genmidi: Add config file for GENMIDI builder.

Config lists the instrument files to use to build the GENMIDI lump.
Some instruments are not yet provided and are nulled out using a dummy
SBI file that doesn't play anything.

lumps/genmidi/instrument.py

@@ -0,0 +1,55 @@
+import os
+import sys
+import sbi_file
+import a2i_file
+
+# Check the specified instrument data is OPL2-compatible and does not
+# use any OPL3 features. Returns an error message, or 'None' if data
+# is valid.
+
+def check_opl2(filename, data):
+	def opl2_warning(message):
+		print >> sys.stderr, "%s: %s" % (filename, message)
+
+	# CHA,B control stereo, but are ignored on OPL2, so it's no problem:
+	#if (data["feedback_fm"] & 0xf0) != 0:
+	#	opl2_warning("Cannot use CHA,B,C,D: %02x" % data["feedback_fm"])
+
+	if data["m_waveform"] > 3:
+		opl2_warning("Modulator uses waveform %i: only 0-3 supported" %
+		             data["m_waveform"])
+	if data["c_waveform"] > 3:
+		opl2_warning("Carrier uses waveform %i: only 0-3 supported" %
+		             data["c_waveform"])
+
+def load_instrument(filename):
+	filename = os.path.join("instruments", filename)
+
+	if filename.endswith(".a2i"):
+		result = a2i_file.read(filename)
+	elif filename.endswith(".sbi"):
+		result = sbi_file.read(filename)
+	else:
+		raise Exception("Unknown instrument file type: '%s'" % filename)
+
+	check_opl2(filename, result)
+
+	return result
+
+class Instrument:
+	def __init__(self, file1, file2=None, note=None):
+		self.instr1 = load_instrument(file1)
+
+		if file2 is not None:
+			self.instr2 = load_instrument(file2)
+		else:
+			self.instr2 = None
+
+		self.fixed_note = note
+
+NullInstrument = Instrument("dummy.sbi")
+
+if __name__ == "__main__":
+	for filename in sys.argv[1:]:
+		Instrument(filename)
+