ContextSnake

Extension

Description

ContextSnake is a Markov chain/generative grammar approach with self-adjusting context depth.

Based on a corpus of examples, a ContextSnake creates a stream of values: For each next value, a number of previous values define the context depth; the longer the context considered, the less likely the context is to occur in its full length. The snake extends the context until it becomes unique, i.e. until it only occurs at a single location in the corpus; then it reduces context depth again so that more than one option becomes available, and picks the next item from these options. (Like a gourmet, it insists on choices.)

Two parameters influence these choices further: minDepth is the minimum context depth below which single choices are accepted), and acceptSingleCond is a flag or function returning a boolean whether to accept single choices even when above minimum depth.

Concept by Gerhard Nierhaus, implementation Alberto de Campo. Comments and suggestions welcome!

Fairytales and poetry can make good material, e.g. we experimented with Schneewittchen (Snow white), Erlkoenig, and others, see ContextSnake_3texts.html. Formulaic segments can become points of 'modulation' between sections.

First code examples: // Demonstrate the algorithm by hand first: // a corpus of two example strings - text sequences are easiest to follow. a = [ "was it a car or a cat i saw", "warsaw" ]; // and a context snake with that corpus. c = ContextSnake(a); // find all locations of the sublist given in corpus; // returns list of index pairs [which example in corpus, starting where in example] c.find("cat"); // "cat" is unique, i.e. only found once in corpus: // in example 0, starting at index 9 c.find("ca"); // "ca" occurs twice, so two locations // do context-snake decisions by hand: c.find("w"); // 4 locations, pick first of them as next (here, by hand): // at [0, 0], next letter is "a"; so the context is now "wa". c.find("wa"); // still 2 locations, pick first c.find("was"); // 1 unique location; reduce context - drop the oldest character c.find("as"); // 1 unique location; reduce context again c.find("s"); // 3 locations; pick the last one c.find("sa"); // 2 locations; pick the latter c.find("saw"); // still 2 locations; but both have no next letter -> end. // The entire string produced by now is: "wasaw".

Class Methods

.new

Instance Methods

.corpus

.starter

.minDepth

.acceptSingleCond

.verbose

.starterLength

.find

.randStarter

.embedInStream

Analysis

ContextSnakes can analyse a sample for different criteria, for example, what the longest sequences are that occur verbatim in the corpus. If the found sequences overlap, then the sample is a possible production from the given corpus. Other methods test whether the sample is valid output of the snake, and whether it is new or not.( a = [ "ContextSnakes can analyse a sample for different criteria, for example, what the longest sequences are that occur verbatim in the corpus.", "If the found sequences overlap, the sample is a possible production from the given corpus.", "Other methods test whether the sample is valid output, and whether it is new or not." ]; c = ContextSnake(a, minDepth: 4); ) 10.do { c.randStarter.asStream.all.join.postcs }; // produce something b = "If the given corpus."; // a short possible production;

.isValidOutput

.vocabulary

.longestSnippets

.isNew

Examples

A very simple music example:

For a musical example with Palestrina melodies, see( (thisProcess.nowExecutingPath.dirname.dirname +/+ "CS_examples/ContextSnake_Palestrina.scd").openOS; ) code:: // A melody which can go up only in steps, and down only in thirds; // can begin with 0, 7, or 6; can end with 7, 4 or 0. b = [ (0..7), [7, 5, 3, 1, 2, 3, 4], [6, 4, 2, 0]]; c = ContextSnake(b, minDepth: 1, starterLength: 1); 10.do { c.asStream.all.postln }; // add pauses between new starts Pbind(\degree, Pseq([Plazy({ c.randStarter }), \_, \_], inf), \dur, 0.2).trace(\degree).play; s.boot; // Add special markers for the beginning and ending to each phrase; // this can be helpful for doing special things on segment borders. ( b = [ (0..7), [7, 5, 3, 1, 2, 3, 4], [6, 4, 2, 0]]; d = b.collect { |ex| ['<'] ++ ex ++ ['>'] }; // add c = ContextSnake(d, minDepth: 1, starterLength: 1); ) ( // beginning and ending symbols create pauses Pbind( \degree, Pn(Plazy({ c.randStarter }), inf), \dur, 0.2 ).trace(\degree).play; )