I represent an index for the messages in a mail database. I acts like a dictionary that maps unique message ID's to IndexFileEntry objects.

The index file is read into memory in its entirety and kept there for the duration of a mail reading session. It should be stored back to disk at the end of the session, or after a major change, such as fetching new mail.

If necessary, it can be completely recreated by scanning the messages file!

	file operations
	delete openOn:messageFile:readLogFlag: readLogMessageFile: rename:

	read-write
	readFrom:messageFile: save writeOn:

	dictionary access	top
	at: Answer the IndexFileEntry for the message with the given ID.
	at:ifAbsent: Answer the IndexFileEntry for the message with the given ID. Evaluate the given block if there is no entry for the given ID.
	at:put: Associate the given IndexFileEntry with the given message ID.
	includesKey: Answer true if my message dictionary contains an entry for the message with the given ID.
	keys Answer a collection of message IDs for the messages in this IndexFile, sorted in ascending timestamp order. Because sorting is expensive, the sorted key list is cached.
	keysCount
	logStream
	privateAt:put: Associate the given IndexFileEntry with the given message ID.
	privateRemove: Remove the entry with the given ID from my Dictionary.
	remove: Remove the entry with the given ID from my Dictionary.

	file operations	top
	delete delete logfile
	openOn:messageFile:readLogFlag: Initialize myself from the file with the given name.
	readLogMessageFile:
	rename: rename log file

	read-write	top
	readFrom:messageFile: Initialize myself from the given text stream. It is assumed that the .index file was written in order of ascending message timestamps, although this method is only less efficient, not incorrect, if this is not the case.
	save Atomically save a representation of this object to its file. The old file is renamed to '<oldname>.bak' before the new file is written. If the write operation fails, the old file may be restored by renaming it. If it succeeds, the .bak file is deleted.
	writeOn: Write my index entries to the given text stream in human-readable form.

	instance creation	top
	openOn:messageFile:readLogFlag: Answer a new instance of me for the given message file, backed by the file with the given name.