# How Voting Systems Work 10, Exercise 2: how often does a method elect someone a quarter of the club ranks last?
# Assumed model: polarised electorates of 25 voters and 5 candidates, 2000 elections, seed 5.
# Needs votelab.py in the same folder.
import random
from votelab import make_electorate, make_ballots, METHODS

rng = random.Random(5)
runs = 2000
bad = {m: 0 for m in METHODS}
for _ in range(runs):
    voters, cands = make_electorate(rng, "polarised")
    b = make_ballots(voters, cands)
    for name, method in METHODS.items():
        w = method(b)
        last = sum(1 for r in b["rank"] if r[-1] == w)
        if last * 4 >= len(b["rank"]):          # at least a quarter of the voters rank the winner last
            bad[name] += 1
for name in METHODS:
    print(f"{name:>9}: winner ranked last by 25% or more in {bad[name]:>3} of {runs} elections ({bad[name] / runs:.1%})")

# Output from a real run:
# plurality: winner ranked last by 25% or more in 381 of 2000 elections (19.1%)
#       irv: winner ranked last by 25% or more in 210 of 2000 elections (10.5%)
#     borda: winner ranked last by 25% or more in  12 of 2000 elections (0.6%)
#  copeland: winner ranked last by 25% or more in 114 of 2000 elections (5.7%)
#  approval: winner ranked last by 25% or more in  39 of 2000 elections (1.9%)
#     score: winner ranked last by 25% or more in  40 of 2000 elections (2.0%)