Print the most severe problem with virtualtext

Fix the ordering of virtualtext so we print the most severe problem on a
line. If two problems are the most severe, we will print the left-most
problem.

autoload/ale/virtualtext.vim

@@ -274,6 +274,32 @@ function! ale#virtualtext#ShowCursorWarningWithDelay() abort
     endif
 endfunction
 
+function! ale#virtualtext#CompareSeverityPerLine(left, right) abort
+    " Compare lines
+    if a:left.lnum < a:right.lnum
+        return -1
+    endif
+
+    if a:left.lnum > a:right.lnum
+        return 1
+    endif
+
+    let l:left_priority = ale#util#GetItemPriority(a:left)
+    let l:right_priority = ale#util#GetItemPriority(a:right)
+
+    " Put highest priority items first.
+    if l:left_priority > l:right_priority
+        return -1
+    endif
+
+    if l:left_priority < l:right_priority
+        return 1
+    endif
+
+    " Put the first seen problem first.
+    return a:left.col - a:right.col
+endfunction
+
 function! ale#virtualtext#SetTexts(buffer, loclist) abort
     if !has('nvim') && s:emulate_virt
         return
@@ -281,17 +307,19 @@ function! ale#virtualtext#SetTexts(buffer, loclist) abort
 
     call ale#virtualtext#Clear(a:buffer)
 
-    let l:filter = ale#Var(a:buffer,'virtualtext_single')
-    let l:seen = {}
+    let l:buffer_list = filter(copy(a:loclist), 'v:val.bufnr == a:buffer')
 
-    for l:item in a:loclist
-        if l:item.bufnr == a:buffer
-            let l:line = max([1, l:item.lnum])
+    if ale#Var(a:buffer,'virtualtext_single')
+        " If we want a single problem per line, sort items on each line by
+        " highest severity and then lowest column position, then de-duplicate
+        " the items by line.
+        call uniq(
+        \   sort(l:buffer_list, function('ale#virtualtext#CompareSeverityPerLine')),
+        \   {a, b -> a.lnum - b.lnum}
+        \)
+    endif
 
-            if !has_key(l:seen,l:line) || l:filter == 0
-                call ale#virtualtext#ShowMessage(a:buffer, l:item)
-                let l:seen[l:line] = 1
-            endif
-        endif
+    for l:item in l:buffer_list
+        call ale#virtualtext#ShowMessage(a:buffer, l:item)
     endfor
 endfunction

doc/ale.txt

@@ -2443,9 +2443,9 @@ g:ale_virtualtext_single                             *g:ale_virtualtext_single*
   appended in turn.
 
   With `single` set to a non-zero value, only the first problem on a line will
-  be printed with virtual text. The problem at the left-most position on a
-  line will be printed. If two problems exist at the same position, the most
-  severe problem will be printed.
+  be printed with virtual text. The most severe problem on a line will be
+  printed. If two problems exist on a line of equal severity, the problem at
+  the left-most position will be printed.
 
 
 g:ale_virtualenv_dir_names                         *g:ale_virtualenv_dir_names*

test/test_virtualtext.vader

@@ -37,6 +37,27 @@ Before:
   \       'col': 5,
   \       'text': 'Line 2 warning 2',
   \     },
+  \     {
+  \       'bufnr': bufnr(''),
+  \       'type': 'W',
+  \       'lnum': 3,
+  \       'col': 3,
+  \       'text': 'Line 3 warning 1',
+  \     },
+  \     {
+  \       'bufnr': bufnr(''),
+  \       'type': 'E',
+  \       'lnum': 3,
+  \       'col': 5,
+  \       'text': 'Line 3 error 1',
+  \     },
+  \     {
+  \       'bufnr': bufnr(''),
+  \       'type': 'E',
+  \       'lnum': 3,
+  \       'col': 6,
+  \       'text': 'Line 3 error 2',
+  \     },
   \   ],
   \ },
   \}
@@ -79,6 +100,7 @@ Execute(Comment text should be detected correctly for HTML files):
 Given python(An example Python file):
   # line 1
   # line 2
+  # line 3
 
 Execute(We should not show virtualtext when disabled):
   if has('patch-9.0.0297') || has('nvim-0.8.0')
@@ -157,11 +179,11 @@ Execute(We should be able to change the virtualtext prefix per-buffer):
     AssertEqual 'B> Line 1 error', ale#virtualtext#GetLastMessageForTests()
   endif
 
-Execute(We should set errors across all lines):
+Execute(We should be able to set messages across all lines):
   if has('patch-9.0.0297') || has('nvim-0.8.0')
     call ale#virtualtext#SetTexts(bufnr(''), g:ale_buffer_info[bufnr('')].loclist)
 
-    AssertEqual '# W: Line 2 warning 2', ale#virtualtext#GetLastMessageForTests()
+    AssertEqual '# E: Line 3 error 2', ale#virtualtext#GetLastMessageForTests()
 
     if has('patch-9.0.0297')
       AssertEqual ['ALEVirtualTextError'], map(prop_list(1), {_, v -> v.type})
@@ -176,12 +198,12 @@ Execute(We should be able to limit virtual messages to the first one only):
   if has('patch-9.0.0297') || has('nvim-0.8.0')
     call ale#virtualtext#SetTexts(bufnr(''), g:ale_buffer_info[bufnr('')].loclist)
 
-    AssertEqual '# W: Line 2 warning 1', ale#virtualtext#GetLastMessageForTests()
+    AssertEqual '# E: Line 3 error 1', ale#virtualtext#GetLastMessageForTests()
 
     if has('patch-9.0.0297')
       AssertEqual ['ALEVirtualTextError'], map(prop_list(1), {_, v -> v.type})
-      AssertEqual ['ALEVirtualTextWarning'],
-      \ map(prop_list(2), {_, v -> v.type})
+      AssertEqual ['ALEVirtualTextWarning'], map(prop_list(2), {_, v -> v.type})
+      AssertEqual ['ALEVirtualTextError'], map(prop_list(3), {_, v -> v.type})
     endif
   endif