diff --git a/R/min_depth_distribution.R b/R/min_depth_distribution.R
index c82bb3e..41db93e 100644
--- a/R/min_depth_distribution.R
+++ b/R/min_depth_distribution.R
@@ -4,11 +4,8 @@ calculate_tree_depth <- function(frame){
     stop("The data frame has to contain columns called 'right daughter' and 'left daughter'!
           It should be a product of the function getTree(..., labelVar = T).")
   }
-  # Both child values of leaf nodes are 0, i.e., lower than min(node_id)
   frame[["depth"]] <- calculate_tree_depth_(
-    node_id = seq_len(nrow(frame)),
-    left_child = frame[["left daughter"]],
-    right_child = frame[["right daughter"]]
+    frame[, c("left daughter", "right daughter")]
   )
   return(frame)
 }
@@ -19,22 +16,30 @@ calculate_tree_depth_ranger <- function(frame){
     stop("The data frame has to contain columns called 'rightChild' and 'leftChild'!
          It should be a product of the function ranger::treeInfo().")
   }
+  # Child nodes are zero based, so we increase them by 1
   frame[["depth"]] <- calculate_tree_depth_(
-    node_id = frame[["nodeID"]],
-    left_child = frame[["leftChild"]],
-    right_child = frame[["rightChild"]]
+    frame[, c("leftChild", "rightChild")] + 1
   )
   return(frame)
 }
 
-# Internal function used to determine the depth of each node
-calculate_tree_depth_ <- function(node_id, left_child, right_child) {
-  n <- length(node_id)
-  depth <- numeric(n)
-  for (i in 2:n) {
-    parent_node <- left_child %in% node_id[i] | right_child %in% node_id[i]
-    depth[i] <- depth[parent_node] + 1
+# Internal function used to determine the depth of each node.
+# The input is a data.frame with left and right child nodes in 1:nrow(childs).
+calculate_tree_depth_ <- function(childs) {
+  childs <- as.matrix(childs)
+  n <- nrow(childs)
+  depth <- rep(NA, times = n)
+  j <- depth[1L] <- 0
+  ix <- 1L  # current nodes, initialized with root node index
+
+  # j loops over tree depth
+  while(anyNA(depth) && j < n) {  # The second condition is never used
+    ix <- as.integer(childs[ix, ])
+    ix <- ix[!is.na(ix) & ix >= 1L]  # leaf nodes do not have childs
+    j <- j + 1
+    depth[ix] <- j
   }
+
   return(depth)
 }