Introduce Encoding class

It represents a multi-byte character encoding. Currently, SjisEncoding
is the only implementation of it.

src/encoding.cpp

@@ -0,0 +1,38 @@
+#include "encoding.h"
+#include "utfsjis.h"
+
+class SjisEncoding : public Encoding {
+public:
+	int mblen(const unsigned char* s)
+	{
+		return is_2byte(*s) ? 2 : 1;
+	}
+
+	int next_codepoint(const unsigned char** s)
+	{
+		int code = *(*s)++;
+		if (is_2byte(code))
+			code = (code << 8) | (uint8)*(*s)++;
+		return sjis_to_unicode(code);
+	}
+
+private:
+	bool is_2byte(unsigned char c) {
+		return (0x81 <= c && c <= 0x9f) || 0xe0 <= c;
+	}
+};
+
+int Encoding::mbslen(const unsigned char* s)
+{
+	int len = 0;
+	while (*s) {
+		s += mblen(s);
+		len++;
+	}
+	return len;
+}
+
+std::unique_ptr<Encoding> Encoding::create()
+{
+	return std::make_unique<SjisEncoding>();
+}