[WIP] This is a first attempt at modernising this plugin.

after/ftplugin/qf.vim

@@ -176,6 +176,8 @@ let b:undo_ftplugin .= "| delcommand Filter"
             \ . "| unlet! b:qf_isLoc"
 
 " decide where to open the location/quickfix window
+" :help g:qf_loclist_window_bottom
+" :help g:qf_window_bottom
 if (b:qf_isLoc == 1 && get(g:, 'qf_loclist_window_bottom', 1))
             \ || (b:qf_isLoc == 0 && get(g:, 'qf_window_bottom', 1))
     wincmd J

autoload/qf.vim

@@ -19,19 +19,6 @@
 let s:save_cpo = &cpo
 set cpo&vim
 
-" open the current entry in th preview window
-function qf#PreviewFileUnderCursor()
-    let cur_list = b:qf_isLoc == 1 ? getloclist('.') : getqflist()
-    let cur_line = getline(line('.'))
-    let cur_file = fnameescape(substitute(cur_line, '|.*$', '', ''))
-    if cur_line =~ '|\d\+'
-        let cur_pos  = substitute(cur_line, '^\(.\{-}|\)\(\d\+\)\(.*\)', '\2', '')
-        execute "pedit +" . cur_pos . " " . cur_file
-    else
-        execute "pedit " . cur_file
-    endif
-endfunction
-
 " helper function
 " returns 1 if the window with the given number is a quickfix window
 "         0 if the window with the given number is not a quickfix window
@@ -73,13 +60,26 @@ function! qf#IsLocWindowOpen(nmbr) abort
     return 0
 endfunction
 
-" returns location list of the current loclist if isLoc is set
-"         qf list otherwise
-function! qf#GetList()
+" returns current location list or quickfix list
+function! qf#GetListItems(idx)
+    let what = { 'idx': a:->get('idx', 0), 'items': 1 }
+
+    if get(b:, 'qf_isLoc', 0)
+        return getloclist(0, what)["items"]
+    else
+        return getqflist(what)["items"]
+    endif
+endfunction
+
+" helper
+" returns the number of items in a loc/qf list
+function! qf#GetListSize()
+    let what = { 'size': 1 }
+
     if get(b:, 'qf_isLoc', 0)
-        return getloclist(0)
+        return getloclist(0, what)["size"]
     else
-        return getqflist()
+        return getqflist(what)["size"]
     endif
 endfunction
 
@@ -107,68 +107,93 @@ function! qf#SetList(newlist, ...)
     endif
 endfunction
 
-function! qf#GetEntryPath(line) abort
-    "                          +- match from the first pipe to the end of line
-    "                          |  declaring EOL explicitly is faster than implicitly
-    "                          |      +- replace match with nothing
-    "                          |      |   +- no flags
-    return substitute(a:line, '|.*$', '', '')
-endfunction
-
 " open the quickfix window if there are valid errors
-function! qf#OpenQuickfix()
+function! qf#OpenQuickfixWindow()
     if get(g:, 'qf_auto_open_quickfix', 1)
         " get user-defined maximum height
         let max_height = get(g:, 'qf_max_height', 10) < 1 ? 10 : get(g:, 'qf_max_height', 10)
 
-        let qf_list = getqflist()
-
-        " shorten paths if applicable
-        if get(g:, 'qf_shorten_path', 0) > 0
-            call setqflist(qf#ShortenPathsInList(qf_list))
-        endif
-
-        execute get(g:, "qf_auto_resize", 1) ? 'cclose|' . min([ max_height, len(qf_list) ]) . 'cwindow' : 'cclose|cwindow'
+        execute get(g:, "qf_auto_resize", 1) ? 'cclose|' . min([ max_height, qf#GetListSize() ]) . 'cwindow' : 'cclose|cwindow'
     endif
 endfunction
 
 " open a location window if there are valid locations
-function! qf#OpenLoclist()
+function! qf#OpenLocationWindow()
     if get(g:, 'qf_auto_open_loclist', 1)
         " get user-defined maximum height
         let max_height = get(g:, 'qf_max_height', 10) < 1 ? 10 : get(g:, 'qf_max_height', 10)
 
-        let loc_list = getloclist(0)
+        execute get(g:, "qf_auto_resize", 1) ? 'lclose|' . min([ max_height, qf#GetListSize() ]) . 'lwindow' : 'lclose|lwindow'
+    endif
+endfunction
 
-        " shorten paths if applicable
-        if get(g:, 'qf_shorten_path', 0) > 0
-            call setloclist(0, qf#ShortenPathsInList(loc_list))
-        endif
+function! qf#QuickfixTextFunc(options)
+    let items = a:options["quickfix"] == 1 ? getqflist() : getloclist(a:options["winid"])
+    return items->map({ key, val -> val->qf#FormatItem() })
+endfunction
+
+function! qf#FormatItem(item)
+    return [
+                \ a:item->qf#FormatFilename(),
+                \ a:item->qf#FormatLocation(),
+                \ a:item->qf#FormatText(),
+                \ ]->join('|')
+endfunction
+
+function! qf#FormatFilename(item)
+    let filename = a:item["bufnr"]->bufname()
 
-        execute get(g:, "qf_auto_resize", 1) ? 'lclose|' . min([ max_height, len(loc_list) ]) . 'lwindow' : 'lclose|lwindow'
+    if has('patch-8.2.1741')
+        return pathshorten(filename, g:->get("qf_shorten_path", 1))
+    else
+        return pathshorten(filename)
     endif
 endfunction
 
-" shorten file paths in given qf/loc list
-function! qf#ShortenPathsInList(list)
-    let index = 0
-    while index < len(a:list)
-        " item is a dict, sample: { lnum: 14, text: 'foo bar', bufnr: 3, ... }
-        let item = a:list[index]
+function! qf#FormatLocation(item)
+    return [
+                \ a:item->get("lnum", 0)->qf#FormatLineNumber(),
+                \ a:item->get("col", 0)->qf#FormatColumn(),
+                \ a:item->get("type", '')->qf#FormatType(),
+                \ a:item->get("nr", 0)->qf#FormatErrorNumber(),
+                \ ]->join('')
+endfunction
 
-        let filepath = bufname(item["bufnr"])
-        let trim_len = get(g:, "qf_shorten_path", 1)
+function! qf#FormatText(item)
+    " return ' ' .. a:item->get("text", '')
+    return a:item["text"]
+endfunction
 
-        " set the 'module' field to customise the visual filename in the qf/loc list (available since 8.0.1782)
-        if has('patch-8.2.1741')
-            let item["module"] = pathshorten(filepath, trim_len)
+function! qf#FormatLineNumber(lnum)
+    return a:lnum != 0 ? a:lnum : '-'
+endfunction
+
+function! qf#FormatColumn(col)
+    return a:col > 0 ? ' col ' .. a:col : ''
+endfunction
+
+function! qf#FormatType(type)
+    let types = {
+                \ 'e': ' error',
+                \ 'i': ' info',
+                \ 'n': ' note',
+                \ 'w': ' warning'
+                \ }
+    return a:type != '' ? types[a:type] : ''
+endfunction
+
+function! qf#FormatErrorNumber(nr)
+    if a:nr > 0
+        if a:nr->string()->len() == 1
+            return '   ' .. a:nr
+        elseif a:nr->string()->len() == 2
+            return '  ' .. a:nr
         else
-            let item["module"] = pathshorten(filepath)
+            return ' ' .. a:nr
         endif
-
-        let index = index + 1
-    endwhile
-    return a:list
+    else
+        return ''
+    endif
 endfunction
 
 let &cpo = s:save_cpo

autoload/qf/do.vim

@@ -19,40 +19,18 @@
 let s:save_cpo = &cpo
 set cpo&vim
 
-" do something with each entry
-" a single function for :Doline and :Dofile both in a quickfix list and
-" a location list
-" falls back to :cdo, :cfdo, :ldo, :lfdo when possible
+" Do something with each entry
+" ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+" Handles :Doline and :Dofile, in quickfix and location lists.
+" Wrapper aound :cdo, :cfdo, :ldo, :lfdo.
 function! qf#do#DoList(line, cmd)
-    if exists("b:qf_isLoc")
-        let prefix = b:qf_isLoc == 1 ? "l" : "c"
-    else
-        let prefix = "c"
-    endif
+    let prefix   = b:->get("qf_isLoc", 1) ? "l" : "c"
+    let modifier = a:line == 1 ? "" : "f"
 
-    if v:version >= 705
-                \ || v:version == 704 && has("patch858")
-        if a:line == 1
-            let modifier = ""
-        else
-            let modifier = "f"
-        endif
-
-        try
-            execute prefix . modifier . "do " . a:cmd
-        catch /^Vim\%((\a\+)\)\=:E\%(553\|42\):/
-        endtry
-    else
-        try
-            silent execute prefix . "first"
-            while 1
-                execute a:cmd
-
-                silent execute a:line == 1 ? prefix . "next" : prefix . "nfile"
-            endwhile
-        catch /^Vim\%((\a\+)\)\=:E\%(553\|42\):/
-        endtry
-    endif
+    try
+        execute prefix . modifier . "do " . a:cmd
+    catch /^Vim\%((\a\+)\)\=:E\%(553\|42\):/
+    endtry
 endfunction
 
 let &cpo = s:save_cpo

autoload/qf/filegroup.vim

@@ -19,40 +19,54 @@
 let s:save_cpo = &cpo
 set cpo&vim
 
-function! s:JumpToFirstItemOfFileChunk() abort
-    let l:chunk_file_path = qf#GetEntryPath(getline('.'))
+function! qf#filegroup#NextFile() abort
+    if exists("b:qf_isLoc")
+        let items = qf#GetListItems(0)
+        let current_index = line('.') - 1
+        let current_bufnr = items[current_index]["bufnr"]
+        let limit = items->len()
 
-    while line('.') - 1 != 0
-                \ && l:chunk_file_path == qf#GetEntryPath(getline(line('.') - 1))
-        normal! k
-    endwhile
+        while current_index < limit && items[current_index]["bufnr"] == current_bufnr
+            let current_index += 1
+        endwhile
 
-    normal! zz
+        if current_index == limit
+            1
+        else
+            execute current_index + 1
+        endif
+    endif
 endfunction
 
-function! s:JumpFileChunk(down) abort
-    let l:start_file_path = qf#GetEntryPath(getline('.'))
-    let l:direction       = a:down ? 'j' : 'k'
-    let l:end             = a:down ? '$' : 1
+function! qf#filegroup#PreviousFile() abort
+    if exists("b:qf_isLoc")
+        let items = qf#GetListItems(0)
+        let current_index = line('.') - 1
+        let current_bufnr = items[current_index]["bufnr"]
+        let limit = 0
 
-    while l:start_file_path
-                \ == qf#GetEntryPath(getline('.'))
-                \    && getline('.') != getline(l:end)
-        execute 'normal! ' . l:direction
-    endwhile
+        while current_index > limit && items[current_index]["bufnr"] == current_bufnr
+            let current_index -= 1
+        endwhile
 
-    call s:JumpToFirstItemOfFileChunk()
-endfunction
+        if current_index == limit
+            normal! G
+        else
+            execute current_index + 1
+        endif
 
-function! qf#filegroup#PreviousFile() abort
-    if exists("b:qf_isLoc")
-        call s:JumpFileChunk(0)
-    endif
-endfunction
+        let current_index = line('.') - 1
+        let current_bufnr = items[current_index]["bufnr"]
 
-function! qf#filegroup#NextFile() abort
-    if exists("b:qf_isLoc")
-        call s:JumpFileChunk(1)
+        while current_index > limit && items[current_index]["bufnr"] == current_bufnr
+            let current_index -= 1
+        endwhile
+
+        if current_index == limit
+            1
+        else
+            execute current_index + 2
+        endif
     endif
 endfunction
 

autoload/qf/preview.vim

@@ -21,16 +21,22 @@ set cpo&vim
 
 " open the current entry in th preview window
 function! qf#preview#PreviewFileUnderCursor()
-    let cur_list = qf#GetList()
-    let cur_line = getline(line('.'))
-    let cur_file = fnameescape(qf#GetEntryPath(cur_line))
+    let winview = winsaveview()
 
-    if cur_line =~ '|\d\+'
-        let cur_pos  = substitute(cur_line, '^\(.\{-}|\)\(\d\+\)\(.*\)', '\2', '')
-        execute "pedit +" . cur_pos . " " . cur_file
+    let current_item        = qf#GetListItems(line('.'))[0]
+    let current_file_name   = current_item["bufnr"]->bufname()
+    let current_file_line   = current_item->get('lnum', 0)
+    let current_file_column = current_item->get('col', 0)
+
+    if current_file_line && current_file_column
+        execute "pedit +" .. current_file_line .. " " .. current_file_name .. "|normal! " .. current_file_column .. "G"
+    elseif current_file_line && !current_file_column
+        execute "pedit +" .. current_file_line .. " " .. current_file_name
     else
-        execute "pedit " . cur_file
+        execute "pedit " .. current_file_name
     endif
+
+    call winrestview(winview)
 endfunction
 
 let &cpo = s:save_cpo

autoload/qf/toggle.vim

@@ -19,7 +19,7 @@
 let s:save_cpo = &cpo
 set cpo&vim
 
-" toggles the quickfix window
+" Toggles the quickfix window.
 function! qf#toggle#ToggleQfWindow(stay) abort
     " save the view if the current window is not a quickfix window
     if get(g:, 'qf_save_win_view', 1)  && !qf#IsQfWindow(winnr())
@@ -38,7 +38,7 @@ function! qf#toggle#ToggleQfWindow(stay) abort
             call winrestview(winview)
         endif
     else
-        execute get(g:, 'qf_auto_resize', 1) ? min([ max_height, len(getqflist()) ]) . 'cwindow' : max_height . 'cwindow'
+        execute get(g:, 'qf_auto_resize', 1) ? min([ max_height, qf#GetListSize() ]) . 'cwindow' : max_height . 'cwindow'
         if qf#IsQfWindowOpen()
             wincmd p
             if !empty(winview)
@@ -51,8 +51,8 @@ function! qf#toggle#ToggleQfWindow(stay) abort
     endif
 endfunction
 
-" toggles the location window associated with the current window
-" or whatever location window has the focus
+" Toggles the location window associated with the current window
+" or whatever location window has the focus.
 function! qf#toggle#ToggleLocWindow(stay) abort
     " save the view if the current window is not a location window
     if get(g:, 'qf_save_win_view', 1) && !qf#IsLocWindow(winnr())
@@ -70,7 +70,7 @@ function! qf#toggle#ToggleLocWindow(stay) abort
             call winrestview(winview)
         endif
     else
-        execute get(g:, 'qf_auto_resize', 1) ? min([ max_height, len(getloclist(0)) ]) . 'lwindow' : max_height . 'lwindow'
+        execute get(g:, 'qf_auto_resize', 1) ? min([ max_height, qf#GetListSize() ]) . 'lwindow' : max_height . 'lwindow'
         if qf#IsLocWindowOpen(0)
             wincmd p
             if !empty(winview)