/*

 * Copyright (c) 2022-2023, the SerenityOS developers.

 *

 * SPDX-License-Identifier: BSD-2-Clause

 */

#include <AK/Base64.h>

#include <AK/ByteBuffer.h>

#include <AK/CharacterTypes.h>

#include <AK/Error.h>

#include <AK/StringBuilder.h>

#include <AK/StringView.h>

#include <AK/Vector.h>

#include <LibWeb/Infra/Base64.h>

#include <LibWeb/Infra/CharacterTypes.h>

namespace Web::Infra {

// https://infra.spec.whatwg.org/#forgiving-base64

ErrorOr<ByteBuffer> decode_forgiving_base64(StringView input)

{

    // 1. Remove all ASCII whitespace from data.

    // FIXME: It is possible to avoid copying input here, it's just a bit tricky to remove the equal signs

    StringBuilder builder;

    for (auto character : input) {

        if (!is_ascii_whitespace(character))

            TRY(builder.try_append(character));

    }

    auto data = builder.string_view();

    // 2. If data’s code point length divides by 4 leaving no remainder, then:

    if (data.length() % 4 == 0) {

        // If data ends with one or two U+003D (=) code points, then remove them from data.

        if (data.ends_with("=="sv))

            data = data.substring_view(0, data.length() - 2);

        else if (data.ends_with('='))

            data = data.substring_view(0, data.length() - 1);

    }

    // 3. If data’s code point length divides by 4 leaving a remainder of 1, then return failure.

    if (data.length() % 4 == 1)

        return Error::from_string_literal("Invalid input length in forgiving base64 decode");

    // 4. If data contains a code point that is not one of

    //     U+002B (+), U+002F (/), ASCII alphanumeric

    // then return failure.

    for (auto point : data) {

        if (point != '+' && point != '/' && !is_ascii_alphanumeric(point))

            return Error::from_string_literal("Invalid character in forgiving base64 decode");

    }

    // 5. Let output be an empty byte sequence.

    // 6. Let buffer be an empty buffer that can have bits appended to it.

    Vector<u8> output;

    u32 buffer = 0;

    auto accumulated_bits = 0;

    auto add_to_buffer = [&](u8 number) {

        VERIFY(number < 64);

        u32 buffer_mask = number;

        if (accumulated_bits == 0)

            buffer_mask <<= 18;

        else if (accumulated_bits == 6)

            buffer_mask <<= 12;

        else if (accumulated_bits == 12)

            buffer_mask <<= 6;

        else if (accumulated_bits == 18)

            buffer_mask <<= 0;

        buffer |= buffer_mask;

        accumulated_bits += 6;

    };

    auto append_bytes = [&]() {

        output.append(static_cast<u8>((buffer & 0xff0000) >> 16));

        output.append(static_cast<u8>((buffer & 0xff00) >> 8));

        output.append(static_cast<u8>(buffer & 0xff));

        buffer = 0;

        accumulated_bits = 0;

    };

    auto alphabet_lookup_table = AK::base64_lookup_table();

    // 7. Let position be a position variable for data, initially pointing at the start of data.

    // 8. While position does not point past the end of data:

    for (auto point : data) {

        // 1. Find the code point pointed to by position in the second column of Table 1: The Base 64 Alphabet of RFC 4648.

        //    Let n be the number given in the first cell of the same row. [RFC4648]

        auto n = alphabet_lookup_table[point];

        VERIFY(n >= 0);

        // 2. Append the six bits corresponding to n, most significant bit first, to buffer.

        add_to_buffer(static_cast<u8>(n));

        // 3. buffer has accumulated 24 bits,

        if (accumulated_bits == 24) {

            // interpret them as three 8-bit big-endian numbers.

            // Append three bytes with values equal to those numbers to output, in the same order, and then empty buffer

            append_bytes();

        }

    }

    // 9. If buffer is not empty, it contains either 12 or 18 bits.

    VERIFY(accumulated_bits == 0 || accumulated_bits == 12 || accumulated_bits == 18);

    // If it contains 12 bits, then discard the last four and interpret the remaining eight as an 8-bit big-endian number.

    if (accumulated_bits == 12)

        output.append(static_cast<u8>((buffer & 0xff0000) >> 16));

    // If it contains 18 bits, then discard the last two and interpret the remaining 16 as two 8-bit big-endian numbers.

    // Append the one or two bytes with values equal to those one or two numbers to output, in the same order.

    if (accumulated_bits == 18) {

        output.append(static_cast<u8>((buffer & 0xff0000) >> 16));

        output.append(static_cast<u8>((buffer & 0xff00) >> 8));

    }

    return ByteBuffer::copy(output);

}

}