Proof Writing & Problem Solving · Superchallenge problem

A truth function $f(P, Q, R)$ assigns true or false to each of the $8$ rows of a truth table in $P, Q, R$, so there are $2^8 = 256$ of them. For how many of them is the statement "if $f(P, Q, R)$, then $f(Q, R, P)$" true in every row (a tautology)?