parsers: add a function to efficiently lowercase ASCII strings

parsers: add a function to efficiently lowercase ASCII strings

We need a way to efficiently lowercase ASCII strings. For example, 'hg status'
needs to build up the fold map -- a map from a canonical case (for OS X,
lowercase) to the actual case of each file and directory in the dirstate.

The current way we do that is to try decoding to ASCII and then calling
lower() on the string, labeled 'orig' below:

    str.decode('ascii')
    return str.lower()

This is pretty inefficient, and it turns out we can do much better.

I also tested out a condition-based approach, labeled 'cond' below:

    (c >= 'A' && c <= 'Z') ? (c + ('a' - 'A')) : c

'cond' turned out to be slower in all cases. A 256-byte lookup table with
invalid values for everything past 127 performed similarly, but this was less
verbose.

On OS X 10.9 with LLVM version 6.0 (clang-600.0.51), the asciilower function
was run against two corpuses.

Corpus 1 (list of files from real-world repo, > 100k files):
orig:   wall 0.428567 comb 0.430000 user 0.430000 sys 0.000000 (best of 24)
cond:   wall 0.077204 comb 0.070000 user 0.070000 sys 0.000000 (best of 100)
lookup: wall 0.060714 comb 0.060000 user 0.060000 sys 0.000000 (best of 100)

Corpus 2 (mozilla-central, 113k files):
orig:   wall 0.238406 comb 0.240000 user 0.240000 sys 0.000000 (best of 42)
cond:   wall 0.040779 comb 0.040000 user 0.040000 sys 0.000000 (best of 100)
lookup: wall 0.037623 comb 0.040000 user 0.040000 sys 0.000000 (best of 100)

On a Linux server-class machine with GCC 4.4.6 20120305 (Red Hat 4.4.6-4):

Corpus 1 (real-world repo, > 100k files):
orig:   wall 0.260899 comb 0.260000 user 0.260000 sys 0.000000 (best of 38)
cond:   wall 0.054818 comb 0.060000 user 0.060000 sys 0.000000 (best of 100)
lookup: wall 0.048489 comb 0.050000 user 0.050000 sys 0.000000 (best of 100)

Corpus 2 (mozilla-central, 113k files):
orig:   wall 0.153082 comb 0.150000 user 0.150000 sys 0.000000 (best of 65)
cond:   wall 0.031007 comb 0.040000 user 0.040000 sys 0.000000 (best of 100)
lookup: wall 0.028793 comb 0.030000 user 0.030000 sys 0.000000 (best of 100)

SSE instructions might help even more, but I didn't experiment with those.

--- a/mercurial/encoding.py	Tue Sep 30 15:58:08 2014 -0700
+++ b/mercurial/encoding.py	Fri Oct 03 18:42:39 2014 -0700
@@ -5,7 +5,7 @@
 # This software may be used and distributed according to the terms of the
 # GNU General Public License version 2 or any later version.
 
-import error
+import error, parsers
 import unicodedata, locale, os
 
 def _getpreferredencoding():
@@ -258,6 +258,15 @@
             return concat(usub.encode(encoding))
     return ellipsis # no enough room for multi-column characters
 
+def asciilower(s):
+    '''convert a string to lowercase if ASCII
+
+    Raises UnicodeDecodeError if non-ASCII characters are found.'''
+    s.decode('ascii')
+    return s.lower()
+
+asciilower = getattr(parsers, 'asciilower', asciilower)
+
 def lower(s):
     "best-effort encoding-aware case-folding of local string s"
     try:

--- a/mercurial/parsers.c	Tue Sep 30 15:58:08 2014 -0700
+++ b/mercurial/parsers.c	Fri Oct 03 18:42:39 2014 -0700
@@ -35,6 +35,27 @@
 	-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1
 };
 
+static char lowertable[128] = {
+	'\x00', '\x01', '\x02', '\x03', '\x04', '\x05', '\x06', '\x07',
+	'\x08', '\x09', '\x0a', '\x0b', '\x0c', '\x0d', '\x0e', '\x0f',
+	'\x10', '\x11', '\x12', '\x13', '\x14', '\x15', '\x16', '\x17',
+	'\x18', '\x19', '\x1a', '\x1b', '\x1c', '\x1d', '\x1e', '\x1f',
+	'\x20', '\x21', '\x22', '\x23', '\x24', '\x25', '\x26', '\x27',
+	'\x28', '\x29', '\x2a', '\x2b', '\x2c', '\x2d', '\x2e', '\x2f',
+	'\x30', '\x31', '\x32', '\x33', '\x34', '\x35', '\x36', '\x37',
+	'\x38', '\x39', '\x3a', '\x3b', '\x3c', '\x3d', '\x3e', '\x3f',
+	'\x40',
+	        '\x61', '\x62', '\x63', '\x64', '\x65', '\x66', '\x67', /* A-G */
+	'\x68', '\x69', '\x6a', '\x6b', '\x6c', '\x6d', '\x6e', '\x6f', /* H-O */
+	'\x70', '\x71', '\x72', '\x73', '\x74', '\x75', '\x76', '\x77', /* P-W */
+	'\x78', '\x79', '\x7a',                                         /* X-Z */
+	                        '\x5b', '\x5c', '\x5d', '\x5e', '\x5f',
+	'\x60', '\x61', '\x62', '\x63', '\x64', '\x65', '\x66', '\x67',
+	'\x68', '\x69', '\x6a', '\x6b', '\x6c', '\x6d', '\x6e', '\x6f',
+	'\x70', '\x71', '\x72', '\x73', '\x74', '\x75', '\x76', '\x77',
+	'\x78', '\x79', '\x7a', '\x7b', '\x7c', '\x7d', '\x7e', '\x7f'
+};
+
 static inline int hexdigit(const char *p, Py_ssize_t off)
 {
 	int8_t val = hextable[(unsigned char)p[off]];
@@ -72,6 +93,39 @@
 	return ret;
 }
 
+static PyObject *asciilower(PyObject *self, PyObject *args)
+{
+	char *str, *newstr;
+	int i, len;
+	PyObject *newobj = NULL;
+
+	if (!PyArg_ParseTuple(args, "s#", &str, &len))
+		goto quit;
+
+	newobj = PyBytes_FromStringAndSize(NULL, len);
+	if (!newobj)
+		goto quit;
+
+	newstr = PyBytes_AS_STRING(newobj);
+
+	for (i = 0; i < len; i++) {
+		char c = str[i];
+		if (c & 0x80) {
+			PyObject *err = PyUnicodeDecodeError_Create(
+				"ascii", str, len, i, (i + 1),
+				"unexpected code byte");
+			PyErr_SetObject(PyExc_UnicodeDecodeError, err);
+			goto quit;
+		}
+		newstr[i] = lowertable[(unsigned char)c];
+	}
+
+	return newobj;
+quit:
+	Py_XDECREF(newobj);
+	return NULL;
+}
+
 /*
  * This code assumes that a manifest is stitched together with newline
  * ('\n') characters.
@@ -2165,6 +2219,7 @@
 	{"parse_manifest", parse_manifest, METH_VARARGS, "parse a manifest\n"},
 	{"parse_dirstate", parse_dirstate, METH_VARARGS, "parse a dirstate\n"},
 	{"parse_index2", parse_index2, METH_VARARGS, "parse a revlog index\n"},
+	{"asciilower", asciilower, METH_VARARGS, "lowercase an ASCII string\n"},
 	{"encodedir", encodedir, METH_VARARGS, "encodedir a path\n"},
 	{"pathencode", pathencode, METH_VARARGS, "fncache-encode a path\n"},
 	{"lowerencode", lowerencode, METH_VARARGS, "lower-encode a path\n"},