Add overload to String.from_utf16 with pointer

spec/std/string/utf16_spec.cr

@@ -43,5 +43,11 @@ describe "String UTF16" do
       input = Slice[0xdc00_u16, 0xd800_u16]
       String.from_utf16(input).should eq("\u{fffd}\u{fffd}")
     end
+
+    it "handles null bytes" do
+      slice = Slice[104_u16, 105_u16, 0_u16, 55296_u16, 56485_u16]
+      String.from_utf16(slice).should eq("hi\0000𐂥")
+      String.from_utf16(slice.to_unsafe).should eq("hi")
+    end
   end
 end

src/string/utf16.cr

@@ -53,7 +53,15 @@ class String
   # slice = Slice[104_u16, 105_u16, 32_u16, 55296_u16, 56485_u16]
   # String.from_utf16(slice) # => "hi 𐂥"
   # ```
-  def self.from_utf16(slice : Slice(UInt16)) : String
+  #
+  # If *slice* is a pointer, the string ends when a zero value is found.
+  #
+  # ```
+  # slice = Slice[104_u16, 105_u16, 0_u16, 55296_u16, 56485_u16]
+  # String.from_utf16(slice)           # => "hi\0000𐂥"
+  # String.from_utf16(slice.to_unsafe) # => "hi"
+  # ```
+  def self.from_utf16(slice : Slice(UInt16) | Pointer(UInt16)) : String
     bytesize = 0
     size = 0
 
@@ -97,4 +105,29 @@ class String
       i += 1
     end
   end
+
+  # Yields each decoded char in the given pointer, stopping at the first null byte.
+  private def self.each_utf16_char(pointer : Pointer(UInt16))
+    loop do
+      byte = pointer.value.to_i
+      break if byte == 0
+
+      if byte < 0xd800 || byte >= 0xe000
+        # One byte
+        codepoint = byte
+      elsif 0xd800 <= byte < 0xdc00 &&
+            0xdc00 <= (pointer + 1).value <= 0xdfff
+        # Surrougate pair
+        pointer = pointer + 1
+        codepoint = ((byte - 0xd800) << 10) + (pointer.value - 0xdc00) + 0x10000
+      else
+        # Invalid byte
+        codepoint = 0xfffd
+      end
+
+      yield codepoint.chr
+
+      pointer = pointer + 1
+    end
+  end
 end