From 8c59db1661aadc56724238ed35792bf6a43ca4b5 Mon Sep 17 00:00:00 2001
From: Sebastian Funk <sebastian.funk@lshtm.ac.uk>
Date: Wed, 10 May 2023 15:41:10 +0100
Subject: [PATCH] optional head/tail

---
 inst/stan/functions/convolve.stan | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/inst/stan/functions/convolve.stan b/inst/stan/functions/convolve.stan
index 746f2ae90..73aafc3b5 100644
--- a/inst/stan/functions/convolve.stan
+++ b/inst/stan/functions/convolve.stan
@@ -8,8 +8,8 @@ vector convolve_with_rev_pmf(vector x, vector y, int len) {
     vector[len] z;
     for (s in 1:len) {
       z[s] = dot_product(
-        x[max(1, (s - ylen + 1)):min(s, xlen)],
-        y[max(1, ylen - s + 1):min(ylen, ylen + xlen - s)]
+        s < ylen ? head(x, min(s, xlen)) : x[(s - ylen + 1):min(s, xlen)],
+        s < xlen ? tail(y, min(s, ylen)) : y[max(1, ylen - s + 1):(ylen + xlen - s)]
       );
     }
    return(z);