UTF-8 part 2: Encoding wide-character strings into UTF-8

We now have Utf8StringEncoder and Utf8Encoder (no state this time)

luna/include/luna/Utf8.h

@@ -15,7 +15,7 @@ class Utf8StringDecoder
 
     Result<usize> code_points() const;
 
-    // The caller must ensure that 'buf' is at least wide_length() + a NULL wide.
+    // The caller must ensure that 'buf' is at least code_points() + a NULL wide.
     Result<void> decode(wchar_t* buf) const;
 
   private:
@@ -23,6 +23,26 @@ class Utf8StringDecoder
     usize m_byte_length;
 };
 
+class Utf8StringEncoder
+{
+  public:
+    Utf8StringEncoder(const wchar_t* str);
+
+    usize code_points() const
+    {
+        return m_code_points;
+    }
+
+    Result<usize> byte_length() const;
+
+    // The caller must ensure that 'buf' is at least byte_length() + a NULL wide.
+    Result<void> encode(char* buf) const;
+
+  private:
+    const wchar_t* m_str;
+    usize m_code_points;
+};
+
 class Utf8StateDecoder
 {
   public:
@@ -36,3 +56,10 @@ class Utf8StateDecoder
     usize m_state_len = 0;
     usize m_state_index = 0;
 };
+
+class Utf8Encoder
+{
+  public:
+    // Does not null-terminate. Returns the number of bytes written.
+    Result<usize> encode(wchar_t c, char buf[4]);
+};

luna/src/Utf8.cpp

@@ -10,7 +10,52 @@ static Result<usize> utf8_char_length(char c)
     return err(EILSEQ);
 }
 
-static Result<wchar_t> encode_utf8_char_into_wide_char(const char* beg, usize& len)
+static Result<usize> wide_char_length_as_utf8(wchar_t c)
+{
+    if (c <= 0x7f) return 1;
+    if (c <= 0x7ff) return 2;
+    if (c <= 0xffff) return 3;
+    if (c <= 0x10ffff) return 4;
+    return err(EILSEQ);
+}
+
+static Result<void> encode_wide_char_as_utf8(wchar_t c, char* result, usize& len)
+{
+    len = TRY(wide_char_length_as_utf8(c));
+
+    u8* buf = (u8*)result;
+
+    if (len == 1)
+    {
+        buf[0] = c & 0x7f;
+        return {};
+    }
+    if (len == 2)
+    {
+        buf[0] = 0b11000000 | ((c & 0x7c0) >> 6);
+        buf[1] = 0b10000000 | (c & 0x3f);
+        return {};
+    }
+    if (len == 3)
+    {
+        buf[0] = 0b11100000 | ((c & 0xf000) >> 12);
+        buf[1] = 0b10000000 | ((c & 0xfc0) >> 6);
+        buf[2] = 0b10000000 | (c & 0x3f);
+        return {};
+    }
+    if (len == 4)
+    {
+        buf[0] = 0b11110000 | ((c & 0x1c0000) >> 18);
+        buf[1] = 0b10000000 | ((c & 0x3f000) >> 12);
+        buf[2] = 0b10000000 | ((c & 0xfc0) >> 6);
+        buf[3] = 0b10000000 | (c & 0x3f);
+        return {};
+    }
+
+    unreachable();
+}
+
+static Result<wchar_t> encode_utf8_as_wide_char(const char* beg, usize& len)
 {
     usize utf8_len = TRY(utf8_char_length(*beg));
     if (utf8_len > len) return err(EILSEQ); // Unterminated sequence
@@ -82,7 +127,7 @@ Result<void> Utf8StringDecoder::decode(wchar_t* buf) const
     while ((usize)(it - m_str) < m_byte_length)
     {
         usize len = m_byte_length - (usize)(it - m_str); // Remaining space
-        *buf = TRY(encode_utf8_char_into_wide_char(it, len));
+        *buf = TRY(encode_utf8_as_wide_char(it, len));
         it += len;
         buf++;
     }
@@ -92,6 +137,41 @@ Result<void> Utf8StringDecoder::decode(wchar_t* buf) const
     return {};
 }
 
+Utf8StringEncoder::Utf8StringEncoder(const wchar_t* str) : m_str(str), m_code_points(wcslen(str))
+{
+}
+
+Result<usize> Utf8StringEncoder::byte_length() const
+{
+    const wchar_t* it = m_str;
+    usize len = 0;
+
+    while (*it)
+    {
+        len += TRY(wide_char_length_as_utf8(*it));
+        it++;
+    }
+
+    return len;
+}
+
+Result<void> Utf8StringEncoder::encode(char* buf) const
+{
+    const wchar_t* it = m_str;
+
+    while (*it)
+    {
+        usize len = 0;
+        TRY(encode_wide_char_as_utf8(*it, buf, len));
+        buf += len;
+        it++;
+    }
+
+    *buf = 0;
+
+    return {};
+}
+
 Utf8StateDecoder::Utf8StateDecoder() : m_state_len(0), m_state_index(0)
 {
 }
@@ -117,7 +197,7 @@ Result<Option<wchar_t>> Utf8StateDecoder::feed(char c)
     if (m_state_index == m_state_len - 1)
     {
         usize len = m_state_len;
-        wchar_t wc = TRY(encode_utf8_char_into_wide_char(m_state, len));
+        wchar_t wc = TRY(encode_utf8_as_wide_char(m_state, len));
         m_state_len = 0;
         return Option<wchar_t>{wc};
     }
@@ -129,3 +209,12 @@ void Utf8StateDecoder::reset()
 {
     m_state_index = m_state_len = 0;
 }
+
+Result<usize> Utf8Encoder::encode(wchar_t c, char buf[4])
+{
+    usize len = 0;
+
+    TRY(encode_wide_char_as_utf8(c, buf, len));
+
+    return len;
+}