#!/usr/bin/env python3

##
## Prints the colours of a fade, one RRGGBB per line, on a wall-clock schedule,
## so the reader gets each colour when it is due. Blends through hue, not
## straight RGB, which keeps the in-between colours bright (red to green passes
## through yellow instead of muddy brown).
##
## Argv: start-RRGGBB end-RRGGBB duration-seconds
##

import colorsys
import sys
import time

STEPS_PER_SECOND = 10


def parse(hex_color):
    return tuple(int(hex_color[i:i + 2], 16) / 255 for i in (0, 2, 4))


def format_rgb(rgb):
    return "".join("%02x" % round(max(0, min(1, channel)) * 255) for channel in rgb)


def blend(a, b, t):
    return a + (b - a) * t


## Takes the shorter way around the colour wheel.
def blend_hue(a, b, t):
    gap = ((b - a + 0.5) % 1.0) - 0.5
    return (a + gap * t) % 1.0


def at_hue(start, end, t):
    hs, ss, vs = colorsys.rgb_to_hsv(*start)
    he, se, ve = colorsys.rgb_to_hsv(*end)
    ## A grey has no meaningful hue, so it borrows the other end's.
    if ss == 0:
        hs = he
    if se == 0:
        he = hs
    return colorsys.hsv_to_rgb(blend_hue(hs, he, t), blend(ss, se, t), blend(vs, ve, t))


def main():
    start, end = parse(sys.argv[1]), parse(sys.argv[2])
    duration = float(sys.argv[3])
    steps = max(1, round(duration * STEPS_PER_SECOND))
    began = time.monotonic()
    last = None
    for step in range(steps + 1):
        due = began + duration * step / steps
        time.sleep(max(0, due - time.monotonic()))
        color = format_rgb(at_hue(start, end, step / steps))
        if color != last:
            print(color, flush=True)
            last = color


if __name__ == "__main__":
    main()