#' Collapse repeated rows to multirow cell
#'
#' @description Collapse same values in columns into multirow cells. This
#' feature does similar things with `group_rows`. However, unlike `group_rows`,
#' it analyzes existing columns, finds out rows that can be grouped together,
#' and make them multirow cells. Note that if you want to use `column_spec` to
#' specify column styles, you should use `column_spec` before `collapse_rows`.
#'
#' @param kable_input Output of `knitr::kable()` with `format` specified
#' @param columns Numeric column positions where rows need to be collapsed.
#'
#' @examples dt <- data.frame(a = c(1, 1, 2, 2), b = c("a", "a", "a", "b"))
#' x <- knitr::kable(dt, "html")
#' collapse_rows(x)
#'
#' @export
collapse_rows <- function(kable_input, columns = NULL) {
  # if (is.null(columns)) {
  #   stop("Please specify numeric positions of columns you want to collapse.")
  # }
  kable_format <- attr(kable_input, "format")
  if (!kable_format %in% c("html", "latex")) {
    message("Currently generic markdown table using pandoc is not supported.")
    return(kable_input)
  }
  if (kable_format == "html") {
    return(collapse_rows_html(kable_input, columns))
  }
  if (kable_format == "latex") {
    return(collapse_rows_latex(kable_input, columns))
  }
}

collapse_rows_html <- function(kable_input, columns) {
  kable_attrs <- attributes(kable_input)
  kable_xml <- read_kable_as_xml(kable_input)
  kable_tbody <- xml_tpart(kable_xml, "tbody")

  kable_dt <- rvest::html_table(xml2::read_html(as.character(kable_input)))[[1]]
  if (is.null(columns)) {
    columns <- seq(1, ncol(kable_dt))
  }
  kable_dt$row_id <- rownames(kable_dt)
  collapse_matrix <- collapse_row_matrix(kable_dt, columns)

  for (i in 1:nrow(collapse_matrix)) {
    matrix_row <- collapse_matrix[i, ]
    names(matrix_row) <- names(collapse_matrix)
    target_row <- xml_child(kable_tbody, i)
    row_node_rm_count <- 0
    for (j in 1:length(matrix_row)) {
      collapsing_col <- as.numeric(sub("x", "", names(matrix_row)[j])) -
        row_node_rm_count
      target_cell <- xml_child(target_row, collapsing_col)
      if (matrix_row[j] == 0) {
        xml_remove(target_cell)
        row_node_rm_count <- row_node_rm_count + 1
      } else if (matrix_row[j] != 1) {
        xml_attr(target_cell, "rowspan") <- matrix_row[j]
        xml_attr(target_cell, "style") <- paste0(
          xml_attr(target_cell, "style"),
          "vertical-align: middle !important;")
      }
    }
  }

  out <- as_kable_xml(kable_xml)
  attributes(out) <- kable_attrs
  return(out)
}

collapse_row_matrix <- function(kable_dt, columns, html = T)  {
  if (html) {
    column_block <- function(x) c(x, rep(0, x - 1))
  } else {
    column_block <- function(x) c(rep(0, x - 1), x)
  }
  mapping_matrix <- list()
  for (i in columns) {
    mapping_matrix[[paste0("x", i)]] <- unlist(lapply(
      rle(kable_dt[, i])$length, column_block))
  }
  mapping_matrix <- data.frame(mapping_matrix)
  return(mapping_matrix)
}

collapse_rows_latex <- function(kable_input, columns) {
  table_info <- magic_mirror(kable_input)
  if (is.null(columns)) {
    columns <- seq(1, table_info$ncol)
  }
  out <- enc2utf8(as.character(kable_input))
  contents <- table_info$contents
  kable_dt <- kable_dt_latex(contents)
  collapse_matrix <- collapse_row_matrix(kable_dt, columns, html = F)

  new_kable_dt <- kable_dt
  new_contents <- c()
  for (j in seq(1:ncol(collapse_matrix))) {
    column_align <- table_info$align_vector_origin[columns[j]]
    column_width <- ifelse(
      is.null(table_info$column_width[[paste0("column_", columns[j])]]),
      "*", table_info$column_width[paste0("column_", columns[j])])
    for (i in seq(1:nrow(collapse_matrix))) {
      new_kable_dt[i, j] <- collapse_new_dt_item(
        kable_dt[i, j], collapse_matrix[i, j], column_width, align = column_align
      )
    }
  }

  midrule_matrix <- collapse_row_matrix(kable_dt, seq(1, table_info$ncol),
                                        html = F)
  midrule_matrix[setdiff(seq(1, table_info$ncol), columns)] <- 1

  ex_bottom <- length(contents) - 1
  contents[2:ex_bottom] <- paste0(contents[2:ex_bottom], "\\\\\\\\")
  if (!table_info$booktabs) {
    contents[2:ex_bottom] <- paste0(contents[2:ex_bottom], "\n\\\\hline")
  }
  for (i in seq(1:nrow(collapse_matrix))) {
    new_contents[i] <- paste0(new_kable_dt[i, ], collapse = " & ")
    if (i != nrow(collapse_matrix)) {
      row_midrule <- midline_groups(which(as.numeric(midrule_matrix[i, ]) > 0),
                                    table_info$booktabs)
      new_contents[i] <- paste0(new_contents[i], "\\\\\\\\\n", row_midrule)
    }
    out <- sub(contents[i + 1], new_contents[i], out)
  }
  out <- gsub("\\\\addlinespace\n", "", out)

  out <- structure(out, format = "latex", class = "knitr_kable")
  table_info$collapse_rows <- TRUE
  attr(out, "kable_meta") <- table_info
  return(out)
}

kable_dt_latex <- function(x) {
  data.frame(do.call(rbind, str_split(x[-1], " & ")), stringsAsFactors = FALSE)
}

collapse_new_dt_item <- function(x, span, width = NULL, align) {
  if (span == 0) return("")
  if (span == 1) return(x)
  out <- paste0(
    "\\\\multirow\\{", -span, "\\}\\{",
    ifelse(is.null(width), "\\*", width),
    "\\}\\{",
    switch(align,
           "l" = "\\\\raggedright\\\\arraybackslash ",
           "c" = "\\\\centering\\\\arraybackslash ",
           "r" = "\\\\raggedleft\\\\arraybackslash "),
    x, "\\}"
  )
  return(out)
}

midline_groups <- function(x, booktabs = T) {
  diffs <- c(1, diff(x))
  start_indexes <- c(1, which(diffs > 1))
  end_indexes <- c(start_indexes-1, length(x))
  ranges <- paste0(x[start_indexes], "-", x[end_indexes])
  if (booktabs) {
    out <- paste0("\\\\cmidrule{", ranges, "}")
  } else {
    out <- paste0("\\\\cline{", ranges, "}")
  }
  out <- paste0(out, collapse = "\n")
  return(out)
}