fix: Multi byte truncate for displayer (#464)

This is needed for calling strdisplaywidth() from Lua loop.

See https://github.com/nvim-telescope/telescope.nvim/issues/414
See a1ed941a78/src/nvim/eval/funcs.c (L9845-L9858)

Co-authored-by: Simon Hauser <Simon-Hauser@outlook.de>

lua/telescope/utils.lua

@@ -202,4 +202,86 @@ function utils.get_os_command_output(cmd)
   return Job:new({ command = command, args = cmd }):sync()
 end
 
+utils.strdisplaywidth = (function()
+  if jit then
+    local ffi = require('ffi')
+    ffi.cdef[[
+      typedef unsigned char char_u;
+      int linetabsize_col(int startcol, char_u *s);
+    ]]
+
+    return function(str, col)
+      local startcol = col or 0
+      local s = ffi.new('char[?]', #str + 1)
+      ffi.copy(s, str)
+      return ffi.C.linetabsize_col(startcol, s) - startcol
+    end
+  else
+    return function(str, col)
+      return #str - (col or 0)
+    end
+  end
+end)()
+
+utils.utf_ptr2len = (function()
+  if jit then
+    local ffi = require('ffi')
+    ffi.cdef[[
+      typedef unsigned char char_u;
+      int utf_ptr2len(const char_u *const p);
+    ]]
+
+    return function(str)
+      local c_str = ffi.new('char[?]', #str + 1)
+      ffi.copy(c_str, str)
+      return ffi.C.utf_ptr2len(c_str)
+    end
+  else
+    return function(str)
+      return str == '' and 0 or 1
+    end
+  end
+end)()
+
+function utils.strcharpart(str, nchar, charlen)
+  local nbyte = 0
+  if nchar > 0 then
+    while nchar > 0 and nbyte < #str do
+      nbyte = nbyte + utils.utf_ptr2len(str:sub(nbyte + 1))
+      nchar = nchar - 1
+    end
+  else
+    nbyte = nchar
+  end
+
+  local len = 0
+  if charlen then
+    while charlen > 0 and nbyte + len < #str do
+      local off = nbyte + len
+      if off < 0 then
+        len = len + 1
+      else
+        len = len + utils.utf_ptr2len(str:sub(off + 1))
+      end
+      charlen = charlen - 1
+    end
+  else
+    len = #str - nbyte
+  end
+
+  if nbyte < 0 then
+    len = len + nbyte
+    nbyte = 0
+  elseif nbyte > #str then
+    nbyte = #str
+  end
+  if len < 0 then
+    len = 0
+  elseif nbyte + len > #str then
+    len = #str - nbyte
+  end
+
+  return str:sub(nbyte + 1, nbyte + len)
+end
+
 return utils