Function decode

fdbrpc/Base64UrlDecode.cpp:55–99

}

int decode(const uint8_t* __restrict codeIn, const int lengthIn, uint8_t* __restrict plaintextOut) noexcept {
	const uint8_t* codechar = codeIn;
	const uint8_t* const codeEnd = codeIn + lengthIn;
	uint8_t* plainchar = plaintextOut;
	uint8_t fragment = 0;

	while (1) {
		// code 1 of 4
		if (codechar == codeEnd) {
			return plainchar - plaintextOut;
		}
		fragment = decodeValue(*codechar++);
		if (fragment == _X)
			return -1;
		*plainchar = (fragment & 0x03f) << 2;
		if (codechar == codeEnd) {
			return -1; // requires at least 2 chars to decode 1 plain byte
		}
		// code 2 of 4
		fragment = decodeValue(*codechar++);
		if (fragment == _X)
			return -1;
		*plainchar++ |= (fragment & 0x030) >> 4;
		if (codechar == codeEnd) {
			return plainchar - plaintextOut;
		}
		*plainchar = (fragment & 0x00f) << 4;
		// code 3 of 4
		fragment = decodeValue(*codechar++);
		if (fragment == _X)
			return -1;
		*plainchar++ |= (fragment >> 2);
		if (codechar == codeEnd) {
			return plainchar - plaintextOut;
		}
		*plainchar = (fragment & 0x003) << 6;
		// code 4 of 4
		fragment = decodeValue(*codechar++);
		if (fragment == _X)
			return -1;
		*plainchar++ |= (fragment & 0x03f);
	}
	/* control should not reach here */
	return plainchar - plaintextOut;
}

int decodedLength(int codeLength) noexcept {
	const auto r = (codeLength & 3);