// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
// See the LICENSE file in the project root for more information.

#nullable disable

using System;
using System.Collections;
using Roslyn.Utilities;

namespace Microsoft.CodeAnalysis.Shared.Utilities;

internal sealed partial class BloomFilter
{
    private const string SerializationFormat = "2";

    public void WriteTo(ObjectWriter writer)
    {
        writer.WriteString(SerializationFormat);
        writer.WriteBoolean(_isCaseSensitive);
        writer.WriteInt32(_hashFunctionCount);
        WriteBitArray(writer, _bitArray);
    }

    private static void WriteBitArray(ObjectWriter writer, BitArray bitArray)
    {
        // Our serialization format doesn't round-trip bit arrays of non-byte lengths
        Contract.ThrowIfTrue(bitArray.Length % 8 != 0);

        writer.WriteInt32(bitArray.Length / 8);

        // This will hold the byte that we will write out after we process every 8 bits. This is
        // LSB, so we push bits into it from the MSB.
        byte b = 0;

        for (var i = 0; i < bitArray.Length; i++)
        {
            if (bitArray[i])
            {
                b = (byte)(0x80 | b >> 1);
            }
            else
            {
                b >>= 1;
            }

            if ((i + 1) % 8 == 0)
            {
                // End of a byte, write out the byte
                writer.WriteByte(b);
            }
        }
    }

    public static BloomFilter ReadFrom(ObjectReader reader)
    {
        var version = reader.ReadString();
        if (!string.Equals(version, SerializationFormat, StringComparison.Ordinal))
        {
            return null;
        }

        var isCaseSensitive = reader.ReadBoolean();
        var hashFunctionCount = reader.ReadInt32();
        var bitArray = ReadBitArray(reader);
        return new BloomFilter(bitArray, hashFunctionCount, isCaseSensitive);
    }

    private static BitArray ReadBitArray(ObjectReader reader)
    {
        // TODO: find a way to use pool
        var length = reader.ReadInt32();
        var bytes = new byte[length];

        for (var i = 0; i < bytes.Length; i++)
        {
            bytes[i] = reader.ReadByte();
        }

        return new BitArray(bytes);
    }
}