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# Convert a simple triplet matrix to svm format
#' @author Peter Ellis
#' @return a character vector of length n = nrow(stm)
#' @import slam
#' @import data.table
calc_stm_svm <- function(stm, y){
# returns a character vector of length y ready for writing in svm format
if(!"simple_triplet_matrix" %in% class(stm)){
stop("stm must be a simple triple matrix")
if(!is.vector(y) | nrow(stm) != length(y)){
stop("y should be a vector of length equal to number of rows of stm")
n <- length(y)
# data.table solution thanks to @roland at
stm2 <- data.table(i = stm$i, j = stm$j, v = stm$v)
res <- stm2[, .(i, jv = paste(j, v, sep = ":"))
][order(i), .(res = paste(jv, collapse = " ")), by = i][["res"]]
out <- paste(y, res)
#' @param stm a simple triplet matrix (class exported slam) of features (ie explanatory variables)
#' @param y a vector of labels. If not provided, a dummy of 1s is provided
#' @param file file to write to.
#' @author Peter Ellis
write_stm_svm <- function(stm, y = rep(1, nrow(stm)), file){
out <- calc_stm_svm(stm, y)
writeLines(out, con = file)