Given a graph such that every directed edge has a sibling, returns a map
from EdgeId to the sibling EdgeId. This method is identical to
GetInEdgeIds() except that (1) it requires edges to have siblings, and
(2) undirected degenerate edges are grouped together in pairs such that
one edge is the sibling of the other. Handles duplicate edges correctly
and is also consistent with GetLeftTurnMap().
REQUIRES: An option is chosen that guarantees sibling pairs:
(options.sibling_pairs() == { REQUIRE, CREATE } ||
options.edge_type() == UNDIRECTED)
Given a graph such that every directed edge has a sibling, returns a map from EdgeId to the sibling EdgeId. This method is identical to GetInEdgeIds() except that (1) it requires edges to have siblings, and (2) undirected degenerate edges are grouped together in pairs such that one edge is the sibling of the other. Handles duplicate edges correctly and is also consistent with GetLeftTurnMap().
REQUIRES: An option is chosen that guarantees sibling pairs: (options.sibling_pairs() == { REQUIRE, CREATE } || options.edge_type() == UNDIRECTED)