Changes

Jump to: navigation, search

ZIM file format

4,346 bytes removed, 18:54, 10 December 2017
no edit summary
|-
| checksumPos || integer || 72 || 8 || pointer to the md5checksum of this file without the checksum itself. This points always 16 bytes before the end of the file.
|-
| geoIndexPos || integer || 80 || 8 || pointer to the geo index (optional). Present if mimeListPos is at least 80.
|}
The URL pointer list is a list of 8 byte offsets to the directory entries.
The directory entries are always ordered by URL (including the namespace prefix). Ordering is simply done by comparing the URL strings in binary.
Since directory entries have variable sizes this is needed for random access.
The offset addresses uncompressed data. The last pointer points to the end of the data area. So there is always one more offset than blobs. Since the first offset points to the start of the first data, the number of offsets can be determined by dividing this offset by 4. The size of one blob is calculated by the difference of two consecutive offsets.
 
== Geo Index ==
 
The geo index contains a spatially indexed data structure so that it is possible to search for articles by their position on the globe. Coordinates are assumed to be WGS84 coordinates, but encoded into binary in the following way: Both latitudes and longitudes use the full span of 32 bit unsigned big endian integers, which means that the south pole has an encoded latitude of
0 and the north pole an encoded latitude of 0xffffffff (actually 0x100000000). This means that the scaling between degree and encoded integers
is different for latitudes and longitudes. The coordinate of Berlin, which is approximately at 52° 31′ N, 13° 23′ E or
52.516667, 13.383333 is encoded as latitude (52.516667 + 90) / 180 * 0x100000000 = 3400580132 and longitude
(13.383333 + 180) / 360 * 0x100000000 = 2307153030.
 
The index actually consists of a list of indices assumed to be ordered by importance. The idea is that the indices
are searched in order until a certain amount of search results is found, such that at low zoom levels, only the
important articles (countries, large cities) are displayed.
 
All offsets in the geo index are counted relative to the start of the geo index. This means that in order to get
absolute file offsets, you have to add the value of the geoIndexPos field in the file header.
 
At geoIndexPos, the geo index starts with the following table:
 
{|{{Prettytable}}
! Field Name !! Type !!Offset!!Length!! Description
|-
| indicesCount || integer || 0 || 4 || number of geo indices (=n)
|-
| <1st offset> || integer || 4 || 4 || offset to the data area of the 1st geo index
|-
| <2nd offset> || integer || 8 || 4 || offset to the data area of the 2nd geo index
|-
| ... || integer || ... || 4 || ...
|-
| <nth offset> || integer ||n*4|| 4 || offset to the data area of the nth geo index
|-
| <end offset> || integer ||(n+1)*4|| 4 || offset to the end of the data area of the nth geo index
|}
 
Each geo index is a binary tree of coordinates alternating between latitudes and longitudes (starting with latitudes)
ending in a list of coordinates and article indices as leaves. A non-leaf node is encoded as follows:
 
{|{{Prettytable}}
! Field Name !! Type !!Offset!!Length!! Description
|-
| value || integer || ? || 4 || discriminator coordinate, cannot be zero (would indicate a leaf node)
|-
| greaterOffset || integer || ? + 4 || 4 || offset to the node containing coordinates with latitude / longitude greater than the discriminator
|-
| <smaller or equal coordinates> || ... || ... || ... || node leading to coordinates with latitude / longitude smaller or equal than the discriminator
|-
| <larger coordinates> || ... || greaterOffset || ... || node leading to coordinates with latitude / longitude greater than the discriminator
|}
 
Leaf nodes containing the actual coordinates and links to the articles are encoded as follows:
 
{|{{Prettytable}}
! Field Name !! Type !!Offset!!Length!! Description
|-
| 0 || integer || ? || 4 || value identifying leaf nodes
|-
| numCoordinates || integer || ? + 4 || 4 || number of coordinates that follow
|-
| <1st latitude> || integer || ? + 8 || 4 || 1st latitude
|-
| <1st longitude> || integer || ? + 12 || 4 || 1st longitude
|-
| <1st title index> || integer || ? + 16 || 4 || 1st title index
|-
| ... || ... || ... || ... || ...
|-
| <nth latitude> || integer || ? + n * 12 - 4 || 4 || nth latitude
|-
| <nth longitude> || integer || ? + n * 12 || 4 || nth longitude
|-
| <nth title index> || integer || ? + n * 12 + 4 || 4 || nth title index
|}
 
The title indices in the above table are article numbers just like in the title pointer list.
 
 
It can be assumed that the index is generated in the following way: All coordinates are sorted
by latitude. The median is stored as discriminator value, the first half of the coordinates is
stored in a "lower node", the second half in an upper node. For each of the two nodes, the
coordinates are sorted by longitude and are split in much the same way until there are
less than a specific number of articles (e.g. 10) left at which point they are
stored as a list in a leaf node.
== Namespaces ==

Navigation menu