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# meta.dat
Dat uses a simple metadata file called `meta.dat`. The purpose of this file is to store the fingerprints of the files in a Dat repository. If you create a `meta.dat` file for a set of files, you can host it on a static HTTP server along with the files and Dat clients will be able to download and verify your files, even if you aren't running a Dat server!
# File format
```
<Header><Entries Index...><Entries...>
```
The format is a header followed by an index of many entries. Entry order is based on the indexing determined by the [Flat In-Order Tree](hyperdrive.md#flat-in-order-trees) algorithm we use in Dat. After the entry index, a concatinated list of entries follows.
### Header format
```
<varint header-length><header protobuf>
```
The header protobuf has this schema:
``` proto
message Header {
required bytes datLink = 1;
required uint64 entries = 2;
optional bool isSigned = 3;
optional string hashType = 4 [default = "sha256"];
optional uint32 hashLength = 5 [default = 32];
optional string signatureType = 6 [default = "ed25519"];
optional uint32 signatureLength = 7 [default = 64];
}
```
### Entry index format
For non-signed entries:
```
<8-byte-chunk-end><chunk-hash>
```
The 8-byte-chunk-end is an unsigned big endian 64 bit integer that should be the absolute position in the file for the **end of the chunk**.
For signed entries in live feeds (only applies to even numbered nodes e.g. leaf nodes):
```
<8-byte-chunk-end><chunk-signature><chunk-hash>
```
For any odd nodes, in either a live or a non-live feed, the non-signed entry format will be used.
## Example
Given a tree like this you might want to look up in a `meta.dat` file the metadata for a specific node:
```
0─┐
1─┐
2─┘ │
3
4─┐ │
5─┘
6─┘
```
If you wanted to look up the metadata for 3, you could read the third (or any!) entry from meta.dat:
First you have to read the varint at the beginning of the file so you know how big the header is:
``` js
var varint = require('varint') // https://github.com/chrisdickinson/varint
var headerLength = varint.decode(firstChunkOfFile)
```
Now you can read the header from the file
``` js
var headerOffset = varint.encodingLength(headerLength)
var headerEndOffset = headerOffset + headerLength
var headerBytes = firstChunkOfFile.slice(headerOffset, headerEndOffset)
```
To decode the header use the protobuf schema. We can use the [protocol-buffers](https://github.com/mafintosh/protocol-buffers) module to do that.
``` js
var messages = require('protocol-buffers')(fs.readFileSync('meta.dat.proto'))
var header = messages.Header.decode(headerBytes)
```
Now we have all the configuration required to calculate an entry offset.
``` js
var entryNumber = 42
var entryOffset = headerEndOffset + entryNumber * (8 + header.hashLength)
```
If you have a signed feed, you have to take into account the extra space required for the signatures in the even nodes.
``` js
var entryOffset = headerLength + entryNumber * (8 + header.hashLength)
+ Math.floor(entryNumber / 2) * header.signatureLength
```
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