Basic port of battle tilemap decompression

scripts/loaders/snes/battle_bgs.gd

@@ -0,0 +1,67 @@
+extends Node
+const LENGTH := 0x500 / 2  # Expanded length 0x500, RLE step produces 0x280 bytes
+
+func decompress_battle_tilemap(buffer: StreamPeerBuffer) -> PoolByteArray:
+	# Decompresses the tilemap for a battle background.
+	# Battle BGs use a type of RLE with 2byte repeat and 1byte incremental repeat.
+	var o1 := PoolByteArray()
+	while len(o1) < LENGTH:
+		var byte := buffer.get_u8()
+		if byte != 0xFF:
+			o1.append(byte)
+		else:  # Byte begins a repeat code
+			var repeat_code := buffer.get_u8()
+			var repeat := repeat_code & 0x3F
+			byte = buffer.get_u8()
+			if repeat_code & 0x80:  # Repeat 2 tiles
+				var b := buffer.get_u8()
+				for i in repeat:
+					o1.append(byte)
+					o1.append(b)
+			else:  # Incremental repeat
+				var inc := buffer.get_u8()
+				if repeat_code & 0x40:  # Negative increment
+					inc = -inc
+				for i in repeat:
+					o1.append(byte + (inc * i))
+	var output := PoolByteArray()
+	for byte in o1:
+		# 0bYXXXXXXX -> 0b0000YY00_1XXXXXXX
+		output.append(byte | 0x80)
+		output.append(((1+(byte>>7)) << 2) & 0xDF)
+	return output
+
+func apply_battle_tilemap_flips(buffer: StreamPeerBuffer, id: int, tilemap: PoolByteArray):
+	if id==0xFF:
+		return tilemap
+	buffer.seek(0x14C736+(id*2))
+	var ptr := 0x140000 + buffer.get_u16()
+	buffer.seek(ptr)
+	var output = tilemap
+	var tile_i := 0
+	while tile_i < LENGTH:
+		var a := buffer.get_u8()
+		if a == 0x00: # Skip N*8 tiles
+			tile_i += buffer.get_u8() * 8
+		else:
+			# Each bit is a tile, 1 means flip horizontally
+			for b in range(7, -1, -1):
+				var tile_flip := (a>>b) & 0x01
+				output[(tile_i*2)+1] |= tile_flip << 6  # Set the OAM flag for this tile
+				tile_i += 1
+	return output
+
+
+class TileMapping:
+	var tile_index: int
+	var palette: int
+	var priority: int
+	var h_flip: int
+	var v_flip: int
+
+	func _init(w: int):
+		self.tile_index = w & 0x03FF
+		self.palette = (w & 0x1C00) >> 10
+		self.priority = (w & 0x2000) >> 13
+		self.h_flip = (w & 0x4000) >> 14
+		self.v_flip = (w & 0x8000) >> 15