For a certain distribution of random variable \(Z\), it has a corresponding cumulative distribution function \(\Psi(x)\). This function measures the probability that \(Z < x\). Since \(\Psi(x)\) is monotonically increasing function, it has an inverse function \(\Psi^{-1}(x)\). It is mathematically approved that for uniform-distributed random variable \(U\) in range (0, 1), \(\Psi^{-1}(U)\) is also a random variable and its cumulative distribution function is \(\Psi(x)\).
We have two versions of Inverse Cumulative Distribution Function (ICDF) with the data type of float or double, details of algorithms could be found from reference papers. Both of them use fractional polynomial approximation method. They both have two branches for value from different input range and they share similar logics. To save DSPs, we combined the two branches and manually binding calculation to the same hardware resource to get the same performance and accuracy. Take ICDF with float data type as an example, the basic approximation formula is (main fractional polynomial approximation part, not all) shown as below:
Although these two conditions have different formulas, they can merge into one formula with parameters configured by which range that \(x\) belongs to. As shown in the diagram below, if parameters took the value at left, it would become the first formula above. If at right, it would become the second formula above.
References: AS 241.