genmidi: Add Python code for reading OPL instruments.

These files load OPL instrument data from SBI (SoundBlaster Instrument)
or A2I (AdlibTracker2 Instrument) format files.

lumps/genmidi/a2i_file.py

@@ -0,0 +1,210 @@
+#!/usr/bin/env python
+
+import sys
+import struct
+
+HEADER_STRING = "_A2ins_"
+
+class BitReader:
+	def __init__(self, data):
+		self.data = data
+		self.index = 0
+		self.byte = 0
+		self.byte_bit = 0
+
+	def read_byte(self):
+		if self.index >= len(self.data):
+			raise IndexError("Reached end of decompress stream " +
+			                 "(%i bytes)" % len(self.data))
+		result, = struct.unpack("B", self.data[self.index:self.index+1])
+		self.index += 1
+		return result
+
+	def read_bit(self):
+		if self.byte_bit <= 0:
+			self.byte = self.read_byte()
+			self.byte_bit = 7
+		else:
+			self.byte_bit -= 1
+
+		if (self.byte & (1 << self.byte_bit)) != 0:
+			result = 1
+		else:
+			result = 0
+
+		return result
+
+	def read_bits(self, n):
+		result = 0
+
+		for i in range(n):
+			result = (result << 1) + self.read_bit()
+
+		return result
+
+def read_gamma(reader):
+	result = 1
+
+	while True:
+		result = (result << 1) | reader.read_bit()
+
+		if reader.read_bit() == 0:
+			break
+
+	return result
+
+def decompress(data, data_len):
+	reader = BitReader(data)
+	result = []
+	lwm = 0
+	last_offset = 0
+
+	# First byte is an implied straight copy.
+	result.append(reader.read_byte())
+
+	while True:
+		if reader.read_bit():
+			if reader.read_bit():
+				if reader.read_bit():
+					# 111 = Copy byte from history,
+					# up to 15 bytes back.
+					#print "111 copy"
+
+					offset = reader.read_bits(4)
+
+					if offset == 0:
+						result.append(0)
+					else:
+						b = result[len(result) - offset]
+						result.append(b)
+
+					lwm = 0
+				else:
+					#print "110 copy"
+					# 110 = Copy 2-3 bytes from
+					# further back in history
+
+					offset = reader.read_byte()
+					count = 2 + (offset & 0x01)
+					offset = offset >> 1
+
+					if offset == 0:
+						break
+
+					index = len(result) - offset
+					for i in range(count):
+						result += result[index:index+1]
+						index += 1
+
+					last_offset = offset
+					lwm = 1
+			else:
+				# 10 = Copy from further away...
+
+				offset = read_gamma(reader)
+
+				if lwm == 0 and offset == 2:
+					#print "10 copy type 1"
+					count = read_gamma(reader)
+					index = len(result) - last_offset
+					for i in range(count):
+						result += result[index:index+1]
+						index += 1
+				else:
+					#print "10 copy type 2"
+					if lwm == 0:
+						offset -= 3
+					else:
+						offset -= 2
+
+					offset = offset * 256 \
+					       + reader.read_byte()
+
+					count = read_gamma(reader)
+
+					if offset >= 32000:
+						count += 1
+					if offset >= 1280:
+						count += 1
+					if offset < 128:
+						count += 2
+
+					index = len(result) - offset
+					for i in range(count):
+						result += result[index:index+1]
+						index += 1
+
+					last_offset = offset
+
+				lwm = 1
+
+		else:
+			#print "Single byte output"
+
+			# 0 = Straight-through byte copy.
+			result.append(reader.read_byte())
+			lwm = 0
+
+		#print "len: %i" % len(result)
+
+	return struct.pack("%iB" % len(result), *result)
+
+FIELDS = [
+	"m_am_vibrato_eg",
+	"c_am_vibrato_eg",
+	"m_ksl_volume",
+	"c_ksl_volume",
+	"m_attack_decay",
+	"c_attack_decay",
+	"m_sustain_release",
+	"c_sustain_release",
+	"m_waveform",
+	"c_waveform",
+	"feedback_fm",
+	"panning",
+	"finetune",
+	"voice_type",
+]
+
+def decode_type_9(data):
+	compressed_len, = struct.unpack("<H", data[0:2])
+	compressed_data = data[2:2+compressed_len]
+	decompressed_data = decompress(compressed_data, compressed_len)
+
+	instr_data = {}
+
+	# Decode main fields:
+
+	for i in range(len(FIELDS)):
+		instr_data[FIELDS[i]], = \
+		    struct.unpack("B", decompressed_data[i:i+1])
+
+	# Decode instrument name
+
+	ps = decompressed_data[14:]
+	instr_data["name"], = struct.unpack("%ip" % len(ps), ps)
+
+	return instr_data
+
+def read(filename):
+	f = open(filename)
+	data = f.read()
+	f.close()
+
+	hdrstr, crc, filever = struct.unpack("<7sHB", data[0:10])
+
+	if hdrstr.lower() != HEADER_STRING.lower():
+		raise Exception("Wrong file header ID string")
+
+	# TODO: CRC?
+
+	if filever == 9:
+		return decode_type_9(data[10:])
+	else:
+		raise Exception("Unsupported file version: %i" % filever)
+
+if __name__ == "__main__":
+	for filename in sys.argv[1:]:
+		print filename
+		print read(filename)
+