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Reverse and re-reverse object key-value maps
var reverseMapFromMap = function(map, callback) {
callback = callback || _.identity;
return _.transform(map, function(reverseMap, one, manyone) {
reverseMap[one] = reversedMap[one] || [];
}, {});
var mapFromReverseMap = function(reverseMap, callback) {
callback = callback || _.identity;
return _.transform(reverseMap, function(map, many, one) {
_.forEach(many, function(manyone) {
map[manyone] = callback(one);
}, {});
//Here's a performance overview of this method versus native and the version not using transform:
//As you can see from the above link, transform method and non-transform method are almost same in performance when it comes to large data, but I chose transform as it looks better to me in terms of readability. Native-only option does not perform so well when operating on large data.

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@trumbitta trumbitta commented Jul 24, 2015

Small typo on #L4?

reversedMap to reverseMap

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