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 offset into the
stop-word
index pointing at the first character of a stop-word entry; similarly,
every directory_offset is an offset into the
directory 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 compactness.
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, respectively; 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 integer
(although it can be any other byte); therefore, it can be used as a
marker to embed extra information into an encoded integer byte
sequence.
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 encodes 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:
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 integers 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:
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:
that is: every word is null-terminated.
Directory Entries¶
Every directory entry in the
directory index++ is of the form:
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
number of words in the file (
W) followed by the file's null-terminated
title.
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.
Every meta-name entry in the
meta-name index is of the form:
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>