FileStructureWalker (pass HgFile, HgFolder to callable; which can ask for VCS data from any file)
External uses: user browses files, selects one and asks for its history 
Params: tip/revision; 
Implementation: manifest

Log --rev
Log <file>
HgDataFile.history() or Changelog.history(file)?


Changelog.all() to return list with placeholder, not-parsed elements (i.e. read only compressedLen field and skip to next record), so that
total number of elements in the list is correct

hg cat
Implementation: logic to find file by name in the repository is the same with Log and other commands


Revlog
What happens when big entry is added to a file - when it detects it can't longer fit into .i and needs .d? Inline flag and .i format changes?

What's hg natural way to see nodeids of specific files (i.e. when I do 'hg --debug manifest -r 11' and see nodeid of some file, and 
then would like to see what changeset this file came from)?

----------
+ support patch from baseRev + few deltas (although done in a way patches are applied one by one instead of accumulated)
+ command-line samples (-R, filenames) (Log & Cat) to show on any repo
+buildfile + run samples
*input stream impl + lifecycle. Step forward with FileChannel and ByteBuffer, although questionable accomplishment (looks bit complicated, cumbersome)
+ dirstate.mtime
+calculate sha1 digest for file to see I can deal with nodeid. +Do this correctly (smaller nodeid - first)
*.hgignored processing
+Nodeid to keep 20 bytes always, Revlog.Inspector to get nodeid array of meaningful data exact size (nor heading 00 bytes, nor 12 extra bytes from the spec)
+DataAccess - implement memory mapped files, 
+Changeset to get index (local revision number)
+RevisionWalker (on manifest) and WorkingCopyWalker (io.File) talking to ? and/or dirstate (StatusCollector and WCSC) 
+RevlogStream - Inflater. Perhaps, InflaterStream instead? branch:wrap-data-access
+repo.status - use same collector class twice, difference as external code. add external walker that keeps collected maps and use it in Log operation to give files+,files-  
+ strip \1\n metadata out from RevlogStream
+ hash/digest long names for fncache

delta merge
DataAccess - collect debug info (buffer misses, file size/total read operations) to find out better strategy to buffer size detection. Compare performance.

Strip off metadata from beg of the stream - DataAccess (with rebase/moveBaseOffset(int)) would be handy

Status operation from GUI - guess, usually on a file/subfolder, hence API should allow for starting path (unlike cmdline, seems useless to implement include/exclide patterns - GUI users hardly enter them, ever)
  -> recently introduced FileWalker may perhaps help solving this (if starts walking from selected folder) for status op against WorkingDir?

? Can I use fncache (names from it - perhaps, would help for Mac issues Alex mentioned)
? Does fncache lists both .i and .d (iow, is it true hashed <long name>.d is different from hashed <long name>.i)
 
??? encodings of fncache, .hgignore, dirstate
??? http://mercurial.selenic.com/wiki/Manifest says "Multiple changesets may refer to the same manifest revision". To me, each changeset 
changes repository, hence manifest should update nodeids of the files it lists, effectively creating new manifest revision.

? hg status, compare revision and local file with kw expansion and eol extension
? subrepos in log, status (-S) and manifest commands

Commands to get CommandContext where they may share various caches (e.g. StatusCollector)

>>>> Effective file read/data access
ReadOperation, Revlog does: repo.getFileSystem().run(this.file, new ReadOperation(), long start=0, long end = -1)
ReadOperation gets buffer (of whatever size, as decided by FS impl), parses it and then  reports if needs more data.
This helps to ensure streams are closed after reading, allows caching (if the same file (or LRU) is read few times in sequence)
and allows buffer management (i.e. reuse. Single buffer for all reads). 
Scheduling multiple operations (in future, to deal with writes - single queue for FS operations - no locks?)

WRITE: Need to register instances that cache files (e.g. dirstate or .hgignore) to FS notifier, so that cache may get cleared if the file changes (i.e. WriteOperation touches it).  

File access:
* NIO and mapped files - should be fast. Although seems to give less control on mem usage. 
* Regular InputStreams and chunked stream on top - allocate List<byte[]>, each (but last) chunk of fixed size (depending on initial file size) 

<<<<<

Tests:
DataAccess - readBytes(length > memBufferSize, length*2 > memBufferSize) - to check impl is capable to read huge chunks of data, regardless of own buffer size

ExecHelper('cmd', OutputParser()).run(). StatusOutputParser, LogOutputParser extends OutputParser. construct java result similar to that of cmd, compare results