def facets(self, edge, other):
"""Iterate over facets obtained by contracting `edge` and
projecting onto `other`.
Each returned item is a triple `(j, k, s)`, where:
- `j` is the index of a `NumberedFatgraph` in `self`;
- `k` is the index of a `NumberedFatgraph` in `other`;
- `s` is the sign by which `self[j].contract(edge)` projects onto `other[k]`.
Only triples for which `s != 0` are returned.
Examples::
>>> p0 = NumberedFatgraphPool(Fatgraph([Vertex([1, 2, 0, 1, 0]), Vertex([3, 3, 2])]))
>>> p1 = NumberedFatgraphPool(Fatgraph([Vertex([0, 1, 0, 1, 2, 2])]))
>>> list(NumberedFatgraphPool.facets(p0, 2, p1))
[(0, 0, 1), (1, 1, 1)]
"""
assert not self.graph.is_loop(edge)
assert self.is_orientable
assert other.is_orientable
g0 = self.graph
g1 = g0.contract(edge)
g2 = other.graph
assert len(g1.boundary_cycles) == len(g2.boundary_cycles)
# compute isomorphism map `f1` from `g1` to `g2`: if there is
# no such isomorphisms, then stop iteration (do this first so
# then we do not waste time on computing if we need to abort
# anyway)
f1 = Fatgraph.isomorphisms(g1, g2).next()
## 1. compute map `phi0` induced on `g0.boundary_cycles` from the
## graph map `f0` which contracts `edge`.
##
(e1, e2) = g0.endpoints(edge)
assert set(g1.boundary_cycles) == set([g0.contract_boundary_cycle(bcy, e1, e2)
for bcy in g0.boundary_cycles]), \
"NumberedFatgraphPool.facets():" \
" Boundary cycles of contracted graph are not the same" \
" as contracted boundary cycles of parent graph:" \
" `%s` vs `%s`" % (g1.boundary_cycles,
[g0.contract_boundary_cycle(bcy, e1, e2)
for bcy in g0.boundary_cycles])
phi0_inv = Permutation((i1, i0) for (i0, i1) in enumerate(
g1.boundary_cycles.index(g0.contract_boundary_cycle(bc0, e1, e2))
for bc0 in g0.boundary_cycles))
## 2. compute map `phi1` induced by isomorphism map `f1` on
## the boundary cycles of `g1` and `g2`.
##
phi1_inv = Permutation((i1, i0) for (i0, i1) in enumerate(
g2.boundary_cycles.index(f1.transform_boundary_cycle(bc1))
for bc1 in g1.boundary_cycles))
assert len(phi1_inv) == len(g1.boundary_cycles)
assert len(phi1_inv) == len(g2.boundary_cycles)
## 3. Compute the composite map `f1^(-1) * f0`.
##
## For every numbering `nb` on `g0`, compute the (index of)
## corresponding numbering on `g2` (under the composition map
## `f1^(-1) * f0`) and return a triple `(index of nb, index of
## push-forward, sign)`.
##
## In the following:
##
## - `j` is the index of a numbering `nb` in `self.numberings`;
## - `k` is the index of the corresponding numbering in `other.numberings`,
## under the composition map `f1^(-1) * f0`;
## - `a` is the the unique automorphism `a` of `other.graph` such that::
##
## self.numberings[j] = pull_back(<permutation induced by `a` applied to> other.numberings[k])
##
## - `s` is the pull-back sign (see below).
##
## The pair `k`,`a` is computed using the
## `NumberedFatgraphPool._index` (which see), applied to each
## of `self.numberings`, rearranged according to the
## permutation of boundary cycles induced by `f1^(-1) * f0`.
##
for (j, (k, a)) in enumerate(
other._index(phi1_inv.rearranged(phi0_inv.rearranged(nb)))
for nb in self.numberings):
## there are three components to the sign `s`:
## - the sign given by the ismorphism `f1`
## - the sign of the automorphism of `g2` that transforms the
## push-forward numbering into the chosen representative in the same orbit
## - the alternating sign from the homology differential
s = f1.compare_orientations() \
* a.compare_orientations() \
* minus_one_exp(g0.edge_numbering[edge])
yield (j, k, s)
def facets(self, edge, other):
"""Iterate over facets obtained by contracting `edge` and
projecting onto `other`.
Each returned item is a triple `(j, k, s)`, where:
- `j` is the index of a `NumberedFatgraph` in `self`;
- `k` is the index of a `NumberedFatgraph` in `other`;
- `s` is the sign by which `self[j].contract(edge)` projects onto `other[k]`.
Only triples for which `s != 0` are returned.
Examples::
>>> p0 = NumberedFatgraphPool(Fatgraph([Vertex([1, 2, 0, 1, 0]), Vertex([3, 3, 2])]))
>>> p1 = NumberedFatgraphPool(Fatgraph([Vertex([0, 1, 0, 1, 2, 2])]))
>>> list(NumberedFatgraphPool.facets(p0, 2, p1))
[(0, 0, 1), (1, 1, 1)]
"""
assert not self.graph.is_loop(edge)
assert self.is_orientable
assert other.is_orientable
g0 = self.graph
g1 = g0.contract(edge)
g2 = other.graph
assert len(g1.boundary_cycles) == len(g2.boundary_cycles)
# compute isomorphism map `f1` from `g1` to `g2`: if there is
# no such isomorphisms, then stop iteration (do this first so
# then we do not waste time on computing if we need to abort
# anyway)
f1 = Fatgraph.isomorphisms(g1,g2).next()
## 1. compute map `phi0` induced on `g0.boundary_cycles` from the
## graph map `f0` which contracts `edge`.
##
(e1, e2) = g0.endpoints(edge)
assert set(g1.boundary_cycles) == set([ g0.contract_boundary_cycle(bcy, e1, e2)
for bcy in g0.boundary_cycles ]), \
"NumberedFatgraphPool.facets():" \
" Boundary cycles of contracted graph are not the same" \
" as contracted boundary cycles of parent graph:" \
" `%s` vs `%s`" % (g1.boundary_cycles,
[ g0.contract_boundary_cycle(bcy, e1, e2)
for bcy in g0.boundary_cycles ])
phi0_inv = Permutation((i1,i0) for (i0,i1) in enumerate(
g1.boundary_cycles.index(g0.contract_boundary_cycle(bc0, e1, e2))
for bc0 in g0.boundary_cycles
))
## 2. compute map `phi1` induced by isomorphism map `f1` on
## the boundary cycles of `g1` and `g2`.
##
phi1_inv = Permutation((i1,i0) for (i0,i1) in enumerate(
g2.boundary_cycles.index(f1.transform_boundary_cycle(bc1))
for bc1 in g1.boundary_cycles
))
assert len(phi1_inv) == len(g1.boundary_cycles)
assert len(phi1_inv) == len(g2.boundary_cycles)
## 3. Compute the composite map `f1^(-1) * f0`.
##
## For every numbering `nb` on `g0`, compute the (index of)
## corresponding numbering on `g2` (under the composition map
## `f1^(-1) * f0`) and return a triple `(index of nb, index of
## push-forward, sign)`.
##
## In the following:
##
## - `j` is the index of a numbering `nb` in `self.numberings`;
## - `k` is the index of the corresponding numbering in `other.numberings`,
## under the composition map `f1^(-1) * f0`;
## - `a` is the the unique automorphism `a` of `other.graph` such that::
##
## self.numberings[j] = pull_back(<permutation induced by `a` applied to> other.numberings[k])
##
## - `s` is the pull-back sign (see below).
##
## The pair `k`,`a` is computed using the
## `NumberedFatgraphPool._index` (which see), applied to each
## of `self.numberings`, rearranged according to the
## permutation of boundary cycles induced by `f1^(-1) * f0`.
##
for (j, (k, a)) in enumerate(other._index(phi1_inv.rearranged(phi0_inv.rearranged(nb)))
for nb in self.numberings):
## there are three components to the sign `s`:
## - the sign given by the ismorphism `f1`
## - the sign of the automorphism of `g2` that transforms the
## push-forward numbering into the chosen representative in the same orbit
## - the alternating sign from the homology differential
s = f1.compare_orientations() \
* a.compare_orientations() \
* minus_one_exp(g0.edge_numbering[edge])
yield (j, k, s)
