combining_two_aggregates.js

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

// Combine them via running them on the same inputs in sequence
// If JavaScript weren't a single threaded run time, we could even do this in parallel!
const combineAggregation = (aggregationA, aggregationB) => ({
  reduction: ([priorA, priorB], record) =>
    [
      aggregationA.operation(priorA, record),
      aggregationB.operation(priorB, record)
    ],
  initialValue: [aggregationA.initialValue, aggregationB.initialValue]
});

// Sum balances by aggregating through _summation_ and _starting at zero_
const sumBalances = makeAggregation(
  (sum, record) => sum + record.balance,
  0
);

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

// Both take the count and sum the balances
const countAndSumBalances =
  combineAggregation(sumBalances, count);

// Now actually run those aggregations!
const getSumOfBalancesAndCount = report =>
  report.reduce(
    countAndSumBalances.operation,
    countAndSumBalances.initialValue
  );