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XPath: Optimized concat (it's now O(n) instead of O(n^2) and there are less allocations)

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git-svn-id: http://pugixml.googlecode.com/svn/trunk@698 99668b35-9821-0410-8761-19e4c4f06640
This commit is contained in:
arseny.kapoulkine 2010-08-29 15:52:36 +00:00
parent b33ac7477c
commit b81027b00d
1 changed files with 61 additions and 8 deletions
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69
src/pugixml.cpp
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@ -7263,6 +7263,66 @@ namespace pugi
}
}
xpath_string eval_string_concat(const xpath_context& c)
{
assert(_type == ast_func_concat);
// count the string number
unsigned int count = 1;
for (xpath_ast_node* n = _right; n; n = n->_next) count++;
// gather all strings
xpath_string static_buffer[4];
xpath_string* buffer = static_buffer;
// allocate on-heap for large concats
if (count > sizeof(static_buffer) / sizeof(static_buffer[0]))
{
buffer = static_cast<xpath_string*>(global_allocate(count * sizeof(xpath_string)));
if (!buffer) return xpath_string(); // $$ out of memory
for (unsigned int i = 0; i < count; ++i) new (&buffer[i]) xpath_string();
}
// compute all strings
_left->eval_string(c).swap(buffer[0]);
size_t pos = 1;
for (xpath_ast_node* n = _right; n; n = n->_next, ++pos) n->eval_string(c).swap(buffer[pos]);
assert(pos == count);
// get total length
size_t length = 0;
for (unsigned int i = 0; i < count; ++i) length += buffer[i].length();
// create final string
char_t* result = static_cast<char_t*>(global_allocate((length + 1) * sizeof(char_t)));
if (result)
{
char_t* ri = result;
for (unsigned int i = 0; i < count; ++i)
for (const char_t* bi = buffer[i].c_str(); *bi; ++bi)
*ri++ = *bi;
*ri = 0;
}
// deallocate strings
if (buffer != static_buffer)
{
for (unsigned int i = 0; i < count; ++i) buffer[i].~xpath_string();
global_deallocate(buffer);
}
// return result
if (!result) return xpath_string(); // $$ out of memory
return xpath_string(result, true);
}
xpath_string eval_string(const xpath_context& c)
{
switch (_type)
@ -7322,14 +7382,7 @@ namespace pugi
return _left->eval_string(c);
case ast_func_concat:
{
xpath_string r = _left->eval_string(c);
for (xpath_ast_node* n = _right; n; n = n->_next)
r.append(n->eval_string(c));
return r;
}
return eval_string_concat(c);
case ast_func_substring_before:
{