Interface AggregatorIf<S, R>

An aggregation (sum, average, …) as four small functions over a state: init → add per value → result. merge combines two partial states – the result is the same as one pass over all values. That makes groups (row grouping, pivot) cheap: the state of a group is the merge of the states of its sub-groups, and an average of averages stays correct ({ sum, count } instead of a number).

The built-in aggregators are in Aggregators; an own aggregator implements this interface and can be used everywhere a name like "sum" is accepted (AggregatorRef).

add(state, …) and merge(a, b) may change state / a and return it (no allocation per row with millions of rows); merge never changes b, so a child state can be merged into several parents. Call init() for a fresh state, e.g. merge(init(), child) for a parent that must not change the child.

Example

// the product of all numbers
const product: AggregatorIf<number, number> = {
  name: "product",
  init: () => 1,
  add: (state, value) => typeof value === "number" ? state * value : state,
  merge: (a, b) => a * b,
  result: state => state
};
aggregate([{ f: 2 }, { f: 3 }, { f: 7 }], "f", product); // 42
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Example

// partial results: the average of two groups is the average of all rows
const avg = Aggregators.avg;
const north = [100, 50, 120].reduce((s, v) => avg.add(s, v), avg.init());
const south = [80, 70, 30].reduce((s, v) => avg.add(s, v), avg.init());
avg.result(avg.merge(north, south)); // 75 – not the mean of 90 and 60
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Since

1.1.94