working to compute a single program that meets the target output

genetic.rb

@@ -1,3 +1,58 @@
 #!/usr/bin/env ruby
 
 require_relative "program"
+
+NUM_PROGRAMS = 100
+EXPECTED_OUTPUT = "HELLOWORLD"
+
+def ord(chr)
+  chr.unpack("C").first
+end
+
+def grade_program(p)
+  output = p.execute
+  grade = (EXPECTED_OUTPUT.size - output.size).abs * 100
+  [EXPECTED_OUTPUT.size, output.size].min.times do |i|
+    grade += (ord(EXPECTED_OUTPUT[i]) - ord(output[i])).abs
+  end
+  grade
+end
+
+def main
+  programs = NUM_PROGRAMS.times.map do
+    p = Program.new
+    [grade_program(p), p]
+  end
+  generation = 0
+  begin
+    last_best = nil
+    while true
+      generation += 1
+      programs += programs.map do |grade, p|
+        p2 = p.clone
+        p2.mutate
+        [grade_program(p2), p2]
+      end
+      programs.sort! do |a, b|
+        a.first <=> b.first
+      end
+      programs = programs.take(NUM_PROGRAMS)
+      if last_best.nil? or programs.first.first < last_best
+        last_best = programs.first.first
+        $stdout.write("\rGrade: #{last_best}, output: #{programs.first.last.execute.inspect}                    ")
+      end
+      if programs.first.first == 0
+        puts "Program found after #{generation} generations:"
+        puts programs.first.last
+        break
+      end
+    end
+  rescue Interrupt
+    puts
+    puts "Canceled at #{generation} generations"
+    puts "The best program (grade #{programs.first.first}) is:"
+    puts programs.first.last
+  end
+end
+
+main

program.rb

@@ -6,15 +6,8 @@ class Program
     end
   end
 
-  private
-
-  def random_instruction
-    case rand(2)
-    when 0
-      [:load, (65 + rand(26)).chr]
-    when 1
-      [:output]
-    end
+  def initialize_copy(other)
+    @instructions = Marshal.load(Marshal.dump(@instructions))
   end
 
   def to_s
@@ -29,5 +22,54 @@ class Program
   end
 
   def execute
+    output = ""
+    register = "A"
+    @instructions.each do |instruction, *params|
+      case instruction
+      when :load
+        register = params.first
+      when :output
+        output += register
+      end
+    end
+    output
+  end
+
+  def mutate
+    case rand(4)
+    when 0 # remove an instruction
+      if @instructions.size > 1
+        i = rand(@instructions.size)
+        @instructions.delete_at(i)
+      end
+    when 1 # insert an instruction
+      i = rand(@instructions.size + 1)
+      @instructions.insert(i, random_instruction)
+    when 2 # replace an instruction
+      i = rand(@instructions.size)
+      @instructions[i] = random_instruction
+    when 3 # modify parameter
+      i = rand(@instructions.size)
+      if @instructions[i].first == :load
+        @instructions[i][1] = mutate_param(@instructions[i][1])
+      end
+    end
+  end
+
+private
+
+  def random_instruction
+    case rand(2)
+    when 0
+      [:load, (65 + rand(26)).chr]
+    when 1
+      [:output]
+    end
+  end
+
+  def mutate_param(old_value)
+    chr = old_value.unpack("C").first
+    chr = (chr + (rand(2) + 1) * [-1, 1][rand(2)]) % 26
+    chr.chr
   end
 end