  We compute the Heegaard Floer homology of $S^3_1(K)$ (the (+1) surgery on the
torus knot $T_{p,q}$) in terms of the semigroup generated by $p$ and $q$, and
we find a compact formula (involving Dedekind sums) for the corresponding
Ozsvath--Szabo d-invariant. We relate the result to known knot invariants of
$T_{p,q}$ as the genus and the Levine--Tristram signatures. Furthermore, we
emphasize the striking resemblance between Heegaard Floer homologies of (+1)
and (-1) surgeries on torus knots. This relation is best seen at the level of
$\tau$ functions.
