# How Voting Systems Work 2, Exercise 1: find the spoilers
# A club of 21 members ranks four candidates (assumed, invented numbers).
# A candidate is a spoiler here if they do not win, but their withdrawal
# changes who wins among the others.
from collections import Counter

profile = (
    [("Ana", "Bo", "Cy", "Di")] * 6
    + [("Bo", "Ana", "Cy", "Di")] * 5
    + [("Cy", "Bo", "Ana", "Di")] * 7
    + [("Di", "Cy", "Bo", "Ana")] * 3
)

def plurality_winner(ballots):
    counts = Counter(b[0] for b in ballots)
    top = max(counts.values())
    return "/".join(sorted(c for c, v in counts.items() if v == top))

def without(ballots, candidate):
    return [tuple(c for c in b if c != candidate) for b in ballots]

base = plurality_winner(profile)
print("first choices:", dict(sorted(Counter(b[0] for b in profile).items())))
print("winner with everyone standing:", base)
for c in sorted(profile[0]):
    if c == base:
        continue
    new = plurality_winner(without(profile, c))
    verdict = "SPOILER" if new != base else "no effect"
    print(f"  {c} withdraws -> winner {new:3s} ({verdict})")

# Output when run:
# first choices: {'Ana': 6, 'Bo': 5, 'Cy': 7, 'Di': 3}
# winner with everyone standing: Cy
#   Ana withdraws -> winner Bo  (SPOILER)
#   Bo withdraws -> winner Ana (SPOILER)
#   Di withdraws -> winner Cy  (no effect)