encoding: implement hex encoding (base16)

Implement hex encoding and decoding (base16) according to RFC 4648.
Add unit tests.

Link: https://www.rfc-editor.org/rfc/rfc4648

lib/std/encoding/hex.c3

@@ -0,0 +1,117 @@
+module std::encoding::hex;
+
+// The implementation is based on https://www.rfc-editor.org/rfc/rfc4648
+
+fn String! encode_buffer(char[] code, char[] buffer)
+{
+	return (String)buffer[:encode_bytes(code, buffer)!];
+}
+
+fn char[]! decode_buffer(char[] code, char[] buffer)
+{
+	return buffer[:decode_bytes(code, buffer)!];
+}
+
+fn String! encode(char[] code, Allocator allocator)
+{
+	char[] data = allocator::alloc_array(allocator, char, encode_len(code.len));
+	return (String)data[:encode_bytes(code, data)!];
+}
+
+fn char[]! decode(char[] code, Allocator allocator)
+{
+	char[] data = allocator::alloc_array(allocator, char, decode_len(code.len));
+	return data[:decode_bytes(code, data)!];
+}
+
+fn String! encode_new(char[] code) @inline => encode(code, allocator::heap());
+fn String! encode_temp(char[] code) @inline => encode(code, allocator::temp());
+fn char[]! decode_new(char[] code) @inline => decode(code, allocator::heap());
+fn char[]! decode_temp(char[] code) @inline => decode(code, allocator::temp());
+
+fault Errors
+{
+	INVALID_CHARACTER,
+}
+
+<*
+ Calculate the size of the encoded data.
+ @param n "Size of the input to be encoded."
+ @return "The size of the input once encoded."
+*>
+fn usz encode_len(usz n) => n * 2;
+
+<*
+ Encode the content of src into dst, which must be properly sized.
+ @param src "The input to be encoded."
+ @param dst "The encoded input."
+ @return "The encoded size."
+ @require dst.len >= encode_len(src.len) "Destination array is not large enough"
+*>
+fn usz! encode_bytes(char[] src, char[] dst)
+{
+	usz j = 0;
+	foreach (v : src) {
+		dst[j] = HEXALPHABET[v >> 4];
+		dst[j + 1] = HEXALPHABET[v & 0x0f];
+		j = j + 2;
+	}
+	return src.len * 2;
+}
+
+<*
+ Calculate the size of the decoded data.
+ @param n "Size of the input to be decoded."
+ @return "The size of the input once decoded."
+*>
+fn usz decode_len(usz n) => n / 2;
+
+<*
+ Decodes src into bytes. Returns the actual number of bytes written to dst.
+
+ Expects that src only contains hexadecimal characters and that src has even
+ length.
+
+ @param src "The input to be decoded."
+ @param dst "The decoded input."
+ @require src.len % 2 == 0 "src is not of even length"
+ @require dst.len >= decode_len(src.len) "Destination array is not large enough"
+ @return! Errors.INVALID_CHARACTER
+*>
+fn usz! decode_bytes(char[] src, char[] dst)
+{
+	usz i, j;
+	char a, b;
+	for (j = 1; j < src.len; j += 2) {
+		a = HEXREVERSE[src[j-1]];
+		b = HEXREVERSE[src[j]];
+		if (a > 0x0f) {
+			return Errors.INVALID_CHARACTER?;
+		}
+		if (b > 0x0f) {
+			return Errors.INVALID_CHARACTER?;
+		}
+		dst[i] = (a << 4) | b;
+		i++;
+	}
+	return i;
+}
+
+const char[*] HEXALPHABET @private = "0123456789abcdef";
+const char[*] HEXREVERSE @private =
+x"ffffffffffffffffffffffffffffffff"
+x"ffffffffffffffffffffffffffffffff"
+x"ffffffffffffffffffffffffffffffff"
+x"00010203040506070809ffffffffffff"
+x"ff0a0b0c0d0e0fffffffffffffffffff"
+x"ffffffffffffffffffffffffffffffff"
+x"ff0a0b0c0d0e0fffffffffffffffffff"
+x"ffffffffffffffffffffffffffffffff"
+x"ffffffffffffffffffffffffffffffff"
+x"ffffffffffffffffffffffffffffffff"
+x"ffffffffffffffffffffffffffffffff"
+x"ffffffffffffffffffffffffffffffff"
+x"ffffffffffffffffffffffffffffffff"
+x"ffffffffffffffffffffffffffffffff"
+x"ffffffffffffffffffffffffffffffff"
+x"ffffffffffffffffffffffffffffffff";