2)
Tr(C*D*A*B)
"""
if pos > 0:
pos = pos % len(self.args[0].args)
else:
pos = -(abs(pos) % len(self.args[0].args))
args = list(self.args[0].args[-pos:] + self.args[0].args[0:-pos])
return Tr(Mul(*(args)))
def _hashable_content(self):
if isinstance(self.args[0], Mul):
args = _cycle_permute(_rearrange_args(self.args[0].args))
else:
args = [self.args[0]]
return tuple(args) + (self.args[1], )
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