namespace WirelessTextSyncer.Windows.Services;

using System.Runtime.InteropServices;

public sealed class KeyboardInjectionService : IKeyboardInjectionService
{
    private const uint InputKeyboard = 1;
    private const uint KeyEventFKeyUp = 0x0002;
    private const uint KeyEventFUnicode = 0x0004;
    private const ushort VirtualKeyBack = 0x08;
    private const ushort VirtualKeyReturn = 0x0D;

    public void InsertText(string text)
    {
        if (string.IsNullOrEmpty(text))
        {
            return;
        }

        foreach (var character in text)
        {
            SendUnicode(character);
        }
    }

    public void Backspace()
    {
        SendVirtualKey(VirtualKeyBack);
    }

    public void Enter()
    {
        SendVirtualKey(VirtualKeyReturn);
    }

    private static void SendUnicode(char character)
    {
        Send([
            CreateUnicodeInput(character, 0),
            CreateUnicodeInput(character, KeyEventFKeyUp)
        ]);
    }

    private static void SendVirtualKey(ushort virtualKey)
    {
        Send([
            CreateVirtualKeyInput(virtualKey, 0),
            CreateVirtualKeyInput(virtualKey, KeyEventFKeyUp)
        ]);
    }

    private static void Send(INPUT[] inputs)
    {
        var sent = SendInput((uint)inputs.Length, inputs, Marshal.SizeOf<INPUT>());
        if (sent != inputs.Length)
        {
            throw new InvalidOperationException("Windows did not accept all keyboard input events.");
        }
    }

    private static INPUT CreateUnicodeInput(char character, uint flags)
    {
        return new INPUT
        {
            type = InputKeyboard,
            U = new InputUnion
            {
                ki = new KEYBDINPUT
                {
                    wScan = character,
                    dwFlags = KeyEventFUnicode | flags
                }
            }
        };
    }

    private static INPUT CreateVirtualKeyInput(ushort virtualKey, uint flags)
    {
        return new INPUT
        {
            type = InputKeyboard,
            U = new InputUnion
            {
                ki = new KEYBDINPUT
                {
                    wVk = virtualKey,
                    dwFlags = flags
                }
            }
        };
    }

    [DllImport("user32.dll", SetLastError = true)]
    private static extern uint SendInput(uint numberOfInputs, INPUT[] inputs, int sizeOfInputStructure);

    [StructLayout(LayoutKind.Sequential)]
    private struct INPUT
    {
        public uint type;
        public InputUnion U;
    }

    [StructLayout(LayoutKind.Explicit)]
    private struct InputUnion
    {
        [FieldOffset(0)]
        public KEYBDINPUT ki;
    }

    [StructLayout(LayoutKind.Sequential)]
    private struct KEYBDINPUT
    {
        public ushort wVk;
        public ushort wScan;
        public uint dwFlags;
        public uint time;
        public UIntPtr dwExtraInfo;
    }
}