R/probability.R

In vgherard/kgrams: Classical k-gram Language Models

#' Language Model Probabilities
#' 
#' Compute sentence probabilities and word continuation conditional 
#' probabilities from a language model
#' 
#' @author Valerio Gherardi
#' @md
#'
#' @param object a character vector for sentence probabilities, 
#' a word-context conditional expression created with the 
#' conditional operator `%|%` (see \link[kgrams]{word_context}).
#' for word continuation probabilities.
#' @param model an object of class \code{language_model}.
#' @param .preprocess a function taking a character vector as input and 
#' returning a character vector as output. Preprocessing transformation  
#' applied to input before computing probabilities
#' @param .tknz_sent a function taking a character vector as input and 
#' returning a character vector as output. Optional sentence tokenization step
#' applied before computing sentence probabilities.
#' @param ... further arguments passed to or from other methods.
#' @return a numeric vector. Probabilities of the sentences or word 
#' continuations.
#' 
#' @details
#' The generic function \code{probability()} is used to obtain both sentence
#' unconditional probabilities (such as Prob("I was starting to feel drunk"))
#' and word continuation conditional probabilities (such as 
#' Prob("you" | "i love")). In plain words, these probabilities answer the 
#' following related but conceptually different questions:
#' 
#' - Sentence probability Prob(s): what is the probability that extracting a
#' single sentence (from a corpus of text, say) we will obtain exactly 's'?
#' - Continuation probability Prob(w|c): what is the probability that a given 
#' context 'c' will be followed exactly by the word 'w'?
#'
#' In order to compute continuation probabilities (i.e. Prob(w|c)), one must
#' create conditional expressions with the infix operator `%|%`, as shown in
#' the examples below. Both \code{probability} and `%|%` are vectorized with
#' respect to words (left hand side of `%|%`), but the context must be a length
#' one character (right hand side of `%|%`).
#' 
#' The input is treated as in \link[kgrams]{query} for what concerns word 
#' tokenization: anything delimited by (one or more) white space(s) is 
#' tokenized as a word. For sentence probabilities, Begin-Of-Sentence and 
#' End-Of-Sentence paddings are implicitly added to the input, but specifying 
#' them explicitly does not produce wrong results as BOS and EOS tokens are 
#' ignored by \code{probability()} (see the examples below). For continuation
#' probabilities, any context of more than \code{N - 1} words (where 
#' \code{N} is the k-gram order the language model) is truncated to the last
#' \code{N - 1} words.
#' 
#' By default, the same \code{.preprocess()} and \code{.tknz_sent()} 
#' functions used during model building are applied to the input, but this can
#' be overriden with arbitrary functions. Notice that the 
#' \code{.tknz_sent} can be useful (for sentence probabilities) if
#' e.g. the input is a length one unprocessed character vector. 
#'
#' @examples 
#' # Usage of probability()
#' 
#' f <- kgram_freqs("a b b a b a b", 2)
#' 
#' m <- language_model(f, "add_k", k = 1)
#' probability(c("a", "b", EOS(), UNK()) %|% BOS(), m) # c(0.4, 0.2, 0.2, 0.2)
#' probability("a" %|% UNK(), m) # not NA
#'
#' @name probability

#' @rdname probability
#' @export
probability <- function(
        object, model, .preprocess = attr(model, ".preprocess"), ... 
        ) 
{
        assert_language_model(model)
        assert_function(.preprocess)
        UseMethod("probability", object)
}
        

#' @rdname probability
#' @export
probability.kgrams_word_context <- function(
        object, model, .preprocess = attr(model, ".preprocess"), ...
        ) 
{
        word <- .preprocess(object$word)
        context <- .preprocess(object$context)
        attr(model, "cpp_obj")$probability(word, context) # return        
}

#' @rdname probability
#' @export
probability.character <- function(
        object, 
        model,
        .preprocess = attr(model, ".preprocess"),
        .tknz_sent = attr(model, ".tknz_sent"),
        ...
        ) 
{
        assert_function(.tknz_sent)
        assert_character_no_NA(object)
        object <- .preprocess(object)
        object <- .tknz_sent(object)
        attr(model, "cpp_obj")$probability_sentence(object) # return
}