create-chain.js

function createChain(words) {
	chain = {};
	for (const word of words) {
		addWordToChain(chain, word);
	}
	return chain;
}

/*
 * Example: Adding "beat" to the chain will
 * update the chain with the following transitions:
 * "" -> "be"         "be" -> "a"         "ea" -> "t"
 */
function addWordToChain(chain, word) {
	updateOccuranceInChain(chain, '', word.slice(0, 2));

	for (var i = 2; i < word.length; i++) {
		prefix = word.substring(i - 2, i);
		updateOccuranceInChain(chain, prefix, word[i]);
	}
}

function updateOccuranceInChain(chain, prefix, occurance) {
	addPrefixIfNew(chain, prefix);
	addPrefixOccuranceIfNew(chain, prefix, occurance);

	chain[prefix][occurance]++;
	chain[prefix]['total']++;
}

function addPrefixIfNew(chain, prefix) {
	if (chain[prefix] == undefined) {
		chain[prefix] = {};
		chain[prefix]['occurances'] = [];
		chain[prefix]['total'] = 0;
	}
}

function addPrefixOccuranceIfNew(chain, prefix, occurance) {
	if (chain[prefix][occurance] == undefined) {
		chain[prefix][occurance] = 0;
		chain[prefix]['occurances'].push(occurance);
	}
}