ecs.js

var entityComponentSystem = function() {
    var my = {}
    my.entities = [];

    var walkEntitiesHelper = function(entities, functionDefinition) {
        _.forEach(entities, 
            function(entity) {
                if (!entity)
                    return;
                var shouldContinue = functionDefinition(entity);
                if (shouldContinue)
                    walkEntitiesHelper(entity.subEntities, functionDefinition);
            });
    }

    var walkEntities = function(functionDefinition) {
        walkEntitiesHelper(my.entities, functionDefinition);
    };

    var runSystem = function(requiredComponents, systemFunction) {
        // Walk the tree
        walkEntities(function(entity) {
            if (entity.hasRequiredComponents(requiredComponents))
                systemFunction(entity);
            return true;
        });
    };

    var runSystemOnce = function(requiredComponents, systemFunction) {
        // Walk the tree
        var shouldContinue = true;
        walkEntities(function(entity) {
            if (entity.hasRequiredComponents(requiredComponents)) {
                systemFunction(entity);
                shouldContinue = false;
            }
            return shouldContinue;
        });
    };

    var addEntity = function(parentEntityName, entityToAdd) {
        if (!parentEntityName) {
            my.entities.push(entityToAdd);
            return;
        }
        var shouldContinue = true;
        walkEntities(function(entity) {
            if (entity.name === parentEntityName) {
                entityToAdd.parent = entity;
                entity.subEntities.push(entityToAdd);
                shouldContinue = false; 
            }
            return shouldContinue;
        });
    };

    var addEntities = function(parentEntityName, entitiesToAdd) {
        _.forEach(entitiesToAdd, function(entity) {
            addEntity(parentEntityName, entity);
        });
    };

    var removeEntities = function(entityName) {
        walkEntities(function(entity) {
            _.remove(
                entity.subEntities,
                function(subEntity) {
                    return subEntity.name === entityName;
                });
            return true;
        });
    };

    var removeEntitiesById = function(entitiesToRemove) {
        var entityIdsToRemove = _(entitiesToRemove)
            .map(function(entity) { return entity.id })
            .keyBy()
            .mapValues(function() { return true; })
            .value();

        walkEntities(function(entity) {
            _.remove(
                entity.subEntities,
                function(subEntity) {
                    return entityIdsToRemove[subEntity.id] === true;
                });
            return true;
        });
    };

    return {
        runSystem: runSystem,
        runSystemOnce: runSystemOnce,
        addEntity: addEntity,
        addEntities: addEntities,
        removeEntities: removeEntities,
        removeEntitiesById: removeEntitiesById
    };
};