add XOR operator and tests

closes #246

README.md

@@ -144,7 +144,7 @@ Also, all functions from Ruby's Math module, including `SIN`, `COS`, `TAN`, etc.
 
 Comparison: `<`, `>`, `<=`, `>=`, `<>`, `!=`, `=`,
 
-Logic: `IF`, `AND`, `OR`, `NOT`, `SWITCH`
+Logic: `IF`, `AND`, `OR`, `XOR`, `NOT`, `SWITCH`
 
 Numeric: `MIN`, `MAX`, `SUM`, `AVG`, `COUNT`, `ROUND`, `ROUNDDOWN`, `ROUNDUP`
 

lib/dentaku/ast.rb

@@ -35,3 +35,4 @@
 require_relative './ast/functions/string_functions'
 require_relative './ast/functions/sum'
 require_relative './ast/functions/switch'
+require_relative './ast/functions/xor'

lib/dentaku/ast/functions/xor.rb

@@ -0,0 +1,48 @@
+require_relative '../function'
+require_relative '../../exceptions'
+
+module Dentaku
+  module AST
+    class Xor < Function
+      def self.min_param_count
+        1
+      end
+
+      def self.max_param_count
+        Float::INFINITY
+      end
+
+      def deferred_args
+        [1, 2]
+      end
+
+      def value(context = {})
+        if @args.empty?
+          raise Dentaku::ArgumentError.for(
+            :too_few_arguments,
+            function_name: 'XOR()', at_least: 1, given: 0
+          ), 'XOR() requires at least one argument'
+        end
+
+        true_arg_count = 0
+        @args.each do |arg|
+          case arg.value(context)
+          when TrueClass
+            true_arg_count += 1
+            break if true_arg_count > 1
+          when FalseClass, nil
+            next
+          else
+            raise Dentaku::ArgumentError.for(
+              :incompatible_type,
+              function_name: 'XOR()', expect: :logical, actual: arg.class
+            ), 'XOR() requires arguments to be logical expressions'
+          end
+        end
+        true_arg_count == 1
+      end
+    end
+  end
+end
+
+Dentaku::AST::Function.register_class(:xor, Dentaku::AST::Xor)

lib/dentaku/parser.rb

@@ -22,6 +22,7 @@ class Parser
 
       and:      AST::And,
       or:       AST::Or,
+      xor:      AST::Xor,
     }.freeze
 
     attr_reader :input, :output, :operations, :arities, :case_sensitive

spec/ast/xor_spec.rb

@@ -0,0 +1,35 @@
+require 'spec_helper'
+require 'dentaku'
+require 'dentaku/ast/functions/or'
+
+describe 'Dentaku::AST::Xor' do
+  let(:calculator) { Dentaku::Calculator.new }
+
+  it 'returns false if all of the arguments are false' do
+    result = Dentaku('XOR(false, false)')
+    expect(result).to eq(false)
+  end
+
+  it 'returns true if only one of the arguments is true' do
+    result = Dentaku('XOR(false, true)')
+    expect(result).to eq(true)
+  end
+
+  it 'returns false if more than one of the arguments is true' do
+    result = Dentaku('XOR(false, true, true)')
+    expect(result).to eq(false)
+  end
+
+  it 'supports nested expressions' do
+    result = Dentaku('XOR(y = 1, x = 1)', x: 1, y: 2)
+    expect(result).to eq(true)
+  end
+
+  it 'raises an error if no arguments are passed' do
+    expect { calculator.evaluate!('XOR()') }.to raise_error(Dentaku::ParseError)
+  end
+
+  it 'raises an error if a non logical argument is passed' do
+    expect { calculator.evaluate!('XOR("r")') }.to raise_error(Dentaku::ArgumentError)
+  end
+end