#!/usr/bin/env node

function roll (bet) {
    //#s [0, 36] w/ 37 potential values total
    var n = Math.floor(Math.random() * 37);
    //return n > 0 && n <= 12 ? 2 * bet : -bet;
    return n > 0 && n <= 18 ? bet : -bet;
}

function play (bank) {
    var bet = 1;
    do {
        var outcome = roll(bet);
        bank += outcome;
        bet = bet * 2 + 1;
    } while (outcome < 0 && bet <= bank);

    return bank;
};

var wins = 0, losses = 0;
for (var j = 0; j < 500000; j++) {
    var principal = 100;
    for (
        var bank = principal;
        bank > 0 && bank / principal < 2;
        bank = play(bank)
    );
    if (bank > principal) wins++
    else losses ++
};
console.log(wins / (wins + losses));