Add a unit test for the loclist comparison function.

autoload/ale/engine.vim

@@ -103,7 +103,7 @@ function! s:HandleExit(job) abort
     " Sort the loclist again.
     " We need a sorted list so we can run a binary search against it
     " for efficient lookup of the messages in the cursor handler.
-    call sort(g:ale_buffer_loclist_map[l:buffer], 's:LocItemCompare')
+    call sort(g:ale_buffer_loclist_map[l:buffer], 'ale#util#LocItemCompare')
 
     if g:ale_set_loclist
         call setloclist(0, g:ale_buffer_loclist_map[l:buffer])
@@ -149,26 +149,6 @@ function! s:FixLocList(buffer, loclist) abort
     endfor
 endfunction
 
-function! s:LocItemCompare(left, right) abort
-    if a:left['lnum'] < a:right['lnum']
-        return -1
-    endif
-
-    if a:left['lnum'] > a:right['lnum']
-        return 1
-    endif
-
-    if a:left['col'] < a:right['col']
-        return -1
-    endif
-
-    if a:left['col'] > a:right['col']
-        return 1
-    endif
-
-    return 0
-endfunction
-
 function! ale#engine#Invoke(buffer, linter) abort
     if has_key(a:linter, 'job')
         " Stop previous jobs for the same linter.

autoload/ale/util.vim

@@ -43,3 +43,23 @@ function! ale#util#GetFunction(string_or_ref) abort
 
     return a:string_or_ref
 endfunction
+
+function! ale#util#LocItemCompare(left, right) abort
+    if a:left['lnum'] < a:right['lnum']
+        return -1
+    endif
+
+    if a:left['lnum'] > a:right['lnum']
+        return 1
+    endif
+
+    if a:left['col'] < a:right['col']
+        return -1
+    endif
+
+    if a:left['col'] > a:right['col']
+        return 1
+    endif
+
+    return 0
+endfunction

test/test_loclist_sorting.vader

@@ -0,0 +1,21 @@
+Before:
+  let g:loclist = [
+  \ {'lnum': 5, 'col': 5},
+  \ {'lnum': 5, 'col': 4},
+  \ {'lnum': 2, 'col': 10},
+  \ {'lnum': 3, 'col': 2},
+  \]
+
+Execute (Sort loclist with comparison function):
+  call sort(g:loclist, 'ale#util#LocItemCompare')
+
+Then (loclist item should be sorted):
+  AssertEqual g:loclist, [
+  \ {'lnum': 2, 'col': 10},
+  \ {'lnum': 3, 'col': 2},
+  \ {'lnum': 5, 'col': 4},
+  \ {'lnum': 5, 'col': 5},
+  \]
+
+After:
+  unlet g:loclist