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Comment:Update comments in fts3.c to more accurately describe the doclist format.
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SHA1: e424a0307359fee6875424c10ecad1a10acfba0e
User & Date: drh 2010-01-08 23:01:33
Context
2010-01-09
07:33
Fix handling of an OOM error in the fts3 offsets() function. Fix a couple of snippet related test cases in e_fts3.test. check-in: 14dc46a7 user: dan tags: trunk
2010-01-08
23:01
Update comments in fts3.c to more accurately describe the doclist format. check-in: e424a030 user: drh tags: trunk
04:50
Added option to restore_jrnl.tcl utility to hex dump journal pages. check-in: 08c545f0 user: shaneh tags: trunk
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Changes to ext/fts3/fts3.c.

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**     * The FTS3 module is being built as an extension
**       (in which case SQLITE_CORE is not defined), or
**
**     * The FTS3 module is being built into the core of
**       SQLite (in which case SQLITE_ENABLE_FTS3 is defined).
*/

/* TODO(shess) Consider exporting this comment to an HTML file or the
** wiki.
*/
/* The full-text index is stored in a series of b+tree (-like)
** structures called segments which map terms to doclists.  The
** structures are like b+trees in layout, but are constructed from the
** bottom up in optimal fashion and are not updatable.  Since trees
** are built from the bottom up, things will be described from the
** bottom up.
**
................................................................................
**         B = 1xxxxxxx    7 bits of data and one flag bit
**
**  7 bits - A
** 14 bits - BA
** 21 bits - BBA
** and so on.
**
** This is identical to how sqlite encodes varints (see util.c).









**
**
**** Document lists ****
** A doclist (document list) holds a docid-sorted list of hits for a
** given term.  Doclists hold docids, and can optionally associate
** token positions and offsets with docids.





**
** A DL_POSITIONS_OFFSETS doclist is stored like this:
**
** array {
**   varint docid;
**   array {                (position list for column 0)
**     varint position;     (delta from previous position plus POS_BASE)
**     varint startOffset;  (delta from previous startOffset)
**     varint endOffset;    (delta from startOffset)
**   }
**   array {
**     varint POS_COLUMN;   (marks start of position list for new column)
**     varint column;       (index of new column)
**     array {
**       varint position;   (delta from previous position plus POS_BASE)
**       varint startOffset;(delta from previous startOffset)
**       varint endOffset;  (delta from startOffset)
**     }
**   }
**   varint POS_END;        (marks end of positions for this document.
** }
**
** Here, array { X } means zero or more occurrences of X, adjacent in
** memory.  A "position" is an index of a token in the token stream
** generated by the tokenizer, while an "offset" is a byte offset,
** both based at 0.  Note that POS_END and POS_COLUMN occur in the
** same logical place as the position element, and act as sentinals
** ending a position list array.














**
** A DL_POSITIONS doclist omits the startOffset and endOffset
** information.  A DL_DOCIDS doclist omits both the position and
** offset information, becoming an array of varint-encoded docids.
**
** On-disk data is stored as type DL_DEFAULT, so we don't serialize
** the type.  Due to how deletion is implemented in the segmentation
................................................................................
        z[iOut++] = z[iIn++];
      }
    }
    z[iOut] = '\0';
  }
}






static void fts3GetDeltaVarint(char **pp, sqlite3_int64 *pVal){
  sqlite3_int64 iVal;
  *pp += sqlite3Fts3GetVarint(*pp, &iVal);
  *pVal += iVal;
}







static void fts3GetDeltaVarint2(char **pp, char *pEnd, sqlite3_int64 *pVal){
  if( *pp>=pEnd ){
    *pp = 0;
  }else{
    fts3GetDeltaVarint(pp, pVal);
  }
}
................................................................................
  *ppCsr = pCsr = (sqlite3_vtab_cursor *)sqlite3_malloc(sizeof(Fts3Cursor));
  if( !pCsr ){
    return SQLITE_NOMEM;
  }
  memset(pCsr, 0, sizeof(Fts3Cursor));
  return SQLITE_OK;
}

/****************************************************************/
/****************************************************************/
/****************************************************************/
/****************************************************************/


/*
** Close the cursor.  For additional information see the documentation
** on the xClose method of the virtual table interface.
*/
static int fulltextClose(sqlite3_vtab_cursor *pCursor){
  Fts3Cursor *pCsr = (Fts3Cursor *)pCursor;







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**     * The FTS3 module is being built as an extension
**       (in which case SQLITE_CORE is not defined), or
**
**     * The FTS3 module is being built into the core of
**       SQLite (in which case SQLITE_ENABLE_FTS3 is defined).
*/




/* The full-text index is stored in a series of b+tree (-like)
** structures called segments which map terms to doclists.  The
** structures are like b+trees in layout, but are constructed from the
** bottom up in optimal fashion and are not updatable.  Since trees
** are built from the bottom up, things will be described from the
** bottom up.
**
................................................................................
**         B = 1xxxxxxx    7 bits of data and one flag bit
**
**  7 bits - A
** 14 bits - BA
** 21 bits - BBA
** and so on.
**
** This is similar in concept to how sqlite encodes "varints" but
** the encoding is not the same.  SQLite varints are big-endian
** are are limited to 9 bytes in length whereas FTS3 varints are
** little-endian and can be upt to 10 bytes in length (in theory).
**
** Example encodings:
**
**     1:    0x01
**   127:    0x7f
**   128:    0x81 0x00
**
**
**** Document lists ****
** A doclist (document list) holds a docid-sorted list of hits for a
** given term.  Doclists hold docids, and can optionally associate
** token positions and offsets with docids.  A position is the index
** of a word within the document.  The first word of the document has
** a position of 0.
**
** FTS3 used to optionally store character offsets using a compile-time
** option.  But that functionality is no longer supported.
**
** A DL_POSITIONS_OFFSETS doclist is stored like this:
**
** array {
**   varint docid;
**   array {                (position list for column 0)
**     varint position;     (delta from previous position plus POS_BASE)


**   }
**   array {
**     varint POS_COLUMN;   (marks start of position list for new column)
**     varint column;       (index of new column)
**     array {
**       varint position;   (delta from previous position plus POS_BASE)


**     }
**   }
**   varint POS_END;        (marks end of positions for this document.
** }
**
** Here, array { X } means zero or more occurrences of X, adjacent in
** memory.  A "position" is an index of a token in the token stream

** generated by the tokenizer. Note that POS_END and POS_COLUMN occur 
** in the same logical place as the position element, and act as sentinals
** ending a position list array.  POS_END is 0.  POS_COLUMN is 1.
** The positions numbers are not stored literally but rather as two more
** the difference from the prior position, or the just the position plus
** 2 for the first position.  Example:
**
**   label:       A B C D E  F  G H   I  J K
**   value:     123 5 9 1 1 14 35 0 234 72 0
**
** The 123 value is the first docid.  For column zero in this document
** there are two matches at positions 3 and 10 (5-2 and 9-2+3).  The 1
** at D signals the start of a new column; the 1 at E indicates that the
** new column is column number 1.  There are two positions at 12 and 45
** (14-2 and 35-2+12).  The 0 at H indicate the end-of-document.  The
** 234 at I is the next docid.  It has one position 72 (72-2) and then
** terminates with the 0 at K.
**
** A DL_POSITIONS doclist omits the startOffset and endOffset
** information.  A DL_DOCIDS doclist omits both the position and
** offset information, becoming an array of varint-encoded docids.
**
** On-disk data is stored as type DL_DEFAULT, so we don't serialize
** the type.  Due to how deletion is implemented in the segmentation
................................................................................
        z[iOut++] = z[iIn++];
      }
    }
    z[iOut] = '\0';
  }
}

/*
** Read a single varint from the doclist at *pp and advance *pp to point
** to the next element of the varlist.  Add the value of the varint
** to *pVal.
*/
static void fts3GetDeltaVarint(char **pp, sqlite3_int64 *pVal){
  sqlite3_int64 iVal;
  *pp += sqlite3Fts3GetVarint(*pp, &iVal);
  *pVal += iVal;
}

/*
** As long as *pp has not reached its end (pEnd), then do the same
** as fts3GetDeltaVarint(): read a single varint and add it to *pVal.
** But if we have reached the end of the varint, just set *pp=0 and
** leave *pVal unchanged.
*/
static void fts3GetDeltaVarint2(char **pp, char *pEnd, sqlite3_int64 *pVal){
  if( *pp>=pEnd ){
    *pp = 0;
  }else{
    fts3GetDeltaVarint(pp, pVal);
  }
}
................................................................................
  *ppCsr = pCsr = (sqlite3_vtab_cursor *)sqlite3_malloc(sizeof(Fts3Cursor));
  if( !pCsr ){
    return SQLITE_NOMEM;
  }
  memset(pCsr, 0, sizeof(Fts3Cursor));
  return SQLITE_OK;
}







/*
** Close the cursor.  For additional information see the documentation
** on the xClose method of the virtual table interface.
*/
static int fulltextClose(sqlite3_vtab_cursor *pCursor){
  Fts3Cursor *pCsr = (Fts3Cursor *)pCursor;