mercurial/pathencode.c

bdiff: early pruning of common prefix before doing expensive computations

It seems quite common that files don't change completely. New lines are often
pretty much appended, and modifications will often only change a small section
of the file which on average will be in the middle.

There can thus be a big win by pruning a common prefix before starting the more
expensive search for longest common substrings.

Worst case, it will scan through a long sequence of similar bytes without
encountering a newline. Splitlines will then have to do the same again ...
twice for each side. If similar lines are found, splitlines will save the
double iteration and hashing of the lines ... plus there will be less lines to
find common substrings in.

This change might in some cases make the algorith pick shorter or less optimal
common substrings. We can't have the cake and eat it.

This make hg --time bundle --base null -r 4.0 go from 14.5 to 15 s - a 3%
increase.

On mozilla-unified:
perfbdiff -m 3041e4d59df2
! wall 0.053088 comb 0.060000 user 0.060000 sys 0.000000 (best of 100) to
! wall 0.024618 comb 0.020000 user 0.020000 sys 0.000000 (best of 116)
perfbdiff 0e9928989e9c --alldata --count 10
! wall 0.702075 comb 0.700000 user 0.700000 sys 0.000000 (best of 15) to
! wall 0.579235 comb 0.580000 user 0.580000 sys 0.000000 (best of 18)

/*
 pathencode.c - efficient path name encoding

 Copyright 2012 Facebook

 This software may be used and distributed according to the terms of
 the GNU General Public License, incorporated herein by reference.
*/

/*
 * An implementation of the name encoding scheme used by the fncache
 * store.  The common case is of a path < 120 bytes long, which is
 * handled either in a single pass with no allocations or two passes
 * with a single allocation.  For longer paths, multiple passes are
 * required.
 */

#define PY_SSIZE_T_CLEAN
#include <Python.h>
#include <assert.h>
#include <ctype.h>
#include <stdlib.h>
#include <string.h>

#include "util.h"

/* state machine for the fast path */
enum path_state {
	START,   /* first byte of a path component */
	A,       /* "AUX" */
	AU,
	THIRD,   /* third of a 3-byte sequence, e.g. "AUX", "NUL" */
	C,       /* "CON" or "COMn" */
	CO,
	COMLPT,  /* "COM" or "LPT" */
	COMLPTn,
	L,
	LP,
	N,
	NU,
	P,       /* "PRN" */
	PR,
	LDOT,    /* leading '.' */
	DOT,     /* '.' in a non-leading position */
	H,       /* ".h" */
	HGDI,    /* ".hg", ".d", or ".i" */
	SPACE,
	DEFAULT  /* byte of a path component after the first */
};

/* state machine for dir-encoding */
enum dir_state {
	DDOT,
	DH,
	DHGDI,
	DDEFAULT
};

static inline int inset(const uint32_t bitset[], char c)
{
	return bitset[((uint8_t)c) >> 5] & (1 << (((uint8_t)c) & 31));
}

static inline void charcopy(char *dest, Py_ssize_t *destlen, size_t destsize,
                            char c)
{
	if (dest) {
		assert(*destlen < destsize);
		dest[*destlen] = c;
	}
	(*destlen)++;
}

static inline void memcopy(char *dest, Py_ssize_t *destlen, size_t destsize,
                           const void *src, Py_ssize_t len)
{
	if (dest) {
		assert(*destlen + len < destsize);
		memcpy((void *)&dest[*destlen], src, len);
	}
	*destlen += len;
}

static inline void hexencode(char *dest, Py_ssize_t *destlen, size_t destsize,
			     uint8_t c)
{
	static const char hexdigit[] = "0123456789abcdef";

	charcopy(dest, destlen, destsize, hexdigit[c >> 4]);
	charcopy(dest, destlen, destsize, hexdigit[c & 15]);
}

/* 3-byte escape: tilde followed by two hex digits */
static inline void escape3(char *dest, Py_ssize_t *destlen, size_t destsize,
			   char c)
{
	charcopy(dest, destlen, destsize, '~');
	hexencode(dest, destlen, destsize, c);
}

static Py_ssize_t _encodedir(char *dest, size_t destsize,
                             const char *src, Py_ssize_t len)
{
	enum dir_state state = DDEFAULT;
	Py_ssize_t i = 0, destlen = 0;

	while (i < len) {
		switch (state) {
		case DDOT:
			switch (src[i]) {
			case 'd':
			case 'i':
				state = DHGDI;
				charcopy(dest, &destlen, destsize, src[i++]);
				break;
			case 'h':
				state = DH;
				charcopy(dest, &destlen, destsize, src[i++]);
				break;
			default:
				state = DDEFAULT;
				break;
			}
			break;
		case DH:
			if (src[i] == 'g') {
				state = DHGDI;
				charcopy(dest, &destlen, destsize, src[i++]);
			}
			else state = DDEFAULT;
			break;
		case DHGDI:
			if (src[i] == '/') {
				memcopy(dest, &destlen, destsize, ".hg", 3);
				charcopy(dest, &destlen, destsize, src[i++]);
			}
			state = DDEFAULT;
			break;
		case DDEFAULT:
			if (src[i] == '.')
				state = DDOT;
			charcopy(dest, &destlen, destsize, src[i++]);
			break;
		}
	}

	return destlen;
}

PyObject *encodedir(PyObject *self, PyObject *args)
{
	Py_ssize_t len, newlen;
	PyObject *pathobj, *newobj;
	char *path;

	if (!PyArg_ParseTuple(args, "O:encodedir", &pathobj))
		return NULL;

	if (PyBytes_AsStringAndSize(pathobj, &path, &len) == -1) {
		PyErr_SetString(PyExc_TypeError, "expected a string");
		return NULL;
	}

	newlen = len ? _encodedir(NULL, 0, path, len + 1) : 1;

	if (newlen == len + 1) {
		Py_INCREF(pathobj);
		return pathobj;
	}

	newobj = PyBytes_FromStringAndSize(NULL, newlen);

	if (newobj) {
		assert(PyBytes_Check(newobj));
		Py_SIZE(newobj)--;
		_encodedir(PyBytes_AS_STRING(newobj), newlen, path,
			   len + 1);
	}

	return newobj;
}

static Py_ssize_t _encode(const uint32_t twobytes[8], const uint32_t onebyte[8],
			  char *dest, Py_ssize_t destlen, size_t destsize,
			  const char *src, Py_ssize_t len,
			  int encodedir)
{
	enum path_state state = START;
	Py_ssize_t i = 0;

	/*
	 * Python strings end with a zero byte, which we use as a
	 * terminal token as they are not valid inside path names.
	 */

	while (i < len) {
		switch (state) {
		case START:
			switch (src[i]) {
			case '/':
				charcopy(dest, &destlen, destsize, src[i++]);
				break;
			case '.':
				state = LDOT;
				escape3(dest, &destlen, destsize, src[i++]);
				break;
			case ' ':
				state = DEFAULT;
				escape3(dest, &destlen, destsize, src[i++]);
				break;
			case 'a':
				state = A;
				charcopy(dest, &destlen, destsize, src[i++]);
				break;
			case 'c':
				state = C;
				charcopy(dest, &destlen, destsize, src[i++]);
				break;
			case 'l':
				state = L;
				charcopy(dest, &destlen, destsize, src[i++]);
				break;
			case 'n':
				state = N;
				charcopy(dest, &destlen, destsize, src[i++]);
				break;
			case 'p':
				state = P;
				charcopy(dest, &destlen, destsize, src[i++]);
				break;
			default:
				state = DEFAULT;
				break;
			}
			break;
		case A:
			if (src[i] == 'u') {
				state = AU;
				charcopy(dest, &destlen, destsize, src[i++]);
			}
			else state = DEFAULT;
			break;
		case AU:
			if (src[i] == 'x') {
				state = THIRD;
				i++;
			}
			else state = DEFAULT;
			break;
		case THIRD:
			state = DEFAULT;
			switch (src[i]) {
			case '.':
			case '/':
			case '\0':
				escape3(dest, &destlen, destsize, src[i - 1]);
				break;
			default:
				i--;
				break;
			}
			break;
		case C:
			if (src[i] == 'o') {
				state = CO;
				charcopy(dest, &destlen, destsize, src[i++]);
			}
			else state = DEFAULT;
			break;
		case CO:
			if (src[i] == 'm') {
				state = COMLPT;
				i++;
			}
			else if (src[i] == 'n') {
				state = THIRD;
				i++;
			}
			else state = DEFAULT;
			break;
		case COMLPT:
			switch (src[i]) {
			case '1': case '2': case '3': case '4': case '5':
			case '6': case '7': case '8': case '9':
				state = COMLPTn;
				i++;
				break;
			default:
				state = DEFAULT;
				charcopy(dest, &destlen, destsize, src[i - 1]);
				break;
			}
			break;
		case COMLPTn:
			state = DEFAULT;
			switch (src[i]) {
			case '.':
			case '/':
			case '\0':
				escape3(dest, &destlen, destsize, src[i - 2]);
				charcopy(dest, &destlen, destsize, src[i - 1]);
				break;
			default:
				memcopy(dest, &destlen, destsize,
					&src[i - 2], 2);
				break;
			}
			break;
		case L:
			if (src[i] == 'p') {
				state = LP;
				charcopy(dest, &destlen, destsize, src[i++]);
			}
			else state = DEFAULT;
			break;
		case LP:
			if (src[i] == 't') {
				state = COMLPT;
				i++;
			}
			else state = DEFAULT;
			break;
		case N:
			if (src[i] == 'u') {
				state = NU;
				charcopy(dest, &destlen, destsize, src[i++]);
			}
			else state = DEFAULT;
			break;
		case NU:
			if (src[i] == 'l') {
				state = THIRD;
				i++;
			}
			else state = DEFAULT;
			break;
		case P:
			if (src[i] == 'r') {
				state = PR;
				charcopy(dest, &destlen, destsize, src[i++]);
			}
			else state = DEFAULT;
			break;
		case PR:
			if (src[i] == 'n') {
				state = THIRD;
				i++;
			}
			else state = DEFAULT;
			break;
		case LDOT:
			switch (src[i]) {
			case 'd':
			case 'i':
				state = HGDI;
				charcopy(dest, &destlen, destsize, src[i++]);
				break;
			case 'h':
				state = H;
				charcopy(dest, &destlen, destsize, src[i++]);
				break;
			default:
				state = DEFAULT;
				break;
			}
			break;
		case DOT:
			switch (src[i]) {
			case '/':
			case '\0':
				state = START;
				memcopy(dest, &destlen, destsize, "~2e", 3);
				charcopy(dest, &destlen, destsize, src[i++]);
				break;
			case 'd':
			case 'i':
				state = HGDI;
				charcopy(dest, &destlen, destsize, '.');
				charcopy(dest, &destlen, destsize, src[i++]);
				break;
			case 'h':
				state = H;
				memcopy(dest, &destlen, destsize, ".h", 2);
				i++;
				break;
			default:
				state = DEFAULT;
				charcopy(dest, &destlen, destsize, '.');
				break;
			}
			break;
		case H:
			if (src[i] == 'g') {
				state = HGDI;
				charcopy(dest, &destlen, destsize, src[i++]);
			}
			else state = DEFAULT;
			break;
		case HGDI:
			if (src[i] == '/') {
				state = START;
				if (encodedir)
					memcopy(dest, &destlen, destsize, ".hg",
						3);
				charcopy(dest, &destlen, destsize, src[i++]);
			}
			else state = DEFAULT;
			break;
		case SPACE:
			switch (src[i]) {
			case '/':
			case '\0':
				state = START;
				memcopy(dest, &destlen, destsize, "~20", 3);
				charcopy(dest, &destlen, destsize, src[i++]);
				break;
			default:
				state = DEFAULT;
				charcopy(dest, &destlen, destsize, ' ');
				break;
			}
			break;
		case DEFAULT:
			while (inset(onebyte, src[i])) {
				charcopy(dest, &destlen, destsize, src[i++]);
				if (i == len)
					goto done;
			}
			switch (src[i]) {
			case '.':
				state = DOT;
				i++;
				break;
			case ' ':
				state = SPACE;
				i++;
				break;
			case '/':
				state = START;
				charcopy(dest, &destlen, destsize, '/');
				i++;
				break;
			default:
				if (inset(onebyte, src[i])) {
					do {
						charcopy(dest, &destlen,
							 destsize, src[i++]);
					} while (i < len &&
						 inset(onebyte, src[i]));
				}
				else if (inset(twobytes, src[i])) {
					char c = src[i++];
					charcopy(dest, &destlen, destsize, '_');
					charcopy(dest, &destlen, destsize,
						 c == '_' ? '_' : c + 32);
				}
				else
					escape3(dest, &destlen, destsize,
						src[i++]);
				break;
			}
			break;
		}
	}
done:
	return destlen;
}

static Py_ssize_t basicencode(char *dest, size_t destsize,
			      const char *src, Py_ssize_t len)
{
	static const uint32_t twobytes[8] = { 0, 0, 0x87fffffe };

	static const uint32_t onebyte[8] = {
		1, 0x2bff3bfa, 0x68000001, 0x2fffffff,
	};

	Py_ssize_t destlen = 0;

	return _encode(twobytes, onebyte, dest, destlen, destsize,
		       src, len, 1);
}

static const Py_ssize_t maxstorepathlen = 120;

static Py_ssize_t _lowerencode(char *dest, size_t destsize,
			       const char *src, Py_ssize_t len)
{
	static const uint32_t onebyte[8] = {
		1, 0x2bfffbfb, 0xe8000001, 0x2fffffff
	};

	static const uint32_t lower[8] = { 0, 0, 0x7fffffe };

	Py_ssize_t i, destlen = 0;

	for (i = 0; i < len; i++) {
		if (inset(onebyte, src[i]))
			charcopy(dest, &destlen, destsize, src[i]);
		else if (inset(lower, src[i]))
			charcopy(dest, &destlen, destsize, src[i] + 32);
		else
			escape3(dest, &destlen, destsize, src[i]);
	}

	return destlen;
}

PyObject *lowerencode(PyObject *self, PyObject *args)
{
	char *path;
	Py_ssize_t len, newlen;
	PyObject *ret;

	if (!PyArg_ParseTuple(args, "s#:lowerencode", &path, &len))
		return NULL;

	newlen = _lowerencode(NULL, 0, path, len);
	ret = PyBytes_FromStringAndSize(NULL, newlen);
	if (ret)
		_lowerencode(PyBytes_AS_STRING(ret), newlen, path, len);

	return ret;
}

/* See store.py:_auxencode for a description. */
static Py_ssize_t auxencode(char *dest, size_t destsize,
			    const char *src, Py_ssize_t len)
{
	static const uint32_t twobytes[8];

	static const uint32_t onebyte[8] = {
		~0U, 0xffff3ffe, ~0U, ~0U, ~0U, ~0U, ~0U, ~0U,
	};

	return _encode(twobytes, onebyte, dest, 0, destsize, src, len, 0);
}

static PyObject *hashmangle(const char *src, Py_ssize_t len, const char sha[20])
{
	static const Py_ssize_t dirprefixlen = 8;
	static const Py_ssize_t maxshortdirslen = 68;
	char *dest;
	PyObject *ret;

	Py_ssize_t i, d, p, lastslash = len - 1, lastdot = -1;
	Py_ssize_t destsize, destlen = 0, slop, used;

	while (lastslash >= 0 && src[lastslash] != '/') {
		if (src[lastslash] == '.' && lastdot == -1)
			lastdot = lastslash;
		lastslash--;
	}

#if 0
	/* All paths should end in a suffix of ".i" or ".d".
           Unfortunately, the file names in test-hybridencode.py
           violate this rule.  */
	if (lastdot != len - 3) {
		PyErr_SetString(PyExc_ValueError,
				"suffix missing or wrong length");
		return NULL;
	}
#endif

	/* If src contains a suffix, we will append it to the end of
	   the new string, so make room. */
	destsize = 120;
	if (lastdot >= 0)
		destsize += len - lastdot - 1;

	ret = PyBytes_FromStringAndSize(NULL, destsize);
	if (ret == NULL)
		return NULL;

	dest = PyBytes_AS_STRING(ret);
	memcopy(dest, &destlen, destsize, "dh/", 3);

	/* Copy up to dirprefixlen bytes of each path component, up to
	   a limit of maxshortdirslen bytes. */
	for (i = d = p = 0; i < lastslash; i++, p++) {
		if (src[i] == '/') {
			char d = dest[destlen - 1];
			/* After truncation, a directory name may end
			   in a space or dot, which are unportable. */
			if (d == '.' || d == ' ')
				dest[destlen - 1] = '_';
			/* The + 3 is to account for "dh/" in the beginning */
			if (destlen > maxshortdirslen + 3)
				break;
			charcopy(dest, &destlen, destsize, src[i]);
			p = -1;
		}
		else if (p < dirprefixlen)
			charcopy(dest, &destlen, destsize, src[i]);
	}

	/* Rewind to just before the last slash copied. */
	if (destlen > maxshortdirslen + 3)
		do {
			destlen--;
		} while (destlen > 0 && dest[destlen] != '/');

	if (destlen > 3) {
		if (lastslash > 0) {
			char d = dest[destlen - 1];
			/* The last directory component may be
			   truncated, so make it safe. */
			if (d == '.' || d == ' ')
				dest[destlen - 1] = '_';
		}

		charcopy(dest, &destlen, destsize, '/');
	}

	/* Add a prefix of the original file's name. Its length
	   depends on the number of bytes left after accounting for
	   hash and suffix. */
	used = destlen + 40;
	if (lastdot >= 0)
		used += len - lastdot - 1;
	slop = maxstorepathlen - used;
	if (slop > 0) {
		Py_ssize_t basenamelen =
			lastslash >= 0 ? len - lastslash - 2 : len - 1;

		if (basenamelen > slop)
			basenamelen = slop;
		if (basenamelen > 0)
			memcopy(dest, &destlen, destsize, &src[lastslash + 1],
				basenamelen);
	}

	/* Add hash and suffix. */
	for (i = 0; i < 20; i++)
		hexencode(dest, &destlen, destsize, sha[i]);

	if (lastdot >= 0)
		memcopy(dest, &destlen, destsize, &src[lastdot],
			len - lastdot - 1);

	assert(PyBytes_Check(ret));
	Py_SIZE(ret) = destlen;

	return ret;
}

/*
 * Avoiding a trip through Python would improve performance by 50%,
 * but we don't encounter enough long names to be worth the code.
 */
static int sha1hash(char hash[20], const char *str, Py_ssize_t len)
{
	static PyObject *shafunc;
	PyObject *shaobj, *hashobj;

	if (shafunc == NULL) {
		PyObject *hashlib, *name = PyBytes_FromString("hashlib");

		if (name == NULL)
			return -1;

		hashlib = PyImport_Import(name);
		Py_DECREF(name);

		if (hashlib == NULL) {
			PyErr_SetString(PyExc_ImportError, "hashlib");
			return -1;
		}
		shafunc = PyObject_GetAttrString(hashlib, "sha1");
		Py_DECREF(hashlib);

		if (shafunc == NULL) {
			PyErr_SetString(PyExc_AttributeError,
					"module 'hashlib' has no "
					"attribute 'sha1'");
			return -1;
		}
	}

	shaobj = PyObject_CallFunction(shafunc, "s#", str, len);

	if (shaobj == NULL)
		return -1;

	hashobj = PyObject_CallMethod(shaobj, "digest", "");
	Py_DECREF(shaobj);
	if (hashobj == NULL)
		return -1;

	if (!PyBytes_Check(hashobj) || PyBytes_GET_SIZE(hashobj) != 20) {
		PyErr_SetString(PyExc_TypeError,
				"result of digest is not a 20-byte hash");
		Py_DECREF(hashobj);
		return -1;
	}

	memcpy(hash, PyBytes_AS_STRING(hashobj), 20);
	Py_DECREF(hashobj);
	return 0;
}

#define MAXENCODE 4096 * 4

static PyObject *hashencode(const char *src, Py_ssize_t len)
{
	char dired[MAXENCODE];
	char lowered[MAXENCODE];
	char auxed[MAXENCODE];
	Py_ssize_t dirlen, lowerlen, auxlen, baselen;
	char sha[20];

	baselen = (len - 5) * 3;
	if (baselen >= MAXENCODE) {
		PyErr_SetString(PyExc_ValueError, "string too long");
		return NULL;
	}

	dirlen = _encodedir(dired, baselen, src, len);
	if (sha1hash(sha, dired, dirlen - 1) == -1)
		return NULL;
	lowerlen = _lowerencode(lowered, baselen, dired + 5, dirlen - 5);
	auxlen = auxencode(auxed, baselen, lowered, lowerlen);
	return hashmangle(auxed, auxlen, sha);
}

PyObject *pathencode(PyObject *self, PyObject *args)
{
	Py_ssize_t len, newlen;
	PyObject *pathobj, *newobj;
	char *path;

	if (!PyArg_ParseTuple(args, "O:pathencode", &pathobj))
		return NULL;

	if (PyBytes_AsStringAndSize(pathobj, &path, &len) == -1) {
		PyErr_SetString(PyExc_TypeError, "expected a string");
		return NULL;
	}

	if (len > maxstorepathlen)
		newlen = maxstorepathlen + 2;
	else
		newlen = len ? basicencode(NULL, 0, path, len + 1) : 1;

	if (newlen <= maxstorepathlen + 1) {
		if (newlen == len + 1) {
			Py_INCREF(pathobj);
			return pathobj;
		}

		newobj = PyBytes_FromStringAndSize(NULL, newlen);

		if (newobj) {
			assert(PyBytes_Check(newobj));
			Py_SIZE(newobj)--;
			basicencode(PyBytes_AS_STRING(newobj), newlen, path,
				    len + 1);
		}
	}
	else
		newobj = hashencode(path, len + 1);

	return newobj;
}