Fix bug in Markov.Chain where reply_chance would get reset, add regression tests for this

* Markov.Chain.add_weight/4 would return a fresh new markov chain,
  instead of using all of the previous values of the chain that was
  being modified. This would result in resetting the reply_chance value
  to the default of 0.01.
* Add tests to make sure this doesn't happen to add_weights in the
  future.

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

lib/contrib/markov/chain.ex

@@ -21,17 +21,17 @@ defmodule Omnibot.Contrib.Markov.Chain do
     end)
   end
 
-  def add_weight(%Chain{chain: chain, order: order}, key, word, increment \\ 1) do
+  def add_weight(chain = %Chain{}, key, word, increment \\ 1) do
-    if length(key) != order do
+    if length(key) != chain.order do
-      raise(ArgumentError, message: "invalid key (length #{length(key)} vs. order #{order})")
+      raise(ArgumentError, message: "invalid key (length #{length(key)} vs. order #{chain.order})")
     end
 
     # %{
     #   ["word1", "word2"] => %{"target" => weight}
     # }
-    chain = Map.update(chain, key, %{word => increment},
+    chain_map = Map.update(chain.chain, key, %{word => increment},
       fn weights -> Map.update(weights, word, increment, &(increment + &1)) end)
-    %Chain{chain: chain, order: order}
+    %Chain{chain | chain: chain_map}
   end
 
   def generate(chain) do

test/contrib/markov/chain_test.exs

@@ -68,6 +68,23 @@ defmodule MarkovChainTest do
     }
   end
 
+  test "chain add_weight does not reset reply_chance" do
+    chain = %Chain {order: 2, reply_chance: 0.0}
+            |> Chain.add_weight(["foo", "bar"], "baz")
+
+    chain = chain |> Chain.add_weight(["foo", "bar"], "baz", 2)
+    assert chain.reply_chance == 0.0
+
+    chain = chain |> Chain.add_weight(["foo", "bar"], "qux")
+    assert chain.reply_chance == 0.0
+
+    chain = chain |> Chain.add_weight(["bar", "baz"], "qux")
+    assert chain.reply_chance == 0.0
+
+    chain = chain |> Chain.add_weight(["bar", "baz"], nil)
+    assert chain.reply_chance == 0.0
+  end
+
   test "chain merge works correctly" do
     chain1 = %Chain {order: 2}
             |> Chain.add_weight(["foo", "bar"], "baz")