In the afternoon, he implemented tagging.

And he looked on his work, and saw that it was good.

src/walkmap/routing.clj

@@ -0,0 +1,18 @@
+(ns walkmap.core
+  "Finding optimal routes to traverse a map."
+    (:require [walkmap.path :as p]
+            [walkmap.polygon :as q]
+            [walkmap.stl :as s]
+            [walkmap.utils :as u]
+            [walkmap.vertex :as v]))
+
+;; Breadth first search is a good algorithm for terrain in which all steps have
+;; equal, but in our world (like the real world), they don't.
+
+;; Reading list:
+;;
+;; https://en.wikipedia.org/wiki/A*_search_algorithm
+;; https://www.redblobgames.com/pathfinding/a-star/introduction.html
+;; https://faculty.nps.edu/ncrowe/opmpaper2.htm
+;;
+;; See https://simon-brooke.github.io/the-great-game/codox/Pathmaking.html

src/walkmap/tag.clj

@@ -0,0 +1,50 @@
+(ns walkmap.tag
+  "Code for tagging, untagging, and finding tags on objects. Note the use of
+  the namespaced keyword, `:walkmap.tag/tags`, denoted in this file `::tags`.
+  This is in an attempt to avoid name clashes with other uses of this key."
+  (:require [clojure.set :refer [difference union]]))
+
+(defn tagged?
+  "True if this `object` is tagged with each of these `tags`."
+  [object & tags]
+  (if
+    (map? object)
+    (if
+      (every? keyword? tags)
+      (let [ot (::tags object)]
+        (and
+          (set? ot)
+          (every? ot tags)
+          true))
+      (throw (IllegalArgumentException.
+               (str "Must be keyword(s): " (map type tags)))))
+    (throw (IllegalArgumentException.
+             (str "Must be a map: " (type object))))))
+
+(defn tag
+  "Return an object like this `object` but with these `tags` added to its tags,
+  if they are not already present."
+  [object & tags]
+  (if
+    (map? object)
+    (if
+      (every? keyword? tags)
+      (assoc object ::tags (union (set tags) (::tags object)))
+      (throw (IllegalArgumentException.
+               (str "Must be keyword(s): " (map type tags)))))
+    (throw (IllegalArgumentException.
+             (str "Must be a map: " (type object))))))
+
+(defn untag
+  "Return an object like this `object` but with these `tags` removed from its
+  tags, if present."
+  [object & tags]
+  (if
+    (map? object)
+    (if
+      (every? keyword? tags)
+      (assoc object ::tags (difference (::tags object) (set tags)))
+      (throw (IllegalArgumentException.
+               (str "Must be keywords: " (map type tags)))))
+    (throw (IllegalArgumentException.
+             (str "Must be a map: " (type object))))))