Don't divide by an interval spanning 0

Author: bschmeck

exercise_2.10.exs

@@ -0,0 +1,39 @@
+defmodule Interval do
+  def make(lower, upper), do: [lower | upper]
+  def lower(interval), do: hd interval
+  def upper(interval), do: tl interval
+
+  def add(x, y), do: make(lower(x) + lower(y), upper(x) + upper(y))
+  
+  def sub(x, y), do: make(lower(x) - upper(y), upper(x) - lower(y))
+  
+  def mul(x, y) do
+    p1 = lower(x) * lower(y)
+    p2 = lower(x) * upper(y)
+    p3 = upper(x) * lower(y)
+    p4 = upper(x) * upper(y)
+
+    make Enum.min([p1, p2, p3, p4]), Enum.max([p1, p2, p3, p4])
+  end
+  
+  def div(x, y) do
+    if span_zero?(y) do
+      raise ArithmeticError, message: "cannot divide by interval spanning zero"
+    end
+    mul(x, make(1 / upper(y), 1 / lower(y)))
+  end
+
+  def span_zero?(interval), do: lower(interval) <= 0 && upper(interval) >= 0
+end
+
+ExUnit.start
+
+defmodule IntervalTests do
+  use ExUnit.Case, async: true
+  
+  test "dividing by zero is an error" do
+    numer = Interval.make 3, 7
+    denom = Interval.make -3, 3
+    assert_raise ArithmeticError, fn -> Interval.div(numer, denom) end
+  end
+end