Verify column positions for vroom_fwf

Fixes #217

NEWS.md

@@ -1,5 +1,7 @@
 # vroom (development version)
 
+* `vroom_fwf()` now verifies that the positions are valid, namely that the begin value is always less than the previous end (#217).
+
 * `vroom_lines()` gains a `locale` argument so you can control the encoding of the file (#218)
 
 * `vroom()` columns now support indexing with NA values (#201)

R/vroom_fwf.R

@@ -31,6 +31,8 @@ vroom_fwf <- function(file,
                       progress = vroom_progress(),
                       .name_repair = "unique") {
 
+  verify_fwf_positions(col_positions)
+
   if (!rlang::is_missing(altrep_opts)) {
     lifecycle::deprecate_warn("1.1.0", "vroom_fwf(altrep_opts = )", "vroom_fwf(altrep = )")
     altrep <- altrep_opts
@@ -154,3 +156,11 @@ fwf_col_names <- function(nm, n) {
   nm[nm_empty] <- paste0("X", seq_len(n))[nm_empty]
   nm
 }
+
+verify_fwf_positions <- function(col_positions) {
+  is_greater <- na.omit(col_positions$begin > col_positions$end)
+  if (any(is_greater)) {
+    bad <- which(is_greater)
+    stop("`col_positions` must have begin less than end.\n* Invalid values at position(s): ", paste0(collapse = ", ", bad), call. = FALSE)
+  }
+}

tests/testthat/test-vroom_fwf.R

@@ -274,3 +274,16 @@ test_that("vroom_fwf() is robust to improper inputs", {
     vroom_fwf("A\n  a\n")
   )
 })
+
+test_that("Errors if begin is greater than end", {
+  positions <- fwf_positions(
+    start = c(1, 3, 5),
+    end = c(3, 1, NA),
+    col_names = c("foo", "bar", "baz")
+  )
+
+  expect_error(
+    vroom_fwf("1  2  3\n", positions),
+    "`col_positions` must have begin less than end"
+  )
+})