Fix fslice crash when a group has only missing order.by values

NEWS.md

@@ -1,5 +1,7 @@
 # collapse 2.1.7
 
+* Fixed a bug in `fslice()` (grouped, `n = 1`, `with.ties = FALSE`) that caused R to crash with a fatal error when a group had only missing values in `order.by`. Thanks @chihyunkim for reporting (#867).
+
 * The *collapse* article is now published in the Journal of Statistical Software: https://doi.org/10.18637/jss.v116.i01. This article is now the primary citation for academic use of *collapse*. It is also a great reference to quickly and thoroughly understand the package. `citation("collapse")` was also updated in this regard. The APA-style citation is:
 
   Krantz, S. (2026). **collapse**: Advanced and fast statistical computing and data transformation in R. *Journal of Statistical Software, 116*(1), 1–38. [https://doi.org/10.18637/jss.v116.i01](https://doi.org/10.18637/jss.v116.i01).

src/small_helper.c

@@ -179,7 +179,7 @@ SEXP gwhich_first(SEXP x, SEXP g, SEXP target) {
     }
     case REALSXP: {
       const double *px = REAL_RO(x), *pt = REAL_RO(target)-1;
-      for(int i = 0; i != l; ++i) if(pres[pg[i]] == 0 && px[i] == pt[pg[i]]) pres[pg[i]] = i+1;
+      for(int i = 0; i != l; ++i) if(pres[pg[i]] == 0 && (px[i] == pt[pg[i]] || (ISNAN(px[i]) && ISNAN(pt[pg[i]])))) pres[pg[i]] = i+1;
       break;
     }
     case STRSXP: {

tests/testthat/test-fslice.R

@@ -89,6 +89,26 @@ test_that("fslice works with grouping", {
   }
 })
 
+test_that("fslice does not crash with all-missing order.by in a group (#867)", {
+  data <- data.frame(group_var = c("a", "a", "b", "b", "c", "c"),
+                      order_var = c(1, 2, 3, NA, NA, NA))
+
+  expect_equal(
+    fslice(data, "group_var", order.by = "order_var", how = "min", n = 1),
+    data.frame(group_var = c("a", "b", "c"), order_var = c(1, 3, NA))
+  )
+
+  expect_equal(
+    fslice(data, "group_var", order.by = "order_var", how = "max", n = 1),
+    data.frame(group_var = c("a", "b", "c"), order_var = c(2, 3, NA))
+  )
+
+  expect_equal(
+    fslice(data, "group_var", order.by = "order_var", how = "min", n = 1, with.ties = TRUE),
+    data.frame(group_var = c("a", "b", "c", "c"), order_var = c(1, 3, NA, NA))
+  )
+})
+
 test_that("fslice works with ties", {
   N <- 1 # c(1, 5, 17)
   for (n in N) {