//
// Copyright (c) 2012 Ian Cowburn
//
using System;
using System.Runtime.InteropServices;

namespace Noddybox.Emulation.EightBit
{
    /// <summary>
    /// Provides a common 8-bit register pattern, which is two 8-bit registers rolled into one.
    /// While this could be done at runtime using <see cref="BitConverter.IsLittleEndian"/> this
    /// method was chosen for speed reasons; interfaces cannot define fields, which is sensible
    /// enoough, but accessing fields directly would allow ref variables to be used with this type's
    /// fields.
    /// </summary>
    [StructLayout(LayoutKind.Explicit)]
    public struct Register16
    {
#if TARGET_IS_BIGENDIAN

        /// <summary>
        /// The 16-bit register.
        /// </summary>
        [FieldOffset(0)]
        public ushort reg;

        /// <summary>
        /// The high 8-bits of the register.
        /// </summary>
        [FieldOffset(0)]
        public byte high;

        /// <summary>
        /// The low 8-bits of the register.
        /// </summary>
        [FieldOffset(1)]
        public byte low;

#else

        /// <summary>
        /// The 16-bit register.
        /// </summary>
        [FieldOffset(0)]
        public ushort reg;

        /// <summary>
        /// The low 8-bits of the register.
        /// </summary>
        [FieldOffset(0)]
        public byte low;

        /// <summary>
        /// The high 8-bits of the register.
        /// </summary>
        [FieldOffset(1)]
        public byte high;

#endif

        /// <summary>
        /// Public constructor.  This is provided to stop the compiler complaining when you use the struct as it
        /// doesn't realise that setting <see cref="reg"/> will also set <see cref="low"/> and <see cref="high"/>.
        /// </summary>
        /// <param name="val">The 16-bit value to assign.</param>
        public Register16(ushort val)
        {
            high = 0;
            low = 0;
            reg = val;
        }

        /// <summary>
        /// Used to verify the compile-time setting at runtime.
        /// </summary>
        public static void Verify()
        {
            Register16 r = new Register16(0);

            r.reg = 0x1234;

            if (r.low != 0x24 || r.high != 0x12)
            {
                throw new Exception("Compile time endian setting incorrect");
            }
        }
    }
}