Fast aligns check

The optimized table formatting algorithm was still O(n) because
calculating aligns required traversal to the separator line on the top
of a table. This heuristic algorithm calculates aligns in O(1) in the
best case by looking at cells in two rows above.

Say, if two above rows are

|      uau | uauyaya         | ya      |
|       ua | uaua            | uaaua   |
| <cursor> |                 |         |

then the aligns can be figured out without need to find the separator
line. If aligns cannot be figured out after this fast check then the
algorithm falls back to O(n) with searching for the separator line.

autoload/vimwiki/tbl.vim

@@ -254,13 +254,6 @@ function! s:get_rows(lnum, ...)
   while lnum <= line('$')
     let line = getline(lnum)
     if s:is_table(line)
-      if lnum == a:lnum && !s:is_separator(line)
-        let cells = vimwiki#tbl#get_cells(line)
-        let clen = len(cells)
-        let max_lens = repeat([0], clen)
-        let aligns = repeat(['left'], clen)
-        let line = s:fmt_row(cells, max_lens, aligns, 0, 0)
-      endif
       call add(rows, [lnum, line])
     else
       break
@@ -275,9 +268,10 @@ function! s:get_rows(lnum, ...)
 endfunction
 
 
-function! s:get_cell_aligns(lnum)
+function! s:get_cell_aligns(lnum, ...)
   let aligns = {}
-  for [lnum, row] in s:get_rows(a:lnum)
+  let depth = a:0 > 0 ? a:1 : 0
+  for [lnum, row] in s:get_rows(a:lnum, depth)
     if s:is_separator(row)
       let cells = vimwiki#tbl#get_cells(row)
       for idx in range(len(cells))
@@ -303,6 +297,46 @@ function! s:get_cell_aligns(lnum)
 endfunction
 
 
+function! s:get_cell_fast_aligns(rows)
+  let aligns = {}
+  let clen = 0
+  for [lnum, row] in a:rows
+    if s:is_separator(row)
+      return s:get_cell_aligns(lnum, 1)
+    endif
+    let cells = vimwiki#tbl#get_cells(row, 1)
+    let clen = len(cells)
+    for idx in range(clen)
+      let cell = cells[idx]
+      if !has_key(aligns, idx)
+        let cs = matchlist(cell, '^\(\s*\)[^[:space:]].\{-}\(\s*\)$')
+        if !empty(cs)
+          let lstart = len(cs[1])
+          let lend = len(cs[2])
+          if lstart > 0 || lend > 0
+            if lstart > 0 && lend > 0
+              let aligns[idx] = 'center'
+            else
+              if lend > 0
+                let aligns[idx] = 'left'
+              elseif lstart > 0
+                let aligns[idx] = 'right'
+              endif
+            endif
+          endif
+        endif
+      endif
+    endfor
+  endfor
+  for idx in range(clen)
+    if !has_key(aligns, idx)
+      return {}
+    endif
+  endfor
+  return aligns
+endfunction
+
+
 function! s:get_cell_max_lens(lnum, ...)
   let max_lens = {}
   let rows = a:0 > 2 ? a:3 : s:get_rows(a:lnum)
@@ -326,6 +360,7 @@ endfunction
 
 function! s:get_aligned_rows(lnum, col1, col2, depth)
   let rows = []
+  let aligns = {}
   let startlnum = 0
   let cells = []
   let max_lens = {}
@@ -335,6 +370,14 @@ function! s:get_aligned_rows(lnum, col1, col2, depth)
     let startlnum = rows[0][0]
     let lrows = len(rows)
     if lrows == a:depth + 1
+      let line = rows[-1][1]
+      if !s:is_separator(line)
+        let lcells = vimwiki#tbl#get_cells(line)
+        let lclen = len(lcells)
+        let lmax_lens = repeat([0], lclen)
+        let laligns = repeat(['left'], lclen)
+        let rows[-1][1] = s:fmt_row(lcells, lmax_lens, laligns, 0, 0)
+      endif
       let i = 1
       for [lnum, row] in rows
         call add(cells, vimwiki#tbl#get_cells(row, i != lrows - 1))
@@ -348,6 +391,8 @@ function! s:get_aligned_rows(lnum, col1, col2, depth)
       endif
       let fst_lens = s:get_cell_max_lens(a:lnum, cells, startlnum, rows[0:0])
       let check_all = max_lens != fst_lens
+      let aligns = s:get_cell_fast_aligns(rows[0:-2])
+      let rows[-1][1] = line
     endif
   endif
   if check_all
@@ -365,7 +410,9 @@ function! s:get_aligned_rows(lnum, col1, col2, depth)
       let max_lens[last_index] = 1
     endif
   endif
+  if empty(aligns)
     let aligns = s:get_cell_aligns(a:lnum)
+  endif
   let result = []
   for [lnum, row] in rows
     if s:is_separator(row)