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