calc.js

function calc(users) {
  return _.reduce(_.map(users, (user) => {
    return _.reduce(_.map(user.puzzles, (puzzle) => {
      return {
        puzzleId: puzzle.puzzleId,
        errors: _.reduce(puzzle.moves, (acc, move) => {
          if (move.isValid) return acc;
          else {
            var fromState = _.get(move, 'fromState');
            return _.merge({}, acc, { [fromState]: _.concat((_.get(acc, fromState) || []), move.toState) });
          }
        }, {})
      };
    }), (report, session) => {
      return _.merge({}, report, {
        [session.puzzleId]: _.reduce(_.union(Object.keys(session.errors), Object.keys((report[session.puzzleId] || {}))), (acc, key) => {
          return _.set(acc, key, _.concat((_.get(report, `${session.puzzleId}.${key}`) || []), (_.get(session, `errors.${key}`) || [])))
        }, {})
      });
    }, {});
  }), (report, user) => {
    return _.reduce(_.union(Object.keys((user || {})), Object.keys(report)), (acc, key) => {
      return _.set(acc, key, _.merge((_.get(report, key) || {}), (_.get(user, key))));
    }, {});
  }, {});
};