+#! /usr/bin/python3
+
+import codecs
+import os
+import struct
+import sys
+import zlib
+
+if len(sys.argv) != 3 or '--help' in sys.argv:
+ sys.stdout.write("""\
+%s: adds a comment to a PNG file
+usage: png-add-comment KEYWORD TEXT < INPUT.PNG > OUTPUT.PNG
+where KEYWORD is the comment type, e.g. "Comment" or "Copyright"
+ and TEXT is the comment's content (encoded in Latin-1).
+""" % sys.argv[0])
+ sys.exit(0)
+
+if os.isatty(1):
+ sys.stderr.write("%s: not writing binary data to a terminal "
+ "(use --help for help)\n")
+ sys.exit(1)
+
+infile = sys.stdin.buffer
+outfile = sys.stdout.buffer
+
+encoded_keyword = codecs.encode(sys.argv[1], "ASCII")
+if len(encoded_keyword) > 79:
+ sys.stderr.write("%s: keyword must be 79 bytes or less\n" % sys.argv[0])
+
+encoded_comment = codecs.encode(sys.argv[2], "Latin-1")
+comment_data = encoded_keyword + bytes([0]) + encoded_comment
+comment_crc = zlib.crc32(b'tEXt' + comment_data, 0)
+comment_chunk = struct.pack('!L', len(comment_data)) + b'tEXt' + comment_data + struct.pack('!L', comment_crc)
+
+def read_fully(stream, n):
+ data = stream.read(n)
+ if len(data) != n:
+ sys.stderr.write("%s: unexpected end of input\n" % sys.argv[0])
+ sys.exit(1)
+ return data
+
+# Copy signature and verify that we're working with a PNG file.
+signature = read_fully(infile, 8)
+if signature != bytes([137, 80, 78, 71, 13, 10, 26, 10]):
+ sys.stderr.write("%s: input is not a PNG file\n" % sys.argv[0])
+ sys.exit(1)
+outfile.write(signature)
+
+comment_written = False
+while True:
+ header = read_fully(infile, 8)
+ chunk_len, chunk_type = struct.unpack('!L 4s', header)
+ chunk_data = read_fully(infile, chunk_len)
+ chunk_crc = read_fully(infile, 4)
+
+ if (chunk_type in (b'iCCP', b'sRGB', b'sBIT', b'gAMA', b'cHRM',
+ b'PLTE', b'tRNS', b'hIST', b'bKGD', b'IDAT',
+ b'IEND')) and not comment_written:
+ outfile.write(comment_chunk)
+ comment_written = True
+
+ outfile.write(header)
+ outfile.write(chunk_data)
+ outfile.write(chunk_crc)
+ crc = struct.unpack('!L', chunk_crc)[0]
+ expected_crc = zlib.crc32(header[4:8] + chunk_data, 0)
+ if crc != expected_crc:
+ sys.stderr.write("%s: bad crc reading PNG chunk\n"
+ % sys.argv[0])
+ if chunk_type == b'IEND':
+ break
+assert comment_written