get_deseq_dataset <- function(sample_table, header, design_formula, tximport, txtype, tx2gene) {

  dir <- ""

  has_header <- !is.null(header)
  use_txi <- !is.null(tximport)
  if (use_txi) {
    if (is.null(tx2gene)) stop("A transcript-to-gene map or a GTF/GFF3 file is required for tximport")
    if (tolower(file_ext(tx2gene)) == "gff") {
      gff_file <- tx2gene
    } else {
      gff_file <- NULL
      tx2gene <- read.table(tx2gene, header = has_header)
    }
  }

  if (!use_txi & has_header) {
      countfiles <- lapply(as.character(sample_table$filename), read.delim, row.names = 1)
      tbl <- do.call("cbind", countfiles)
      colnames(tbl) <- rownames(sample_table) # take sample ids from header

      # check for htseq report lines (from DESeqDataSetFromHTSeqCount function)
      old_special_names <- c(
        "no_feature",
        "ambiguous",
        "too_low_aQual",
        "not_aligned",
        "alignment_not_unique"
      )
      special_rows <- (substr(rownames(tbl), 1, 1) == "_") | rownames(tbl) %in% old_special_names
      tbl <- tbl[!special_rows, , drop = FALSE]

      dds <- DESeqDataSetFromMatrix(
        countData = tbl,
        colData = subset(sample_table, select = -filename),
        design = design_formula
      )
  } else if (!use_txi & !has_header) {

    # construct the object from HTSeq files
    dds <- DESeqDataSetFromHTSeqCount(
      sampleTable = sample_table,
      directory = dir,
      design = design_formula
    )
    colnames(dds) <- row.names(sample_table)

  } else {
      # construct the object using tximport
      library("tximport")
      txi_files <- as.character(sample_table$filename)
      labs <- row.names(sample_table)
      names(txi_files) <- labs
      if (!is.null(gff_file)) {
        # first need to make the tx2gene table
        # this takes ~2-3 minutes using Bioconductor functions
        suppressPackageStartupMessages({
          library("GenomicFeatures")
        })
        txdb <- makeTxDbFromGFF(gff_file)
        k <- keys(txdb, keytype = "TXNAME")
        tx2gene <- select(txdb, keys = k, columns = "GENEID", keytype = "TXNAME")
        # Remove 'transcript:' from transcript IDs (when gff_file is a GFF3 from Ensembl and the transcript does not have a Name)
        tx2gene$TXNAME <- sub("^transcript:", "", tx2gene$TXNAME)  # nolint
      }
      try(txi <- tximport(txi_files, type = txtype, tx2gene = tx2gene))
      if (!exists("txi")) {
        # Remove version from transcript IDs in tx2gene...
        tx2gene$TXNAME <- sub("\\.[0-9]+$", "", tx2gene$TXNAME)  # nolint
        # ...and in txi_files
        txi <- tximport(txi_files, type = txtype, tx2gene = tx2gene, ignoreTxVersion = TRUE)
      }
      dds <- DESeqDataSetFromTximport(
        txi,
        subset(sample_table, select = -c(filename)),
        design_formula
      )
  }
  return(dds)
}