degesch: use hopefully better colors for nicks

- exclude white from the 16-color range
 - use colors from the 256-color cube when available

degesch.c

@@ -2438,20 +2438,47 @@ formatter_parse_mirc (struct formatter *self, const char *s)
 
 // - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
 
+static int *
+filter_color_cube_for_acceptable_nick_colors (size_t *len)
+{
+	static int table[6 * 6 * 6];
+	size_t len_counter = 0;
+	for (int x = 0; x < 6 * 6 * 6; x++)
+	{
+		int r =  x / 36;
+		int g = (x / 6) % 6;
+		int b = (x % 6);
+
+		// Use the luma value of colours within the cube to filter colours that
+		// look okay-ish on terminals with both black and white backgrounds
+		double luma = 0.2126 * r / 6. + 0.7152 * g / 6. + 0.0722 * b / 6.;
+		if (luma >= .3 && luma <= .5)
+			table[len_counter++] = 16 + x;
+	}
+	*len = len_counter;
+	return table;
+}
+
 static void
 formatter_parse_nick (struct formatter *self, char *s)
 {
 	char *nick = irc_cut_nickname (s);
-	int color = siphash_wrapper (nick, strlen (nick)) % 8;
+	int color = siphash_wrapper (nick, strlen (nick)) % 7;
+
+	// Never use the black colour, could become transparent on black terminals;
+	// white is similarly excluded from the range
+	if (color == COLOR_BLACK)
+		color = -1;
+
+	size_t len;
+	// TODO: precompute this table
+	int *colors = filter_color_cube_for_acceptable_nick_colors (&len);
+	color |= colors[siphash_wrapper (nick, strlen (nick)) % len] << 16;
 
 	// We always use the default color for ourselves
 	if (self->s && irc_is_this_us (self->s, nick))
 		color = -1;
 
-	// Never use the black colour, could become transparent on black terminals
-	if (color == COLOR_BLACK)
-		color = -1;
-
 	FORMATTER_ADD_ITEM (self, FG_COLOR, .color = color);
 
 	char *x = irc_to_utf8 (self->ctx, nick);