using System;
using System.Buffers.Binary;

namespace AcDream.Core.Net.Messages;

/// <summary>
/// Shared reader for the AC <c>String16L</c> wire format used by every
/// inbound and outbound chat / name / message body.
///
/// <para>
/// Wire shape:
/// <code>
///   u16 length      // byte count, NOT char count
///   byte[] text     // CP1252-encoded bytes, no terminator
///   pad to 4-byte boundary
/// </code>
/// </para>
///
/// <para>
/// Codec is Windows-1252 (CP1252), matching retail and holtburger's
/// <c>encoding_rs::WINDOWS_1252</c>. Registration of the
/// <c>CodePagesEncodingProvider</c> is handled by
/// <see cref="Encodings"/>.
/// </para>
/// </summary>
internal static class StringReader
{
    /// <summary>
    /// Read a String16L from <paramref name="source"/> at <paramref name="pos"/>,
    /// advancing <paramref name="pos"/> past the length, body, and 4-byte
    /// alignment padding. Throws <see cref="FormatException"/> on truncation.
    /// </summary>
    public static string ReadString16L(ReadOnlySpan<byte> source, ref int pos)
    {
        if (source.Length - pos < 2) throw new FormatException("truncated String16L length");
        ushort length = BinaryPrimitives.ReadUInt16LittleEndian(source.Slice(pos));
        pos += 2;
        if (source.Length - pos < length) throw new FormatException("truncated String16L body");
        string result = Encodings.Windows1252.GetString(source.Slice(pos, length));
        pos += length;
        int recordSize = 2 + length;
        int padding = (4 - (recordSize & 3)) & 3;
        pos += padding;
        return result;
    }
}