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/** @file prefix_compressed_strings.h
* @brief Handle encoding and decoding prefix-compressed lists of strings
*/
/* Copyright (C) 2004,2005,2006,2007,2008,2009,2010 Olly Betts
*
* This program is free software; you can redistribute it and/or
* modify it under the terms of the GNU General Public License as
* published by the Free Software Foundation; either version 2 of the
* License, or (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program; if not, write to the Free Software
* Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301
* USA
*/
#ifndef XAPIAN_INCLUDED_PREFIX_COMPRESSED_STRINGS_H
#define XAPIAN_INCLUDED_PREFIX_COMPRESSED_STRINGS_H
#include <xapian/error.h>
#include <algorithm>
#include <string>
// We XOR the length values with this so that they are more likely to coincide
// with lower case ASCII letters, which are likely to be common. This means
// that zlib should do a better job of compressing tag values - in tests, this
// gave 5% better compression.
#define MAGIC_XOR_VALUE 96
class PrefixCompressedStringItor {
const unsigned char * p;
size_t left;
std::string current;
PrefixCompressedStringItor(const unsigned char * p_, size_t left_,
const std::string ¤t_)
: p(p_), left(left_), current(current_) { }
public:
explicit PrefixCompressedStringItor(const std::string & s)
: p(reinterpret_cast<const unsigned char *>(s.data())),
left(s.size()) {
if (left) {
operator++();
} else {
p = NULL;
}
}
const std::string & operator*() const {
return current;
}
PrefixCompressedStringItor operator++(int) {
const unsigned char * old_p = p;
size_t old_left = left;
std::string old_current = current;
operator++();
return PrefixCompressedStringItor(old_p, old_left, old_current);
}
PrefixCompressedStringItor & operator++() {
if (left == 0) {
p = NULL;
} else {
if (!current.empty()) {
current.resize(*p++ ^ MAGIC_XOR_VALUE);
--left;
}
size_t add;
if (left == 0 || (add = *p ^ MAGIC_XOR_VALUE) >= left)
throw Xapian::DatabaseCorruptError("Bad spelling data (too little left)");
current.append(reinterpret_cast<const char *>(p + 1), add);
p += add + 1;
left -= add + 1;
}
return *this;
}
bool at_end() const {
return p == NULL;
}
};
class PrefixCompressedStringWriter {
std::string current;
std::string & out;
public:
explicit PrefixCompressedStringWriter(std::string & out_) : out(out_) { }
void append(const std::string & word) {
// If this isn't the first entry, see how much of the previous one
// we can reuse.
if (!current.empty()) {
size_t len = std::min(current.size(), word.size());
size_t i;
for (i = 0; i < len; ++i) {
if (current[i] != word[i]) break;
}
out += char(i ^ MAGIC_XOR_VALUE);
out += char((word.size() - i) ^ MAGIC_XOR_VALUE);
out.append(word.data() + i, word.size() - i);
} else {
out += char(word.size() ^ MAGIC_XOR_VALUE);
out += word;
}
current = word;
}
};
struct PrefixCompressedStringItorGt {
/// Return true if and only if a's string is strictly greater than b's.
bool operator()(const PrefixCompressedStringItor *a,
const PrefixCompressedStringItor *b) const {
return (**a > **b);
}
};
#endif // XAPIAN_INCLUDED_PREFIX_COMPRESSED_STRINGS_H
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