aggregating_aggregates.js

// Again, producing an aggregation
const makeAggregation = (operation, initial) => ({
  operation: operation,
  initialValue: initial
});

// Adjust the combination a bit to be more general
// We take _all the aggregations_ and apply them to the prior values _in their index_ and the next value _in their index_
const combineAggregationsReduction = (combined, next) => 
  makeAggregation((prior, nextValue) =>
    // A bit finicky, admittedly
    [
      …combined.operation(
       prior.slice(0, -1),
       nextValue
      ),
      next.operation(
        prior[prior.length - 1],
        nextValue
      )
    ],
    […combined.initialValue, next.initialValue]
  );

// We need to wrap the first aggregate's reduction and initial value in arrays
// so that we can combine it correctly with the rest.
const liftToArray = aggregate => 
  makeAggregation(
    ([a], b) => [aggregate.operation(a, b)],
    [aggregate.initialValue]
   );

// Reduce our list of aggregates into one aggregate
const combineAggregation = aggregates =>
  aggregates.slice(1).reduce(
    combineAggregationsReduction,
    liftToArray(aggregates[0])
  );

// Which we can use to combine multiple aggregates via…
// Again, sum balances by aggregating through _summation_ and _starting at zero_
const sumBalances = makeAggregation(
  (sum, record) => sum + record.balance,
  0
);

// Again, count via just adding 1 every time we see another record
const count = makeAggregation(
  (sum, record) => sum + 1,
  0
);

// Take the product over "variation" whatever that is
const productOfVariation =
  makeAggregation(
    (product, record) => product * record.variation,
    1
);

// Record the _last_ id in the report via just saving the latest seen
const lastIdSeen =
  makeAggregation(
    (prior, next) => next.id,
    null 
  );

// And we now run all of the aggregates at once
const getAggregates = reports => {
  const aggregates = combineAggregation([
    count,
    sumBalances,
    productOfVariation,
    lastIdSeen
  ]);
  
  return reports.reduce(
    aggregates.operation,
    aggregates.initialValue
  );
};