bracket-gen.js

var choose = function(a, b, teams) {
    if (teams[a] && teams[b]) {
        return teams[a] < teams[b] ? a : b;
    } else if (teams[a]) {
        return a;
    } else if (teams[b]) {
        return b;
    } else {
        return Math.random() < 0.5 ? a : b;
    }
};

Array.prototype.bracketShift = function(index, teams) {
    var leftSet = this[index];
    return this.ipush(leftSet.pairs().map(function(pair) {
        return choose(pair[0], pair[1], teams);
    }));
};

Array.prototype.bracket = function(teams) {
    var l = (this.length / 2) - 1;
    var loop = function(acc, set) { return acc === l ? set : loop(acc + 1, set.bracketShift(acc, teams)); };
    return loop(0, [this]);
};


// In use

var teams = ['vikings', 'lions', 'bears', '49ers', 'cubs', 'seahawks', 'giants', 'seagulls'];
var bracket = teams.bracket({'vikings': 1, 'seagulls': 2});