using System.Text;
using Nonemm.Digital;

namespace Nonemm.Digital.Tests;

/// The transmit buffer with the engine holding the text. The engine here is a
/// stand-in that behaves the way MMTTY's help says MMTTY does: it takes
/// characters, a backspace removes the last one it has not transmitted yet, and
/// it says how many are left. Whether the real engine does that is what
/// `tools/Nonemm.EngineProbe` is for.
public class EngineTypeAheadTests
{
    private static readonly TimeSpan Patience = TimeSpan.FromSeconds(5);
    private static readonly TimeSpan Poll = TimeSpan.FromMilliseconds(5);

    private readonly FakeEngine engine = new();

    private EngineTypeAhead Buffer() => new(engine, Poll);

    private static async Task WaitForAsync(Func<bool> ready)
    {
        DateTime giveUp = DateTime.UtcNow + Patience;
        while (!ready() && DateTime.UtcNow < giveUp)
        {
            await Task.Delay(2);
        }
    }

    /// The whole message goes to the engine as fast as the engine takes it,
    /// which is the point: an engine with characters in hand never transmits
    /// idle in the middle of a message.
    [Fact]
    public async Task AMessageGoesToTheEngineWhole()
    {
        using EngineTypeAhead buffer = Buffer();

        buffer.Append("CQ TEST");

        await WaitForAsync(() => engine.Waiting == "CQ TEST");
        Assert.Equal("CQ TEST", engine.Waiting);
    }

    /// What the engine has transmitted moves out of the box, along with the one
    /// character held back because the count is a poll old.
    [Fact]
    public async Task WhatTheEngineHasTransmittedIsNotPendingAnyMore()
    {
        using EngineTypeAhead buffer = Buffer();
        buffer.Append("CQ TEST");
        await WaitForAsync(() => engine.Waiting == "CQ TEST");

        engine.Transmit(3);

        await WaitForAsync(() => buffer.Sent.Length >= 4);
        Assert.Equal("CQ T", buffer.Sent);
        Assert.Equal("EST", buffer.Pending);
    }

    [Fact]
    public async Task AnEditTakesBackWhatHasNotBeenTransmitted()
    {
        using EngineTypeAhead buffer = Buffer();
        buffer.Append("OM5X 599 001");
        await WaitForAsync(() => engine.Waiting.Length == 12);
        engine.Transmit(5);
        await WaitForAsync(() => buffer.Sent.Length >= 6);

        buffer.Rewrite("99 002", TransmitBuffer.NoCursor);

        await WaitForAsync(() => engine.Waiting.EndsWith("599 002", StringComparison.Ordinal));
        engine.Transmit(engine.Waiting.Length);
        await WaitForAsync(() => !buffer.IsSending);
        Assert.Equal("OM5X 599 002", engine.Transmitted);
    }

    /// The character the engine was last known to be holding is treated as
    /// gone. Taking it back would race with the engine transmitting it, and a
    /// backspace the engine refuses would put the text after it on the air
    /// twice.
    [Fact]
    public async Task TheGuardCharacterIsNotTakenBack()
    {
        using EngineTypeAhead buffer = Buffer();
        buffer.Append("ABCDEF");
        await WaitForAsync(() => engine.Waiting == "ABCDEF");
        engine.Transmit(2);
        await WaitForAsync(() => buffer.Sent == "ABC");

        buffer.Rewrite("", TransmitBuffer.NoCursor);

        await WaitForAsync(() => engine.Waiting == "C");
        Assert.Equal("C", engine.Waiting);
        Assert.Equal("", buffer.Pending);
    }

    [Fact]
    public async Task NothingGoesOutFromBehindTheCursor()
    {
        using EngineTypeAhead buffer = Buffer();

        buffer.Rewrite("CQ TEST", 2);

        await WaitForAsync(() => engine.Waiting == "CQ");
        await Task.Delay(30);
        Assert.Equal("CQ", engine.Waiting);
        // the first queued character is the guard, so it shows as gone
        Assert.Equal("Q TEST", buffer.Pending);
    }

    [Fact]
    public async Task TheRestGoesOutWhenTheCursorMovesOn()
    {
        using EngineTypeAhead buffer = Buffer();
        buffer.Rewrite("CQ TEST", 2);
        await WaitForAsync(() => engine.Waiting == "CQ");

        buffer.Cursor = TransmitBuffer.NoCursor;

        await WaitForAsync(() => engine.Waiting == "CQ TEST");
        Assert.Equal("CQ TEST", engine.Waiting);
    }

    [Fact]
    public async Task TheBufferSaysWhenTheMessageHasGoneOut()
    {
        using EngineTypeAhead buffer = Buffer();
        int drained = 0;
        buffer.Drained += (_, _) => Interlocked.Increment(ref drained);
        buffer.Append("TU");
        await WaitForAsync(() => engine.Waiting == "TU");

        engine.Transmit(2);

        await WaitForAsync(() => Volatile.Read(ref drained) == 1);
        Assert.Equal(1, Volatile.Read(ref drained));
        Assert.Equal("TU", buffer.Sent);
    }

    /// The engine unkeying means its buffer is empty, whatever the last count
    /// said.
    [Fact]
    public async Task AnIdleEngineHasTransmittedEverythingItHeld()
    {
        using EngineTypeAhead buffer = Buffer();
        buffer.Append("TU");
        await WaitForAsync(() => engine.Waiting == "TU");
        await Task.Delay(20);

        buffer.EngineIdle();

        Assert.Equal("TU", buffer.Sent);
        Assert.Equal("", buffer.Pending);
    }

    /// MMTTY, as its help describes it: characters go in, a backspace takes the
    /// last one back while it has not been transmitted, and the count is what
    /// is left.
    private sealed class FakeEngine : EngineBuffer
    {
        private readonly Lock gate = new();
        private readonly StringBuilder waiting = new();
        private readonly StringBuilder transmitted = new();

        public event EventHandler<int>? Buffered;

        /// What the engine holds and has not transmitted.
        public string Waiting
        {
            get
            {
                lock (gate)
                {
                    return waiting.ToString();
                }
            }
        }

        public string Transmitted
        {
            get
            {
                lock (gate)
                {
                    return transmitted.ToString();
                }
            }
        }

        public Task TypeAsync(char character, CancellationToken cancellation = default)
        {
            lock (gate)
            {
                if (character != '\b')
                {
                    waiting.Append(character);
                }
                else if (waiting.Length > 0)
                {
                    waiting.Remove(waiting.Length - 1, 1);
                }
            }
            return Task.CompletedTask;
        }

        public Task AskBufferedAsync(string property = "", CancellationToken cancellation = default)
        {
            int left;
            lock (gate)
            {
                left = waiting.Length;
            }
            Buffered?.Invoke(this, left);
            return Task.CompletedTask;
        }

        /// The engine transmitting, driven by the test rather than a clock.
        public void Transmit(int count)
        {
            lock (gate)
            {
                int going = Math.Min(count, waiting.Length);
                transmitted.Append(waiting.ToString(0, going));
                waiting.Remove(0, going);
            }
        }
    }
}