Document struct equivalence hack and implications of EFF_XY and

EFF_GROUP.

include/file.h

@@ -55,11 +55,19 @@ struct empfile {
     struct castr *cadef;	/* ca defs selection list */
 };
 
+/*
+ * struct empfile flags
+ *
+ * EFF_XY and EFF_GROUP assert that coordinates / group of such a
+ * file's record can be safely obtained by dereferencing its memory
+ * address cast to struct genitem *
+ */
 #define EFF_XY		bit(0)	/* has location */
 #define EFF_MEM		bit(1)	/* stored entirely in-memory */
 #define EFF_OWNER	bit(2)	/* has concept of owner */
 #define EFF_GROUP	bit(3)	/* has concept of group */
 
+/* file types, types 0..EF_MAX-1 are empfile[] indexes */
 #define EF_BAD		-1	/* illegal file type */
 #define EF_SECTOR	0
 #define EF_SHIP		1

include/land.h

@@ -47,6 +47,7 @@
 #define LND_MINMOBCOST  0.200
 
 struct lndstr {
+    /* initial part must match struct genitem */
     short ef_type;
     natid lnd_own;		/* owner's country num */
     short lnd_uid;		/* unit id (land unit) */
@@ -60,6 +61,7 @@ struct lndstr {
     coord lnd_opx, lnd_opy;	/* Op sector coords */
     short lnd_mission;		/* mission code */
     short lnd_radius;		/* mission radius */
+    /* end of part matching struct genitem */
     s_char lnd_flags;		/* unit flags (unused) */
     short lnd_ship;		/* pointer to transporting ship */
     s_char lnd_harden;		/* for missiles */

include/nuke.h

@@ -37,11 +37,13 @@
 #define	N_MAXNUKE	15
 
 struct nukstr {
+    /* initial part must match struct genitem */
     short ef_type;
     natid nuk_own;
     short nuk_uid;
     coord nuk_x;		/* current loc of device */
     coord nuk_y;
+    /* end of part matching struct genitem */
     s_char nuk_n;		/* number of nukes in list */
     short nuk_ship;		/* currently aboard ship (unused) */
     short nuk_trade;		/* index into trade file (unused) */

include/plane.h

@@ -46,6 +46,7 @@
  * exactly equivalent with shp_x to shp_fleet
  */
 struct plnstr {
+    /* initial part must match struct genitem */
     short ef_type;
     natid pln_own;		/* owning country */
     short pln_uid;		/* plane unit id */
@@ -60,6 +61,7 @@ struct plnstr {
     coord pln_opy;		/* Op sector coords */
     short pln_mission;		/* mission code */
     short pln_radius;		/* mission radius */
+    /* end of part matching struct genitem */
     u_char pln_range;		/* total distance, not radius */
     u_char pln_range_max;	/* max range for this plane */
     short pln_ship;		/* pointer to carrier */

include/sect.h

@@ -42,11 +42,13 @@
 /* The order of the following elements is there to match up with genitem */
 
 struct sctstr {
+    /* initial part must match struct genitem */
     short ef_type;
     natid sct_own;		/* owner's country num */
     short sct_elev;		/* elevation/depth */
     coord sct_x;		/* x coord of sector */
     coord sct_y;		/* y coord of sector */
+    /* end of part matching struct genitem */
     u_char sct_type;		/* sector type */
     u_char sct_effic;		/* 0% to 100% */
     short sct_mobil;		/* mobility units */

include/ship.h

@@ -60,6 +60,7 @@
 #define	MAXSHPNAMLEN	24
 
 struct shpstr {
+    /* initial part must match struct genitem */
     short ef_type;
     natid shp_own;		/* owner's country num */
     short shp_uid;		/* unit id (ship #) */
@@ -73,6 +74,7 @@ struct shpstr {
     coord shp_opx, shp_opy;	/* Op sector coords */
     short shp_mission;		/* mission code */
     short shp_radius;		/* mission radius */
+    /* end of part matching struct genitem */
     u_char shp_nplane;		/* number of planes on board */
     u_char shp_nland;		/* number of land units on board */
     short shp_armor;