Provide an Empire clock with ETU resolution that is properly

synchronized to updates (the one based on s_p_etu isn't synchronized):
(gamestr): New members game_turn, game_tick, game_rt.
(game_ca): New selectors for them.  Restrict tick and rt to deity for
now.  We'd need xdump update the Empire clock to make them safe for
players.
(game_record_update): New.
(update_main): Call it.
(game_tick_tick, game_tick_to_now, game_step_a_tick): New, not yet
used.

src/lib/common/game.c

@@ -0,0 +1,137 @@
+/*
+ *  Empire - A multi-player, client/server Internet based war game.
+ *  Copyright (C) 1986-2007, Dave Pare, Jeff Bailey, Thomas Ruschak,
+ *                           Ken Stevens, Steve McClure
+ *
+ *  This program is free software; you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation; either version 2 of the License, or
+ *  (at your option) any later version.
+ *
+ *  This program is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License
+ *  along with this program; if not, write to the Free Software
+ *  Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
+ *
+ *  ---
+ *
+ *  See files README, COPYING and CREDITS in the root of the source
+ *  tree for related information and legal notices.  It is expected
+ *  that future projects/authors will amend these files as needed.
+ *
+ *  ---
+ *
+ *  game.c: Game file access
+ * 
+ *  Known contributors to this file:
+ *     Markus Armbruster, 2007
+ */
+
+/*
+ * On Empire Time:
+ *
+ * A turn is terminated by an update and consists of etu_per_update
+ * ETUs.  The Empire clock counts turns and ETUs.  When updates move
+ * around in real time (schedule change, downtime, etc.), the Empire
+ * clock automatically adapts the length of an ETU in seconds
+ * accordingly.
+ */
+
+#include <config.h>
+
+#include <math.h>
+#include "file.h"
+#include "game.h"
+#include "optlist.h"
+#include "prototypes.h"
+#include "server.h"
+
+/*
+ * Record an update in the game file, the current time is NOW.
+ */
+void
+game_record_update(time_t now)
+{
+    struct gamestr *game = getgamep();
+
+    game->game_turn++;
+    game->game_tick = 0;
+    game->game_rt = now;
+    putgame();
+}
+
+/*
+ * Return current duration of an ETU in seconds.
+ * Note: may return HUGE_VAL when the Empire clock is not ticking.
+ */
+static double
+secs_per_etu(struct gamestr *game)
+{
+    double secs;
+
+    if (!update_time[0])
+	return HUGE_VAL;	/* no update scheduled */
+
+    secs = update_time[0] - game->game_rt;
+    if (secs < 0)
+	return HUGE_VAL;	/* update seems to be late */
+    return secs / (etu_per_update - game->game_tick);
+}
+
+/*
+ * Update the Empire clock according to the current real time.
+ * Return the game struct.
+ */
+struct gamestr *
+game_tick_tick(void)
+{
+    struct gamestr *game = getgamep();
+    double dsecs, s_p_etu;
+    int detu;
+
+    dsecs = time(NULL) - game->game_rt;
+    if (CANT_HAPPEN(dsecs < 0))
+	dsecs = 0;
+    s_p_etu = secs_per_etu(game);
+    detu = (int)(dsecs / s_p_etu);
+    if (detu > 0) {
+	if (CANT_HAPPEN(game->game_tick + detu > etu_per_update))
+	    detu = etu_per_update - game->game_tick;
+	game->game_tick += detu;
+	game->game_rt += detu * s_p_etu;
+	putgame();
+    }
+
+    return game;
+}
+
+/*
+ * Set ETU timestamp *TICK to the current ETU time.
+ * Return by how many ETUs it was increased.
+ */
+int
+game_tick_to_now(short *tick)
+{
+    return game_step_a_tick(game_tick_tick(), tick);
+}
+
+/*
+ * Set ETU timestamp *TICK to the ETU time recorded in the game struct.
+ * The Empire clock is not updated.
+ * Return by how many ETUs it was increased.
+ */
+int
+game_step_a_tick(struct gamestr *game, short *tick)
+{
+    int d;
+
+    d = game->game_tick - *tick;
+    if (CANT_HAPPEN(d < 0))
+	d = 0;
+    *tick = game->game_tick;
+    return d;
+}