Allow dt > 1

R/filter-support.R

@@ -53,12 +53,18 @@ check_dt <- function(dt, allow_null = FALSE, name = deparse(substitute(dt)),
   if (dt <= 0) {
     cli::cli_abort("Expected '{name}' to be greater than 0")
   }
-  if (dt > 1) {
-    cli::cli_abort("Expected '{name}' to be at most 1")
-  }
-  if (!rlang::is_integerish(1 / dt)) {
-    cli::cli_abort("Expected '{name}' to be the inverse of an integer",
-                   arg = "dt", call = call)
+  if (dt >= 1) {
+    if (!rlang::is_integerish(dt) || is.infinite(dt)) {
+      cli::cli_abort(
+        "Expected '{name}' to be an integer, if greater than  1",
+        arg = "dt", call = call)
+    }
+  } else {
+    if (!rlang::is_integerish(1 / dt)) {
+      cli::cli_abort(
+        "Expected '{name}' to be the inverse of an integer, if less than 1",
+        arg = "dt", call = call)
+    }
   }
   dt
 }

inst/include/dust2/r/helpers.hpp

@@ -236,11 +236,15 @@ inline double check_dt(cpp11::list r_time_control, bool enabled = true,
     cpp11::stop("Expected 'dt' to be greater than 0");
   }
   if (dt > 1) {
-    cpp11::stop("Expected 'dt' to be at most 1");
+    if (!is_integer_like(dt, eps)) {
+      cpp11::stop("Expected 'dt' to be an integer, if above 1");
+    }
   }
-  const auto inv_dt = 1 / dt;
-  if (!is_integer_like(inv_dt, eps)) {
-    cpp11::stop("Expected 'dt' to be the inverse of an integer");
+  if (dt < 1) {
+    const auto inv_dt = 1 / dt;
+    if (!is_integer_like(inv_dt, eps)) {
+      cpp11::stop("Expected 'dt' to be the inverse of an integer, if below 1");
+    }
   }
   return dt;
 }

tests/testthat/test-filter-support.R

@@ -1,15 +1,18 @@
 test_that("can validate that 'dt' is reasonable", {
   expect_no_error(check_dt(1))
   expect_no_error(check_dt(1 / 5))
+  expect_no_error(check_dt(2))
   expect_no_error(check_dt(NULL, allow_null = TRUE))
   expect_error(check_dt(-1, name = "dt"),
                "Expected 'dt' to be greater than 0")
-  expect_error(check_dt(2, name = "dt"),
-               "Expected 'dt' to be at most 1")
+  expect_error(check_dt(2.5, name = "dt"),
+               "Expected 'dt' to be an integer, if greater than 1")
   expect_error(check_dt(1 / 3.5, name = "dt"),
                "Expected 'dt' to be the inverse of an integer")
   expect_error(check_dt(NULL, name = "dt"),
                "'dt' must be a scalar")
+  expect_error(check_dt(Inf, name = "dt"),
+               "Expected 'dt' to be an integer, if greater than 1")
 })
 
 

tests/testthat/test-walk.R

@@ -85,7 +85,7 @@ test_that("provided dt is reasonable", {
     "Expected 'dt' to be greater than 0")
   expect_error(
     dust_system_create(walk(), pars, n_particles = 10, dt = 1.5),
-    "Expected 'dt' to be at most 1")
+    "Expected 'dt' to be an integer, if greater than 1")
   expect_error(
     dust_system_create(walk(), pars, n_particles = 10, dt = sqrt(2) / 2),
     "Expected 'dt' to be the inverse of an integer")
@@ -111,6 +111,18 @@ test_that("time aligns to grid when dt is less than 1", {
     fixed = TRUE)
 })
 
+test_that("time aligns to grid when dt is more than 1", {
+  pars <- list(sd = 1, random_initial = TRUE)
+  obj <- dust_system_create(walk(), pars, n_particles = 10, dt = 2.0,
+                            time = 0.0)
+  expect_equal(dust_system_time(obj), 0.0)
+
+  expect_error(
+    dust_system_create(walk(), pars, n_particles = 10, dt = 2.0,
+                            time = 1.0),
+    "'time' must be a multiple of 'dt'")
+})
+
 
 ## Not a test at all of the system, but of the error handling in
 ## helpers.hpp