runningPercentileBy()

Calculate the running percentile of BigDecimal objects mapped from a Stream<INPUT> via a mappingFunction.

Implementation Notes

This implementation is suitable for mapping an arbitrary Stream<INPUT> to BigDecimal via a mappingFunction; for a version that operates directly on a Stream<BigDecimal>, see runningPercentile(). By default, nulls are ignored and play no part in calculations, see treatNullAs() and treatNullAsZero() below for ways to change this behavior. The default MathContext for all calculations is MathContext.DECIMAL64, but this can be overridden (see withMathContext(), below).

Each emitted value is the percentile of every element seen so far, so the last value emitted is the percentile of the whole stream. Values that fall between two ranks are linearly interpolated (the R-7 method, NumPy’s default). The percentile is converted to BigDecimal from its decimal representation, so 99.9 is exactly 99.9. The rank and interpolation are calculated exactly, and only the result is rounded to the MathContext.

Every value seen is retained in sorted order, so memory use grows with the length of the stream. For long or unbounded streams, consider movingPercentileBy() instead.

Signatures

runningPercentileBy(double percentile, Function<INPUT, BigDecimal> mappingFunction)

percentile - The percentile to calculate, between 0 and 100, inclusive
mappingFunction - A non-null function to map stream INPUT elements into BigDecimal for calculation

Additional Methods

Method	Purpose
`treatNullAsZero()`	When encountering a `null` value in a stream, treat it as `BigDecimal.ZERO` instead.
`treatNullAs(BigDecimal replacement)`	When encountering a `null` value in a stream, treat it as the given `replacement` value instead.
`treatNullAsOne()`	When encountering a `null` value in a stream, treat it as `BigDecimal.ONE` instead.
`withMathContext(MathContext mathContext)`	Replace the `MathContext` used for all mathematical operations performed by this gatherer.
`withOriginal()`	Include the original input value from the stream in addition to the calculated value in a `WithOriginal` record. See example.

See runningPercentile() for examples of each of these.

Examples

Running 90th percentile, mapped from an object

record NamedValue(String name, BigDecimal value) {}

Stream
    .of(
        new NamedValue("first",  new BigDecimal("1.0")),
        new NamedValue("second", new BigDecimal("2.0")),
        new NamedValue("third",  new BigDecimal("10.0"))
    )
    .gather(Gatherers4j.runningPercentileBy(90, NamedValue::value))
    .toList();

// [
//   BigDecimal("1.0"),
//   BigDecimal("1.9"),
//   BigDecimal("8.4")
// ]

Emitting a record containing the original and calculated values

record NamedValue(String name, BigDecimal value) {}

Stream
    .of(
        new NamedValue("first",  new BigDecimal("1.0")),
        new NamedValue("second", new BigDecimal("2.0")),
        new NamedValue("third",  new BigDecimal("10.0"))
    )
    .gather(Gatherers4j.runningPercentileBy(90, NamedValue::value).withOriginal())
    .toList();

// [
//   WithOriginal[original=NamedValue[name=first, value=1.0], calculated=1.0],
//   WithOriginal[original=NamedValue[name=second, value=2.0], calculated=1.9],
//   WithOriginal[original=NamedValue[name=third, value=10.0], calculated=8.4]
// ]