Add method to subtract IPv4 subnets

lib/ipaddress/ipv4.rb

@@ -516,7 +516,7 @@ def <=>(oth)
       to_u32 <=> oth.to_u32
     end
     alias eql? ==
-    
+
     #
     # Returns the number of IP addresses included
     # in the network. It also counts the network
@@ -805,6 +805,42 @@ def subnet(subprefix)
       end
     end
 
+    #
+    # Subtract other from the current range
+    # Returns list of ranges that excludes :other
+    #
+    # Example:
+    #
+    #   a = IPAddress '10.0.0.0/16'
+    #   b = IPAddress '10.0.128.0/18'
+    #   c = IPAddress '192.168.0.0/16'
+    #
+    #   a.subtract(b)
+    #   => 
+    #   [#<IPAddress::IPv4:0x0000000111e61a88 @address="10.0.0.0", @allocator=0, @octets=[10, 0, 0, 0], @prefix=18, @u32=167772160>,
+    #    #<IPAddress::IPv4:0x0000000111e61470 @address="10.0.64.0", @allocator=0, @octets=[10, 0, 64, 0], @prefix=18, @u32=167788544>,
+    #    #<IPAddress::IPv4:0x0000000111e60840 @address="10.0.192.0", @allocator=0, @octets=[10, 0, 192, 0], @prefix=18, @u32=167821312>] 
+    #   b.subtract(a)
+    #    => []
+    #   a.subtract(c)
+    #    => [#<IPAddress::IPv4:0x0000000107f08978 @address="10.0.0.0", @allocator=0, @octets=[10, 0, 0, 0], @prefix=16, @u32=167772160>] 
+    #
+    def subtract(other)
+      raise ArgumentError unless other.is_a? self.class
+      result = []
+      if self.include? other
+        split_bits = 2**(self.prefix - other.prefix)
+        other_alike_subnets = self.split split_bits
+        raise "Prefix of subnet and split doesn't match" unless other_alike_subnets.first.prefix == other.prefix
+
+        other_alike_subnets.delete other
+        result = other_alike_subnets
+      elsif !other.include? self
+        result << self.clone
+      end
+      result
+    end
+
     #
     # Returns the difference between two IP addresses
     # in unsigned int 32 bits format

test/ipaddress/ipv4_test.rb

@@ -491,6 +491,16 @@ def test_method_subnet
     arr = ["172.16.10.0/24"]
     assert_equal arr, @network.subnet(24).map {|s| s.to_string}
   end
+
+  def test_method_subtract
+    a = IPAddress '10.0.0.0/16'
+    b = IPAddress '10.0.128.0/18'
+    c = IPAddress '192.168.0.0/16'
+
+    assert !a.subtract(b).include?(b)
+    assert b.subtract(a).empty?
+    assert_equal a, a.subtract(c).first
+  end
 
   def test_method_supernet
     assert_raises(ArgumentError) {@ip.supernet(24)}