Add a zip_with function.

spec/util_spec.cr

@@ -0,0 +1,29 @@
+require "./spec_helper"
+
+describe Array do
+  describe "#union" do
+    it "works with no elements" do
+      ([] of Set(Int32)).union.should eq(Set(Int32).new)
+    end
+
+    it "correctly performs a union" do
+      [Set { 1 }, Set { 2 }, Set {1, 2, 3}].union.should eq(Set { 1,2,3})
+    end
+  end
+
+  describe "#intersect" do
+    expect_raises(Enumerable::EmptyError) do
+      ([] of Set(Int32)).intersect
+    end
+
+    it "correctly performs an intersection" do
+      [Set {1}, Set {1,2}, Set {1,2,3}].intersect.should eq(Set{1})
+    end
+  end
+
+  describe "#zip_with" do
+    it "performs as expected" do
+      [1,2,3].zip_with([2,3,4]) { |l,r| l*r }.should eq([2,6,12])
+    end
+  end
+end

src/advent/util.cr

@@ -1,4 +1,12 @@
 class Array(T)
+  def zip_with(other : Array(R), &block : T, R -> U) forall R, U
+    dest = [] of U
+    zip(other) do |l, r|
+      dest << yield l, r
+    end
+    dest
+  end
+
   def union
     reduce(T.new) do |l, r|
       l | r