VarAction2: Detect and replace reverse subtract via temp storage

src/newgrf.cpp

@@ -5739,6 +5739,21 @@ static void NewSpriteGroup(ByteReader *buf)
 							replace_with_constant_load(0);
 						} else {
 							switch (adjust.operation) {
+								case DSGA_OP_SUB:
+									if (adjust.variable == 0x7D && adjust.shift_num == 0 && adjust.and_mask == 0xFFFFFFFF) {
+										DeterministicSpriteGroupAdjust &prev = group->adjusts[group->adjusts.size() - 2];
+										if (group->adjusts.size() >= 3 && prev.operation == DSGA_OP_RST) {
+											const DeterministicSpriteGroupAdjust &prev2 = group->adjusts[group->adjusts.size() - 3];
+											if (prev2.operation == DSGA_OP_STO && prev2.type == DSGA_TYPE_NONE && prev2.variable == 0x1A &&
+													prev2.shift_num == 0 && prev2.and_mask == adjust.parameter) {
+												/* Convert: store, load var, subtract stored --> (dead) store, reverse subtract var */
+												prev.operation = DSGA_OP_RSUB;
+												group->adjusts.pop_back();
+												break;
+											}
+										}
+									}
+									break;
 								case DSGA_OP_SMIN:
 									if (adjust.variable == 0x1A && adjust.shift_num == 0 && adjust.and_mask == 1) {
 										DeterministicSpriteGroupAdjust &prev = group->adjusts[group->adjusts.size() - 2];

src/newgrf_spritegroup.cpp

@@ -189,6 +189,7 @@ static U EvalAdjustT(const DeterministicSpriteGroupAdjust &adjust, ScopeResolver
 		case DSGA_OP_SGE:  return ((S)last_value >= (S)value) ? 1 : 0;
 		case DSGA_OP_SLE:  return ((S)last_value <= (S)value) ? 1 : 0;
 		case DSGA_OP_SGT:  return ((S)last_value >  (S)value) ? 1 : 0;
+		case DSGA_OP_RSUB: return value - last_value;
 		default:           return value;
 	}
 }
@@ -556,6 +557,7 @@ static const char *_dsg_op_special_names[] {
 	"SGE",
 	"SLE",
 	"SGT",
+	"RSUB",
 };
 static_assert(lengthof(_dsg_op_special_names) == DSGA_OP_SPECIAL_END - DSGA_OP_TERNARY);
 

src/newgrf_spritegroup.h

@@ -168,6 +168,7 @@ enum DeterministicSpriteGroupAdjustOperation {
 	DSGA_OP_SGE,            ///< (signed) a >= b ? 1 : 0,
 	DSGA_OP_SLE,            ///< (signed) a <= b ? 1 : 0,
 	DSGA_OP_SGT,            ///< (signed) a > b ? 1 : 0,
+	DSGA_OP_RSUB,           ///< b - a
 
 	DSGA_OP_SPECIAL_END,
 };