games/doom: fix desyncing demo (thanks qwx)

the code used P_Random()-P_Random() in some places which has
undefined evaluation order resulting in the wrong pseudo random
numbers being returned causing demo playback to desync.

this change adds P_Random2() function which returns the right
delta-random number and uses it in place of P_Random()-P_Random()
expression.

1 files changed, 6 insertions, 0 deletions

diff --git a/sys/src/games/doom/m_random.c b/sys/src/games/doom/m_random.c
index 09f857d0b..957bee316 100644
--- a/sys/src/games/doom/m_random.c
+++ b/sys/src/games/doom/m_random.c
@@ -60,6 +60,12 @@ int P_Random (void)
     return rndtable[prndindex];
 }
 
+int P_Random2 (void)
+{
+    int tmp = P_Random();
+    return tmp - P_Random();
+}
+
 int M_Random (void)
 {
     rndindex = (rndindex+1)&0xff;