Base64.cpp

/*
 * Copyright (c) 2020-2022, Andreas Kling <kling@serenityos.org>
 *
 * SPDX-License-Identifier: BSD-2-Clause
 */

#include <AK/Assertions.h>
#include <AK/Base64.h>
#include <AK/Error.h>
#include <AK/StringBuilder.h>
#include <AK/Types.h>
#include <AK/Vector.h>

namespace AK {

size_t calculate_base64_decoded_length(StringView input)
{
    auto length = input.length() * 3 / 4;

    if (input.ends_with("="sv))
        --length;
    if (input.ends_with("=="sv))
        --length;

    return length;
}

size_t calculate_base64_encoded_length(ReadonlyBytes input)
{
    return ((4 * input.size() / 3) + 3) & ~3;
}

static ErrorOr<ByteBuffer> decode_base64_impl(StringView input, ReadonlySpan<i16> alphabet_lookup_table)
{
    input = input.trim_whitespace();

    if (input.length() % 4 != 0)
        return Error::from_string_literal("Invalid length of Base64 encoded string");

    auto get = [&](size_t offset, bool* is_padding) -> ErrorOr<u8> {
        if (offset >= input.length())
            return 0;

        auto ch = static_cast<unsigned char>(input[offset]);
        if (ch == '=') {
            if (!is_padding)
                return Error::from_string_literal("Invalid '=' character outside of padding in base64 data");
            *is_padding = true;
            return 0;
        }

        i16 result = alphabet_lookup_table[ch];
        if (result < 0)
            return Error::from_string_literal("Invalid character in base64 data");
        VERIFY(result < 256);
        return { result };
    };

    ByteBuffer output;
    TRY(output.try_resize(calculate_base64_decoded_length(input)));

    size_t input_offset = 0;
    size_t output_offset = 0;

    while (input_offset < input.length()) {
        bool in2_is_padding = false;
        bool in3_is_padding = false;

        u8 const in0 = TRY(get(input_offset++, nullptr));
        u8 const in1 = TRY(get(input_offset++, nullptr));
        u8 const in2 = TRY(get(input_offset++, &in2_is_padding));
        u8 const in3 = TRY(get(input_offset++, &in3_is_padding));

        output[output_offset++] = (in0 << 2) | ((in1 >> 4) & 3);

        if (!in2_is_padding)
            output[output_offset++] = ((in1 & 0xf) << 4) | ((in2 >> 2) & 0xf);

        if (!in3_is_padding)
            output[output_offset++] = ((in2 & 0x3) << 6) | in3;
    }

    return output;
}

static ErrorOr<String> encode_base64_impl(ReadonlyBytes input, ReadonlySpan<char> alphabet)
{
    Vector<u8> output;
    TRY(output.try_ensure_capacity(calculate_base64_encoded_length(input)));

    auto get = [&](size_t const offset, bool* need_padding = nullptr) -> u8 {
        if (offset >= input.size()) {
            if (need_padding)
                *need_padding = true;
            return 0;
        }
        return input[offset];
    };

    for (size_t i = 0; i < input.size(); i += 3) {
        bool is_8bit = false;
        bool is_16bit = false;

        u8 const in0 = get(i);
        u8 const in1 = get(i + 1, &is_16bit);
        u8 const in2 = get(i + 2, &is_8bit);

        u8 const index0 = (in0 >> 2) & 0x3f;
        u8 const index1 = ((in0 << 4) | (in1 >> 4)) & 0x3f;
        u8 const index2 = ((in1 << 2) | (in2 >> 6)) & 0x3f;
        u8 const index3 = in2 & 0x3f;

        output.unchecked_append(alphabet[index0]);
        output.unchecked_append(alphabet[index1]);
        output.unchecked_append(is_16bit ? '=' : alphabet[index2]);
        output.unchecked_append(is_8bit ? '=' : alphabet[index3]);
    }

    return String::from_utf8_without_validation(output);
}

ErrorOr<ByteBuffer> decode_base64(StringView input)
{
    static constexpr auto lookup_table = base64_lookup_table();
    return decode_base64_impl(input, lookup_table);
}

ErrorOr<ByteBuffer> decode_base64url(StringView input)
{
    static constexpr auto lookup_table = base64url_lookup_table();
    return decode_base64_impl(input, lookup_table);
}

ErrorOr<String> encode_base64(ReadonlyBytes input)
{
    return encode_base64_impl(input, base64_alphabet);
}
ErrorOr<String> encode_base64url(ReadonlyBytes input)
{
    return encode_base64_impl(input, base64url_alphabet);
}

}