LibTextCodec: Start fleshing out a simple text codec library

We're starting with a very basic decoding API and only ISO-8859-1 and
UTF-8 decoding (and UTF-8 decoding is really a no-op since String is
expected to be UTF-8.)

Libraries/LibTextCodec/Decoder.cpp

@@ -0,0 +1,73 @@
+/*
+ * Copyright (c) 2020, Andreas Kling <kling@serenityos.org>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include <AK/String.h>
+#include <AK/StringBuilder.h>
+#include <LibTextCodec/Decoder.h>
+
+namespace TextCodec {
+
+Latin1Decoder& latin1_decoder()
+{
+    static Latin1Decoder* decoder;
+    if (!decoder)
+        decoder = new Latin1Decoder;
+    return *decoder;
+}
+
+UTF8Decoder& utf8_decoder()
+{
+    static UTF8Decoder* decoder;
+    if (!decoder)
+        decoder = new UTF8Decoder;
+    return *decoder;
+}
+
+Decoder* decoder_for(const String& encoding)
+{
+    if (encoding.equals_ignoring_case("iso-8859-1"))
+        return &latin1_decoder();
+    if (encoding.equals_ignoring_case("utf-8"))
+        return &utf8_decoder();
+    return nullptr;
+}
+
+String UTF8Decoder::to_utf8(const StringView& input)
+{
+    return input;
+}
+
+String Latin1Decoder::to_utf8(const StringView& input)
+{
+    StringBuilder builder(input.length());
+    for (size_t i = 0; i < input.length(); ++i) {
+        u8 ch = input[i];
+        builder.append(ch >= 0x80 ? '?' : ch);
+    }
+    return builder.to_string();
+}
+
+}