RSS colors

RED_KEYS =  ((0., 0.),
             (0.35, 0.),
             (0.66, 1.),
             (0.89, 1.),
             (1., 0.5))
GREEN_KEYS = ((0., 0.),
              (0.125, 0.),
              (0.375, 1.),
              (0.64, 1.),
              (0.91, 0.),
              (1., 0.))
BLUE_KEYS = ((0., 0.5),
             (0.11, 1.),
             (0.34, 1.),
             (0.65, 0.),
             (1., 0.))

def interpolate(x, keys):
    assert 0 <= x <= 1
    assert keys[0][0] == 0
    assert keys[-1][0] == 1
    for i in range(len(keys)):
        if x < keys[i][0] or i == len(keys)-1:
            ax, ay = float(x) - keys[i-1][0], keys[i-1][1]
            bx, by = keys[i][0] - float(x), keys[i][1]
            return (bx*ay + ax*by) / (ax+bx)

def get_color_normalized(x):
    red = interpolate(x, RED_KEYS)
    green = interpolate(x, GREEN_KEYS)
    blue = interpolate(x, BLUE_KEYS)
    return (red, green, blue)

def clamp(x, xmin, xmax):
    return min(xmax, max(xmin, x))

def get_color(x, xmin, xmax):
    x = float(clamp(x, xmin, xmax))
    return get_color_normalized((x - xmin) / (xmax - xmin))

def get_color_for_rss(rss):
    return get_color(rss, -100, -30)

if __name__ == '__main__':
    print get_color_for_rss(-110)
    print get_color_for_rss(-100)
    print get_color_for_rss(-34)
    print get_color_for_rss(-30)
    print get_color_for_rss(-20)