ShiritoriHelper.user.js

// ==UserScript==
// @name           shiritori helper
// @namespace      http://fuba.moaningnerds.org/
// @include        http://gdd-2010-quiz-japan.appspot.com/problems?problem=shiritori*
// @require https://gist.github.com/184276.txt
// ==/UserScript==

String.prototype.head = function () {
    return this.charAt(0);
};

String.prototype.tail = function () {
    return this.charAt(this.length - 1);
};

String.prototype.x = function (number) {
    var result = '';
    for (var i = 0; i< number; i++) {
        result += this;
    }
    return result;
};

Array.prototype.diff = function (diffTargetObject, log) {
    var loghash = {};
    for (var i = 0; i < log.length; i++) {
        loghash[log[i]] = 1;
    }
    
    var newArray = [];
    this.forEach(function (word) {
        if (!(diffTargetObject[word] || loghash[word])) {
            newArray.push(word);
        }
    });
    
    return newArray;
};

Array.prototype.getHeads = function (head) {
    var heads = {};
    var regexp = new RegExp('^'+head);
    this.forEach(function (word) {
        if (regexp.test(word)) {
            heads[word] = word.tail();
        }
    });
    return heads;
};

Array.prototype.hasLoop = function () {
    var is_mine = false;
    var log_me = {};
    var log_him = {};
    
    for (var i = 0; i < this.length; i++) {
        var head = this[i].head();
        if (is_mine) {
            if (log_me[head]) return 1;
            log_me[head] = 1;
            is_mine = false;
        }
        else {
            if (log_him[head]) return 2;
            log_him[head] = 1;
            is_mine = true;
        }
    }
    
    return 0;
};

var maxDepth = 4;

function doShiritori (list, history, log, targettext, depth) {
    var result = '';
    if (depth > maxDepth) return result;
    
    var nexts = list.diff(history, log).getHeads(targettext.tail());
    
    for (var word in nexts) {
        var newLog = log.concat(word);
        var hasLoop = newLog.hasLoop();
        if (hasLoop == 0) {
            if (depth == maxDepth) {
                //result += newLog.join(' > ') + "\n";
            }
            
            result += doShiritori(
                list.diff(history, log),
                history,
                log.concat(word),
                word,
                depth + 1
            );
        }
        else if (hasLoop == 1) {
            result += '[DANGER]' + newLog.join(' > ') + "\n";
        }
        else {
            result += '[ SAFE ]' + newLog.join(' > ') + "\n";
        }
    }
    
    return result;
}

function main () {
    var defaultList = [];
    $X('//ul[1]/li/text()').forEach(function(node){
        defaultList.push(node.textContent);
    });
    
    var defaultHistory = {};
    $X('//ul[2]/li/text()').forEach(
        function(node){
            defaultHistory[node.textContent] = 1;
        }
    );
    var targets = $X('//input[@name="answer0"]').
        map(function(node){return node.value});
    
    var pre = document.createElement('pre');
    for (var i = 0; i < targets.length; i++) {
        if (targets[i] != 'reset') {
            pre.innerHTML = doShiritori(
                defaultList.diff(defaultHistory, []),
                defaultHistory,
                [],
                targets[i],
                1
            );
        }
    }
    $X('id("main")')[0].appendChild(pre);
}

main();