Filter::mean() cannot compute a negative mean

[edgar-bonet]

The program

#include <Filter.h>

void setup() {
    Filter filter(3);
    filter.write(-3);
    filter.write(-5);
    filter.write(-7);
    Serial.begin(9600);
    Serial.print("mean(-3, -5, -7) = ");
    Serial.println(filter.mean());
}

void loop(){}

outputs

mean(-3, -5, -7) = 143165572

This is due to the (scaled) mean being computed as

long mean = sum / _values.available();

where sum is of type long and _values.available() is unsigned long. According to the C++ rules on usual arithmetic conversions, evaluating the above expression involves an implicit cast of sum to unsigned long, hence the wrap around.

The obvious fix is to change _values.available() to be of any type among long, int, short or unsigned short.