replace in-memory cache with a persistent json cache

we just store the raw response from overpass

pkgs/maps/tiles.fnl

@@ -58,7 +58,7 @@
       ]
      (table.concat  "\n"))))
 
-(fn canvas [elements offset-x offset-y]
+(fn canvas [elements]
   (let [nodes {}
         lines {}]
     (each [_ e (ipairs elements)]
@@ -75,30 +75,43 @@
              [ tx ty  ])))))
     lines))
 
-(fn polylines-from-net [x y zoom]
-  (let [(lat lon) (tile->latlon x y zoom)
-        o  (overpass lat lon zoom)
+
+(fn file-exists? [name]
+  (match (io.open name :r)
+    f (do (f:close) true)
+    _ false))
+
+(fn unparsed-from-disk [x y zoom fetch-fn]
+  (let [k (..  x "_" y "_" zoom)
+        pathname (.. "/tmp/tiles/" k ".json")]
+    (if (file-exists? pathname)
+        (with-open [i (io.open pathname :r)]
+          (i:read "*a"))
+        (with-open [j (io.open pathname :w)]
+          (let [g (fetch-fn)]
+            (j:write g)
+            g)))))
+
+(fn unparsed-for-xyz [x y zoom]
+    (let [(lat lon) (tile->latlon x y zoom)
+        o (overpass lat lon zoom)
         r
         (req.new_from_uri
          "https://overpass-api.de/api/interpreter")
         query  { :data o }]
     (tset r.headers ":method" "POST")
     (r:set_body (dict_to_query query))
-    (let [(headers stream) (r:go)
-          (tx ty) (latlon->tile lat lon zoom)
-          data (json.decode (stream:get_body_as_string))]
-      (canvas data.elements (math.floor tx) (math.floor ty)))))
+    (let [(headers stream) (r:go)]
+      (stream:get_body_as_string))))
 
-(local cache {})
+(fn polylines-from-net [x y zoom]
+  (let [s (unparsed-from-disk
+           x y zoom
+           (fn []
+             (unparsed-for-xyz x y zoom)))
+        ;_ (print :unoparsed (s:sub 1 40))
+        data (json.decode s)]
+    (canvas data.elements)))
 
-(fn polylines [x y zoom]
-  (let [k (..  x "/" y "/" zoom)
-        lines (. cache k)]
-    (print k (not (not lines)))
-    (if lines
-        (do (print "cached! " x y zoom) lines)
-        (let [lines (polylines-from-net x y zoom)]
-          (tset cache k lines)
-          lines))))
 
-{ : polylines : latlon->tile }
+{ :polylines polylines-from-net : latlon->tile }