swish++.index(5) File Formats Manual swish++.index(5)
NAME
swish++.index - SWISH++ index file format
SYNOPSIS
long num_words;
off_t word_offset[ num_words ];
long num_stop_words;
off_t stop_word_offset[ num_stop_words ];
long num_directories;
off_t directory_offset[ num_directories ];
long num_files;
off_t file_offset[ num_files ];
long num_meta_names;
off_t meta_name_offset[ num_meta_names ];
word index
stop-word index
directory index++
file index
meta-name index
DESCRIPTION
The index file format used by SWISH++ is as shown above. Every
word_offset is an offset into the word index pointing at the first
character of a word entry; similarly, every stop_word_offset is an off-
set into the stop-word index pointing at the first character of a stop-
word entry; similarly, every directory_offset is an offset into the di-
rectory index++ pointing at the first character of a directory entry;
similarly, every file_offset is an offset into the file index pointing
at the first byte of a file entry; finally, every meta_name_offset is
an offset into the meta-name index pointing at the first character of a
meta-name entry.
The index file is written as it is so that it can be mapped into memory
via the mmap(2) Unix system call enabling ``instantaneous'' access.
Encoded Integers
All integers in an index file are stored in an encoded format for com-
pactness. An integer is encoded to use a varying number of bytes. For
a given byte, only the lower 7 bits are used for data; the high bit, if
set, is used to indicate whether the integer continues into the next
byte. The encoded integer is in big-endian byte order. For example,
the integers 0-127 are encoded as single bytes of \x00-\x7F, respec-
tively; the integer 128 is encoded as the two bytes of \x8100.
Note that the byte \x80 will never be the first byte of an encoded in-
teger (although it can be any other byte); therefore, it can be used as
a marker to embed extra information into an encoded integer byte se-
quence.
Encoded Integer Lists
Because the high bit of the last byte of an encoded integer is always
clear, lists of encoded integers can be stored one right after the
other with no separators. For example, the byte sequence \x010203 en-
codes the three separate integers 1, 2, and 3; the byte sequence
\x81002A8101 encodes the three integers 128, 42, and 129.
Word Entries
Every word entry in the word index is of the form:
word0{data}...
that is: a null-terminated word followed by one or more data entries
where a data entry is:
{F}{O}{R}[{list}...]{marker}
that is: a file-index (F) followed by the number of occurrences in the
file (O) followed by a rank (R) followed by zero or more lists of inte-
gers followed by a marker byte. The file-index is an index into the
file_offset table; the marker byte is one of:
00 Another data entry follows.
80 No more data entries follow.
A list is:
{type}{I}...x80
that is: a type followed by one or more integers (I) followed by an
\x80 byte where type defines the type of list, i.e., what the integers
mean. Currently, there is only one type of list:
01 Meta-ID list. Meta-IDs are unique integers identifying which meta
name(s) a word is associated with in the meta-name index.
02 Word-position list. Every word in a file has a position: the first
word is 1, the second word is 2, etc. Each word position is stored
as a delta from the previous position for compactness.
Stop-Word Entries
Every stop-word entry in the stop-word index is of the form:
stop-word0
that is: every word is null-terminated.
Directory Entries
Every directory entry in the directory index++ is of the form:
directory-path0
that is: a null-terminated full pathname of a directory (not including
the trailing slash). The pathnames are relative to where the indexing
was performed (unless absolute paths were used).
File Entries
Every file entry in the file index is of the form:
{D}file-name0{S}{W}file-title0
that is: the file's directory index++ (D) followed by a null-terminated
file name followed by the file's size in bytes (S) followed by the num-
ber of words in the file (W) followed by the file's null-terminated ti-
tle.
For an HTML or XHTML file, the title is what is between <TITLE> ...
</TITLE> pairs; for an MP3 file, the title is the value of title field;
for a mail or news file, the title is the value of the Subject header;
for a LaTeX file, the title is the argument of the \title command; for
a Unix manual page file, the title is the contents of the first line
within the NAME section. If a file is not one of those types of files,
or is but does not have a title, the title is simply the file (not
path) name.
Meta-Name Entries
Every meta-name entry in the meta-name index is of the form:
meta-name0{I}
that is: a null-terminated meta-name followed by the ID (I).
CAVEATS
Generated index files are machine-dependent (size of data types and
byte-order).
SEE ALSO
index++(1), search++(1)
AUTHOR
Paul J. Lucas <pauljlucas@mac.com>
SWISH++ March 29, 2004 swish++.index(5)
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