Add "index.py", which is similar to the Davex "index" command. It will construct

the INDEXED.HELP file at build time.

Work in progress. It has a few loose ends (see TODOs), and it doesn't do the traditional
compression yet (5 bits %1xxxx for the 16 most popular characters, 8 bits for the other
characters, 1..127).

build/index.py

@@ -0,0 +1,96 @@
+#!/usr/bin/python3
+
+# [TODO] extract filename from pathname
+# [TODO] check that we will not overflow the index into the text data
+# [TODO] add the extra index entries if requested
+# [TODO] what is the 4-byte length field in each entry? I don't think it's ever used, but traditionally the index command has set it to something.
+
+# index create indexed.help <index-size-in-bytes>
+# index add    indexed.help <text-file-to-add> [additional-alias-for-this-file]
+
+# Format of an indexed file is:
+#
+# +000   $00 DvxIdx $00
+# +008  Offset_to_text  (4 bytes)
+# +012  Offset_to_index (4 bytes)
+# +016  Offset_to_free_index_space (4 bytes)
+# +020  <reserved, 32 bytes>
+# +052  start of index
+#
+# Index is:
+#   length-pfx'd string, offset(4), length(4)
+#    :
+#   $00
+#
+# Text chunks are terminated by a $00. The data
+# is compressed, with 16 common characters using
+# only 5 bits (%1xxxx), and the remaining ones
+# using 8 bits (%0xxxxxxx).
+
+import sys, struct
+
+verb = sys.argv[1]
+indexFile = sys.argv[2]
+
+if verb == "create":
+	indexSize = int(sys.argv[3])
+	print("We will create with size = " + str(indexSize))
+	offsetToIndex = 52
+	offsetOfText = offsetToIndex + indexSize
+
+	outFile = open(indexFile, "wb")
+	outFile.write(bytes("\x00DvxIdx\x00", "utf8"))
+	outFile.write(struct.pack('i', offsetOfText))
+	outFile.write(struct.pack('i', offsetToIndex))
+	outFile.write(struct.pack('i', offsetToIndex))  # start of free space in index
+	for pad in range(8):
+		outFile.write(struct.pack('i', 0))	# 8 four-byte integers: reserved
+
+	outFile.seek(offsetOfText - 1)
+	outFile.write(struct.pack('b', 0))
+	exit(0)
+
+if verb == "add":
+	outFile = open(indexFile, "r+b")
+	addFilePath = sys.argv[3]
+	addFile = open (addFilePath, "r")
+	addLines = addFile.readlines()
+	addFile.close()
+
+	# End-of-file offset is where we will put the new chunk of compressed text.
+	outFile.seek(0, 2) # end of file
+	offsetToNewText = outFile.tell()
+
+	# Seek to free index space
+	outFile.seek(16)
+	data = outFile.read(4)
+	freeIndexSpace = int.from_bytes(data, "little")
+	print("freeIndexSpace = ", freeIndexSpace)
+
+	# Write new index entry: length-byte, string name, 4-byte offset to start of text, 4-byte length of (compressed?) text
+	entryName = addFilePath		# [TODO] extract just the filename portion
+	outFile.seek(freeIndexSpace)
+	outFile.write(struct.pack('b', len(entryName)))
+	outFile.write(bytes(entryName, "utf8"))
+	outFile.write(struct.pack('i', offsetToNewText))
+	outFile.write(struct.pack('i', 0x77777777))	# [TODO] write the length
+
+	# Optionally write additional index entries for the same text we are adding.
+
+	# Update freeIndexSpace in file
+	freeIndexSpace = outFile.tell()
+	outFile.seek(16)
+	outFile.write(struct.pack('i', freeIndexSpace))
+
+	# Write the new text
+	outFile.seek(offsetToNewText)
+	for line in addLines:
+		for ch in line.rstrip():
+		    outFile.write(bytes(ch, "utf8"))
+		outFile.write(struct.pack('b', 13)) # carriage return
+	outFile.write(struct.pack('b', 0))
+	outFile.close()
+	exit(0)
+
+print("Unknown verb: " + verb)
+exit(1)