  The Fast Fourier Transform (FFT) over a finite field $\mathbb{F}_q$ computes
evaluations of a given polynomial of degree less than $n$ at a specifically
chosen set of $n$ distinct evaluation points in $\mathbb{F}_q$. If $q$ or $q-1$
is a smooth number, then the divide-and-conquer approach leads to the fastest
known FFT algorithms. Depending on the type of group that the set of evaluation
points forms, these algorithms are classified as multiplicative (Math of Comp.
1965) and additive (FOCS 2014) FFT algorithms. In this work, we provide a
unified framework for FFT algorithms that include both multiplicative and
additive FFT algorithms as special cases, and beyond: our framework also works
when $q+1$ is smooth, while all known results require $q$ or $q-1$ to be
smooth. For the new case where $q+1$ is smooth (this new case was not
considered before in literature as far as we know), we show that if $n$ is a
divisor of $q+1$ that is $B$-smooth for a real $B>0$, then our FFT needs
$O(Bn\log n)$ arithmetic operations in $\mathbb{F}_q$. Our unified framework is
a natural consequence of introducing the algebraic function fields into the
study of FFT.
