solution to https://adventofcode.com/2016/day/1 part 2, uses reduce(), nearly functional

d1p2.py

#!/usr/bin/env python3

# solution to https://adventofcode.com/2016/day/1 part 2
# in a functional style with python 3.3+ enhanced accumulate() and dropwhile()
# Mark Jenkins <mark@markjenkins.ca>

from itertools import accumulate, dropwhile, chain

MOVEMENTS = ((0,1), (1,0), (0,-1), (-1,0))
DIRECTION_CHANGES = {"R": 1, "L": -1}
def next_pos(state, token):
    displacement, direction = state
    change_dir = DIRECTION_CHANGES[token[0]]
    change_magnitude = int(token[1:])
    new_direction = (direction + change_dir) % len(MOVEMENTS)
    move = (change_magnitude*MOVEMENTS[new_direction][0], 
            change_magnitude*MOVEMENTS[new_direction][1] )
    new_displacement = (displacement[0] + move[0], displacement[1] + move[1])
    locations_visited = frozenset(
        (displacement[0] + i*MOVEMENTS[new_direction][0],
         displacement[1] + i*MOVEMENTS[new_direction][1])
        for i in range(1, change_magnitude+1)
        )
    return new_displacement, new_direction, locations_visited

def next_pos_with_history(state, token):
    (displacement, direction,
     last_locations_visited, old_locations_visited) = state
    combined_locations_visited = old_locations_visited.union(
        last_locations_visited)

    new_displacement, new_direction, new_locations_visited = \
        next_pos((displacement, direction),token)
    return (new_displacement, new_direction, 
            new_locations_visited, combined_locations_visited)

initial_state = ((0, 0), 0, frozenset(), frozenset())

history_accumulation = accumulate( 
    chain( (initial_state,), input().split(", ") ),
    next_pos_with_history
) # accumulate


def no_repeat_of_history(state):
    (displacement, direction,
     last_locations_visited, old_locations_visited) = state
    return not any( new_loc in old_locations_visited 
                    for new_loc in last_locations_visited)

# find the first repeat of history
success=next(dropwhile(no_repeat_of_history, history_accumulation), None)

if success == None:
    print("no repeat of location found")
else:
    assert( not no_repeat_of_history(success) )
    (displacement, direction,
     last_locations_visited, old_locations_visited) = success
    repeat_location = next(
        dropwhile( lambda new_loc: new_loc not in old_locations_visited,
                   last_locations_visited ) # dropwhile
    ) # next
    print( abs(repeat_location[0]) + abs(repeat_location[1]) )