evaluate nodes with a stack #2

# evaluate nodes with a stack -- functional algorithm

from collections import namedtuple
#------------------------------------------------------------------------------80

PullOp = namedtuple("PullOp", "nid slot_id")
Node = namedtuple("Node", "slots defaultArgs")
Undefined = Ellipsis

nodeDB = {1: Node(slots=tuple(), defaultArgs=tuple()),
          2: Node(slots=(PullOp(1,0),), defaultArgs=(Undefined,)),
          3: Node(slots=(PullOp(2,1),), defaultArgs=(Undefined,)),
          4: Node(slots=(PullOp(2,0), PullOp(3,2)), defaultArgs=(Undefined, Undefined)),
          5: Node(slots=(PullOp(3,0),), defaultArgs=(Undefined,)),
          6: Node(slots=(PullOp(4,0),), defaultArgs=(Undefined,))
}

eval_stack = {2: ('a', 'b'), 4: (1, -1)}
cc_stack = {(4,1): 'cc-x'}

nodes = {3: (Undefined,), 4: (Undefined, Undefined)}
nodes_eval = (3, 4)


def fresh_slot_value(nid, slot_id, slot_value):
    ''' (PURE)
    returns a fresh slot value from cc-stack, eval-stack or default
    arguments, if the slot value is Undefined.

    The function needs read-only access to cc-stack, eval-stack, and nodeDB.
    '''

    # if current slot-value is already 'defined' by a prior evaluation,
    # then no need to change
    if slot_value is not Undefined:
        return slot_value

    #ELSE: if current slot-value is 'undefined'

    # 1) obtain value from cc-stack, if slot is in cc-stack
    push_op = (nid, slot_id) # push operation (from cc-stack)
    if push_op in cc_stack:
        return cc_stack[(nid, slot_id)] # do push-operation

    # 2) obtain a value from eval-stack, if slot in eval-stack
    pull_op = nodeDB[nid].slots[slot_id] # pull operation (from eval-stack)
    if (pull_op.nid in eval_stack
        and len(eval_stack[pull_op.nid])-1 >= pull_op.slot_id):
        return eval_stack[pull_op.nid][pull_op.slot_id] # do pull-operation

    # otherwise take the default argument
    return nodeDB[nid].defaultArgs[slot_id]
    #END if


new_nodes_eval = {nid:
              tuple(fresh_slot_value(nid, slot_id, nodes[nid][slot_id])
                    # for each slot of the current node
                    for slot_id, _ in enumerate(nodeDB[nid].slots)
                    )
              # for all nodes in eval-nodes
              for nid in nodes
              }

print(new_nodes_eval)