encode.cpp

/*
 * Copyright 2025 Siddhartha Menon
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#include <array>
#include <cstdint>
#include <iostream>
#include <istream>

// Read in three bytes of binary and output four base64 characters
void encode(std::istream& src) {
    const char* alphabet =
        "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=";

    while (true) {
        std::array<uint8_t, 3> input{};
        src.read(reinterpret_cast<char*>(input.data()), 3);
        size_t data_size = src.gcount();

        if (data_size == 0) {
            return;
        }

        std::array<char, 4> output;

        const uint32_t packed_data =
            0x0 + (input[0] << 16) + (input[1] << 8) + input[2];

        size_t i = 0;
        for (; i < data_size + 1; ++i) {
            output[i] = alphabet[(packed_data >> (18 - 6 * i)) & 0x3F];
        }

        // Pad the rest
        for (; i < output.size(); ++i) {
            output[i] = alphabet[64];
        }

        std::cout.write(output.data(), 4);

        if (data_size != 3) {
            return;
        }
    }
}