str2nr_nr2str.vim

function! s:str2nr(str, base) abort
  let xs = map(range(0x30, 0x39) + range(0x41, 0x5A), 'nr2char(v:val)')
  let n = 0
  let total = 0
  if a:base == 1
    return len(a:str) - 1
  else
    for c in reverse(split(a:str, '\zs'))
      let total += index(xs, c) * pow(a:base, n)
      let n += 1
    endfor
    return total
  endif
endfunction

function! s:nr2str(n, base) abort
  let xs = map(range(0x30, 0x39) + range(0x41, 0x5A), 'nr2char(v:val)')
  let rest = a:n
  let str = ''
  while 0 < rest
    let str = xs[rest % a:base] . str
    let rest = rest / a:base
  endwhile
  return str
endfunction

echo s:str2nr('101', 2)
" 5.0
echo s:nr2str(129, 36)
" 3L