Convert values between RGB hex codes and xterm-256 color codes.

colortrans.vim

" Port of colortrans.py
" https://gist.github.com/MicahElliott/719710
" Largely untested. Hopefully Micah did that :-)

let s:shorttorgb = [
    \ '000000', '800000', '008000', '808000', '000080', '800080', '008080',
    \ 'c0c0c0', '808080', 'ff0000', '00ff00', 'ffff00', '0000ff', 'ff00ff',
    \ '00ffff', 'ffffff', '000000', '00005f', '000087', '0000af', '0000d7',
    \ '0000ff', '005f00', '005f5f', '005f87', '005faf', '005fd7', '005fff',
    \ '008700', '00875f', '008787', '0087af', '0087d7', '0087ff', '00af00',
    \ '00af5f', '00af87', '00afaf', '00afd7', '00afff', '00d700', '00d75f',
    \ '00d787', '00d7af', '00d7d7', '00d7ff', '00ff00', '00ff5f', '00ff87',
    \ '00ffaf', '00ffd7', '00ffff', '5f0000', '5f005f', '5f0087', '5f00af',
    \ '5f00d7', '5f00ff', '5f5f00', '5f5f5f', '5f5f87', '5f5faf', '5f5fd7',
    \ '5f5fff', '5f8700', '5f875f', '5f8787', '5f87af', '5f87d7', '5f87ff',
    \ '5faf00', '5faf5f', '5faf87', '5fafaf', '5fafd7', '5fafff', '5fd700',
    \ '5fd75f', '5fd787', '5fd7af', '5fd7d7', '5fd7ff', '5fff00', '5fff5f',
    \ '5fff87', '5fffaf', '5fffd7', '5fffff', '870000', '87005f', '870087',
    \ '8700af', '8700d7', '8700ff', '875f00', '875f5f', '875f87', '875faf',
    \ '875fd7', '875fff', '878700', '87875f', '878787', '8787af', '8787d7',
    \ '8787ff', '87af00', '87af5f', '87af87', '87afaf', '87afd7', '87afff',
    \ '87d700', '87d75f', '87d787', '87d7af', '87d7d7', '87d7ff', '87ff00',
    \ '87ff5f', '87ff87', '87ffaf', '87ffd7', '87ffff', 'af0000', 'af005f',
    \ 'af0087', 'af00af', 'af00d7', 'af00ff', 'af5f00', 'af5f5f', 'af5f87',
    \ 'af5faf', 'af5fd7', 'af5fff', 'af8700', 'af875f', 'af8787', 'af87af',
    \ 'af87d7', 'af87ff', 'afaf00', 'afaf5f', 'afaf87', 'afafaf', 'afafd7',
    \ 'afafff', 'afd700', 'afd75f', 'afd787', 'afd7af', 'afd7d7', 'afd7ff',
    \ 'afff00', 'afff5f', 'afff87', 'afffaf', 'afffd7', 'afffff', 'd70000',
    \ 'd7005f', 'd70087', 'd700af', 'd700d7', 'd700ff', 'd75f00', 'd75f5f',
    \ 'd75f87', 'd75faf', 'd75fd7', 'd75fff', 'd78700', 'd7875f', 'd78787',
    \ 'd787af', 'd787d7', 'd787ff', 'd7af00', 'd7af5f', 'd7af87', 'd7afaf',
    \ 'd7afd7', 'd7afff', 'd7d700', 'd7d75f', 'd7d787', 'd7d7af', 'd7d7d7',
    \ 'd7d7ff', 'd7ff00', 'd7ff5f', 'd7ff87', 'd7ffaf', 'd7ffd7', 'd7ffff',
    \ 'ff0000', 'ff005f', 'ff0087', 'ff00af', 'ff00d7', 'ff00ff', 'ff5f00',
    \ 'ff5f5f', 'ff5f87', 'ff5faf', 'ff5fd7', 'ff5fff', 'ff8700', 'ff875f',
    \ 'ff8787', 'ff87af', 'ff87d7', 'ff87ff', 'ffaf00', 'ffaf5f', 'ffaf87',
    \ 'ffafaf', 'ffafd7', 'ffafff', 'ffd700', 'ffd75f', 'ffd787', 'ffd7af',
    \ 'ffd7d7', 'ffd7ff', 'ffff00', 'ffff5f', 'ffff87', 'ffffaf', 'ffffd7',
    \ 'ffffff', '080808', '121212', '1c1c1c', '262626', '303030', '3a3a3a',
    \ '444444', '4e4e4e', '585858', '626262', '6c6c6c', '767676', '808080',
    \ '8a8a8a', '949494', '9e9e9e', 'a8a8a8', 'b2b2b2', 'bcbcbc', 'c6c6c6',
    \ 'd0d0d0', 'dadada', 'e4e4e4', 'eeeeee'
\ ]

let s:striphash = {s -> s[0] == '#' ? s[1:] : s}
let s:rgbtoshort = {}
let s:i = 0
while s:i < len(s:shorttorgb)
    let s:rgb = s:shorttorgb[s:i]
    let s:rgbtoshort[s:rgb] = s:i
    let s:i += 1
endwhile

fun! g:Shorttorgb(short)
    return s:shorttorgb[str2nr(a:short)]
endfun

" Find the closest xterm-256 approximation to the given RGB value.
" @param rgb: Hex code representing an RGB value, eg, 'abcdef'
" @returns: String between 0 and 255, compatible with xterm.
fun! g:Rgbtoshort(rgb)
    let rgb = s:striphash(a:rgb)
    let incs = [0x00, 0x5f, 0x87, 0xaf, 0xd7, 0xff]
    let parts = [str2nr(rgb[0:1], 16), str2nr(rgb[2:3], 16),
            \ str2nr(rgb[4:5], 16)]
    let res = []
    for part in parts
        let i = 0
        while i < len(incs) - 1
            let [s, b] = [incs[i], incs[i + 1]]  " smaller, bigger
            if s <= part && part <= b
                let s1 = abs(s - part)
                let b1 = abs(b - part)
                if s1 < b1
                    let closest = s
                else
                    let closest = b
                endif
                call add(res, closest)
                break
            endif
            let i += 1
        endwhile
    endfor
    let approxrgb = join(map(copy(res), {i, x -> printf('%02x', x)}), '')
    let shortequiv = s:rgbtoshort[approxrgb]
    return [shortequiv, approxrgb]
endfun