draw map once only and copy it to screen in on_draw

This massively reduces cpu usage, however it doesn't yet work
if we've moved far enough that we'd need to fetch new tiles.

pkgs/maps/default.nix

@@ -2,6 +2,7 @@
 , pkg-config
 , buildPackages
 , callPackage
+, cairo
 , clutter
 , fetchFromGitHub
 , fetchurl
@@ -52,8 +53,8 @@ in stdenv.mkDerivation {
   buildInputs = [
     lua
     gtk3.dev
+    cairo.dev
     gobject-introspection
-    osm-gps-map
     glib-networking
   ];
   nativeBuildInputs = [

pkgs/maps/main.fnl

@@ -10,6 +10,7 @@
         : Gdk
         : Gio
         : GLib
+        : cairo
         }
        (require :lgi))
 
@@ -70,48 +71,74 @@ label.readout {
   (collect [k v (pairs table2) &into table1]
     k v))
 
-;; given  lat/lon
-;; we want the tile containing lat to be roughly centred
-;; on the screen, and enough tiles either side of it
-;; to fill the width of the screen plus a bit
+(var map-surface nil)
 
-(fn cairo-the-map [self g]
-  (let [{ : lat : lon : zoom } app-state
-        num-tiles-x (+ 0 (math.ceil (/ map-width tile-size)))
-        num-tiles-y (+ 0 (math.ceil (/ map-height tile-size)))
+(fn map-bounds [lat lon zoom]
+  (let [num-tiles-x (+ 1 (math.ceil (/ map-width tile-size)))
+        num-tiles-y (+ 1 (math.ceil (/ map-height tile-size)))
         (tile-x tile-y) (tiles.latlon->tile app-state.lat app-state.lon app-state.zoom)
         min-tile-x (math.floor (- tile-x (/ num-tiles-x 2)))
-        max-tile-x (math.ceil (+ tile-x (/ num-tiles-x 2)))
+        max-tile-x (+ min-tile-x num-tiles-x 4)
         min-tile-y (math.floor (- tile-y (/ num-tiles-y 2)))
-        max-tile-y (math.ceil  (+ tile-y (/ num-tiles-y 2)))
-
-        left-edge (- tile-x (/ map-width 2 tile-size))
-        top-edge (- tile-y (/ map-height 2 tile-size))
+        max-tile-y (+ min-tile-y num-tiles-y 4)]
+    {
+     :min { :x min-tile-x :y min-tile-y }
+     :max { :x max-tile-x :y max-tile-y }
+     : num-tiles-x : num-tiles-y
+     }))
 
+(fn cairo-the-map [window]
+  (let [{ : lat : lon : zoom } app-state
+        { : num-tiles-x : num-tiles-y &as bounds } (map-bounds lat lon zoom)
         lines []]
 
-    (for [x min-tile-x max-tile-x]
-      (for [y min-tile-y max-tile-y]
+    (for [x bounds.min.x bounds.max.x]
+      (for [y bounds.min.y bounds.max.y]
         (merge lines (tiles.polylines x y zoom))))
 
-    ;; put tile-x and tile-y in the centre of the visible area.
-    (g:translate (* tile-size (- min-tile-x left-edge))
-                 (* tile-size (- min-tile-y top-edge)))
+    (let [map-surface
+          (window:create_similar_surface
+           cairo.Content.COLOR
+           (* tile-size (+ 4 num-tiles-x))
+           (* tile-size (+ 4 num-tiles-y)))
+          g (cairo.Context.create map-surface)]
+
+      (g:set_source_rgb 1 1 1)
+      (g:rectangle 0 0 (* tile-size num-tiles-x) (* tile-size num-tiles-y))
+      (g:fill)
+
+      (g:set_source_rgb 0.2 0.2 0.6)
+      (g:set_line_width 2)
+      (each [_ line (pairs lines)]
+        (case line
+          [[sx sy] & more]
+          (do
+            (g:move_to (* tile-size (- sx bounds.min.x))
+                       (* tile-size (- sy bounds.min.y)))
+            (each [_ [x y] (ipairs more)]
+              (let [x1 (* tile-size (- x bounds.min.x))
+                    y1 (* tile-size (- y bounds.min.y))]
+                (g:line_to x1 y1))))))
+      (g:stroke)
+      map-surface)))
+
+
+(fn on-osm-draw [widget g]
+  (when (not map-surface)
+    (let [window (widget:get_window)]
+      (set map-surface (cairo-the-map window))))
+
+  (let [(tile-x tile-y) (tiles.latlon->tile app-state.lat app-state.lon app-state.zoom)
+        bounds (map-bounds tile-x tile-y)
+        offset-x (- (* tile-size (- tile-x bounds.min.x)) (/ map-width 2))
+        offset-y (- (* tile-size (- tile-y bounds.min.y)) (/ map-height 2))]
+
+    (g:set_source_surface map-surface (- offset-x) (- offset-y))
+    (g:set_operator cairo.Operator.SOURCE)
+    (g:rectangle 0 0 map-width map-height)
+    (g:fill)))
+
 
-    (g:set_source_rgb 0.2 0.2 0.4)
-    (g:set_line_width 3)
-    (each [_ line (pairs lines)]
-      (case line
-        [[sx sy] & more]
-        (do
-          (g:move_to (* tile-size (- sx min-tile-x))
-                     (* tile-size (- sy min-tile-y)))
-          (each [_ [x y] (ipairs more)]
-            (let [x1 (* tile-size (- x min-tile-x))
-                  y1 (* tile-size (- y min-tile-y))]
-              (g:line_to x1 y1))))))
-    (g:stroke)
-    true))
 
 (fn register-widget [name widget]
   (tset state-widgets name widget)
@@ -122,7 +149,7 @@ label.readout {
    :osm
    (Gtk.DrawingArea {
                      :width map-width :height map-height
-                     :on_draw  cairo-the-map
+                     :on_draw on-osm-draw
                      })))
 
 (fn readout [name text]