Handle non-positive amounts (#55)

lib/query.rb

@@ -14,7 +14,10 @@ class Query
   def amount
     return unless @params[:amount]
 
-    @params[:amount].to_f
+    value = @params[:amount].to_f
+    raise ArgumentError, "invalid amount" unless value.positive?
+
+    value
   end
 
   def base

spec/edge_cases_spec.rb

@@ -23,6 +23,11 @@ describe "the server" do
     _(last_response).must_be(:unprocessable?)
   end
 
+  it "will not process an invalid amount" do
+    get "/latest?amount=0&from=USD&to=EUR"
+    _(last_response).must_be(:unprocessable?)
+  end
+
   it "will not process a date before 2000" do
     get "/1999-01-01"
     _(last_response).must_be(:not_found?)

spec/query_spec.rb

@@ -13,6 +13,11 @@ describe Query do
     _(query.amount).must_equal(100.0)
   end
 
+  it "requires a positive amount" do
+    _ { Query.new(amount: "0").to_h }.must_raise(ArgumentError)
+    _ { Query.new(amount: "-1").to_h }.must_raise(ArgumentError)
+  end
+
   it "defaults amount to nothing" do
     query = Query.new
     _(query.amount).must_be_nil