diff --git a/ChangeLog b/ChangeLog
index 189c84f379..4cdb594db2 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,15 @@
+Wed Jan 30 12:26:59 2008  Tanaka Akira  <akr@fsij.org>
+
+	* enc/utf_16be.c (UTF16_IS_SURROGATE_FIRST): avoid branch.
+	  (UTF16_IS_SURROGATE_SECOND): ditto.
+	  (UTF16_IS_SURROGATE): defined.
+	  (utf16be_mbc_enc_len): validation implemented.
+
+	* enc/utf_16le.c (UTF16_IS_SURROGATE_FIRST): avoid branch.
+	  (UTF16_IS_SURROGATE_SECOND): ditto.
+	  (UTF16_IS_SURROGATE): defined.
+	  (utf16le_mbc_enc_len): validation implemented.
+
 Wed Jan 30 12:06:43 2008  Tadayoshi Funaba  <tadf@dotrb.org>
 
 	* bignum.c (rb_cstr_to_inum): '0_2' is a valid representatin.
diff --git a/enc/utf_16be.c b/enc/utf_16be.c
index ab5ee1d4a3..5311ba3641 100644
--- a/enc/utf_16be.c
+++ b/enc/utf_16be.c
@@ -29,8 +29,9 @@
 
 #include "regenc.h"
 
-#define UTF16_IS_SURROGATE_FIRST(c)    (c >= 0xd8 && c <= 0xdb)
-#define UTF16_IS_SURROGATE_SECOND(c)   (c >= 0xdc && c <= 0xdf)
+#define UTF16_IS_SURROGATE_FIRST(c)    (((c) & 0xfc) == 0xd8)
+#define UTF16_IS_SURROGATE_SECOND(c)   (((c) & 0xfc) == 0xdc)
+#define UTF16_IS_SURROGATE(c)          (((c) & 0xf8) == 0xd8)
 
 static const int EncLen_UTF16[] = {
   2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2,
@@ -55,7 +56,28 @@ static int
 utf16be_mbc_enc_len(const UChar* p, const OnigUChar* e ARG_UNUSED,
 		    OnigEncoding enc ARG_UNUSED)
 {
-  return EncLen_UTF16[*p];
+  int byte = p[0];
+  if (!UTF16_IS_SURROGATE(byte)) {
+    if (2 <= e-p)
+      return ONIGENC_CONSTRUCT_MBCLEN_CHARFOUND(2);
+    else
+      return ONIGENC_CONSTRUCT_MBCLEN_NEEDMORE(1);
+  }
+  if (UTF16_IS_SURROGATE_FIRST(byte)) {
+    switch (e-p) {
+      case 1: return ONIGENC_CONSTRUCT_MBCLEN_NEEDMORE(3);
+      case 2: return ONIGENC_CONSTRUCT_MBCLEN_NEEDMORE(2);
+      case 3:
+        if (UTF16_IS_SURROGATE_SECOND(p[2]))
+          return ONIGENC_CONSTRUCT_MBCLEN_NEEDMORE(1);
+        break;
+      default:
+        if (UTF16_IS_SURROGATE_SECOND(p[2]))
+          return ONIGENC_CONSTRUCT_MBCLEN_CHARFOUND(4);
+        break;
+    }
+  }
+  return ONIGENC_CONSTRUCT_MBCLEN_INVALID();
 }
 
 static int
diff --git a/enc/utf_16le.c b/enc/utf_16le.c
index 355d9c23eb..33bfe08f47 100644
--- a/enc/utf_16le.c
+++ b/enc/utf_16le.c
@@ -29,8 +29,9 @@
 
 #include "regenc.h"
 
-#define UTF16_IS_SURROGATE_FIRST(c)    (c >= 0xd8 && c <= 0xdb)
-#define UTF16_IS_SURROGATE_SECOND(c)   (c >= 0xdc && c <= 0xdf)
+#define UTF16_IS_SURROGATE_FIRST(c)    (((c) & 0xfc) == 0xd8)
+#define UTF16_IS_SURROGATE_SECOND(c)   (((c) & 0xfc) == 0xdc)
+#define UTF16_IS_SURROGATE(c)          (((c) & 0xf8) == 0xd8)
 
 static const int EncLen_UTF16[] = {
   2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2,
@@ -52,10 +53,23 @@ static const int EncLen_UTF16[] = {
 };
 
 static int
-utf16le_mbc_enc_len(const UChar* p, const OnigUChar* e ARG_UNUSED,
+utf16le_mbc_enc_len(const UChar* p, const OnigUChar* e,
 		    OnigEncoding enc ARG_UNUSED)
 {
-  return EncLen_UTF16[*(p+1)];
+  int len = e-p, byte;
+  if (len < 2)
+    return ONIGENC_CONSTRUCT_MBCLEN_NEEDMORE(1);
+  byte = p[1];
+  if (!UTF16_IS_SURROGATE(byte)) {
+    return ONIGENC_CONSTRUCT_MBCLEN_CHARFOUND(2);
+  }
+  if (UTF16_IS_SURROGATE_FIRST(byte)) {
+    if (len < 4)
+      return ONIGENC_CONSTRUCT_MBCLEN_NEEDMORE(4-len);
+    if (UTF16_IS_SURROGATE_SECOND(p[3]))
+      return ONIGENC_CONSTRUCT_MBCLEN_CHARFOUND(4);
+  }
+  return ONIGENC_CONSTRUCT_MBCLEN_INVALID();
 }
 
 static int
diff --git a/test/ruby/test_utf16.rb b/test/ruby/test_utf16.rb
index 485a926db1..05236348d6 100644
--- a/test/ruby/test_utf16.rb
+++ b/test/ruby/test_utf16.rb
@@ -48,8 +48,59 @@ EOT
   # tests start
 
   def test_utf16be_valid_encoding
-    s = "\xd8\x00\xd8\x00".force_encoding("utf-16be")
-    assert_equal(false, s.valid_encoding?, "#{encdump s}.valid_encoding?")
+    [
+      "\x00\x00",
+      "\xd7\xff",
+      "\xd8\x00\xdc\x00",
+      "\xdb\xff\xdf\xff",
+      "\xe0\x00",
+      "\xff\xff",
+    ].each {|s|
+      s.force_encoding("utf-16be")
+      assert_equal(true, s.valid_encoding?, "#{encdump s}.valid_encoding?")
+    }
+    [
+      "\x00",
+      "\xd7",
+      "\xd8\x00",
+      "\xd8\x00\xd8\x00",
+      "\xdc\x00",
+      "\xdc\x00\xd8\x00",
+      "\xdc\x00\xdc\x00",
+      "\xe0",
+      "\xff",
+    ].each {|s|
+      s.force_encoding("utf-16be")
+      assert_equal(false, s.valid_encoding?, "#{encdump s}.valid_encoding?")
+    }
+  end
+
+  def test_utf16le_valid_encoding
+    [
+      "\x00\x00",
+      "\xff\xd7",
+      "\x00\xd8\x00\xdc",
+      "\xff\xdb\xff\xdf",
+      "\x00\xe0",
+      "\xff\xff",
+    ].each {|s|
+      s.force_encoding("utf-16le")
+      assert_equal(true, s.valid_encoding?, "#{encdump s}.valid_encoding?")
+    }
+    [
+      "\x00",
+      "\xd7",
+      "\x00\xd8",
+      "\x00\xd8\x00\xd8",
+      "\x00\xdc",
+      "\x00\xdc\x00\xd8",
+      "\x00\xdc\x00\xdc",
+      "\xe0",
+      "\xff",
+    ].each {|s|
+      s.force_encoding("utf-16le")
+      assert_equal(false, s.valid_encoding?, "#{encdump s}.valid_encoding?")
+    }
   end
 
   def test_strftime