Replace :bmx-gap-probability with :bmx-gene-length-limit

src/propeller/genome.cljc

@@ -7,16 +7,14 @@ They hold the genetic material for an `individual`. In the initial population, w
 
 (defn make-random-plushy
   "Creates and returns a new plushy made of random instructions."
-  [{:keys [instructions max-initial-plushy-size bmx? bmx-gap-probability]}]
-  (if bmx?
-    (repeatedly
-     (rand-int max-initial-plushy-size)
-     #(if (< (rand) bmx-gap-probability)
-        :gap
-        (utils/random-instruction instructions)))
-    (repeatedly
-     (rand-int max-initial-plushy-size)
-     #(utils/random-instruction instructions))))
+  [{:keys [instructions max-initial-plushy-size bmx? bmx-gene-length-limit]}]
+  (let [plushy (repeatedly (rand-int max-initial-plushy-size)
+                           #(utils/random-instruction instructions))]
+    (if bmx?
+      (-> plushy
+          (utils/fill-empty-genes instructions)
+          (utils/enforce-gene-length-limit bmx-gene-length-limit))
+      plushy)))
 
 (defn plushy->push-internal
   [plushy argmap]

src/propeller/problems/boolean/mul3.cljc

@@ -313,7 +313,7 @@
      :single-thread-mode         false
      :bmx?                       true
      :bmx-exchange-rate          0.5
-     :bmx-gap-probability        0.1
+     :bmx-gene-length-limit      10
      :bmx-gap-change-probability 0.01
      :bmx-complementary?         true}
     (apply hash-map (map #(if (string? %) (read-string %) %) args)))))

src/propeller/session.cljc

@@ -57,7 +57,9 @@
 #_(genome/plushy->push
     (genome/make-random-plushy 
      {:instructions (instructions/get-stack-instructions #{:float :integer :exec :boolean}) 
-      :bmx-gap-probability 20}))
+      :max-initial-plushy-size 100 
+      :bmx? true
+      :bmx-gene-length-limit 10}))
 
 ;; One way of running a genetic programming problem defined in the project
 ;; is to require the problem's namespace and then call `gp/gp` using the

src/propeller/utils.cljc

@@ -209,3 +209,25 @@
                                 (random-instruction instructions)
                                 gene))
                             (extract-genes plushy)))))
+
+(defn break-up
+  "A utility function for bmx-related genetic operators. Returns the provided
+  :gap-free plushy with gaps randomly inserted to ensure that no gene is longer
+  than the provided limit."
+  [gene limit]
+  (if (> (count gene) limit)
+    (let [i (inc (rand-int (dec (count gene))))]
+      (concat (break-up (take i gene) limit)
+              [:gap]
+              (break-up (drop i gene) limit)))
+    gene))
+
+(defn enforce-gene-length-limit
+  "A utility function for bmx-related genetic operators. Returns the provided
+   plushy with any over-length genes broken into non-empty pieces, recursively
+   until all genes obey the limit."
+  [plushy limit]
+  (flatten (interpose :gap
+                      (mapv (fn [gene]
+                              (break-up gene limit))
+                            (extract-genes plushy)))))

src/propeller/variation.cljc

@@ -244,7 +244,8 @@ The function `new-individual` returns a new individual produced by selection and
          (->  (bmx plushy1 plushy2 bmx-exchange-rate max-distance argmap)
               (uniform-gap-addition gap-change-prob)
               (uniform-gap-deletion gap-change-prob)
-              (utils/fill-empty-genes (:instructions argmap))))
+              (utils/fill-empty-genes (:instructions argmap))
+              (utils/enforce-gene-length-limit (:bmx-gene-length-limit argmap))))
        ;
        :umad ;; uniform mutation by addition and deletion, see uniform-deletion for the
                ;; adjustment that makes this size neutral on average
@@ -276,7 +277,8 @@ The function `new-individual` returns a new individual produced by selection and
               (uniform-gap-deletion gap-change-prob)
               (uniform-addition (:instructions argmap) umad-rate)
               (uniform-deletion umad-rate)
-              (utils/fill-empty-genes (:instructions argmap))))
+              (utils/fill-empty-genes (:instructions argmap))
+              (utils/enforce-gene-length-limit (:bmx-gene-length-limit argmap))))
        ;
        :rumad ;; responsive UMAD, uses a deletion rate computed from the actual
                 ;; number of additions made