ReciprocalFloatFunction implements a reciprocal function f(x) = a/(mx+b), based on the float value of a field or function as exported by {@link org.apache.lucene.queries.function.ValueSource}.
When a and b are equal, and x>=0, this function has a maximum value of 1 that drops as x increases. Increasing the value of a and b together results in a movement of the entire function to a flatter part of the curve.
These properties make this an idea function for boosting more recent documents.
Example: recip(ms(NOW,mydatefield),3.16e-11,1,1)
A multiplier of 3.16e-11 changes the units from milliseconds to years (since there are about 3.16e10 milliseconds per year). Thus, a very recent date will yield a value close to 1/(0+1) or 1, a date a year in the past will get a multiplier of about 1/(1+1) or 1/2, and date two years old will yield 1/(2+1) or 1/3.
@see org.apache.lucene.queries.function.FunctionQuery