Added a set of rules which attempt to model natural processes in the

environment.

src/mw_engine/natural_rules.clj

@@ -0,0 +1,120 @@
+(ns mw-engine.natural-rules
+  (:use mw-engine.utils
+        mw-engine.world))
+
+;; rules describing the natural ecosystem
+
+(def treeline 10)
+
+;; one in fifty chance of lightning strike
+(def lightning-probability 50)
+
+;; rules describing vegetation
+(def vegetation-rules
+  (list 
+    ;; Randomly, birds plant tree seeds into pasture.
+    (fn [cell world] (cond (and (= (:state cell) :pasture)(< (rand 10) 1))(merge cell {:state :scrub})))
+    ;; Scrub below the treeline grows gradually into forest, providing browsing pressure is not to high
+    (fn [cell world] 
+      (cond (and 
+              (= (:state cell) :scrub)
+              ;; browsing limit really ought to vary with soil fertility, but...
+              (< (+ (population cell :deer)(or (:sheep cell) 0)) 6)
+              (< (:altitude cell) treeline)) 
+        (merge cell {:state :scrub2})))
+    (fn [cell world] (cond (= (:state cell) :scrub2) (merge cell {:state :forest})))
+    ;; Forest on fertile land at low altitude grows to climax
+    (fn [cell world] 
+      (cond 
+        (and 
+          (= (:state cell) :forest) 
+          (> (:fertility cell) 10)) 
+        (merge cell {:state :climax})))
+    ;; Climax forest occasionally catches fire (e.g. lightning strikes)
+    (fn [cell world] (cond (and (= (:state cell) :climax)(< (rand lightning-probability) 1)) (merge cell {:state :fire})))
+    ;; Climax forest neighbouring fires is likely to catch fire
+    (fn [cell world]
+      (cond 
+        (and (= (:state cell) :climax)
+             (< (rand 3) 1)
+             (not (empty? (get-neighbours-with-state world (:x cell) (:y cell) 1 :fire))))
+        (merge cell {:state :fire})))
+    ;; After fire we get waste
+    (fn [cell world] (cond (= (:state cell) :fire) (merge cell {:state :waste})))
+    ;; And after waste we get pioneer species; if there's a woodland seed 
+    ;; source, it's going to be scrub, otherwise grassland.
+    (fn [cell world]
+      (cond
+        (and (= (:state cell) :waste)
+             (not 
+               (empty? 
+                 (flatten 
+                   (list 
+                     (get-neighbours-with-state world (:x cell) (:y cell) 1 :scrub2)
+                     (get-neighbours-with-state world (:x cell) (:y cell) 1 :forest)
+                     (get-neighbours-with-state world (:x cell) (:y cell) 1 :climax))))))
+        (merge cell {:state :scrub})))
+    (fn [cell world]
+      (cond (= (:state cell) :waste)
+        (merge cell {:state :pasture})))
+    ;; Forest increases soil fertility
+    (fn [cell world]
+      (cond (member? (:state cell) '(:forest :climax))
+        (merge cell {:fertility (+ (:fertility cell) 1)})))
+  ))
+
+;; rules describing animal behaviour
+(def predation-rules
+  (list
+    ;; deer arrive occasionally at the edge of the map.
+    (fn [cell world]
+      (cond (and (< (count (get-neighbours world cell)) 8)
+                 (< (rand 50) 1)
+                 (= (population cell :deer) 0))
+        (merge cell {:deer 2})))
+    ;; if there are too many deer for the fertility of the area to sustain, 
+    ;; some die or move on.
+    (fn [cell world]
+      (cond (> (* (population cell :deer) 10) (:fertility cell))
+        (merge cell {:deer (int (/ (:fertility cell) 10))})))
+    ;; deer gradually spread through the world by breeding or migrating.
+    (fn [cell world]
+      (let [n (apply + (map #(population % :deer) (get-neighbours world cell)))]
+        (cond (and
+                (= (population cell :deer) 0)
+                (>= n 2))
+          (merge cell {:deer (int (/ n 2))}))))
+    ;; deer breed.
+    (fn [cell world]
+      (cond 
+        (>= (population cell :deer) 2)
+        (merge cell {:deer (int (* (:deer cell) 4))})))
+    ;; wolves arrive occasionally at the edge of the map.
+;;    (fn [cell world]
+;;      (cond (and (< (count (get-neighbours world cell)) 8)
+;;                 (< (rand 50) 1)
+;;                 (= (population cell :wolves) 0))
+;;        (merge cell {:wolves 2})))
+    ;; if there are not enough deer to sustain the population of wolves, 
+    ;; some wolves die or move on.
+    (fn [cell world]
+      (cond (> (population cell :wolves) (population cell :deer))
+        (merge cell {:wolves 0})))
+    ;; wolves gradually spread through the world by breeding or migrating.
+    (fn [cell world]
+      (let [n (apply + (map #(population % :wolves) (get-neighbours world cell)))]
+        (cond (and
+                (= (population cell :wolves) 0)
+                (>= n 2))
+          (merge cell {:wolves 2}))))
+    ;; wolves breed.
+ ;;   (fn [cell world]
+ ;;     (cond 
+ ;;       (>= (population cell :wolves) 2)
+ ;;       (merge cell {:wolves (int (* (:wolves cell) 2))})))
+    ;; wolves eat deer
+    (fn [cell world]
+      (merge cell {:deer (- (population cell :deer) (population cell :wolves))}))
+    ))
+
+(def natural-rules (flatten (list vegetation-rules predation-rules)))