SpiralP/trickle

Lua bitstream

Trickle

Trickle is a bitstream for Lua. When writing data to a trickle stream, you must specify its size in bits. The stream will automatically pack this data into compressed bytes, which often leads to greatly reduced network bandwidth. This is useful in networking applications where it is necessary to reduce network traffic, such as realtime multiplayer games.

Usage

local trickle = require 'trickle'
local stream = trickle.create()

stream:write(7, '3bits')
stream:write(true, 'bool')
stream:write(1.8, 'float')

-- send over the network
-- to convert a stream to a string, call tostring(stream)
-- to convert a string to a stream, call trickle.create(string)

stream:read('3bits') -- 7
stream:read('bool') -- true
stream:read('float') -- 1.8

Signatures

A signature describes a high level template for a message. It can be used to reduce duplication in reading and writing of messages, and allows reading and writing of tables of data without having to write each individual field in a specific order. First, define a signature:

local signature = {
  { 'playerId', '4bits' },
  { 'x', '8bits' },
  { 'y', '8bits' },
  { 'direction', '9bits' } -- direction in degrees
}

Then, pack a message into a trickle stream using the signature:

local playerData = {
  playerId = 1,
  x = 63,
  y = 41,
  direction = 270
}

stream:pack(playerData, signature)

unpack may be used to decode the stream:

local playerData = stream:unpack(signature)

Delta Groups

Let's say we only want to send the position of the player if it has changed. Right now this isn't possible with our signature, because we have to send 8 bits for the x position and 8 bits for the y position to keep the structure of the signature uniform for the receiving end. Delta groups provide a mechanism to specify these optional groups of data.

To declare a delta group, specify a delta key in the signature table:

local signature = {
  { 'playerId', '4bits'},
  { 'x', '8bits' },
  { 'y', '8bits' },
  { 'direction', '9bits' },
  delta = {
    { 'x', 'y' },
    'direction'
  }
}

What we've done is defined two optional pieces of our message: a group for the position of the player and a group for the direction of the player. Now, if we pack a message like this:

local playerData = {
  playerId = 1,
  x = nil,
  y = nil,
  direction = nil
}

stream:pack(playerData, signature)

trickle will only pack the 4 bit player id into the message, which saves 25 bits! Internally, a single bit is added to the beginning of the message for each delta group, signifying whether or not its data is present. This means that if a delta group contains more than one key (such as the 'x' and 'y' group above), either all of the data must be present or all of it must be nil. This, for example, would result in an error:

local playerData = {
  playerId = 1,
  x = 17,
  y = nil,
  direction = 90
}

stream:pack(playerData, signature) -- Error: Only part of message delta group "x, y" was provided.

Documentation

• trickle.create([str]) - Creates a stream. If str is provided then the contents of the stream are set to this initial value.
• tostring(stream) - Serialize the stream's data to a string.
• stream:clear() - Resets the stream to contain no data.
• stream:truncate() - If a byte has been partially written, it will not be present in the stream yet. truncate pads the rest of the byte with null data and adds it to the stream's data. This is mostly for internal use, and is automatically called when converting the stream to a string.

Writing

• stream:write(value, type) - Writes a value to the stream. type can either be string, float, bool, or <n>bits for integers, where n is the number of bits the integer should use.
• stream:writeString(str) - Writes a string to the stream. Uses one byte per character.
• stream:writeBool(bool) - Writes a boolean to the stream. Uses one bit.
• stream:writeFloat(float) - Writes a float to the stream. Note that the float is currently serialized as a string. This is suboptimal and should be improved in the future.
• stream:writeBits(value, n) - Writes a number to the stream, using n bits.

Reading

• stream:read(type) - Reads and returns a value from the stream.
• stream:readString() - Reads and returns a string.
• stream:readBool() - Reads and returns a boolean.
• stream:readFloat() - Reads and returns a float.
• stream:readBits(n) - Readss n bits from the string and returns them as a number.

Signatures

• stream:pack(data, signature) - Packs data into the stream according to the structure specified by signature.
• stream:unpack(signature) - Uses signature to read data from the string, returning a table with the data.

License

MIT, see LICENSE for details.