Bidirectional context model for adaptive compression

A computer readable medium including instructions that, when executed by a processing system, cause the processing system to perform a method for encoding data, the method comprising:

• in response to a symbol retrieved from an input stream having a sequence of symbols, determining whether a current context can encode the retrieved symbol, wherein the current context is a context associated with a previously emitted symbol from the input stream;
• emitting a code representing an escape down token in the current context to a code stream if the current context cannot encode the retrieved symbol, wherein the emitted code representing the escape down token is used by a decoder to switch from a higher order context to a lower order context during decoding the code stream;
• emitting a code representing the retrieved symbol in the current context to the code stream if the current context can encode the retrieved symbol;
• and
• optionally escaping to a context having a higher order by emitting a code representing an escape up token in the code stream, wherein a higher order context is a child context to a lower order context as a parent context, wherein the emitted code representing the escape up token is used by the decoder to switch from a lower order context to a higher order context during decoding the code stream.

A computer implemented method for decoding data, the method comprising:

• in response to a code stream and a literal stream, determining whether a current context can code anything other than an escape down token and/or an escape up token;
• consuming a code from the code stream if the current context can code anything other than the escape down token and/or the escape up token;
• escaping up from the current context to a higher order context to be a next current context if the consumed code represents an escape up token in the current context;
• and
• emitting a literal from the literal stream to an output stream if the current context cannot code anything other than the escape down token and/or the escape up token and if the current context is a root context.

A computer readable medium including instructions that, when executed by a processing system, cause the processing system to perform a method for decoding data, the method comprising:

• in response to a code stream and a literal stream, determining whether a current context can code anything other than an escape down token and/or an escape up token;
• consuming a code from the code stream if the current context can code anything other than the escape down token and/or the escape up token;
• escaping up from the current context to a higher order context to be a next current context if the consumed code represents an escape up token in the current context;
• and
• emitting a literal from the literal stream to an output stream if the current context cannot code anything other than the escape down token and/or the escape up token and if the current context is a root context.