Add Xapian::DOC_ASSUME_VALID flag

[xapian.git] / xapian-core / api / omdatabase.cc

/* omdatabase.cc: External interface for running queries
 *
 * Copyright 1999,2000,2001 BrightStation PLC
 * Copyright 2001,2002 Ananova Ltd
 * Copyright 2002,2003,2004,2005,2006,2007,2008,2009,2010,2011,2013,2014,2016 Olly Betts
 * Copyright 2006,2008 Lemur Consulting Ltd
 *
 * 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
 */

#include <config.h>

#include "autoptr.h"

#include <xapian/constants.h>
#include <xapian/error.h>
#include <xapian/positioniterator.h>
#include <xapian/postingiterator.h>
#include <xapian/termiterator.h>
#include <xapian/unicode.h>

#include "omassert.h"
#include "debuglog.h"
#include "backends/alltermslist.h"
#include "backends/multi/multi_alltermslist.h"
#include "backends/multi/multi_postlist.h"
#include "backends/multi/multi_termlist.h"
#include "backends/multivaluelist.h"
#include "backends/database.h"
#include "editdistance.h"
#include "expand/ortermlist.h"
#include "internaltypes.h"
#include "noreturn.h"
#include "pack.h"

#include <algorithm>
#include <cstdlib> // For abs().
#include <cstring>
#include <vector>

using namespace std;
using Xapian::Internal::intrusive_ptr;

XAPIAN_NORETURN(static void docid_zero_invalid());
static void docid_zero_invalid()
{
    throw Xapian::InvalidArgumentError("Document ID 0 is invalid");
}

XAPIAN_NORETURN(static void no_subdatabases());
static void no_subdatabases()
{
    throw Xapian::InvalidOperationError("No subdatabases");
}

XAPIAN_NORETURN(static void empty_metadata_key());
static void empty_metadata_key()
{
    throw Xapian::InvalidArgumentError("Empty metadata keys are invalid");
}

inline size_t
sub_db(Xapian::docid did, size_t n_dbs)
{
    return (did - 1) % n_dbs;
}

inline size_t
sub_docid(Xapian::docid did, size_t n_dbs)
{
    return (did - 1) / n_dbs + 1;
}

namespace Xapian {

Database::Database()
{
    LOGCALL_CTOR(API, "Database", NO_ARGS);
}

Database::Database(Database::Internal *internal_)
{
    LOGCALL_CTOR(API, "Database", internal_);
    intrusive_ptr<Database::Internal> newi(internal_);
    internal.push_back(newi);
}

Database::Database(const Database &other)
{
    LOGCALL_CTOR(API, "Database", other);
    internal = other.internal;
}

void
Database::operator=(const Database &other)
{
    LOGCALL_VOID(API, "Database::operator=", other);
    internal = other.internal;
}

Database::~Database()
{
    LOGCALL_DTOR(API, "Database");
}

bool
Database::reopen()
{
    LOGCALL(API, bool, "Database::reopen", NO_ARGS);
    bool maybe_changed = false;
    vector<intrusive_ptr<Database::Internal> >::iterator i;
    for (i = internal.begin(); i != internal.end(); ++i) {
        if ((*i)->reopen())
            maybe_changed = true;
    }
    RETURN(maybe_changed);
}

void
Database::close()
{
    LOGCALL_VOID(API, "Database::close", NO_ARGS);
    vector<intrusive_ptr<Database::Internal> >::iterator i;
    for (i = internal.begin(); i != internal.end(); ++i) {
        (*i)->close();
    }
}

void
Database::add_database(const Database & database)
{
    LOGCALL_VOID(API, "Database::add_database", database);
    if (this == &database) {
        LOGLINE(API, "Database added to itself");
        throw Xapian::InvalidArgumentError("Can't add a Database to itself");
    }
    vector<intrusive_ptr<Database::Internal> >::const_iterator i;
    for (i = database.internal.begin(); i != database.internal.end(); ++i) {
        internal.push_back(*i);
    }
}

PostingIterator
Database::postlist_begin(const string &tname) const
{
    LOGCALL(API, PostingIterator, "Database::postlist_begin", tname);

    // Don't bother checking that the term exists first.  If it does, we
    // just end up doing more work, and if it doesn't, we save very little
    // work.

    // Handle the common case of a single database specially.
    if (internal.size() == 1)
        RETURN(PostingIterator(internal[0]->open_post_list(tname)));

    if (rare(internal.empty()))
        RETURN(PostingIterator());

    vector<LeafPostList *> pls;
    try {
        vector<intrusive_ptr<Database::Internal> >::const_iterator i;
        for (i = internal.begin(); i != internal.end(); ++i) {
            pls.push_back((*i)->open_post_list(tname));
            pls.back()->next();
        }
        Assert(pls.begin() != pls.end());
    } catch (...) {
        vector<LeafPostList *>::iterator i;
        for (i = pls.begin(); i != pls.end(); ++i) {
            delete *i;
            *i = 0;
        }
        throw;
    }

    RETURN(PostingIterator(new MultiPostList(pls, *this)));
}

TermIterator
Database::termlist_begin(Xapian::docid did) const
{
    LOGCALL(API, TermIterator, "Database::termlist_begin", did);
    if (did == 0)
        docid_zero_invalid();

    unsigned int multiplier = internal.size();
    if (rare(multiplier == 0))
        no_subdatabases();
    TermList *tl;
    if (multiplier == 1) {
        // There's no need for the MultiTermList wrapper in the common case
        // where we're only dealing with a single database.
        tl = internal[0]->open_term_list(did);
    } else {
        Assert(multiplier != 0);
        Xapian::doccount n = (did - 1) % multiplier; // which actual database
        Xapian::docid m = (did - 1) / multiplier + 1; // real docid in that database

        tl = new MultiTermList(internal[n]->open_term_list(m), *this, n);
    }
    RETURN(TermIterator(tl));
}

TermIterator
Database::allterms_begin(const std::string & prefix) const
{
    LOGCALL(API, TermIterator, "Database::allterms_begin", NO_ARGS);
    TermList * tl;
    if (rare(internal.size() == 0)) {
        tl = NULL;
    } else if (internal.size() == 1) {
        tl = internal[0]->open_allterms(prefix);
    } else {
        tl = new MultiAllTermsList(internal, prefix);
    }
    RETURN(TermIterator(tl));
}

bool
Database::has_positions() const
{
    LOGCALL(API, bool, "Database::has_positions", NO_ARGS);
    // If any sub-database has positions, the combined database does.
    vector<intrusive_ptr<Database::Internal> >::const_iterator i;
    for (i = internal.begin(); i != internal.end(); ++i) {
        if ((*i)->has_positions()) RETURN(true);
    }
    RETURN(false);
}

PositionIterator
Database::positionlist_begin(Xapian::docid did, const string &tname) const
{
    LOGCALL(API, PositionIterator, "Database::positionlist_begin", did | tname);
    if (tname.empty())
        throw InvalidArgumentError("Zero length terms are invalid");
    if (did == 0)
        docid_zero_invalid();

    unsigned int multiplier = internal.size();
    if (rare(multiplier == 0))
        no_subdatabases();
    Xapian::doccount n = (did - 1) % multiplier; // which actual database
    Xapian::docid m = (did - 1) / multiplier + 1; // real docid in that database
    RETURN(PositionIterator(internal[n]->open_position_list(m, tname)));
}

Xapian::doccount
Database::get_doccount() const
{
    LOGCALL(API, Xapian::doccount, "Database::get_doccount", NO_ARGS);
    Xapian::doccount docs = 0;
    vector<intrusive_ptr<Database::Internal> >::const_iterator i;
    for (i = internal.begin(); i != internal.end(); ++i) {
        docs += (*i)->get_doccount();
    }
    RETURN(docs);
}

Xapian::docid
Database::get_lastdocid() const
{
    LOGCALL(API, Xapian::docid, "Database::get_lastdocid", NO_ARGS);
    Xapian::docid did = 0;

    unsigned int multiplier = internal.size();
    for (Xapian::doccount i = 0; i < multiplier; ++i) {
        Xapian::docid did_i = internal[i]->get_lastdocid();
        if (did_i) did = std::max(did, (did_i - 1) * multiplier + i + 1);
    }
    RETURN(did);
}

Xapian::doclength
Database::get_avlength() const
{
    LOGCALL(API, Xapian::doclength, "Database::get_avlength", NO_ARGS);
    Xapian::doccount docs = 0;
    totlen_t totlen = 0;

    vector<intrusive_ptr<Database::Internal> >::const_iterator i;
    for (i = internal.begin(); i != internal.end(); ++i) {
        docs += (*i)->get_doccount();
        totlen += (*i)->get_total_length();
    }
    LOGLINE(UNKNOWN, "get_avlength() = " << totlen << " / " << docs <<
            " (from " << internal.size() << " dbs)");

    if (docs == 0) RETURN(0.0);
    RETURN(totlen / double(docs));
}

Xapian::doccount
Database::get_termfreq(const string & tname) const
{
    LOGCALL(API, Xapian::doccount, "Database::get_termfreq", tname);
    if (tname.empty()) RETURN(get_doccount());

    Xapian::doccount tf = 0;
    vector<intrusive_ptr<Database::Internal> >::const_iterator i;
    for (i = internal.begin(); i != internal.end(); ++i) {
        Xapian::doccount sub_tf;
        (*i)->get_freqs(tname, &sub_tf, NULL);
        tf += sub_tf;
    }
    RETURN(tf);
}

Xapian::termcount
Database::get_collection_freq(const string & tname) const
{
    LOGCALL(API, Xapian::termcount, "Database::get_collection_freq", tname);
    if (tname.empty()) RETURN(get_doccount());

    Xapian::termcount cf = 0;
    vector<intrusive_ptr<Database::Internal> >::const_iterator i;
    for (i = internal.begin(); i != internal.end(); ++i) {
        Xapian::termcount sub_cf;
        (*i)->get_freqs(tname, NULL, &sub_cf);
        cf += sub_cf;
    }
    RETURN(cf);
}

Xapian::doccount
Database::get_value_freq(Xapian::valueno slot) const
{
    LOGCALL(API, Xapian::doccount, "Database::get_value_freq", slot);

    Xapian::doccount vf = 0;
    vector<intrusive_ptr<Database::Internal> >::const_iterator i;
    for (i = internal.begin(); i != internal.end(); ++i) {
        vf += (*i)->get_value_freq(slot);
    }
    RETURN(vf);
}

string
Database::get_value_lower_bound(Xapian::valueno slot) const
{
    LOGCALL(API, string, "Database::get_value_lower_bound", slot);

    if (rare(internal.empty())) RETURN(string());

    vector<intrusive_ptr<Database::Internal> >::const_iterator i;
    i = internal.begin();
    string full_lb = (*i)->get_value_lower_bound(slot);
    while (++i != internal.end()) {
        string lb = (*i)->get_value_lower_bound(slot);
        if (lb < full_lb) full_lb = lb;
    }
    RETURN(full_lb);
}

std::string
Database::get_value_upper_bound(Xapian::valueno slot) const
{
    LOGCALL(API, std::string, "Database::get_value_upper_bound", slot);

    std::string full_ub;
    vector<intrusive_ptr<Database::Internal> >::const_iterator i;
    for (i = internal.begin(); i != internal.end(); ++i) {
        std::string ub = (*i)->get_value_upper_bound(slot);
        if (ub > full_ub)
            full_ub = ub;
    }
    RETURN(full_ub);
}

Xapian::termcount
Database::get_doclength_lower_bound() const
{
    LOGCALL(API, Xapian::termcount, "Database::get_doclength_lower_bound", NO_ARGS);

    if (rare(internal.empty())) RETURN(0);

    Xapian::termcount full_lb = 0;
    vector<intrusive_ptr<Database::Internal> >::const_iterator i;
    for (i = internal.begin(); i != internal.end(); ++i) {
        // Skip sub-databases which are empty or only contain documents with
        // doclen==0.
        if ((*i)->get_total_length() != 0) {
            Xapian::termcount lb = (*i)->get_doclength_lower_bound();
            if (full_lb == 0 || lb < full_lb) full_lb = lb;
        }
    }
    RETURN(full_lb);
}

Xapian::termcount
Database::get_doclength_upper_bound() const
{
    LOGCALL(API, Xapian::termcount, "Database::get_doclength_upper_bound", NO_ARGS);

    Xapian::termcount full_ub = 0;
    vector<intrusive_ptr<Database::Internal> >::const_iterator i;
    for (i = internal.begin(); i != internal.end(); ++i) {
        Xapian::termcount ub = (*i)->get_doclength_upper_bound();
        if (ub > full_ub) full_ub = ub;
    }
    RETURN(full_ub);
}

Xapian::termcount
Database::get_wdf_upper_bound(const string & term) const
{
    LOGCALL(API, Xapian::termcount, "Database::get_wdf_upper_bound", term);
    if (term.empty()) RETURN(0);

    Xapian::termcount full_ub = 0;
    vector<intrusive_ptr<Database::Internal> >::const_iterator i;
    for (i = internal.begin(); i != internal.end(); ++i) {
        Xapian::termcount ub = (*i)->get_wdf_upper_bound(term);
        if (ub > full_ub) full_ub = ub;
    }
    RETURN(full_ub);
}

ValueIterator
Database::valuestream_begin(Xapian::valueno slot) const
{
    LOGCALL(API, ValueIterator, "Database::valuestream_begin", slot);
    if (internal.size() == 0)
        RETURN(ValueIterator());
    if (internal.size() != 1)
        RETURN(ValueIterator(new MultiValueList(internal, slot)));
    RETURN(ValueIterator(internal[0]->open_value_list(slot)));
}

Xapian::termcount
Database::get_doclength(Xapian::docid did) const
{
    LOGCALL(API, Xapian::termcount, "Database::get_doclength", did);
    if (did == 0)
        docid_zero_invalid();

    unsigned int multiplier = internal.size();
    if (rare(multiplier == 0))
        no_subdatabases();
    Xapian::doccount n = (did - 1) % multiplier; // which actual database
    Xapian::docid m = (did - 1) / multiplier + 1; // real docid in that database
    RETURN(internal[n]->get_doclength(m));
}

Xapian::termcount
Database::get_unique_terms(Xapian::docid did) const
{
    LOGCALL(API, Xapian::termcount, "Database::get_unique_terms", did);
    if (did == 0)
        docid_zero_invalid();
    unsigned int multiplier = internal.size();
    if (rare(multiplier == 0))
        no_subdatabases();
    Xapian::doccount n = (did - 1) % multiplier; // which actual database
    Xapian::docid m = (did - 1) / multiplier + 1; // real docid in that database
    RETURN(internal[n]->get_unique_terms(m));
}

Document
Database::get_document(Xapian::docid did) const
{
    LOGCALL(API, Document, "Database::get_document", did);
    if (did == 0)
        docid_zero_invalid();

    unsigned int multiplier = internal.size();
    if (rare(multiplier == 0))
        no_subdatabases();
    Xapian::doccount n = (did - 1) % multiplier; // which actual database
    Xapian::docid m = (did - 1) / multiplier + 1; // real docid in that database

    // Open non-lazily so we throw DocNotFoundError if the doc doesn't exist.
    RETURN(Document(internal[n]->open_document(m, false)));
}

Document
Database::get_document(Xapian::docid did, unsigned flags) const
{
    LOGCALL(API, Document, "Database::get_document", did|flags);
    if (did == 0)
        docid_zero_invalid();

    unsigned int multiplier = internal.size();
    if (rare(multiplier == 0))
        no_subdatabases();
    Xapian::doccount n = (did - 1) % multiplier; // which actual database
    Xapian::docid m = (did - 1) / multiplier + 1; // real docid in that database

    bool assume_valid = flags & Xapian::DOC_ASSUME_VALID;
    RETURN(Document(internal[n]->open_document(m, assume_valid)));
}

bool
Database::term_exists(const string & tname) const
{
    LOGCALL(API, bool, "Database::term_exists", tname);
    if (tname.empty()) {
        RETURN(get_doccount() != 0);
    }
    vector<intrusive_ptr<Database::Internal> >::const_iterator i;
    for (i = internal.begin(); i != internal.end(); ++i) {
        if ((*i)->term_exists(tname)) RETURN(true);
    }
    RETURN(false);
}

void
Database::keep_alive()
{
    LOGCALL_VOID(API, "Database::keep_alive", NO_ARGS);
    vector<intrusive_ptr<Database::Internal> >::const_iterator i;
    for (i = internal.begin(); i != internal.end(); ++i) {
        (*i)->keep_alive();
    }
}

string
Database::get_description() const
{
    /// @todo display contents of the database
    return "Database()";
}

// We sum the character frequency histogram absolute differences to compute a
// lower bound on the edit distance.  Rather than counting each Unicode code
// point uniquely, we use an array with VEC_SIZE elements and tally code points
// modulo VEC_SIZE which can only reduce the bound we calculate.
//
// There will be a trade-off between how good the bound is and how large and
// array is used (a larger array takes more time to clear and sum over).  The
// value 64 is somewhat arbitrary - it works as well as 128 for the testsuite
// but that may not reflect real world performance.  FIXME: profile and tune.

#define VEC_SIZE 64

static int
freq_edit_lower_bound(const vector<unsigned> & a, const vector<unsigned> & b)
{
    int vec[VEC_SIZE];
    memset(vec, 0, sizeof(vec));
    vector<unsigned>::const_iterator i;
    for (i = a.begin(); i != a.end(); ++i) {
        ++vec[(*i) % VEC_SIZE];
    }
    for (i = b.begin(); i != b.end(); ++i) {
        --vec[(*i) % VEC_SIZE];
    }
    unsigned int total = 0;
    for (size_t j = 0; j < VEC_SIZE; ++j) {
        total += abs(vec[j]);
    }
    // Each insertion or deletion adds at most 1 to total.  Each transposition
    // doesn't change it at all.  But each substitution can change it by 2 so
    // we need to divide it by 2.  Rounding up is OK, since the odd change must
    // be due to an actual edit.
    return (total + 1) / 2;
}

// Word must have a trigram score at least this close to the best score seen
// so far.
#define TRIGRAM_SCORE_THRESHOLD 2

string
Database::get_spelling_suggestion(const string &word,
                                  unsigned max_edit_distance) const
{
    LOGCALL(API, string, "Database::get_spelling_suggestion", word | max_edit_distance);
    if (word.size() <= 1) return string();
    AutoPtr<TermList> merger;
    for (size_t i = 0; i < internal.size(); ++i) {
        TermList * tl = internal[i]->open_spelling_termlist(word);
        LOGLINE(SPELLING, "Sub db " << i << " tl = " << (void*)tl);
        if (tl) {
            if (merger.get()) {
                merger.reset(new OrTermList(merger.release(), tl));
            } else {
                merger.reset(tl);
            }
        }
    }
    if (!merger.get()) RETURN(string());

    // Convert word to UTF-32.
    // Extra brackets needed to avoid this being misparsed as a function
    // prototype.
    vector<unsigned> utf32_word((Utf8Iterator(word)), Utf8Iterator());

    vector<unsigned> utf32_term;

    Xapian::termcount best = 1;
    string result;
    int edist_best = max_edit_distance;
    Xapian::doccount freq_best = 0;
    Xapian::doccount freq_exact = 0;
    while (true) {
        TermList *ret = merger->next();
        if (ret) merger.reset(ret);

        if (merger->at_end()) break;

        string term = merger->get_termname();
        Xapian::termcount score = merger->get_wdf();

        LOGLINE(SPELLING, "Term \"" << term << "\" ngram score " << score);
        if (score + TRIGRAM_SCORE_THRESHOLD >= best) {
            if (score > best) best = score;

            // There's no point considering a word where the difference
            // in length is greater than the smallest number of edits we've
            // found so far.

            // First check the length of the encoded UTF-8 version of term.
            // Each UTF-32 character is 1-4 bytes in UTF-8.
            if (abs(long(term.size()) - long(word.size())) > edist_best * 4) {
                LOGLINE(SPELLING, "Lengths much too different");
                continue;
            }

            // Now convert to UTF-32, and compare the true lengths more
            // strictly.
            utf32_term.assign(Utf8Iterator(term), Utf8Iterator());

            if (abs(long(utf32_term.size()) - long(utf32_word.size()))
                    > edist_best) {
                LOGLINE(SPELLING, "Lengths too different");
                continue;
            }

            if (freq_edit_lower_bound(utf32_term, utf32_word) > edist_best) {
                LOGLINE(SPELLING, "Rejected by character frequency test");
                continue;
            }

            int edist = edit_distance_unsigned(&utf32_term[0],
                                               int(utf32_term.size()),
                                               &utf32_word[0],
                                               int(utf32_word.size()),
                                               edist_best);
            LOGLINE(SPELLING, "Edit distance " << edist);

            if (edist <= edist_best) {
                Xapian::doccount freq = 0;
                for (size_t j = 0; j < internal.size(); ++j)
                    freq += internal[j]->get_spelling_frequency(term);

                LOGLINE(SPELLING, "Freq " << freq << " best " << freq_best);
                // Even if we have an exact match, there may be a much more
                // frequent potential correction which will still be
                // interesting.
                if (edist == 0) {
                    freq_exact = freq;
                    continue;
                }

                if (edist < edist_best || freq > freq_best) {
                    LOGLINE(SPELLING, "Best so far: \"" << term <<
                                      "\" edist " << edist << " freq " << freq);
                    result = term;
                    edist_best = edist;
                    freq_best = freq;
                }
            }
        }
    }
    if (freq_best < freq_exact)
        RETURN(string());
    RETURN(result);
}

TermIterator
Database::spellings_begin() const
{
    LOGCALL(API, TermIterator, "Database::spellings_begin", NO_ARGS);
    AutoPtr<TermList> merger;
    for (size_t i = 0; i < internal.size(); ++i) {
        TermList * tl = internal[i]->open_spelling_wordlist();
        if (tl) {
            if (merger.get()) {
                merger.reset(new FreqAdderOrTermList(merger.release(), tl));
            } else {
                merger.reset(tl);
            }
        }
    }
    RETURN(TermIterator(merger.release()));
}

TermIterator
Database::synonyms_begin(const std::string &term) const
{
    LOGCALL(API, TermIterator, "Database::synonyms_begin", term);
    AutoPtr<TermList> merger;
    for (size_t i = 0; i < internal.size(); ++i) {
        TermList * tl = internal[i]->open_synonym_termlist(term);
        if (tl) {
            if (merger.get()) {
                merger.reset(new OrTermList(merger.release(), tl));
            } else {
                merger.reset(tl);
            }
        }
    }
    RETURN(TermIterator(merger.release()));
}

TermIterator
Database::synonym_keys_begin(const std::string &prefix) const
{
    LOGCALL(API, TermIterator, "Database::synonym_keys_begin", prefix);
    AutoPtr<TermList> merger;
    for (size_t i = 0; i < internal.size(); ++i) {
        TermList * tl = internal[i]->open_synonym_keylist(prefix);
        if (tl) {
            if (merger.get()) {
                merger.reset(new OrTermList(merger.release(), tl));
            } else {
                merger.reset(tl);
            }
        }
    }
    RETURN(TermIterator(merger.release()));
}

string
Database::get_metadata(const string & key) const
{
    LOGCALL(API, string, "Database::get_metadata", key);
    if (rare(key.empty()))
        empty_metadata_key();
    if (internal.empty()) RETURN(std::string());
    RETURN(internal[0]->get_metadata(key));
}

Xapian::TermIterator
Database::metadata_keys_begin(const std::string &prefix) const
{
    LOGCALL(API, Xapian::TermIterator, "Database::metadata_keys_begin", NO_ARGS);
    if (internal.empty()) RETURN(TermIterator());
    RETURN(TermIterator(internal[0]->open_metadata_keylist(prefix)));
}

std::string
Database::get_uuid() const
{
    LOGCALL(API, std::string, "Database::get_uuid", NO_ARGS);
    string uuid;
    for (size_t i = 0; i < internal.size(); ++i) {
        string sub_uuid = internal[i]->get_uuid();
        // If any of the sub-databases have no uuid, we can't make a uuid for
        // the combined database.
        if (sub_uuid.empty())
            RETURN(sub_uuid);
        if (!uuid.empty()) uuid += ':';
        uuid += sub_uuid;
    }
    RETURN(uuid);
}

Xapian::rev
Database::get_revision() const
{
    LOGCALL(API, Xapian::rev, "Database::get_revision", NO_ARGS);
    size_t n_dbs = internal.size();
    if (rare(n_dbs != 1))
        throw Xapian::InvalidOperationError("Database::get_revision() requires "
                                            "exactly one subdatabase");
    const string& s = internal[0]->get_revision_info();
    const char* p = s.data();
    const char* end = p + s.size();
    Xapian::rev revision;
    if (!unpack_uint(&p, end, &revision))
        throw Xapian::UnimplementedError("Database::get_revision() only "
                                         "supported for chert and glass");
    return revision;
}

///////////////////////////////////////////////////////////////////////////

WritableDatabase::WritableDatabase() : Database()
{
    LOGCALL_CTOR(API, "WritableDatabase", NO_ARGS);
}

WritableDatabase::WritableDatabase(Database::Internal *internal_)
        : Database(internal_)
{
    LOGCALL_CTOR(API, "WritableDatabase", internal_);
}

WritableDatabase::WritableDatabase(const WritableDatabase &other)
        : Database(other)
{
    LOGCALL_CTOR(API, "WritableDatabase", other);
}

void
WritableDatabase::operator=(const WritableDatabase &other)
{
    LOGCALL_VOID(API, "WritableDatabase::operator=", other);
    Database::operator=(other);
}

WritableDatabase::~WritableDatabase()
{
    LOGCALL_DTOR(API, "WritableDatabase");
}

void
WritableDatabase::commit()
{
    LOGCALL_VOID(API, "WritableDatabase::commit", NO_ARGS);
    size_t n_dbs = internal.size();
    if (rare(n_dbs == 0))
        no_subdatabases();
    for (size_t i = 0; i != n_dbs; ++i)
        internal[i]->commit();
}

void
WritableDatabase::begin_transaction(bool flushed)
{
    LOGCALL_VOID(API, "WritableDatabase::begin_transaction", flushed);
    size_t n_dbs = internal.size();
    if (rare(n_dbs == 0))
        no_subdatabases();
    for (size_t i = 0; i != n_dbs; ++i)
        internal[i]->begin_transaction(flushed);
}

void
WritableDatabase::commit_transaction()
{
    LOGCALL_VOID(API, "WritableDatabase::commit_transaction", NO_ARGS);
    size_t n_dbs = internal.size();
    if (rare(n_dbs == 0))
        no_subdatabases();
    for (size_t i = 0; i != n_dbs; ++i)
        internal[i]->commit_transaction();
}

void
WritableDatabase::cancel_transaction()
{
    LOGCALL_VOID(API, "WritableDatabase::cancel_transaction", NO_ARGS);
    size_t n_dbs = internal.size();
    if (rare(n_dbs == 0))
        no_subdatabases();
    for (size_t i = 0; i != n_dbs; ++i)
        internal[i]->cancel_transaction();
}


Xapian::docid
WritableDatabase::add_document(const Document & document)
{
    LOGCALL(API, Xapian::docid, "WritableDatabase::add_document", document);
    size_t n_dbs = internal.size();
    if (rare(n_dbs == 0))
        no_subdatabases();
    if (n_dbs == 1)
        RETURN(internal[0]->add_document(document));

    // Which database will the next never used docid be in?
    Xapian::docid did = get_lastdocid() + 1;
    if (rare(did == 0)) {
        throw Xapian::DatabaseError("Run out of docids - you'll have to use copydatabase to eliminate any gaps before you can add more documents");
    }
    // We want exactly did to be used, not a lower docid if that subdb isn't
    // using the docid before it, so call replace_document() not
    // add_document().
    size_t i = sub_db(did, n_dbs);
    internal[i]->replace_document(sub_docid(did, n_dbs), document);
    RETURN(did);
}

void
WritableDatabase::delete_document(Xapian::docid did)
{
    LOGCALL_VOID(API, "WritableDatabase::delete_document", did);
    if (rare(did == 0))
        docid_zero_invalid();

    size_t n_dbs = internal.size();
    if (rare(n_dbs == 0))
        no_subdatabases();
    size_t i = sub_db(did, n_dbs);
    internal[i]->delete_document(sub_docid(did, n_dbs));
}

void
WritableDatabase::delete_document(const std::string & unique_term)
{
    LOGCALL_VOID(API, "WritableDatabase::delete_document", unique_term);
    if (unique_term.empty())
        throw InvalidArgumentError("Empty termnames are invalid");
    size_t n_dbs = internal.size();
    if (rare(n_dbs == 0))
        no_subdatabases();
    for (size_t i = 0; i != n_dbs; ++i)
        internal[i]->delete_document(unique_term);
}

void
WritableDatabase::replace_document(Xapian::docid did, const Document & document)
{
    LOGCALL_VOID(API, "WritableDatabase::replace_document", did | document);
    if (did == 0)
        docid_zero_invalid();
    size_t n_dbs = internal.size();
    if (rare(n_dbs == 0))
        no_subdatabases();
    size_t i = sub_db(did, n_dbs);
    internal[i]->replace_document(sub_docid(did, n_dbs), document);
}

Xapian::docid
WritableDatabase::replace_document(const std::string & unique_term,
                                   const Document & document)
{
    LOGCALL(API, Xapian::docid, "WritableDatabase::replace_document", unique_term | document);
    if (unique_term.empty())
        throw InvalidArgumentError("Empty termnames are invalid");
    size_t n_dbs = internal.size();
    if (rare(n_dbs == 0))
        no_subdatabases();
    if (n_dbs == 1)
        RETURN(internal[0]->replace_document(unique_term, document));

    Xapian::PostingIterator postit = postlist_begin(unique_term);
    // If no unique_term in the database, this is just an add_document().
    if (postit == postlist_end(unique_term)) {
        // Which database will the next never used docid be in?
        size_t i = sub_db(get_lastdocid() + 1, n_dbs);
        RETURN(internal[i]->add_document(document));
    }

    Xapian::docid retval = *postit;
    size_t i = sub_db(retval, n_dbs);
    internal[i]->replace_document(sub_docid(retval, n_dbs), document);

    // Delete any other occurrences of unique_term.
    while (++postit != postlist_end(unique_term)) {
        Xapian::docid did = *postit;
        i = sub_db(did, n_dbs);
        internal[i]->delete_document(sub_docid(did, n_dbs));
    }

    return retval;
}

void
WritableDatabase::add_spelling(const std::string & word,
                               Xapian::termcount freqinc) const
{
    LOGCALL_VOID(API, "WritableDatabase::add_spelling", word | freqinc);
    if (rare(internal.empty()))
        no_subdatabases();
    // FIXME: Is adding to the first subdatabase sensible?
    internal[0]->add_spelling(word, freqinc);
}

void
WritableDatabase::remove_spelling(const std::string & word,
                                  Xapian::termcount freqdec) const
{
    LOGCALL_VOID(API, "WritableDatabase::remove_spelling", word | freqdec);
    size_t n_dbs = internal.size();
    if (rare(n_dbs == 0))
        no_subdatabases();
    for (size_t i = 0; i < n_dbs; ++i) {
        internal[i]->remove_spelling(word, freqdec);
    }
}

void
WritableDatabase::add_synonym(const std::string & term,
                              const std::string & synonym) const
{
    LOGCALL_VOID(API, "WritableDatabase::add_synonym", term | synonym);
    if (rare(internal.empty()))
        no_subdatabases();
    // FIXME: Is adding to the first subdatabase sensible?
    internal[0]->add_synonym(term, synonym);
}

void
WritableDatabase::remove_synonym(const std::string & term,
                                 const std::string & synonym) const
{
    LOGCALL_VOID(API, "WritableDatabase::remove_synonym", term | synonym);
    size_t n_dbs = internal.size();
    if (rare(n_dbs == 0))
        no_subdatabases();
    for (size_t i = 0; i < n_dbs; ++i) {
        internal[i]->remove_synonym(term, synonym);
    }
}

void
WritableDatabase::clear_synonyms(const std::string & term) const
{
    LOGCALL_VOID(API, "WritableDatabase::clear_synonyms", term);
    size_t n_dbs = internal.size();
    if (rare(n_dbs == 0))
        no_subdatabases();
    for (size_t i = 0; i < n_dbs; ++i) {
        internal[i]->clear_synonyms(term);
    }
}

void
WritableDatabase::set_metadata(const string & key, const string & value)
{
    LOGCALL_VOID(API, "WritableDatabase::set_metadata", key | value);
    if (rare(key.empty()))
        empty_metadata_key();
    if (rare(internal.empty()))
        no_subdatabases();
    internal[0]->set_metadata(key, value);
}

string
WritableDatabase::get_description() const
{
    /// @todo display contents of the writable database
    return "WritableDatabase()";
}

}