Index: trunk/src/layer_grp.c
===================================================================
--- trunk/src/layer_grp.c	(revision 10443)
+++ trunk/src/layer_grp.c	(revision 10444)
@@ -678,7 +678,28 @@
 	return -1;
 }
 
+int pcb_layergrp_dist(pcb_board_t *pcb, pcb_layergrp_id_t gid1, pcb_layergrp_id_t gid2, pcb_layer_type_t mask, int *diff)
+{
+	int gid, d, cnt;
 
+	if ((gid1 < 0) || (gid2 < 0))
+		return -1;
+	if (gid1 == gid2) {
+		*diff = 0;
+		return 0;
+	}
+
+	d = (gid1 < gid2) ? +1 : -1;
+	cnt = 0;
+	for(gid = gid1; gid != gid2; gid += d) {
+		if (pcb->LayerGroups.grp[gid].type & mask)
+			cnt++;
+	}
+	*diff = cnt;
+	return 0;
+}
+
+
 static pcb_layergrp_id_t pcb_layergrp_get_cached(pcb_board_t *pcb, pcb_layer_id_t *cache, unsigned int loc, unsigned int typ)
 {
 	pcb_layergrp_t *g;
Index: trunk/src/layer_grp.h
===================================================================
--- trunk/src/layer_grp.h	(revision 10443)
+++ trunk/src/layer_grp.h	(revision 10444)
@@ -115,7 +115,13 @@
 /* Return the array index of lid within the grp's lid list or -1 if not on the list */
 int pcb_layergrp_index_in_grp(pcb_layergrp_t *grp, pcb_layer_id_t lid);
 
+/* Calculate the distance between gid1 and gid2 in the stack, in number of
+   layer groups matching mask. The result is placed in *diff. Returns 0 on success.
+   Typical use case: gid1 is an internal copper layer, gid2 is the top copper layer,
+   mask is PCB_LYT_COPPER -> *diff is the copper index of the internal layer from top */
+int pcb_layergrp_dist(pcb_board_t *pcb, pcb_layergrp_id_t gid1, pcb_layergrp_id_t gid2, pcb_layer_type_t mask, int *diff);
 
+
 /* Enable/disable inhibition of layer changed events during layer group updates */
 void pcb_layergrp_inhibit_inc(void);
 void pcb_layergrp_inhibit_dec(void);