Index: trunk/src/layer.c
===================================================================
--- trunk/src/layer.c	(revision 19102)
+++ trunk/src/layer.c	(revision 19103)
@@ -38,6 +38,8 @@
 #include "layer_ui.h"
 #include "layer_vis.h"
 #include "rtree.h"
+#include "funchash.h"
+#include "funchash_core.h"
 #include "obj_pstk_inlines.h"
 #include "list_common.h"
 
@@ -467,6 +469,35 @@
 	return pcb_layer_rename_(&data->Layer[layer], pcb_strdup(color));
 }
 
+int pcb_layer_set_purpose__(pcb_layer_t *ly, char *purpose)
+{
+	free(ly->purpose);
+	if (purpose == NULL) {
+		ly->purpose = NULL;
+		ly->purpi = F_user;
+	}
+	else {
+		ly->purpose = purpose;
+		ly->purpi = pcb_funchash_get(purpose, NULL);
+		if (ly->purpi < 0)
+			ly->purpi = F_user;
+	}
+	return 0;
+}
+
+int pcb_layer_set_purpose_(pcb_layer_t *ly, char *purpose)
+{
+	int ret = pcb_layer_set_purpose__(ly, purpose);
+	pcb_event(PCB_EVENT_LAYERS_CHANGED, NULL);
+	return ret;
+}
+
+int pcb_layer_set_purpose(pcb_layer_t *ly, const char *purpose)
+{
+	return pcb_layer_set_purpose_(ly, pcb_strdup(purpose));
+}
+
+
 #undef APPEND
 
 static int is_last_top_copper_layer(pcb_board_t *pcb, int layer)
Index: trunk/src/layer.h
===================================================================
--- trunk/src/layer.h	(revision 19102)
+++ trunk/src/layer.h	(revision 19103)
@@ -115,6 +115,8 @@
 	const char *name;              /* layer name */
 
 	pcb_layer_combining_t comb;    /* how to combine this layer with other layers in the group */
+	char *purpose;                 /* what a doc/mech layer is used for */
+	int purpi;                     /* integer version of purpose from the funtion hash (cache) */
 
 	/* for bound layers these point to the board layer's*/
 	pcb_rtree_t *line_tree, *text_tree, *polygon_tree, *arc_tree;
@@ -268,7 +270,12 @@
 int pcb_layer_rename_(pcb_layer_t *Layer, char *Name);
 int pcb_layer_recolor_(pcb_layer_t *Layer, char *color);
 
+/* Change the purpose field and recalc purpi (not undoable) */
+int pcb_layer_set_purpose__(pcb_layer_t *ly, char *purpose); /* no strdup, no event */
+int pcb_layer_set_purpose_(pcb_layer_t *ly, char *purpose); /* no strdup, send layer change event */
+int pcb_layer_set_purpose(pcb_layer_t *ly, const char *purpose); /* strdup, send event */
 
+
 /* index is 0..PCB_MAX_LAYER-1.  If old_index is -1, a new layer is
    inserted at that index.  If new_index is -1, the specified layer is
    deleted.  Returns non-zero on error, zero if OK.  */