namespace DodoSSH.Client.Shell.Keyboard;

/// <summary>
/// What a latched Ctrl or Alt does to the key pressed after it.
/// </summary>
/// <remarks>
/// <para>
/// <b>The encodings xterm uses, and nothing cleverer.</b> Ctrl with a character is that character's
/// control form — <c>c &amp; 0x1F</c>, the rule caret notation names. Alt is an Escape in front, which is
/// what readline's <c>M-b</c> and <c>M-f</c> and every Emacs binding expect from a terminal. Ctrl or Alt with
/// a cursor key is the modified CSI form, <c>CSI 1 ; m X</c>, which is how Ctrl+← jumps a word in bash and
/// zsh.
/// </para>
/// <para>
/// <b>Anything else goes through untouched.</b> Ctrl with a pasted line or with a function key has no
/// encoding a shell agrees on, and mangling one into something else would be worse than ignoring the
/// latch. The latch still releases, because the person pressed the key it was latched for.
/// </para>
/// </remarks>
internal static class TerminalModifiers
{
    /// <summary>Applies whichever modifiers are latched to one key's worth of bytes.</summary>
    internal static byte[] Apply(byte[] payload, bool control, bool alt)
    {
        ArgumentNullException.ThrowIfNull(payload);

        if (!control && !alt)
        {
            return payload;
        }

        // ESC [ X for the six cursor keys that have a modified form: the arrows, Home and End.
        if (payload is [0x1B, (byte)'[', var final] && final is (byte)'A' or (byte)'B' or (byte)'C'
                or (byte)'D' or (byte)'H' or (byte)'F')
        {
            var modifier = 1 + (alt ? 2 : 0) + (control ? 4 : 0);

            return [0x1B, (byte)'[', (byte)'1', (byte)';', (byte)('0' + modifier), final];
        }

        var result = payload;

        if (control && payload is [var single])
        {
            result = [ControlForm(single)];
        }

        return alt ? [0x1B, .. result] : result;
    }

    /// <summary>The control character for one byte, or the byte itself where there is none.</summary>
    /// <remarks>Space as NUL is the one addition to caret notation, and every terminal makes it.</remarks>
    private static byte ControlForm(byte c) =>
        c == (byte)' ' ? (byte)0x00
        : c < 0x80 && KeySequence.TryControl((char)c, out var control) ? control
        : c;
}