Fix edge cases for starts_with?, ends_with? and contains?

Author: alco

lib/elixir/lib/string.ex

@@ -811,12 +811,16 @@ defmodule String do
   """
   @spec starts_with?(t, t | [t]) :: boolean
 
-  def starts_with?(string, prefixes) do
-    case :binary.match(string, prefixes) do
-      :nomatch -> false
-      {0, _}   -> true
-      _        -> false
-    end
+  def starts_with?(_, "") do
+    true
+  end
+
+  def starts_with?(string, prefix) when is_binary(prefix) do
+    match?({0,_}, :binary.match(string, prefix))
+  end
+
+  def starts_with?(string, prefixes) when is_list(prefixes) do
+    Enum.any?(prefixes, starts_with?(string, &1))
   end
 
   @doc """
@@ -847,7 +851,7 @@ defmodule String do
 
   @doc """
   Returns true if `string` contains match, otherwise false.
-  `matches` can be either a single match of a list of matches.
+  `matches` can be either a single string or a list of strings.
 
   ## Examples
 
@@ -859,13 +863,13 @@ defmodule String do
       false
 
   """
-  def contains?(string, matches) when is_list(matches) do
-    case :binary.match(string, matches) do
-      :nomatch -> false
-      _        -> true
-    end
-  end
+  @spec contains?(t, t | [t]) :: boolean
 
-  def contains?(string, match), do: contains?(string, [ match ])
+  def contains?(_, "") do
+    true
+  end
 
+  def contains?(string, matches) do
+    :nomatch != :binary.match(string, matches)
+  end
 end

lib/elixir/test/elixir/string_test.exs

@@ -281,6 +281,16 @@ defmodule StringTest do
   end
 
   test :starts_with? do
+    assert String.starts_with? "", ""
+    assert String.starts_with? "", ["", "a"]
+    assert String.starts_with? "b", ["", "a"]
+
+    assert String.starts_with? "abc", ""
+    assert String.starts_with? "abc", [""]
+
+    refute String.starts_with? "", "abc"
+    refute String.starts_with? "", [" "]
+
     assert String.starts_with? "hello", "he"
     assert String.starts_with? "hello", "hello"
     assert String.starts_with? "hello", ["hellö", "hell"]
@@ -292,6 +302,16 @@ defmodule StringTest do
   end
 
   test :ends_with? do
+    assert String.ends_with? "", ""
+    assert String.ends_with? "", ["", "a"]
+    refute String.ends_with? "", ["a", "b"]
+
+    assert String.ends_with? "abc", ""
+    assert String.ends_with? "abc", ["", "x"]
+
+    refute String.ends_with? "", "abc"
+    refute String.ends_with? "", [" "]
+
     assert String.ends_with? "hello", "lo"
     assert String.ends_with? "hello", "hello"
     assert String.ends_with? "hello", ["hell", "lo", "xx"]
@@ -304,6 +324,13 @@ defmodule StringTest do
   end
 
   test :contains? do
+    assert String.contains? "", ""
+    assert String.contains? "abc", ""
+    assert String.contains? "abc", ["", "x"]
+
+    refute String.contains? "", " "
+    refute String.contains? "", "a"
+
     assert String.contains? "elixir of life", "of"
     assert String.contains? "エリクシア", "シ"
     assert String.contains? "elixir of life", ["mercury", "life"]