D3990: linelog: add a Python implementation of the linelog datastructure
durin42 (Augie Fackler)
phabricator at mercurial-scm.org
Wed Aug 1 18:47:23 EDT 2018
durin42 updated this revision to Diff 9759.
durin42 marked 4 inline comments as done.
REPOSITORY
rHG Mercurial
CHANGES SINCE LAST UPDATE
https://phab.mercurial-scm.org/D3990?vs=9681&id=9759
REVISION DETAIL
https://phab.mercurial-scm.org/D3990
AFFECTED FILES
contrib/wix/help.wxs
mercurial/help/internals/linelog.txt
mercurial/linelog.py
tests/test-linelog.py
CHANGE DETAILS
diff --git a/tests/test-linelog.py b/tests/test-linelog.py
new file mode 100644
--- /dev/null
+++ b/tests/test-linelog.py
@@ -0,0 +1,173 @@
+from __future__ import absolute_import, print_function
+
+import difflib
+import random
+import unittest
+
+from mercurial import linelog
+
+maxlinenum = 0xffffff
+maxb1 = 0xffffff
+maxdeltaa = 10
+maxdeltab = 10
+
+def _genedits(seed, endrev):
+ lines = []
+ random.seed(seed)
+ rev = 0
+ for rev in range(0, endrev):
+ n = len(lines)
+ a1 = random.randint(0, n)
+ a2 = random.randint(a1, min(n, a1 + maxdeltaa))
+ b1 = random.randint(0, maxb1)
+ b2 = random.randint(b1, b1 + maxdeltab)
+ blines = [(rev, idx) for idx in range(b1, b2)]
+ lines[a1:a2] = blines
+ yield lines, rev, a1, a2, b1, b2
+
+class linelogtests(unittest.TestCase):
+ def testlinelogencodedecode(self):
+ program = [linelog._eof(0, 0),
+ linelog._jge(41, 42),
+ linelog._jump(0, 43),
+ linelog._eof(0, 0),
+ linelog._jl(44, 45),
+ linelog._line(46, 47),
+ ]
+ ll = linelog.linelog(program, maxrev=100)
+ enc = ll.encode()
+ # round-trips okay
+ self.assertEqual(linelog.linelog.fromdata(enc)._program, ll._program)
+ self.assertEqual(linelog.linelog.fromdata(enc), ll)
+ # This encoding matches the encoding used by hg-experimental's
+ # linelog file, or is supposed to if it doesn't.
+ self.assertEqual(enc, ('\x00\x00\x01\x90\x00\x00\x00\x06'
+ '\x00\x00\x00\xa4\x00\x00\x00*'
+ '\x00\x00\x00\x00\x00\x00\x00+'
+ '\x00\x00\x00\x00\x00\x00\x00\x00'
+ '\x00\x00\x00\xb1\x00\x00\x00-'
+ '\x00\x00\x00\xba\x00\x00\x00/'))
+
+ def testsimpleedits(self):
+ ll = linelog.linelog()
+ # Initial revision: add lines 0, 1, and 2
+ ll.replacelines(1, 0, 0, 0, 3)
+ self.assertEqual([(l.rev, l.linenum) for l in ll.annotate(1)],
+ [(1, 0),
+ (1, 1),
+ (1, 2),
+ ])
+ # Replace line 1 with a new line
+ ll.replacelines(2, 1, 2, 1, 2)
+ self.assertEqual([(l.rev, l.linenum) for l in ll.annotate(2)],
+ [(1, 0),
+ (2, 1),
+ (1, 2),
+ ])
+ # delete a line out of 2
+ ll.replacelines(3, 1, 2, 0, 0)
+ self.assertEqual([(l.rev, l.linenum) for l in ll.annotate(3)],
+ [(1, 0),
+ (1, 2),
+ ])
+ # annotation of 1 is unchanged
+ self.assertEqual([(l.rev, l.linenum) for l in ll.annotate(1)],
+ [(1, 0),
+ (1, 1),
+ (1, 2),
+ ])
+ ll.annotate(3) # set internal state to revision 3
+ start = ll.getoffset(0)
+ end = ll.getoffset(1)
+ self.assertEqual(ll.getalllines(start, end), [
+ (1, 0),
+ (2, 1),
+ (1, 1),
+ ])
+ self.assertEqual(ll.getalllines(), [
+ (1, 0),
+ (2, 1),
+ (1, 1),
+ (1, 2),
+ ])
+
+ def testparseclinelogfile(self):
+ # This data is what the replacements in testsimpleedits
+ # produce when fed to the original linelog.c implementation.
+ data = ('\x00\x00\x00\x0c\x00\x00\x00\x0f'
+ '\x00\x00\x00\x00\x00\x00\x00\x02'
+ '\x00\x00\x00\x05\x00\x00\x00\x06'
+ '\x00\x00\x00\x06\x00\x00\x00\x00'
+ '\x00\x00\x00\x00\x00\x00\x00\x07'
+ '\x00\x00\x00\x06\x00\x00\x00\x02'
+ '\x00\x00\x00\x00\x00\x00\x00\x00'
+ '\x00\x00\x00\t\x00\x00\x00\t'
+ '\x00\x00\x00\x00\x00\x00\x00\x0c'
+ '\x00\x00\x00\x08\x00\x00\x00\x05'
+ '\x00\x00\x00\x06\x00\x00\x00\x01'
+ '\x00\x00\x00\x00\x00\x00\x00\x05'
+ '\x00\x00\x00\x0c\x00\x00\x00\x05'
+ '\x00\x00\x00\n\x00\x00\x00\x01'
+ '\x00\x00\x00\x00\x00\x00\x00\t')
+ llc = linelog.linelog.fromdata(data)
+ self.assertEqual([(l.rev, l.linenum) for l in llc.annotate(1)],
+ [(1, 0),
+ (1, 1),
+ (1, 2),
+ ])
+ self.assertEqual([(l.rev, l.linenum) for l in llc.annotate(2)],
+ [(1, 0),
+ (2, 1),
+ (1, 2),
+ ])
+ self.assertEqual([(l.rev, l.linenum) for l in llc.annotate(3)],
+ [(1, 0),
+ (1, 2),
+ ])
+ # Check we emit the same bytecode.
+ ll = linelog.linelog()
+ # Initial revision: add lines 0, 1, and 2
+ ll.replacelines(1, 0, 0, 0, 3)
+ # Replace line 1 with a new line
+ ll.replacelines(2, 1, 2, 1, 2)
+ # delete a line out of 2
+ ll.replacelines(3, 1, 2, 0, 0)
+ diff = '\n ' + '\n '.join(difflib.unified_diff(
+ ll.debugstr().splitlines(), llc.debugstr().splitlines(),
+ 'python', 'c', lineterm=''))
+ self.assertEqual(ll._program, llc._program, 'Program mismatch: ' + diff)
+ # Done as a secondary step so we get a better result if the
+ # program is where the mismatch is.
+ self.assertEqual(ll, llc)
+ self.assertEqual(ll.encode(), data)
+
+ def testanothersimplecase(self):
+ ll = linelog.linelog()
+ ll.replacelines(3, 0, 0, 0, 2)
+ ll.replacelines(4, 0, 2, 0, 0)
+ self.assertEqual([(l.rev, l.linenum) for l in ll.annotate(4)],
+ [])
+ self.assertEqual([(l.rev, l.linenum) for l in ll.annotate(3)],
+ [(3, 0), (3, 1)])
+ # rev 2 is empty because contents were only ever introduced in rev 3
+ self.assertEqual([(l.rev, l.linenum) for l in ll.annotate(2)],
+ [])
+
+ def testrandomedits(self):
+ # Inspired by original linelog tests.
+ seed = random.random()
+ numrevs = 2000
+ ll = linelog.linelog()
+ # Populate linelog
+ for lines, rev, a1, a2, b1, b2 in _genedits(seed, numrevs):
+ ll.replacelines(rev, a1, a2, b1, b2)
+ ar = ll.annotate(rev)
+ self.assertEqual(ll.annotateresult, lines)
+ # Verify we can get back these states by annotating each rev
+ for lines, rev, a1, a2, b1, b2 in _genedits(seed, numrevs):
+ ar = ll.annotate(rev)
+ self.assertEqual([(l.rev, l.linenum) for l in ar], lines)
+
+if __name__ == '__main__':
+ import silenttestrunner
+ silenttestrunner.main(__name__)
diff --git a/mercurial/linelog.py b/mercurial/linelog.py
new file mode 100644
--- /dev/null
+++ b/mercurial/linelog.py
@@ -0,0 +1,414 @@
+# linelog - efficient cache for annotate data
+#
+# Copyright 2018 Google LLC.
+#
+# This software may be used and distributed according to the terms of the
+# GNU General Public License version 2 or any later version.
+"""linelog is an efficient cache for annotate data inspired by SCCS Weaves.
+
+SCCS Weaves are an implementation of
+https://en.wikipedia.org/wiki/Interleaved_deltas. See
+mercurial/help/internals/linelog.txt for an exploration of SCCS weaves
+and how linelog works in detail.
+
+Here's a hacker's summary: a linelog is a program which is executed in
+the context of a revision. Executing the program emits information
+about lines, including the revision that introduced them and the line
+number in the file at the introducing revision. When an insertion or
+deletion is performed on the file, a jump instruction is used to patch
+in a new body of annotate information.
+"""
+from __future__ import absolute_import, print_function
+
+import abc
+import struct
+
+from mercurial import (
+ pycompat,
+)
+from .thirdparty import (
+ attr,
+)
+
+_llentry = struct.Struct('>II')
+
+class LineLogError(Exception):
+ """Error raised when something bad happens internally in linelog."""
+
+ at attr.s
+class lineinfo(object):
+ # Introducing revision of this line.
+ rev = attr.ib()
+ # Line number for this line in its introducing revision.
+ linenum = attr.ib()
+ # Private. Offset in the linelog program of this line. Used internally.
+ _offset = attr.ib()
+
+ at attr.s
+class annotateresult(object):
+ rev = attr.ib()
+ lines = attr.ib()
+ _eof = attr.ib()
+
+ def __iter__(self):
+ return iter(self.lines)
+
+class _llinstruction(object):
+
+ __metaclass__ = abc.ABCMeta
+
+ @abc.abstractmethod
+ def __init__(self, op1, op2):
+ pass
+
+ @abc.abstractmethod
+ def __str__(self):
+ pass
+
+ def __repr__(self):
+ return str(self)
+
+ @abc.abstractmethod
+ def __eq__(self, other):
+ pass
+
+ @abc.abstractmethod
+ def encode(self):
+ """Encode this instruction to the binary linelog format."""
+
+ @abc.abstractmethod
+ def execute(self, rev, pc, emit):
+ """Execute this instruction.
+
+ Args:
+ rev: The revision we're annotating.
+ pc: The current offset in the linelog program.
+ emit: A function that accepts a single lineinfo object.
+
+ Returns:
+ The new value of pc. Returns None if exeuction should stop
+ (that is, we've found the end of the file.)
+ """
+
+class _jge(_llinstruction):
+ """If the current rev is greater than or equal to op1, jump to op2."""
+
+ def __init__(self, op1, op2):
+ self._cmprev = op1
+ self._target = op2
+
+ def __str__(self):
+ return 'JGE %d %d' % (self._cmprev, self._target)
+
+ def __eq__(self, other):
+ return (type(self) == type(other)
+ and self._cmprev == other._cmprev
+ and self._target == other._target)
+
+ def encode(self):
+ return _llentry.pack(self._cmprev << 2, self._target)
+
+ def execute(self, rev, pc, emit):
+ if rev >= self._cmprev:
+ return self._target
+ return pc + 1
+
+class _jump(_llinstruction):
+ """Unconditional jumps are expressed as a JGE with op1 set to 0."""
+
+ def __init__(self, op1, op2):
+ if op1 != 0:
+ raise LineLogError("malformed JUMP, op1 must be 0, got %d" % op1)
+ self._target = op2
+
+ def __str__(self):
+ return 'JUMP %d' % (self._target)
+
+ def __eq__(self, other):
+ return (type(self) == type(other)
+ and self._target == other._target)
+
+ def encode(self):
+ return _llentry.pack(0, self._target)
+
+ def execute(self, rev, pc, emit):
+ return self._target
+
+class _eof(_llinstruction):
+ """EOF is expressed as a JGE that always jumps to 0."""
+
+ def __init__(self, op1, op2):
+ if op1 != 0:
+ raise LineLogError("malformed EOF, op1 must be 0, got %d" % op1)
+ if op2 != 0:
+ raise LineLogError("malformed EOF, op2 must be 0, got %d" % op2)
+
+ def __str__(self):
+ return 'EOF'
+
+ def __eq__(self, other):
+ return type(self) == type(other)
+
+ def encode(self):
+ return _llentry.pack(0, 0)
+
+ def execute(self, rev, pc, emit):
+ return None
+
+class _jl(_llinstruction):
+ """If the current rev is less than op1, jump to op2."""
+
+ def __init__(self, op1, op2):
+ self._cmprev = op1
+ self._target = op2
+
+ def __str__(self):
+ return 'JL %d %d' % (self._cmprev, self._target)
+
+ def __eq__(self, other):
+ return (type(self) == type(other)
+ and self._cmprev == other._cmprev
+ and self._target == other._target)
+
+ def encode(self):
+ return _llentry.pack(1 | (self._cmprev << 2), self._target)
+
+ def execute(self, rev, pc, emit):
+ if rev < self._cmprev:
+ return self._target
+ return pc + 1
+
+class _line(_llinstruction):
+ """Emit a line."""
+
+ def __init__(self, op1, op2):
+ # This line was introduced by this revision number.
+ self._rev = op1
+ # This line had the specified line number in the introducing revision.
+ self._origlineno = op2
+
+ def __str__(self):
+ return 'LINE %d %d' % (self._rev, self._origlineno)
+
+ def __eq__(self, other):
+ return (type(self) == type(other)
+ and self._rev == other._rev
+ and self._origlineno == other._origlineno)
+
+ def encode(self):
+ return _llentry.pack(2 | (self._rev << 2), self._origlineno)
+
+ def execute(self, rev, pc, emit):
+ emit(lineinfo(self._rev, self._origlineno, pc))
+ return pc + 1
+
+def _decodeone(data, offset):
+ """Decode a single linelog instruction from an offset in a buffer."""
+ try:
+ op1, op2 = _llentry.unpack_from(data, offset)
+ except struct.error as e:
+ raise LineLogError('reading an instruction failed: %r' % e)
+ opcode = op1 & 0b11
+ op1 = op1 >> 2
+ if opcode == 0:
+ if op1 == 0:
+ if op2 == 0:
+ return _eof(op1, op2)
+ return _jump(op1, op2)
+ return _jge(op1, op2)
+ elif opcode == 1:
+ return _jl(op1, op2)
+ elif opcode == 2:
+ return _line(op1, op2)
+ raise NotImplementedError('Unimplemented opcode %r' % opcode)
+
+class linelog(object):
+ """Efficient cache for per-line history information."""
+
+ def __init__(self, program=None, maxrev=0):
+ if program is None:
+ # We pad the program with an extra leading EOF so that our
+ # offsets will match the C code exactly. This means we can
+ # interoperate with the C code.
+ program = [_eof(0, 0), _eof(0, 0)]
+ self._program = program
+ self._lastannotate = None
+ self._maxrev = maxrev
+
+ def __eq__(self, other):
+ return (type(self) == type(other)
+ and self._program == other._program
+ and self._maxrev == other._maxrev)
+
+ def __repr__(self):
+ return '<linelog at %s: maxrev=%d size=%d>' % (
+ hex(id(self)), self._maxrev, len(self._program))
+
+ def debugstr(self):
+ fmt = '%%%dd %%s' % len(str(len(self._program)))
+ return '\n'.join(
+ fmt % (idx, i) for idx, i in enumerate(self._program[1:], 1))
+
+ @classmethod
+ def fromdata(cls, buf):
+ if len(buf) % _llentry.size != 0:
+ raise LineLogError(
+ "invalid linelog buffer size %d (must be a multiple of %d)" % (
+ len(buf), _llentry.size))
+ expected = len(buf) / _llentry.size
+ fakejge = _decodeone(buf, 0)
+ if isinstance(fakejge, _jump):
+ maxrev = 0
+ else:
+ maxrev = fakejge._cmprev
+ numentries = fakejge._target
+ if expected != numentries:
+ raise LineLogError("corrupt linelog data: claimed"
+ " %d entries but given data for %d entries" % (
+ expected, numentries))
+ instructions = [_eof(0, 0)]
+ for offset in pycompat.xrange(1, numentries):
+ instructions.append(_decodeone(buf, offset * _llentry.size))
+ return cls(instructions, maxrev=maxrev)
+
+ def encode(self):
+ hdr = _jge(self._maxrev, len(self._program)).encode()
+ return hdr + ''.join(i.encode() for i in self._program[1:])
+
+ def clear(self):
+ self._program = []
+ self._maxrev = 0
+ self._lastannotate = None
+
+ def replacelines(self, rev, a1, a2, b1, b2):
+ """Replace lines [a1, a2) with lines [b1, b2)."""
+ if self._lastannotate:
+ # TODO(augie): make replacelines() accept a revision at
+ # which we're editing as well as a revision to mark
+ # responsible for the edits. In hg-experimental it's
+ # stateful like this, so we're doing the same thing to
+ # retain compatibility with absorb until that's imported.
+ ar = self._lastannotate
+ else:
+ ar = self.annotate(rev)
+ # ar = self.annotate(self._maxrev)
+ if a1 > len(ar.lines):
+ raise LineLogError(
+ '%d contains %d lines, tried to access line %d' % (
+ rev, len(ar.lines), a1))
+ elif a1 == len(ar.lines):
+ # Simulated EOF instruction since we're at EOF, which
+ # doesn't have a "real" line.
+ a1inst = _eof(0, 0)
+ a1info = lineinfo(0, 0, ar._eof)
+ else:
+ a1info = ar.lines[a1]
+ a1inst = self._program[a1info._offset]
+ oldproglen = len(self._program)
+ appendinst = self._program.append
+
+ # insert
+ if b1 < b2:
+ # Determine the jump target for the JGE at the start of
+ # the new block.
+ tgt = oldproglen + (b2 - b1 + 1)
+ # Jump to skip the insert if we're at an older revision.
+ appendinst(_jl(rev, tgt))
+ for linenum in pycompat.xrange(b1, b2):
+ appendinst(_line(rev, linenum))
+ # delete
+ if a1 < a2:
+ if a2 > len(ar.lines):
+ raise LineLogError(
+ '%d contains %d lines, tried to access line %d' % (
+ rev, len(ar.lines), a2))
+ elif a2 == len(ar.lines):
+ endaddr = ar._eof
+ else:
+ endaddr = ar.lines[a2]._offset
+ if a2 > 0 and rev < self._maxrev:
+ # If we're here, we're deleting a chunk of an old
+ # commit, so we need to be careful and not touch
+ # invisible lines between a2-1 and a2 (IOW, lines that
+ # are added later).
+ endaddr = ar.lines[a2 - 1]._offset + 1
+ appendinst(_jge(rev, endaddr))
+ # copy instruction from a1
+ appendinst(a1inst)
+ # if a1inst isn't a jump or EOF, then we need to add an unconditional
+ # jump back into the program here.
+ if not isinstance(a1inst, (_jump, _eof)):
+ appendinst(_jump(0, a1info._offset + 1))
+ # Patch instruction at a1, which makes our patch live.
+ self._program[a1info._offset] = _jump(0, oldproglen)
+ # For compat with the C version, re-annotate rev so that
+ # self.annotateresult is cromulent.. We could fix up the
+ # annotateresult in place (which is how the C version works),
+ # but for now we'll pass on that and see if it matters in
+ # practice.
+ self.annotate(max(self._lastannotate.rev, rev))
+ if rev > self._maxrev:
+ self._maxrev = rev
+
+ def annotate(self, rev):
+ pc = 1
+ lines = []
+ # Sanity check: if len(lines) is longer than len(program), we
+ # hit an infinite loop in the linelog program somehow and we
+ # should stop.
+ while pc is not None and len(lines) < len(self._program):
+ inst = self._program[pc]
+ lastpc = pc
+ pc = inst.execute(rev, pc, lines.append)
+ if pc is not None:
+ raise LineLogError(
+ 'Probably hit an infinite loop in linelog. Program:\n' +
+ self.debugstr())
+ ar = annotateresult(rev, lines, lastpc)
+ self._lastannotate = ar
+ return ar
+
+ @property
+ def maxrev(self):
+ return self._maxrev
+
+ # Stateful methods which depend on the value of the last
+ # annotation run. This API is for compatiblity with the original
+ # linelog, and we should probably consider refactoring it.
+ @property
+ def annotateresult(self):
+ """Return the last annotation result. C linelog code exposed this."""
+ return [(l.rev, l.linenum) for l in self._lastannotate.lines]
+
+ def getoffset(self, line):
+ return self._lastannotate.lines[line]._offset
+
+ def getalllines(self, start=0, end=0):
+ """Get all lines that ever occurred in [start, end).
+
+ Passing start == end == 0 means "all lines ever".
+
+ This works in terms of *internal* program offsets, not line numbers.
+ """
+ pc = start or 1
+ lines = []
+ # only take as many steps as there are instructions in the
+ # program - if we don't find an EOF or our stop-line before
+ # then, something is badly broken.
+ for step in pycompat.xrange(len(self._program)):
+ inst = self._program[pc]
+ nextpc = pc + 1
+ if isinstance(inst, _jump):
+ nextpc = inst._target
+ elif isinstance(inst, _eof):
+ return lines
+ elif isinstance(inst, (_jl, _jge)):
+ pass
+ elif isinstance(inst, _line):
+ lines.append((inst._rev, inst._origlineno))
+ else:
+ raise LineLogError("Illegal instruction %r" % inst)
+ if nextpc == end:
+ return lines
+ pc = nextpc
+ raise LineLogError("Failed to perform getalllines")
diff --git a/mercurial/help/internals/linelog.txt b/mercurial/help/internals/linelog.txt
new file mode 100644
--- /dev/null
+++ b/mercurial/help/internals/linelog.txt
@@ -0,0 +1,251 @@
+linelog is a storage format inspired by the "Interleaved deltas" idea. See
+https://en.wikipedia.org/wiki/Interleaved_deltas for its introduction.
+
+0. SCCS Weave
+
+ To understand what linelog is, first we have a quick look at a simplified
+ (with header removed) SCCS weave format, which is an implementation of the
+ "Interleaved deltas" idea.
+
+0.1 Basic SCCS Weave File Format
+
+ A SCCS weave file consists of plain text lines. Each line is either a
+ special instruction starting with "^A" or part of the content of the real
+ file the weave tracks. There are 3 important operations, where REV denotes
+ the revision number:
+
+ ^AI REV, marking the beginning of an insertion block introduced by REV
+ ^AD REV, marking the beginning of a deletion block introduced by REV
+ ^AE REV, marking the end of the block started by "^AI REV" or "^AD REV"
+
+ Note on revision numbers: For any two different revision numbers, one must
+ be an ancestor of the other to make them comparable. This enforces linear
+ history. Besides, the comparison functions (">=", "<") should be efficient.
+ This means, if revisions are strings like git or hg, an external map is
+ required to convert them into integers.
+
+ For example, to represent the following changes:
+
+ REV 1 | REV 2 | REV 3
+ ------+-------+-------
+ a | a | a
+ b | b | 2
+ c | 1 | c
+ | 2 |
+ | c |
+
+ A possible weave file looks like:
+
+ ^AI 1
+ a
+ ^AD 3
+ b
+ ^AI 2
+ 1
+ ^AE 3
+ 2
+ ^AE 2
+ c
+ ^AE 1
+
+ An "^AE" does not always match its nearest operation ("^AI" or "^AD"). In
+ the above example, "^AE 3" does not match the nearest "^AI 2" but "^AD 3".
+ Therefore we need some extra information for "^AE". The SCCS weave uses a
+ revision number. It could also be a boolean value about whether it is an
+ insertion or a deletion (see section 0.4).
+
+0.2 Checkout
+
+ The "checkout" operation is to retrieve file content at a given revision,
+ say X. It's doable by going through the file line by line and:
+
+ - If meet ^AI rev, and rev > X, find the corresponding ^AE and jump there
+ - If meet ^AD rev, and rev <= X, find the corresponding ^AE and jump there
+ - Ignore ^AE
+ - For normal lines, just output them
+
+0.3 Annotate
+
+ The "annotate" operation is to show extra metadata like the revision number
+ and the original line number a line comes from.
+
+ It's basically just a "Checkout". For the extra metadata, they can be stored
+ side by side with the line contents. Alternatively, we can infer the
+ revision number from "^AI"s.
+
+ Some SCM tools have to calculate diffs on the fly and thus are much slower
+ on this operation.
+
+0.4 Tree Structure
+
+ The word "interleaved" is used because "^AI" .. "^AE" and "^AD" .. "^AE"
+ blocks can be interleaved.
+
+ If we consider insertions and deletions separately, they can form tree
+ structures, respectively.
+
+ +--- ^AI 1 +--- ^AD 3
+ | +- ^AI 2 | +- ^AD 2
+ | | | |
+ | +- ^AE 2 | +- ^AE 2
+ | |
+ +--- ^AE 1 +--- ^AE 3
+
+ More specifically, it's possible to build a tree for all insertions, where
+ the tree node has the structure "(rev, startline, endline)". "startline" is
+ the line number of "^AI" and "endline" is the line number of the matched
+ "^AE". The tree will have these properties:
+
+ 1. child.rev > parent.rev
+ 2. child.startline > parent.startline
+ 3. child.endline < parent.endline
+
+ A similar tree for all deletions can also be built with the first property
+ changed to:
+
+ 1. child.rev < parent.rev
+
+0.5 Malformed Cases
+
+ The following cases are considered malformed in our implementation:
+
+ 1. Interleaved insertions, or interleaved deletions.
+ It can be rewritten to a non-interleaved tree structure.
+
+ ^AI/D x ^AI/D x
+ ^AI/D y -> ^AI/D y
+ ^AE x ^AE y
+ ^AE y ^AE x
+
+ 2. Nested insertions, where the inner one has a smaller revision number.
+ It can be rewritten to a non-nested form.
+
+ ^AI x + 1 ^AI x + 1
+ ^AI x -> ^AE x + 1
+ ^AE x ^AI x
+ ^AE x + 1 ^AE x
+
+ 3. Insertion or deletion inside another deletion, where the outer deletion
+ block has a smaller revision number.
+
+ ^AD x ^AD x
+ ^AI/D x + 1 -> ^AE x
+ ^AE x + 1 ^AI/D x + 1
+ ^AE x ^AE x
+
+ Some of them may be valid in other implementations for special purposes. For
+ example, to "revive" a previously deleted block in a newer revision.
+
+0.6 Cases Can Be Optimized
+
+ It's always better to get things nested. For example, the left is more
+ efficient than the right while they represent the same content:
+
+ +--- ^AD 2 +- ^AD 1
+ | +- ^AD 1 | LINE A
+ | | LINE A +- ^AE 1
+ | +- ^AE 1 +- ^AD 2
+ | LINE B | LINE B
+ +--- ^AE 2 +- ^AE 2
+
+ Our implementation sometimes generates the less efficient data. To always
+ get the optimal form, it requires extra code complexity that seems unworthy.
+
+0.7 Inefficiency
+
+ The file format can be slow because:
+
+ - Inserting a new line at position P requires rewriting all data after P.
+ - Finding "^AE" requires walking through the content (O(N), where N is the
+ number of lines between "^AI/D" and "^AE").
+
+1. Linelog
+
+ The linelog is a binary format that dedicates to speed up mercurial (or
+ git)'s "annotate" operation. It's designed to avoid issues mentioned in
+ section 0.7.
+
+1.1 Content Stored
+
+ Linelog is not another storage for file contents. It only stores line
+ numbers and corresponding revision numbers, instead of actual line content.
+ This is okay for the "annotate" operation because usually the external
+ source is fast to checkout the content of a file at a specific revision.
+
+ A typical SCCS weave is also fast on the "grep" operation, which needs
+ random accesses to line contents from different revisions of a file. This
+ can be slow with linelog's no-line-content design. However we could use
+ an extra map ((rev, line num) -> line content) to speed it up.
+
+ Note the revision numbers in linelog should be independent from mercurial
+ integer revision numbers. There should be some mapping between linelog rev
+ and hg hash stored side by side, to make the files reusable after being
+ copied to another machine.
+
+1.2 Basic Format
+
+ A linelog file consists of "instruction"s. An "instruction" can be either:
+
+ - JGE REV ADDR # jump to ADDR if rev >= REV
+ - JL REV ADDR # jump to ADDR if rev < REV
+ - LINE REV LINENUM # append the (LINENUM+1)-th line in revision REV
+
+ For example, here is the example linelog representing the same file with
+ 3 revisions mentioned in section 0.1:
+
+ SCCS | Linelog
+ Weave | Addr : Instruction
+ ------+------+-------------
+ ^AI 1 | 0 : JL 1 8
+ a | 1 : LINE 1 0
+ ^AD 3 | 2 : JGE 3 6
+ b | 3 : LINE 1 1
+ ^AI 2 | 4 : JL 2 7
+ 1 | 5 : LINE 2 2
+ ^AE 3 |
+ 2 | 6 : LINE 2 3
+ ^AE 2 |
+ c | 7 : LINE 1 2
+ ^AE 1 |
+ | 8 : END
+
+ This way, "find ^AE" is O(1) because we just jump there. And we can insert
+ new lines without rewriting most part of the file by appending new lines and
+ changing a single instruction to jump to them.
+
+ The current implementation uses 64 bits for an instruction: The opcode (JGE,
+ JL or LINE) takes 2 bits, REV takes 30 bits and ADDR or LINENUM takes 32
+ bits. It also stores the max revision number and buffer size at the first
+ 64 bits for quick access to these values.
+
+1.3 Comparing with Mercurial's revlog format
+
+ Apparently, linelog is very different from revlog: linelog stores rev and
+ line numbers, while revlog has line contents and other metadata (like
+ parents, flags). However, the revlog format could also be used to store rev
+ and line numbers. For example, to speed up the annotate operation, we could
+ also pre-calculate annotate results and just store them using the revlog
+ format.
+
+ Therefore, linelog is actually somehow similar to revlog, with the important
+ trade-off that it only supports linear history (mentioned in section 0.1).
+ Essentially, the differences are:
+
+ a) Linelog is full of deltas, while revlog could contain full file
+ contents sometimes. So linelog is smaller. Revlog could trade
+ reconstruction speed for file size - best case, revlog is as small as
+ linelog.
+ b) The interleaved delta structure allows skipping large portion of
+ uninteresting deltas so linelog's content reconstruction is faster than
+ the delta-only version of revlog (however it's possible to construct
+ a case where interleaved deltas degrade to plain deltas, so linelog
+ worst case would be delta-only revlog). Revlog could trade file size
+ for reconstruction speed.
+ c) Linelog implicitly maintains the order of all lines it stores. So it
+ could dump all the lines from all revisions, with a reasonable order.
+ While revlog could also dump all line additions, it requires extra
+ computation to figure out the order putting those lines - that's some
+ kind of "merge".
+
+ "c" makes "hg absorb" easier to implement and makes it possible to do
+ "annotate --deleted".
diff --git a/contrib/wix/help.wxs b/contrib/wix/help.wxs
--- a/contrib/wix/help.wxs
+++ b/contrib/wix/help.wxs
@@ -46,6 +46,7 @@
<File Id="internals.censor.txt" Name="censor.txt" />
<File Id="internals.changegroups.txt" Name="changegroups.txt" />
<File Id="internals.config.txt" Name="config.txt" />
+ <File Id="internals.linelog.txt" Name="linelog.txt" />
<File Id="internals.requirements.txt" Name="requirements.txt" />
<File Id="internals.revlogs.txt" Name="revlogs.txt" />
<File Id="internals.wireprotocol.txt" Name="wireprotocol.txt" />
To: durin42, #hg-reviewers, indygreg
Cc: indygreg, mercurial-devel
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