add target in collapse_rows #484
diff --git a/R/collapse_rows.R b/R/collapse_rows.R
index 4ae0145..e520b40 100644
--- a/R/collapse_rows.R
+++ b/R/collapse_rows.R
@@ -7,7 +7,7 @@
 #' specify column styles, you should use `column_spec` before `collapse_rows`.
 #'
 #' @param kable_input Output of `knitr::kable()` with `format` specified
-#' @param columns A numeric value or vector indicating in which column(s) rows 
+#' @param columns A numeric value or vector indicating in which column(s) rows
 #' need to be collapsed.
 #' @param valign Select from "top", "middle"(default), "bottom". The reason why
 #' "top" is not default is that the multirow package on CRAN win-builder is
@@ -35,7 +35,8 @@
                           row_group_label_position = c('identity', 'stack'),
                           custom_latex_hline = NULL,
                           row_group_label_fonts = NULL,
-                          headers_to_remove = NULL) {
+                          headers_to_remove = NULL,
+                          target = NULL) {
   kable_format <- attr(kable_input, "format")
   if (!kable_format %in% c("html", "latex")) {
     warning("Please specify format in kable. kableExtra can customize either ",
@@ -44,8 +45,13 @@
     return(kable_input)
   }
   valign <- match.arg(valign, c("middle", "top", "bottom"))
+  if (!is.null(target)) {
+    if (length(target) > 1 && is.integer(target)) {
+      stop("target can only be a length 1 integer")
+    }
+  }
   if (kable_format == "html") {
-    return(collapse_rows_html(kable_input, columns, valign))
+    return(collapse_rows_html(kable_input, columns, valign, target))
   }
   if (kable_format == "latex") {
     latex_hline <- match.arg(latex_hline, c("full", "major", "none", "custom"))
@@ -57,22 +63,27 @@
   }
 }
 
-collapse_rows_html <- function(kable_input, columns, valign) {
+collapse_rows_html <- function(kable_input, columns, valign, target) {
   kable_attrs <- attributes(kable_input)
-  kable_xml <- read_kable_as_xml(kable_input)
+  kable_xml <- 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))
   }
+  if (!is.null(target)) {
+    if (!target %in% columns) {
+      stop("target has to be within the range of columns")
+    }
+  }
   if (!is.null(kable_attrs$header_above)) {
     kable_dt_col_names <- unlist(kable_dt[kable_attrs$header_above, ])
     kable_dt <- kable_dt[-(1:kable_attrs$header_above),]
     names(kable_dt) <- kable_dt_col_names
   }
-  kable_dt$row_id <- seq(nrow(kable_dt))
-  collapse_matrix <- collapse_row_matrix(kable_dt, columns)
+  # kable_dt$row_id <- seq(nrow(kable_dt))
+  collapse_matrix <- collapse_row_matrix(kable_dt, columns, target = target)
 
   for (i in 1:nrow(collapse_matrix)) {
     matrix_row <- collapse_matrix[i, ]
@@ -101,17 +112,34 @@
   return(out)
 }
 
-collapse_row_matrix <- function(kable_dt, columns, html = T)  {
+split_factor <- function(x) {
+  group_idx <- seq(1, length(x))
+  return(factor(unlist(lapply(group_idx, function(i) {rep(i, x[i])}))))
+}
+
+collapse_row_matrix <- function(kable_dt, columns, html = T, target = NULL)  {
   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])$lengths, column_block))
+  if (is.null(target)) {
+    for (i in columns) {
+      mapping_matrix[[paste0("x", i)]] <- unlist(lapply(
+        rle(kable_dt[, i])$lengths, column_block))
+    }
+  } else {
+    target_group = split_factor(rle(kable_dt[, target])$lengths)
+    for (i in columns) {
+      column_split = split(kable_dt[, i], target_group)
+      mapping_matrix[[paste0("x", i)]] <- unlist(lapply(
+        column_split, function(sp) {
+          lapply(rle(sp)$length, column_block)
+        }))
+    }
   }
+
   mapping_matrix <- data.frame(mapping_matrix)
   return(mapping_matrix)
 }
diff --git a/inst/NEWS.md b/inst/NEWS.md
index 354a7f0..630cd7a 100644
--- a/inst/NEWS.md
+++ b/inst/NEWS.md
@@ -9,6 +9,10 @@
 
 * Removed xcolor from latex dependency list
 
+* `collapse_rows` has a new option `target` to choose the target column in 
+`collapse_rows` (#484)
+
+
 kableExtra 1.1.0
 --------------------------------------------------------------------------------