# movingPercentileBy()

> Create a Stream that represents the moving percentile of `BigDecimal` objects mapped from a `Stream<INPUT>` via a `mappingFunction`, looking back `windowSize` elements.

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Site [llms.txt](/gatherers4j/llms.txt)

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### 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 [`movingPercentile()`](/gatherers4j/gatherers/mathematical/movingpercentile/).
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`](https://docs.oracle.com/en/java/javase/25/docs/api/java.base/java/math/MathContext.html#DECIMAL64), but this can be overridden (see `withMathContext()`, below).

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`.

**Signatures**

`movingPercentileBy(double percentile, int windowSize, Function<INPUT, BigDecimal> mappingFunction)`
* `percentile` - The percentile to calculate, between 0 and 100, inclusive
* `windowSize` - How many trailing elements to calculate the percentile over, must be greater than 1
* `mappingFunction` - A non-null function to map stream `INPUT` elements into `BigDecimal` for calculation

**Additional Methods**

| Method                                     | Purpose                                                                                                                                                                                                                                                                                                              |
|--------------------------------------------|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| `excludePartialValues()`                   | When calculating the moving percentile, and the full size of the window has not yet been reached, the gatherer should suppress emitting values until the lookback window is full.                                                                                                                                    |
| `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`](https://github.com/tginsberg/gatherers4j/blob/main/src/main/java/com/ginsberg/gatherers4j/dto/WithOriginal.java) record. [See example.](#emitting-a-record-containing-the-original-and-calculated-values) |

See [`movingPercentile()`](/gatherers4j/gatherers/mathematical/movingpercentile/) for examples of each of these.

### Examples

#### Moving 90th percentile of window size 3, mapped from an object

```java
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")),
        new NamedValue("fourth", new BigDecimal("2.0"))
    )
    .gather(Gatherers4j.movingPercentileBy(90, 3, NamedValue::value))
    .toList();

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

#### Emitting a record containing the original and calculated values

```java
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.movingPercentileBy(90, 2, 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=9.2]
// ]
```