(svn r605) -newgrf: Framework for supporting variational spritegroups . Deterministic only at the moment, but random ones support shouldn't be that difficult now It doesn't do anything, but makes these actions actually possible (pasky).

sprite.h

@@ -7,7 +7,7 @@
  * depots or stations): */
 
 typedef struct DrawTileSeqStruct {
-	int8 delta_x;
+	int8 delta_x; // 0x80 is sequence terminator
 	int8 delta_y;
 	int8 delta_z;
 	byte width,height;
@@ -20,12 +20,16 @@ typedef struct DrawTileSprites {
 	DrawTileSeqStruct const *seq;
 } DrawTileSprites;
 
+// Iterate through all DrawTileSeqStructs in DrawTileSprites.
 #define foreach_draw_tile_seq(idx, list) for (idx = list; ((byte) idx->delta_x) != 0x80; idx++)
 
 
 /* This is for custom sprites: */
 
-struct SpriteGroup {
+
+struct SpriteGroup;
+
+struct RealSpriteGroup {
 	// XXX: Would anyone ever need more than 16 spritesets? Maybe we should
 	// use even less, now we take whole 8kb for custom sprites table, oh my!
 	byte sprites_per_set; // means number of directions - 4 or 8
@@ -43,4 +47,74 @@ struct SpriteGroup {
 	uint16 loading[16]; // sprite ids
 };
 
+/* Shared by deterministic and random groups. */
+enum VarSpriteGroupScope {
+	VSG_SCOPE_SELF,
+	// Engine of consists for vehicles, city for stations.
+	VSG_SCOPE_PARENT,
+};
+
+struct DeterministicSpriteGroupRanges;
+
+struct DeterministicSpriteGroup {
+	// Take this variable:
+	enum VarSpriteGroupScope var_scope;
+	byte variable;
+
+	// Do this with it:
+	byte shift_num;
+	byte and_mask;
+
+	// Then do this with it:
+	enum DeterministicSpriteGroupOperation {
+		DSG_OP_NONE,
+		DSG_OP_DIV,
+		DSG_OP_MOD,
+	} operation;
+	byte add_val;
+	byte divmod_val;
+	
+	// And apply it to this:
+	byte num_ranges;
+	struct DeterministicSpriteGroupRanges *ranges; // Dynamically allocated
+
+	// Dynamically allocated, this is the sole owner
+	struct SpriteGroup *default_group;
+};
+
+struct SpriteGroup {
+	enum SpriteGroupType {
+		SGT_REAL,
+		SGT_DETERMINISTIC,
+		SGT_RANDOM, /* TODO */
+	} type;
+
+	union {
+		struct RealSpriteGroup real;
+		struct DeterministicSpriteGroup determ;
+	} g;
+};
+
+struct DeterministicSpriteGroupRanges {
+	struct SpriteGroup group;
+	byte range_low;
+	byte range_high;
+};
+
+/* This is a temporary helper for SpriteGroup users not supporting variational
+ * sprite groups yet - it just traverses those cowardly, always taking the
+ * default choice until it hits a real sprite group, returning it. */
+static struct RealSpriteGroup *TriviallyGetRSG(struct SpriteGroup *sg);
+
+
+
+/**** Inline functions ****/
+
+static INLINE struct RealSpriteGroup *TriviallyGetRSG(struct SpriteGroup *sg)
+{
+	if (sg->type == SGT_REAL)
+		return &sg->g.real;
+	return TriviallyGetRSG(sg->g.determ.default_group);
+}
+
 #endif