From c5dd9701497601c207c73afef54e4532fc2f6688 Mon Sep 17 00:00:00 2001
From: dndang23 <dndang23@amherst.edu>
Date: Wed, 28 Dec 2022 17:14:54 -0500
Subject: [PATCH] Implemented epsilon-lexicase selection into propeller

---
 src/propeller/gp.cljc        | 38 +++++++++++++++++++++++++++++++++++-
 src/propeller/selection.cljc | 21 +++++++++++++++++++-
 2 files changed, 57 insertions(+), 2 deletions(-)

diff --git a/src/propeller/gp.cljc b/src/propeller/gp.cljc
index 72ec5af..8d291ae 100644
--- a/src/propeller/gp.cljc
+++ b/src/propeller/gp.cljc
@@ -13,6 +13,38 @@
             [propeller.push.instructions.string]
             [propeller.push.instructions.vector]))
 
+(defn mean [coll]
+  (let [sum (apply + coll)
+        count (count coll)]
+    (if (pos? count)
+      (/ sum (float count))
+      0)))
+
+(defn median [coll]
+  (let [sorted (sort coll)
+        cnt (count sorted)
+        halfway (quot cnt 2.0)]
+    (if (odd? cnt)
+      (nth sorted halfway)
+      (let [bottom (dec halfway)
+            bottom-val (nth sorted bottom)
+            top-val (nth sorted halfway)]
+        (mean [bottom-val top-val])))))
+
+(defn median-absolute-deviation
+  [coll]
+  (let [median-val (median coll)]
+    (median (map #(Math/abs (- % median-val)) coll))))
+
+(defn epsilon-list
+  [pop]
+  (let [error-list (map :errors pop)
+        length (count (:errors (first pop)))]
+    (loop [epsilons [] i 0]
+      (if (= i length)
+        epsilons
+        (recur (conj epsilons (median-absolute-deviation (map #(nth % i) error-list))) (inc i))))))
+
 (defn report
   "Reports information each generation."
   [pop generation argmap]
@@ -50,7 +82,11 @@
                                  (mapper
                                    (partial error-function argmap (:training-data argmap))
                                    population))
-          best-individual (first evaluated-pop)]
+          best-individual (first evaluated-pop)
+          argmap (if (= (:parent-selection argmap) :epsilon-lexicase)
+                           (assoc argmap :epsilons (epsilon-list evaluated-pop))
+                           argmap)]
+      (prn argmap)
       (if (:custom-report argmap)
         ((:custom-report argmap) evaluated-pop generation argmap)
         (report evaluated-pop generation argmap))
diff --git a/src/propeller/selection.cljc b/src/propeller/selection.cljc
index 487ecf2..2ab96d9 100755
--- a/src/propeller/selection.cljc
+++ b/src/propeller/selection.cljc
@@ -21,9 +21,28 @@
                        survivors)
                (rest cases))))))
 
+(defn epsilon-lexicase-selection
+  "Selects an individual from the population using epsilon-lexicase selection."
+  [pop argmap]
+  (let [epsilons (:epsilons argmap)]
+    (loop [survivors pop
+           cases (shuffle (range (count (:errors (first pop)))))]
+      (if (or (empty? cases)
+              (empty? (rest survivors)))
+        (rand-nth survivors)
+
+        (let [min-err-for-case (apply min (map #(nth % (first cases))
+                                               (map :errors survivors)))
+              epsilon (nth epsilons (first cases))]
+
+          (recur (filter #(<= (Math/abs (- (nth (:errors %) (first cases)) min-err-for-case)) epsilon)
+                         survivors)
+                 (rest cases)))))))
+
 (defn select-parent
   "Selects a parent from the population using the specified method."
   [pop argmap]
   (case (:parent-selection argmap)
     :tournament (tournament-selection pop argmap)
-    :lexicase (lexicase-selection pop argmap)))
+    :lexicase (lexicase-selection pop argmap)
+    :epsilon-lexicase (epsilon-lexicase-selection pop argmap)))