backends/flint/flint_postlist.cc

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/* flint_postlist.cc: Postlists in a flint database
 *
 * Copyright 1999,2000,2001 BrightStation PLC
 * Copyright 2002,2003,2004,2005,2007 Olly Betts
 * Copyright 2007 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 "flint_postlist.h"

#include "flint_cursor.h"
#include "flint_database.h"
#include "flint_utils.h"
#include "noreturn.h"
#include "omdebug.h"
#include "utils.h"

Xapian::doccount
FlintPostListTable::get_termfreq(const string & term) const
{
    string key = make_key(term);
    string tag;
    if (!get_exact_entry(key, tag)) return 0;

    Xapian::doccount termfreq;
    const char * p = tag.data();
    FlintPostList::read_number_of_entries(&p, p + tag.size(), &termfreq, NULL);
    return termfreq;
}

Xapian::termcount
FlintPostListTable::get_collection_freq(const string & term) const
{
    string key = make_key(term);
    string tag;
    if (!get_exact_entry(key, tag)) return 0;

    Xapian::termcount collfreq;
    const char * p = tag.data();
    FlintPostList::read_number_of_entries(&p, p + tag.size(), NULL, &collfreq);
    return collfreq;
}

// How big should chunks in the posting list be?  (They
// will grow slightly bigger than this, but not more than a
// few bytes extra) - FIXME: tune this value to try to
// maximise how well blocks are used.  Or performance.
// Or indexing speed.  Or something...
const unsigned int CHUNKSIZE = 2000;

class PostlistChunkWriter {
    public:
        PostlistChunkWriter(const string &orig_key_,
                            bool is_first_chunk_,
                            const string &tname_,
                            bool is_last_chunk_);

        void append(FlintTable * table, Xapian::docid did,
                    Xapian::termcount wdf, flint_doclen_t doclen);

        void raw_append(Xapian::docid first_did_, Xapian::docid current_did_,
                        const string & s) {
            Assert(!started);
            first_did = first_did_;
            current_did = current_did_;
            if (!s.empty()) {
                chunk.append(s);
                started = true;
            }
        }

        void flush(FlintTable *table);

    private:
        string orig_key;
        string tname;
        bool is_first_chunk;
        bool is_last_chunk;
        bool started;

        Xapian::docid first_did;
        Xapian::docid current_did;

        string chunk;
};

// Static functions

XAPIAN_NORETURN(static void report_read_error(const char * position));
static void report_read_error(const char * position)
{
    if (position == 0) {
        // data ran out
        throw Xapian::DatabaseCorruptError("Data ran out unexpectedly when reading posting list.");
    }
    // overflow
    throw Xapian::RangeError("Value in posting list too large.");
}

static inline bool get_tname_from_key(const char **src, const char *end,
                               string &tname)
{
    return unpack_string_preserving_sort(src, end, tname);
}

static inline bool
check_tname_in_key_lite(const char **keypos, const char *keyend, const string &tname)
{
    string tname_in_key;

    // Read the termname.
    if (!get_tname_from_key(keypos, keyend, tname_in_key)) {
        report_read_error(*keypos);
    }

    // This should only fail if the postlist doesn't exist at all.
    return tname_in_key == tname;
}

static inline bool
check_tname_in_key(const char **keypos, const char *keyend, const string &tname)
{
    if (*keypos == keyend) return false;

    return check_tname_in_key_lite(keypos, keyend, tname);
}

static Xapian::docid
read_start_of_first_chunk(const char ** posptr,
                          const char * end,
                          Xapian::doccount * number_of_entries_ptr,
                          Xapian::termcount * collection_freq_ptr)
{
    DEBUGCALL_STATIC(DB, Xapian::docid, "read_start_of_first_chunk",
                     (const void *)posptr << ", " <<
                     (const void *)end << ", " <<
                     (void *)number_of_entries_ptr << ", " <<
                     (void *)collection_freq_ptr);

    FlintPostList::read_number_of_entries(posptr, end,
                           number_of_entries_ptr, collection_freq_ptr);
    if (number_of_entries_ptr)
        DEBUGLINE(DB, "number_of_entries = " << *number_of_entries_ptr);
    if (collection_freq_ptr)
        DEBUGLINE(DB, "collection_freq = " << *collection_freq_ptr);

    Xapian::docid did;
    // Read the docid of the first entry in the posting list.
    if (!unpack_uint(posptr, end, &did))
        report_read_error(*posptr);
    ++did;
    DEBUGLINE(DB, "doc_id = " << did);
    RETURN(did);
}

static inline void read_did_increase(const char ** posptr,
                              const char * end,
                              Xapian::docid * did_ptr)
{
    Xapian::docid did_increase;
    if (!unpack_uint(posptr, end, &did_increase)) report_read_error(*posptr);
    *did_ptr += did_increase + 1;
}

static inline void read_wdf_and_length(const char ** posptr,
                                const char * end,
                                Xapian::termcount * wdf_ptr,
                                flint_doclen_t * doclength_ptr)
{
    if (!unpack_uint(posptr, end, wdf_ptr)) report_read_error(*posptr);
    if (!unpack_uint(posptr, end, doclength_ptr)) report_read_error(*posptr);
}

static Xapian::docid
read_start_of_chunk(const char ** posptr,
                    const char * end,
                    Xapian::docid first_did_in_chunk,
                    bool * is_last_chunk_ptr)
{
    DEBUGCALL_STATIC(DB, Xapian::docid, "read_start_of_chunk",
                     reinterpret_cast<const void*>(posptr) << ", " <<
                     reinterpret_cast<const void*>(end) << ", " <<
                     first_did_in_chunk << ", " <<
                     reinterpret_cast<const void*>(is_last_chunk_ptr));

    // Read whether this is the last chunk
    if (!unpack_bool(posptr, end, is_last_chunk_ptr))
        report_read_error(*posptr);
    if (is_last_chunk_ptr)
        DEBUGLINE(DB, "is_last_chunk = " << *is_last_chunk_ptr);

    // Read what the final document ID in this chunk is.
    Xapian::docid increase_to_last;
    if (!unpack_uint(posptr, end, &increase_to_last))
        report_read_error(*posptr);
    ++increase_to_last;
    Xapian::docid last_did_in_chunk = first_did_in_chunk + increase_to_last;
    DEBUGLINE(DB, "last_did_in_chunk = " << last_did_in_chunk);
    RETURN(last_did_in_chunk);
}

static string make_wdf_and_length(Xapian::termcount wdf, flint_doclen_t doclength)
{
    return pack_uint(wdf) + pack_uint(doclength);
}

static void write_start_of_chunk(string & chunk,
                                 unsigned int start_of_chunk_header,
                                 unsigned int end_of_chunk_header,
                                 bool is_last_chunk,
                                 Xapian::docid first_did_in_chunk,
                                 Xapian::docid last_did_in_chunk)
{
    Assert((size_t)(end_of_chunk_header - start_of_chunk_header) <= chunk.size());
    Assert(last_did_in_chunk >= first_did_in_chunk);
    Xapian::docid increase_to_last = last_did_in_chunk - first_did_in_chunk;

    chunk.replace(start_of_chunk_header,
                  end_of_chunk_header - start_of_chunk_header,
                  pack_bool(is_last_chunk) + pack_uint(increase_to_last - 1));
    // FIXME - storing increase_to_last - 1 is bogus as this value is
    // -1 when a postlist chunk has a single entry!  Luckily the code
    // works despite this, but it's ugly.
}

class PostlistChunkReader {
    public:
        PostlistChunkReader(Xapian::docid first_did, const string & data_)
            : data(data_), pos(data.data()), end(pos + data.length()), at_end(data.empty()), did(first_did)
        {
            if (!at_end) read_wdf_and_length(&pos, end, &wdf, &doclength);
        }

        Xapian::docid get_docid() const {
            return did;
        }
        Xapian::termcount get_wdf() const {
            return wdf;
        }
        flint_doclen_t get_doclength() const {
            DEBUGCALL(DB, flint_doclen_t, "PostlistChunkReader::get_doclength", "");
            RETURN(doclength);
        }

        bool is_at_end() const {
            return at_end;
        }

        void next();

    private:
        string data;

        const char *pos;
        const char *end;

        bool at_end;

        Xapian::docid did;
        Xapian::termcount wdf;
        flint_doclen_t doclength;
};

void
PostlistChunkReader::next()
{
    if (pos == end) {
        at_end = true;
    } else {
        read_did_increase(&pos, end, &did);
        read_wdf_and_length(&pos, end, &wdf, &doclength);
    }
}

PostlistChunkWriter::PostlistChunkWriter(const string &orig_key_,
                                         bool is_first_chunk_,
                                         const string &tname_,
                                         bool is_last_chunk_)
        : orig_key(orig_key_),
          tname(tname_), is_first_chunk(is_first_chunk_),
          is_last_chunk(is_last_chunk_),
          started(false)
{
    DEBUGCALL(DB, void, "PostlistChunkWriter::PostlistChunkWriter",
              orig_key_ << ", " << is_first_chunk_ << ", " << tname_ << ", " <<
              is_last_chunk_);
}

void
PostlistChunkWriter::append(FlintTable * table, Xapian::docid did,
                            Xapian::termcount wdf, flint_doclen_t doclen)
{
    if (!started) {
        started = true;
        first_did = did;
    } else {
        Assert(did > current_did);
        // Start a new chunk if this one has grown to the threshold.
        if (chunk.size() >= CHUNKSIZE) {
            bool save_is_last_chunk = is_last_chunk;
            is_last_chunk = false;
            flush(table);
            is_last_chunk = save_is_last_chunk;
            is_first_chunk = false;
            first_did = did;
            chunk = "";
            orig_key = FlintPostListTable::make_key(tname, first_did);
        } else {
            chunk.append(pack_uint(did - current_did - 1));
        }
    }
    current_did = did;
    chunk.append(make_wdf_and_length(wdf, doclen));
}

static inline string
make_start_of_first_chunk(Xapian::termcount entries,
                          Xapian::termcount collectionfreq,
                          Xapian::docid new_did)
{
    return pack_uint(entries) + pack_uint(collectionfreq) + pack_uint(new_did - 1);
}

static inline string
make_start_of_chunk(bool new_is_last_chunk,
                    Xapian::docid new_first_did,
                    Xapian::docid new_final_did)
{
    Assert(new_final_did >= new_first_did);
    return pack_bool(new_is_last_chunk) +
            pack_uint(new_final_did - new_first_did - 1);
}

void
PostlistChunkWriter::flush(FlintTable *table)
{
    DEBUGCALL(DB, void, "PostlistChunkWriter::flush", table);

    /* This is one of the more messy parts involved with updating posting
     * list chunks.
     * 
     * Depending on circumstances, we may have to delete an entire chunk
     * or file it under a different key, as well as possibly modifying both
     * the previous and next chunk of the postlist.
     */

    if (!started) {
        /* This chunk is now empty so disappears entirely.
         *
         * If this was the last chunk, then the previous chunk
         * must have its "is_last_chunk" flag updated.
         *
         * If this was the first chunk, then the next chunk must
         * be transformed into the first chunk.  Messy!
         */
        DEBUGLINE(DB, "PostlistChunkWriter::flush(): deleting chunk");
        Assert(!orig_key.empty());
        if (is_first_chunk) {
            DEBUGLINE(DB, "PostlistChunkWriter::flush(): deleting first chunk");
            if (is_last_chunk) {
                /* This is the first and the last chunk, ie the only
                 * chunk, so just delete the tag.
                 */
                table->del(orig_key);
                return;
            }

            /* This is the messiest case.  The first chunk is to
             * be removed, and there is at least one chunk after
             * it.  Need to rewrite the next chunk as the first
             * chunk.
             */
            AutoPtr<FlintCursor> cursor(table->cursor_get());

            if (!cursor->find_entry(orig_key)) {
                throw Xapian::DatabaseCorruptError("The key we're working on has disappeared");
            }

            // Extract existing counts from the first chunk so we can reinsert
            // them into the block we're renaming.
            Xapian::termcount num_ent, coll_freq;
            {
                cursor->read_tag();
                const char *tagpos = cursor->current_tag.data();
                const char *tagend = tagpos + cursor->current_tag.size();

                (void)read_start_of_first_chunk(&tagpos, tagend,
                                                &num_ent, &coll_freq);
            }

            // Seek to the next chunk.
            cursor->next();
            if (cursor->after_end()) {
                throw Xapian::DatabaseCorruptError("Expected another key but found none");
            }
            const char *kpos = cursor->current_key.data();
            const char *kend = kpos + cursor->current_key.size();
            if (!check_tname_in_key(&kpos, kend, tname)) {
                throw Xapian::DatabaseCorruptError("Expected another key with the same term name but found a different one");
            }

            // Read the new first docid
            Xapian::docid new_first_did;
            if (!unpack_uint_preserving_sort(&kpos, kend,
                                             &new_first_did)) {
                report_read_error(kpos);
            }

            cursor->read_tag();
            const char *tagpos = cursor->current_tag.data();
            const char *tagend = tagpos + cursor->current_tag.size();

            // Read the chunk header
            bool new_is_last_chunk;
            Xapian::docid new_last_did_in_chunk =
                read_start_of_chunk(&tagpos, tagend, new_first_did,
                                    &new_is_last_chunk);

            string chunk_data(tagpos, tagend);

            // First remove the renamed tag
            table->del(cursor->current_key);

            // And now write it as the first chunk
            string tag;
            tag = make_start_of_first_chunk(num_ent, coll_freq, new_first_did);
            tag += make_start_of_chunk(new_is_last_chunk,
                                              new_first_did,
                                              new_last_did_in_chunk);
            tag += chunk_data;
            table->add(orig_key, tag);
            return;
        }

        DEBUGLINE(DB, "PostlistChunkWriter::flush(): deleting secondary chunk");
        /* This isn't the first chunk.  Check whether we're the last
         * chunk.
         */

        // Delete this chunk
        table->del(orig_key);

        if (is_last_chunk) {
            DEBUGLINE(DB, "PostlistChunkWriter::flush(): deleting secondary last chunk");
            // Update the previous chunk's is_last_chunk flag.
            AutoPtr<FlintCursor> cursor(table->cursor_get());

            /* Should not find the key we just deleted, but should
             * find the previous chunk. */
            if (cursor->find_entry(orig_key)) {
                throw Xapian::DatabaseCorruptError("Flint key not deleted as we expected");
            }
            // Make sure this is a chunk with the right term attached.
            const char * keypos = cursor->current_key.data();
            const char * keyend = keypos + cursor->current_key.size();
            if (!check_tname_in_key(&keypos, keyend, tname)) {
                throw Xapian::DatabaseCorruptError("Couldn't find chunk before delete chunk");
            }

            bool is_prev_first_chunk = (keypos == keyend);

            // Now update the last_chunk
            cursor->read_tag();
            string tag = cursor->current_tag;

            const char *tagpos = tag.data();
            const char *tagend = tagpos + tag.size();

            // Skip first chunk header
            Xapian::docid first_did_in_chunk;
            if (is_prev_first_chunk) {
                first_did_in_chunk = read_start_of_first_chunk(&tagpos, tagend,
                                          0, 0);
            } else {
                if (!unpack_uint_preserving_sort(&keypos, keyend,
                                                 &first_did_in_chunk))
                    report_read_error(keypos);
            }
            bool wrong_is_last_chunk;
            string::size_type start_of_chunk_header = tagpos - tag.data();
            Xapian::docid last_did_in_chunk =
                read_start_of_chunk(&tagpos, tagend, first_did_in_chunk,
                                    &wrong_is_last_chunk);
            string::size_type end_of_chunk_header = tagpos - tag.data();

            // write new is_last flag
            write_start_of_chunk(tag,
                                 start_of_chunk_header,
                                 end_of_chunk_header,
                                 true, // is_last_chunk
                                 first_did_in_chunk,
                                 last_did_in_chunk);
            table->add(cursor->current_key, tag);
        }
    } else {
        DEBUGLINE(DB, "PostlistChunkWriter::flush(): updating chunk which still has items in it");
        /* The chunk still has some items in it.  Two major subcases:
         * a) This is the first chunk.
         * b) This isn't the first chunk.
         *
         * The subcases just affect the chunk header.
         */
        string tag;

        /* First write the header, which depends on whether this is the
         * first chunk.
         */
        if (is_first_chunk) {
            /* The first chunk.  This is the relatively easy case,
             * and we just have to write this one back to disk.
             */
            DEBUGLINE(DB, "PostlistChunkWriter::flush(): rewriting the first chunk, which still has items in it");
            string key = FlintPostListTable::make_key(tname);
            bool ok = table->get_exact_entry(key, tag);
            (void)ok;
            Assert(ok);
            Assert(!tag.empty());

            Xapian::termcount num_ent, coll_freq;
            {
                const char * tagpos = tag.data();
                const char * tagend = tagpos + tag.size();
                (void)read_start_of_first_chunk(&tagpos, tagend,
                                                &num_ent, &coll_freq);
            }

            tag = make_start_of_first_chunk(num_ent, coll_freq, first_did);

            tag += make_start_of_chunk(is_last_chunk, first_did, current_did);
            tag += chunk;
            table->add(key, tag);
            return;
        }

        DEBUGLINE(DB, "PostlistChunkWriter::flush(): updating secondary chunk which still has items in it");
        /* Not the first chunk.
         *
         * This has the easy sub-sub-case:
         *   The first entry in the chunk hasn't changed
         * ...and the hard sub-sub-case:
         *   The first entry in the chunk has changed.  This is
         *   harder because the key for the chunk changes, so
         *   we've got to do a switch.
         */

        // First find out the initial docid
        const char *keypos = orig_key.data();
        const char *keyend = keypos + orig_key.size();
        if (!check_tname_in_key(&keypos, keyend, tname)) {
            throw Xapian::DatabaseCorruptError("Have invalid key writing to postlist");
        }
        Xapian::docid initial_did;
        if (!unpack_uint_preserving_sort(&keypos, keyend, &initial_did)) {
            report_read_error(keypos);
        }
        string new_key;
        if (initial_did != first_did) {
            /* The fiddlier case:
             * Create a new tag with the correct key, and replace
             * the old one.
             */
            new_key = FlintPostListTable::make_key(tname, first_did);
            table->del(orig_key);
        } else {
            new_key = orig_key;
        }

        // ...and write the start of this chunk.
        tag = make_start_of_chunk(is_last_chunk, first_did, current_did);

        tag += chunk;
        table->add(new_key, tag);
    }
}

void FlintPostList::read_number_of_entries(const char ** posptr,
                                   const char * end,
                                   Xapian::doccount * number_of_entries_ptr,
                                   Xapian::termcount * collection_freq_ptr)
{
    if (!unpack_uint(posptr, end, number_of_entries_ptr))
        report_read_error(*posptr);
    if (!unpack_uint(posptr, end, collection_freq_ptr))
        report_read_error(*posptr);
}

FlintPostList::FlintPostList(Xapian::Internal::RefCntPtr<const FlintDatabase> this_db_,
                             const string & tname_)
        : this_db(this_db_),
          tname(tname_),
          have_started(false),
          cursor(this_db->postlist_table.cursor_get()),
          is_at_end(false)
{
    DEBUGCALL(DB, void, "FlintPostList::ostList",
              this_db_.get() << ", " << tname_);
    string key = FlintPostListTable::make_key(tname);
    int found = cursor->find_entry(key);
    if (!found) {
        number_of_entries = 0;
        is_at_end = true;
        pos = 0;
        end = 0;
        first_did_in_chunk = 0;
        last_did_in_chunk = 0;
        return;
    }
    cursor->read_tag();
    pos = cursor->current_tag.data();
    end = pos + cursor->current_tag.size();

    did = read_start_of_first_chunk(&pos, end, &number_of_entries, NULL);
    first_did_in_chunk = did;
    last_did_in_chunk = read_start_of_chunk(&pos, end, first_did_in_chunk,
                                            &is_last_chunk);
    read_wdf_and_length(&pos, end, &wdf, &doclength);
}

FlintPostList::~FlintPostList()
{
    DEBUGCALL(DB, void, "FlintPostList::~FlintPostList", "");
}

bool
FlintPostList::next_in_chunk()
{
    DEBUGCALL(DB, bool, "FlintPostList::next_in_chunk", "");
    if (pos == end) RETURN(false);

    read_did_increase(&pos, end, &did);
    read_wdf_and_length(&pos, end, &wdf, &doclength);

    // Either not at last doc in chunk, or pos == end, but not both.
    Assert(did <= last_did_in_chunk);
    Assert(did < last_did_in_chunk || pos == end);
    Assert(pos != end || did == last_did_in_chunk);

    RETURN(true);
}

void
FlintPostList::next_chunk()
{
    DEBUGCALL(DB, void, "FlintPostList::next_chunk", "");
    if (is_last_chunk) {
        is_at_end = true;
        return;
    }

    cursor->next();
    if (cursor->after_end()) {
        is_at_end = true;
        throw Xapian::DatabaseCorruptError("Unexpected end of posting list for `" +
                                     tname + "'");
    }
    const char * keypos = cursor->current_key.data();
    const char * keyend = keypos + cursor->current_key.size();
    // Check we're still in same postlist
    if (!check_tname_in_key_lite(&keypos, keyend, tname)) {
        is_at_end = true;
        throw Xapian::DatabaseCorruptError("Unexpected end of posting list for `" +
                                     tname + "'");
    }

    Xapian::docid newdid;
    if (!unpack_uint_preserving_sort(&keypos, keyend, &newdid)) {
        report_read_error(keypos);
    }
    if (newdid <= did) {
        throw Xapian::DatabaseCorruptError("Document ID in new chunk of postlist (" +
                om_tostring(newdid) +
                ") is not greater than final document ID in previous chunk (" +
                om_tostring(did) + ")");
    }
    did = newdid;

    cursor->read_tag();
    pos = cursor->current_tag.data();
    end = pos + cursor->current_tag.size();

    first_did_in_chunk = did;
    last_did_in_chunk = read_start_of_chunk(&pos, end, first_did_in_chunk,
                                            &is_last_chunk);
    read_wdf_and_length(&pos, end, &wdf, &doclength);
}

PositionList *
FlintPostList::read_position_list()
{
    DEBUGCALL(DB, PositionList *, "FlintPostList::read_position_list", "");
    positionlist.read_data(&this_db->position_table, did, tname);
    RETURN(&positionlist);
}

PositionList *
FlintPostList::open_position_list() const
{
    DEBUGCALL(DB, PositionList *, "FlintPostList::open_position_list", "");
    RETURN(new FlintPositionList(&this_db->position_table, did, tname));
}

PostList *
FlintPostList::next(Xapian::weight w_min)
{
    DEBUGCALL(DB, PostList *, "FlintPostList::next", w_min);
    (void)w_min; // no warning

    if (!have_started) {
        have_started = true;
    } else {
        if (!next_in_chunk()) next_chunk();
    }

    DEBUGLINE(DB, string("Moved to ") <<
              (is_at_end ? string("end.") : string("docid, wdf, doclength = ") +
               om_tostring(did) + ", " + om_tostring(wdf) + ", " +
               om_tostring(doclength)));

    RETURN(NULL);
}

bool
FlintPostList::current_chunk_contains(Xapian::docid desired_did)
{
    DEBUGCALL(DB, bool, "FlintPostList::current_chunk_contains", desired_did);
    if (desired_did >= first_did_in_chunk &&
        desired_did <= last_did_in_chunk) {
        RETURN(true);
    }
    RETURN(false);
}

void
FlintPostList::move_to_chunk_containing(Xapian::docid desired_did)
{
    DEBUGCALL(DB, void,
              "FlintPostList::move_to_chunk_containing", desired_did);
    (void)cursor->find_entry(FlintPostListTable::make_key(tname, desired_did));
    Assert(!cursor->after_end());

    const char * keypos = cursor->current_key.data();
    const char * keyend = keypos + cursor->current_key.size();
    // Check we're still in same postlist
    if (!check_tname_in_key_lite(&keypos, keyend, tname)) {
        // This should only happen if the postlist doesn't exist at all.
        is_at_end = true;
        is_last_chunk = true;
        return;
    }
    is_at_end = false;

    cursor->read_tag();
    pos = cursor->current_tag.data();
    end = pos + cursor->current_tag.size();

    if (keypos == keyend) {
        // In first chunk
#ifdef XAPIAN_DEBUG
        Xapian::doccount old_number_of_entries = number_of_entries;
        did = read_start_of_first_chunk(&pos, end, &number_of_entries, NULL);
        Assert(old_number_of_entries == number_of_entries);
#else
        did = read_start_of_first_chunk(&pos, end, NULL, NULL);
#endif
    } else {
        // In normal chunk
        if (!unpack_uint_preserving_sort(&keypos, keyend, &did)) {
            report_read_error(keypos);
        }
    }

    first_did_in_chunk = did;
    last_did_in_chunk = read_start_of_chunk(&pos, end, first_did_in_chunk,
                                            &is_last_chunk);
    read_wdf_and_length(&pos, end, &wdf, &doclength);

    // Possible, since desired_did might be after end of this chunk and before
    // the next.
    if (desired_did > last_did_in_chunk) next_chunk();
}

bool
FlintPostList::move_forward_in_chunk_to_at_least(Xapian::docid desired_did)
{
    DEBUGCALL(DB, bool,
              "FlintPostList::move_forward_in_chunk_to_at_least", desired_did);
    if (desired_did > last_did_in_chunk) {
        pos = end;
        RETURN(false);
    }
    while (did < desired_did) {
        // FIXME: perhaps we don't need to decode the wdf and document length
        // for documents we're skipping past.
        bool at_end_of_chunk = !next_in_chunk();
        if (at_end_of_chunk) RETURN(false);
    }
    RETURN(true);
}

PostList *
FlintPostList::skip_to(Xapian::docid desired_did, Xapian::weight w_min)
{
    DEBUGCALL(DB, PostList *,
              "FlintPostList::skip_to", desired_did << ", " << w_min);
    (void)w_min; // no warning
    // We've started now - if we hadn't already, we're already positioned
    // at start so there's no need to actually do anything.
    have_started = true;

    // Don't skip back, and don't need to do anything if already there.
    if (is_at_end || desired_did <= did) RETURN(NULL);

    // Move to correct chunk
    if (!current_chunk_contains(desired_did)) {
        move_to_chunk_containing(desired_did);
        // Might be at_end now, so we need to check before trying to move
        // forward in chunk.
        if (is_at_end) RETURN(NULL);
    }

    // Move to correct position in chunk
    bool have_document = move_forward_in_chunk_to_at_least(desired_did);
#ifdef XAPIAN_DEBUG
    Assert(have_document);
#else
    (void)have_document;
#endif

    DEBUGLINE(DB, string("Skipped to ") <<
              (is_at_end ? string("end.") : string("docid, wdf, doclength = ") +
               om_tostring(did) + ", " + om_tostring(wdf) + ", " +
               om_tostring(doclength) + "."));

    RETURN(NULL);
}

string
FlintPostList::get_description() const
{
    return tname + ":" + om_tostring(number_of_entries);
}

// Returns the last did to allow in this chunk.
Xapian::docid
FlintPostListTable::get_chunk(const string &tname,
          Xapian::docid did, bool adding,
          PostlistChunkReader ** from, PostlistChunkWriter **to)
{
    // Get chunk containing entry
    string key = make_key(tname, did);

    // Find the right chunk
    AutoPtr<FlintCursor> cursor(cursor_get());

    cursor->find_entry(key);
    Assert(!cursor->after_end());

    const char * keypos = cursor->current_key.data();
    const char * keyend = keypos + cursor->current_key.size();

    if (!check_tname_in_key(&keypos, keyend, tname)) {
        // Postlist for this termname doesn't exist.
        if (!adding)
            throw Xapian::DatabaseCorruptError("Attempted to delete or modify an entry in a non-existent posting list for " + tname);

        *from = NULL;
        *to = new PostlistChunkWriter("", true, tname, true);
        return Xapian::docid(-1);
    }

    // See if we're appending - if so we can shortcut by just copying
    // the data part of the chunk wholesale.
    bool is_first_chunk = (keypos == keyend);

    cursor->read_tag();
    const char * pos = cursor->current_tag.data();
    const char * end = pos + cursor->current_tag.size();
    Xapian::docid first_did_in_chunk;
    if (is_first_chunk) {
        first_did_in_chunk = read_start_of_first_chunk(&pos, end, NULL, NULL);
    } else {
        if (!unpack_uint_preserving_sort(&keypos, keyend,
                                         &first_did_in_chunk)) {
            report_read_error(keypos);
        }
    }

    bool is_last_chunk;
    Xapian::docid last_did_in_chunk;
    last_did_in_chunk = read_start_of_chunk(&pos, end, first_did_in_chunk, &is_last_chunk);
    *to = new PostlistChunkWriter(cursor->current_key, is_first_chunk, tname,
                                  is_last_chunk);
    if (did > last_did_in_chunk) {
        // This is the shortcut.  Not very pretty, but I'll leave refactoring
        // until I've a clearer picture of everything which needs to be done.
        // (FIXME)
        *from = NULL;
        (*to)->raw_append(first_did_in_chunk, last_did_in_chunk,
                          string(pos, end)); 
    } else {
        *from = new PostlistChunkReader(first_did_in_chunk, string(pos, end));
    }
    if (is_last_chunk) return Xapian::docid(-1);

    // Find first did of next tag.
    cursor->next();
    if (cursor->after_end()) {
        throw Xapian::DatabaseCorruptError("Expected another key but found none");
    }
    const char *kpos = cursor->current_key.data();
    const char *kend = kpos + cursor->current_key.size();
    if (!check_tname_in_key(&kpos, kend, tname)) {
        throw Xapian::DatabaseCorruptError("Expected another key with the same term name but found a different one");
    }

    // Read the new first docid
    Xapian::docid first_did_of_next_chunk;
    if (!unpack_uint_preserving_sort(&kpos, kend, &first_did_of_next_chunk)) {
        report_read_error(kpos);
    }
    return first_did_of_next_chunk - 1;
}

void
FlintPostListTable::merge_changes(
    const map<string, map<Xapian::docid, pair<char, Xapian::termcount> > > & mod_plists,
    const map<Xapian::docid, Xapian::termcount> & doclens,
    const map<string, pair<Xapian::termcount_diff, Xapian::termcount_diff> > & freq_deltas)
{
    DEBUGCALL(DB, void, "FlintPostListTable::merge_changes", "mod_plists, doclens, freq_deltas");
    map<string, map<Xapian::docid, pair<char, Xapian::termcount> > >::const_iterator i;
    for (i = mod_plists.begin(); i != mod_plists.end(); ++i) {
        if (i->second.empty()) continue;
        string tname = i->first;
        {
            // Rewrite the first chunk of this posting list with the updated
            // termfreq and collfreq.
            map<string, pair<Xapian::termcount_diff, Xapian::termcount_diff> >::const_iterator deltas = freq_deltas.find(tname);
            Assert(deltas != freq_deltas.end());

            string current_key = make_key(tname);
            string tag;
            (void)get_exact_entry(current_key, tag);

            // Read start of first chunk to get termfreq and collfreq.
            const char *pos = tag.data();
            const char *end = pos + tag.size();
            Xapian::termcount termfreq, collfreq;
            Xapian::docid firstdid, lastdid;
            bool islast;
            if (pos == end) {
                termfreq = 0;
                collfreq = 0;
                firstdid = 0;
                lastdid = 0;
                islast = true;
            } else {
                firstdid = read_start_of_first_chunk(&pos, end,
                                                     &termfreq, &collfreq);
                // Handle the generic start of chunk header.
                lastdid = read_start_of_chunk(&pos, end, firstdid, &islast);
            }

            termfreq += deltas->second.first;
            if (termfreq == 0) {
                // All postings deleted!  So we can shortcut by zapping the
                // posting list.
                if (islast) {
                    // Only one entry for this posting list.
                    del(current_key);
                    continue;
                }
                AutoPtr<FlintCursor> cursor(cursor_get());
                bool found = cursor->find_entry(current_key);
                Assert(found);
                if (!found) continue; // Reduce damage!
                while (cursor->del()) {
                    const char *kpos = cursor->current_key.data();
                    const char *kend = kpos + cursor->current_key.size();
                    if (!check_tname_in_key_lite(&kpos, kend, tname)) break;
                }
                continue;
            }
            collfreq += deltas->second.second;

            // Rewrite start of first chunk to update termfreq and collfreq.
            string newhdr = make_start_of_first_chunk(termfreq, collfreq, firstdid);
            newhdr += make_start_of_chunk(islast, firstdid, lastdid);
            if (pos == end) {
                add(current_key, newhdr);
            } else {
                Assert((size_t)(pos - tag.data()) <= tag.size());
                tag.replace(0, pos - tag.data(), newhdr);
                add(current_key, tag);
            }
        }
        map<Xapian::docid, pair<char, Xapian::termcount> >::const_iterator j;
        j = i->second.begin();
        Assert(j != i->second.end()); // This case is caught above.

        Xapian::docid max_did;
        PostlistChunkReader *from;
        PostlistChunkWriter *to;
        max_did = get_chunk(tname, j->first, j->second.first == 'A',
                            &from, &to);
        for ( ; j != i->second.end(); ++j) {
            Xapian::docid did = j->first;

next_chunk:
            DEBUGLINE(DB, "Updating tname=" << tname << ", did=" << did);
            if (from) while (!from->is_at_end()) {
                Xapian::docid copy_did = from->get_docid();
                if (copy_did >= did) {
                    if (copy_did == did) {
                        Assert(j->second.first != 'A');
                        from->next();
                    }
                    break;
                }
                to->append(this, copy_did,
                           from->get_wdf(), from->get_doclength());
                from->next();
            }
            if ((!from || from->is_at_end()) && did > max_did) {
                delete from;
                to->flush(this);
                delete to;
                max_did = get_chunk(tname, did, false, &from, &to);
                goto next_chunk;
            }

            if (j->second.first != 'D') {
                map<Xapian::docid, Xapian::termcount>::const_iterator k = doclens.find(did);
                Assert(k != doclens.end());
                Xapian::termcount new_doclen = k->second;
                Xapian::termcount new_wdf = j->second.second;

                to->append(this, did, new_wdf, new_doclen);
            }
        }

        if (from) {
            while (!from->is_at_end()) {
                to->append(this, from->get_docid(),
                           from->get_wdf(), from->get_doclength());
                from->next();
            }
            delete from;
        }
        to->flush(this);
        delete to;
    }
}

---