Finding reciprocally connected nodes in a graph using map-reduce, psuedocode

Reciprogra.txt

def mapper(input):
  for each edge node1 -> node2 in input:
    emit (key: node1, node1 -> node2)
    emit (key: node2, node2 <- node1)

def reducer(key: node, edges):
  for each distinct edge node1 -> node2 in edges:
    if node1 <= node2:
      if node2 <- node1 in edges:
        emit(node1, node2)

# --> improvement: filter against communicating redundant connections between map and reduce

def mapper(input):
  # only emit in/out edges to the smaller node
  # (reduces transport costs)
  for each edge node1 -> node2 in input:
    if node1 <= node2:
      emit (key: node1, node1 -> node2)
    if node2 >= node1:
      emit (key: node1, node1 <- node2)

def reducer(key: node, edges):
  for each distinct edge node1 -> node2 in edges:
    if node1 <- node2 in edges:
      emit(node1, node2)
      emit(node2, node1)

"""
--> could improve (but it's getting late):
   only communicate if it's an in or out edge, not 
   the two nodes comprising the edge, plus the 
   smaller node as key - since "node1" is a duplicate
   information between mapper and reducer, instead
   a single bit could depict the direction of the edge
"""