test.vim

profile start profile.log
profile func *strwidth*

set rtp+=/path/to/vital.vim/

let s:V = vital#of('vital')
let s:P = s:V.import('Prelude')

function! Error_strwidthpart()
endfunction

function! Test_strwidthpart()
  let strmin = 0
  let strmax = 300
  let strstep = 5
  let outmin = -10
  let outmax = 300
  let outstep = 5
  for i in range(strmin, strmax, strstep)
    let str = repeat('あa', i)
    for j in range(outmin, outmax, outstep)
      let out1 = s:P.strwidthpart(str, i)
      let out2 = s:P.new_strwidthpart(str, i)
      if out1 !=# out2
        call Error_strwidthpart()
      endif
      let out1 = s:P.strwidthpart_reverse(str, i)
      let out2 = s:P.new_strwidthpart_reverse(str, i)
      if out1 !=# out2
        call Error_strwidthpart()
      endif
    endfor
  endfor
endfunction
call Test_strwidthpart()

exit

vim -u NONE -i NONE -N -e -s -S test.vim

profile result: https://gist.github.com/itchyny/21937d6847eb87f6104a