add biomes

include/tools.h

@@ -6,6 +6,8 @@
 
 #include "globals.h"
 
+#define mod_abs(a, b) ((a % b + b) % b)
+
 ssize_t recv_all (int client_fd, void *buf, size_t n, uint8_t require_first);
 ssize_t send_all (int fd, const void *buf, size_t len);