CarpetIOBasic: Add output criterion "divisor"

Support output criterion "divisor" in CarpetIOBasic.
---
 Carpet/CarpetIOBasic/param.ccl      |    1 +
 Carpet/CarpetIOBasic/src/iobasic.cc |   17 +++++++++++++++++
 2 files changed, 18 insertions(+), 0 deletions(-)

2 files changed, 18 insertions, 0 deletions

diff --git a/Carpet/CarpetIOBasic/param.ccl b/Carpet/CarpetIOBasic/param.ccl
index 252e873e8..8d105ff6a 100644
--- a/Carpet/CarpetIOBasic/param.ccl
+++ b/Carpet/CarpetIOBasic/param.ccl
@@ -28,6 +28,7 @@ KEYWORD outInfo_criterion "Criterion to select scalar output intervals, override
 {
   "never"     :: "Never output"
   "iteration" :: "Output every so many iterations"
+  "divisor"   :: "Output if iteration mod divisor == 0."
   "time"      :: "Output every that much coordinate time"
 } "iteration"
 
diff --git a/Carpet/CarpetIOBasic/src/iobasic.cc b/Carpet/CarpetIOBasic/src/iobasic.cc
index 2d0dcb895..2c413e3c2 100644
--- a/Carpet/CarpetIOBasic/src/iobasic.cc
+++ b/Carpet/CarpetIOBasic/src/iobasic.cc
@@ -562,6 +562,23 @@ namespace CarpetIOBasic {
         output_this_iteration = false;
       }
 
+    } else if (CCTK_EQUALS (myoutcriterion, "divisor")) {
+
+      int myoutevery = outInfo_every;
+      if (myoutevery == -2) {
+        myoutevery = out_every;
+      }
+      if (myoutevery <= 0) {
+        // output is disabled
+        output_this_iteration = false;
+      } else if ((cctk_iteration % myoutevery) == 0 ) {
+        // we already decided to output this iteration
+        output_this_iteration = true;
+      } else {
+        // we want no output at this iteration
+        output_this_iteration = false;
+      }
+
     } else if (CCTK_EQUALS (myoutcriterion, "time")) {
 
       CCTK_REAL myoutdt = outInfo_dt;