/*

 * 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);

}

}