Fix `String#byteslice` with `MRB_UTF8_STRING` and some edge cases

Example:

  $ bin/mruby -e '
    p "あa".byteslice(1)
    p "bar".byteslice(3)
    p "bar".byteslice(4..0)
  '

  Before this patch:

    "a"
    ""
    RangeError (4..0 out of range)

  After this patch (same as Ruby):

    "\x81"
    nil
    nil

1 files changed, 51 insertions, 0 deletions

diff --git a/mrbgems/mruby-string-ext/test/string.rb b/mrbgems/mruby-string-ext/test/string.rb
index 02777e594..bf633bcef 100644
--- a/mrbgems/mruby-string-ext/test/string.rb
+++ b/mrbgems/mruby-string-ext/test/string.rb
@@ -26,10 +26,61 @@ end
 
 assert('String#byteslice') do
   str1 = "hello"
+  str2 = "\u3042ab"  # "\xE3\x81\x82ab"
+
+  assert_equal("h", str1.byteslice(0))
   assert_equal("e", str1.byteslice(1))
+  assert_equal(nil, str1.byteslice(5))
   assert_equal("o", str1.byteslice(-1))
+  assert_equal(nil, str1.byteslice(-6))
+  assert_equal("\xE3", str2.byteslice(0))
+  assert_equal("\x81", str2.byteslice(1))
+  assert_equal(nil, str2.byteslice(5))
+  assert_equal("b", str2.byteslice(-1))
+  assert_equal(nil, str2.byteslice(-6))
+
+  assert_equal("", str1.byteslice(0, 0))
+  assert_equal(str1, str1.byteslice(0, 6))
+  assert_equal("el", str1.byteslice(1, 2))
+  assert_equal("", str1.byteslice(5, 1))
+  assert_equal("o", str1.byteslice(-1, 6))
+  assert_equal(nil, str1.byteslice(-6, 1))
+  assert_equal(nil, str1.byteslice(0, -1))
+  assert_equal("", str2.byteslice(0, 0))
+  assert_equal(str2, str2.byteslice(0, 6))
+  assert_equal("\x81\x82", str2.byteslice(1, 2))
+  assert_equal("", str2.byteslice(5, 1))
+  assert_equal("b", str2.byteslice(-1, 6))
+  assert_equal(nil, str2.byteslice(-6, 1))
+  assert_equal(nil, str2.byteslice(0, -1))
+
   assert_equal("ell", str1.byteslice(1..3))
   assert_equal("el", str1.byteslice(1...3))
+  assert_equal("h", str1.byteslice(0..0))
+  assert_equal("", str1.byteslice(5..0))
+  assert_equal("o", str1.byteslice(4..5))
+  assert_equal(nil, str1.byteslice(6..0))
+  assert_equal("", str1.byteslice(-1..0))
+  assert_equal("llo", str1.byteslice(-3..5))
+  assert_equal("\x81\x82a", str2.byteslice(1..3))
+  assert_equal("\x81\x82", str2.byteslice(1...3))
+  assert_equal("\xE3", str2.byteslice(0..0))
+  assert_equal("", str2.byteslice(5..0))
+  assert_equal("b", str2.byteslice(4..5))
+  assert_equal(nil, str2.byteslice(6..0))
+  assert_equal("", str2.byteslice(-1..0))
+  assert_equal("\x82ab", str2.byteslice(-3..5))
+
+  assert_raise(ArgumentError) { str1.byteslice }
+  assert_raise(ArgumentError) { str1.byteslice(1, 2, 3) }
+  assert_raise(TypeError) { str1.byteslice("1") }
+  assert_raise(TypeError) { str1.byteslice("1", 2) }
+  assert_raise(TypeError) { str1.byteslice(1, "2") }
+  assert_raise(TypeError) { str1.byteslice(1..2, 3) }
+
+  skip unless Object.const_defined?(:Float)
+  assert_equal("o", str1.byteslice(4.0))
+  assert_equal("\x82ab", str2.byteslice(2.0, 3.0))
 end
 
 assert('String#dump') do