Add missing row separator encoding conversion (#69)

The conversion logic is borrowed from ruby/ruby's io.c:
https://github.com/ruby/ruby/blob/40391faeab608665da87a05c686c074f91a5a206/io.c#L4059-L4079

Fix #68

Reported by IWAMOTO Kouichi. Thanks!!!

ext/stringio/stringio.c

@@ -1143,38 +1143,57 @@ struct getline_arg {
 };
 
 static struct getline_arg *
-prepare_getline_args(struct getline_arg *arg, int argc, VALUE *argv)
+prepare_getline_args(struct StringIO *ptr, struct getline_arg *arg, int argc, VALUE *argv)
 {
-    VALUE str, lim, opts;
+    VALUE rs, lim, opts;
     long limit = -1;
     int respect_chomp;
 
-    argc = rb_scan_args(argc, argv, "02:", &str, &lim, &opts);
-    respect_chomp = argc == 0 || !NIL_P(str);
+    argc = rb_scan_args(argc, argv, "02:", &rs, &lim, &opts);
+    respect_chomp = argc == 0 || !NIL_P(rs);
     switch (argc) {
       case 0:
-	str = rb_rs;
+	rs = rb_rs;
 	break;
 
       case 1:
-	if (!NIL_P(str) && !RB_TYPE_P(str, T_STRING)) {
-	    VALUE tmp = rb_check_string_type(str);
+        if (!NIL_P(rs) && !RB_TYPE_P(rs, T_STRING)) {
+            VALUE tmp = rb_check_string_type(rs);
 	    if (NIL_P(tmp)) {
-		limit = NUM2LONG(str);
-		str = rb_rs;
+                limit = NUM2LONG(rs);
+                rs = rb_rs;
 	    }
 	    else {
-		str = tmp;
+                rs = tmp;
 	    }
 	}
 	break;
 
       case 2:
-	if (!NIL_P(str)) StringValue(str);
+        if (!NIL_P(rs)) StringValue(rs);
 	if (!NIL_P(lim)) limit = NUM2LONG(lim);
 	break;
     }
-    arg->rs = str;
+    if (!NIL_P(rs)) {
+        rb_encoding *enc_rs, *enc_io;
+        enc_rs = rb_enc_get(rs);
+        enc_io = get_enc(ptr);
+        if (enc_rs != enc_io &&
+            (rb_enc_str_coderange(rs) != ENC_CODERANGE_7BIT ||
+             (RSTRING_LEN(rs) > 0 && !rb_enc_asciicompat(enc_io)))) {
+            if (rs == rb_rs) {
+                rs = rb_enc_str_new(0, 0, enc_io);
+                rb_str_buf_cat_ascii(rs, "\n");
+                rs = rs;
+            }
+            else {
+                rb_raise(rb_eArgError, "encoding mismatch: %s IO with %s RS",
+                         rb_enc_name(enc_io),
+                         rb_enc_name(enc_rs));
+            }
+        }
+    }
+    arg->rs = rs;
     arg->limit = limit;
     arg->chomp = 0;
     if (!NIL_P(opts)) {
@@ -1302,15 +1321,15 @@ strio_getline(struct getline_arg *arg, struct StringIO *ptr)
 static VALUE
 strio_gets(int argc, VALUE *argv, VALUE self)
 {
+    struct StringIO *ptr = readable(self);
     struct getline_arg arg;
     VALUE str;
 
-    if (prepare_getline_args(&arg, argc, argv)->limit == 0) {
-	struct StringIO *ptr = readable(self);
+    if (prepare_getline_args(ptr, &arg, argc, argv)->limit == 0) {
 	return rb_enc_str_new(0, 0, get_enc(ptr));
     }
 
-    str = strio_getline(&arg, readable(self));
+    str = strio_getline(&arg, ptr);
     rb_lastline_set(str);
     return str;
 }
@@ -1347,16 +1366,16 @@ static VALUE
 strio_each(int argc, VALUE *argv, VALUE self)
 {
     VALUE line;
+    struct StringIO *ptr = readable(self);
     struct getline_arg arg;
 
-    StringIO(self);
     RETURN_ENUMERATOR(self, argc, argv);
 
-    if (prepare_getline_args(&arg, argc, argv)->limit == 0) {
+    if (prepare_getline_args(ptr, &arg, argc, argv)->limit == 0) {
 	rb_raise(rb_eArgError, "invalid limit: 0 for each_line");
     }
 
-    while (!NIL_P(line = strio_getline(&arg, readable(self)))) {
+    while (!NIL_P(line = strio_getline(&arg, ptr))) {
 	rb_yield(line);
     }
     return self;
@@ -1374,15 +1393,15 @@ static VALUE
 strio_readlines(int argc, VALUE *argv, VALUE self)
 {
     VALUE ary, line;
+    struct StringIO *ptr = readable(self);
     struct getline_arg arg;
 
-    StringIO(self);
-    ary = rb_ary_new();
-    if (prepare_getline_args(&arg, argc, argv)->limit == 0) {
+    if (prepare_getline_args(ptr, &arg, argc, argv)->limit == 0) {
 	rb_raise(rb_eArgError, "invalid limit: 0 for readlines");
     }
 
-    while (!NIL_P(line = strio_getline(&arg, readable(self)))) {
+    ary = rb_ary_new();
+    while (!NIL_P(line = strio_getline(&arg, ptr))) {
 	rb_ary_push(ary, line);
     }
     return ary;

test/stringio/test_stringio.rb

@@ -88,6 +88,14 @@ def test_gets
     assert_string("", Encoding::UTF_8, StringIO.new("foo").gets(0))
   end
 
+  def test_gets_utf_16
+    stringio = StringIO.new("line1\nline2\nline3\n".encode("utf-16le"))
+    assert_equal("line1\n".encode("utf-16le"), stringio.gets)
+    assert_equal("line2\n".encode("utf-16le"), stringio.gets)
+    assert_equal("line3\n".encode("utf-16le"), stringio.gets)
+    assert_nil(stringio.gets)
+  end
+
   def test_gets_chomp
     assert_equal(nil, StringIO.new("").gets(chomp: true))
     assert_equal("", StringIO.new("\n").gets(chomp: true))