An insane function!

Find the number of functions $f$ from $\{0, 1, 2, 3, 4, 5, 6\}$ to the integers such that $f(0) = 0$ , $f(6) = 12$ , and

$|x - y|$ $\leq$ $|f(x) - f(y)|$ $\leq$ $3|x - y|$

for all $x$ and $y$ in $\{0, 1, 2, 3, 4, 5, 6\}$ .