truecolor_to_256.py

"""Convert truecolor escape sequence to 256 color escape sequence

* \x1b[38;2;{red};{green};{blue}m -> \x1b[38;5;{color}m (foreground)
* \x1b[48;2;{red};{green};{blue}m -> \x1b[48;5;{color}m (background)

where 0 <= red, green, blue <= 255 represents 24-bit color
and 16 <= color <= 231

The logic is based on the colour_find_rgb() in tmux/colour.c
Original copyright notice is:

    Copyright (c) 2008 Nicholas Marriott <nicholas.marriott@gmail.com>
    Copyright (c) 2016 Avi Halachmi <avihpit@yahoo.com>

    Permission to use, copy, modify, and distribute this software for any
    purpose with or without fee is hereby granted, provided that the above
    copyright notice and this permission notice appear in all copies.

    THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
    WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
    MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
    ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
    WHATSOEVER RESULTING FROM LOSS OF MIND, USE, DATA OR PROFITS, WHETHER
    IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING
    OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
"""
import re


def to_6(n):
    "256 color to 6"
    if n < 48:
        return 0
    if n < 114:
        return 1
    return (n-35) // 40


def from_6(q):
    "6 to 256"
    q2c = [0, 95, 135, 175, 215, 255]
    return q2c[q]


def dist_sq(a, b):
    return sum((p-q)*(p-q) for p, q in zip(a, b))


def truecolor_to_256(s):
    pattern = re.compile(r"\x1b\[([34]8);2;(\d+);(\d+);(\d+)((;\d+)*)m")

    def repl(m):
        r, g, b = map(int, (m[2], m[3], m[4]))
        # 6x6x6
        qr = to_6(r)
        qg = to_6(g)
        qb = to_6(b)
        # grey
        grey_avg = (r + g + b) // 3
        if grey_avg > 238:
            grey_idx = 23
        else:
            grey_idx = (grey_avg - 3) // 10
        grey = grey_idx*10 + 8
        # use closer one
        d = dist_sq(map(from_6, (qr, qg, qb)), (r, g, b))
        if dist_sq((grey, grey, grey), (r, g, b)) < d:
            c = grey_idx + 232
        else:
            c = qr*36 + qg*6 + qb + 16
        return f'\x1b[{m[1]};5;{c}{m[5]}m'
    return re.sub(pattern, repl, s)


if __name__ == '__main__':
    import sys
    sys.stdout.write(truecolor_to_256(sys.stdin.read()))