Get one character under cursor; with also multibyte character.

gistfile11

" For test (Do quickrun on the target character!):
" aiu
" あいう

function! s:char2bits(char)
    let bits = map(range(8), '0')
    let cur_keta = len(bits) - 1
    let nr = char2nr(a:char)
    while cur_keta >= 0
        let cur_num = float2nr(pow(2, cur_keta))
        if nr >= cur_num
            let nr -= cur_num
            let bits[cur_keta] = 1
        endif
        let cur_keta -= 1
    endwhile
    return reverse(bits)
endfunction

function! s:is_trail_char(char)
    let bits = s:char2bits(a:char)
    return bits[0] && !bits[1]
endfunction

function! s:get_current_and_next_index(str, cur_idx)
    let cur_idx = a:cur_idx
    let str = a:str

    let end_idx = cur_idx + 1
    while s:is_trail_char(str[end_idx])
        let end_idx += 1
    endwhile

    " NOTE: `cur_idx` is always first index.
    return [cur_idx, end_idx]
endfunction

function! s:get_current_character()
    if mode() ==# 'c'
        let str = getcmdline()
        let cur_idx = getcmdpos() - 1
    elseif mode() ==# 'n'
        let str = getline('.')
        let cur_idx = col('.') - 1
    else
        echoerr 'not supported'
    endif

    let [begin, end] = s:get_current_and_next_index(str, cur_idx)
    return strpart(str, begin, end - begin)
endfunction

echom s:get_current_character()