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checksum.py

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+from __future__ import print_function
+
+import sys
+
+def calc_checksum(data, old):
+	a = 0
+	c = 0
+	for b in data:
+		a = a + b + c
+		c = 0
+		if a > 255:
+			a = a - 256
+			c = 1
+	if not old:
+		a += c
+	return a
+
+def usage():
+	print("Usage: python checksum.py {--old|--new} <file.bin> [<checksum> <offset>]")
+	exit(1)
+
+# main
+
+if len(sys.argv) != 3 and len(sys.argv) != 5:
+	usage()
+
+if sys.argv[1] == "--old":
+	old = True
+elif sys.argv[1] == "--new":
+	old = False
+else:
+	usage()
+
+filename = sys.argv[2]
+
+data = bytearray(open(filename, 'rb').read())
+
+checksum = calc_checksum(data, old)
+
+if len(sys.argv) != 5:
+	print(filename + ": " + hex(checksum))
+	exit(0)
+
+desired_checksum = int(sys.argv[3], 16)
+offset = int(sys.argv[4], 16)
+
+if checksum < desired_checksum:
+	data[offset] = desired_checksum - checksum
+else:
+	data[offset] = 0xff - (checksum - desired_checksum)
+
+file = open(filename, "wb")
+file.write(data)