using System.Text;
using Nonemm.Digital;

namespace Nonemm.Digital.Tests;

/// The text waiting to go out, fed to the engine a few characters at a time.
/// The pump is run at a baud rate no radio uses so the tests do not wait for
/// RTTY.
public class TypeAheadTests
{
    private static readonly TimeSpan Patience = TimeSpan.FromSeconds(5);

    /// Fast enough that a message goes out in milliseconds, slow enough that a
    /// test can still catch the pump partway through.
    private const double Fast = 7500;

    /// A character time of 100 ms, so a test can tell the characters sent ahead
    /// from the ones that wait for the engine.
    private const double Slow = 75;

    private readonly StringBuilder went = new();

    private string Sent
    {
        get
        {
            lock (went)
            {
                return went.ToString();
            }
        }
    }

    private TypeAhead Buffer(double baud = Fast) =>
        new(
            (character, _) =>
            {
                lock (went)
                {
                    went.Append(character);
                }
                return Task.CompletedTask;
            },
            baud);

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

    [Fact]
    public async Task AMessageGoesOut()
    {
        using TypeAhead buffer = Buffer();

        buffer.Append("CQ TEST");

        await WaitForAsync(() => Sent == "CQ TEST");
        Assert.Equal("CQ TEST", Sent);
        Assert.Equal("CQ TEST", buffer.Sent);
        Assert.Equal("", buffer.Pending);
    }

    [Fact]
    public async Task WhatHasGoneOutIsNotPendingAnyMore()
    {
        using TypeAhead buffer = Buffer(baud: 40);

        buffer.Append("CQ TEST DE OM5M");

        await WaitForAsync(() => Sent.Length >= 2);
        Assert.StartsWith(buffer.Sent, "CQ TEST DE OM5M", StringComparison.Ordinal);
        Assert.Equal("CQ TEST DE OM5M", buffer.Sent + buffer.Pending);
    }

    [Fact]
    public async Task TheOperatorRewritesWhatHasNotGoneOut()
    {
        using TypeAhead buffer = Buffer(baud: 60);
        buffer.Append("OM5X 599 001");

        await WaitForAsync(() => buffer.Sent.Length >= 5);
        buffer.Rewrite("599 002", TypeAhead.NoCursor);

        await WaitForAsync(() => buffer.Pending.Length == 0);
        Assert.EndsWith("599 002", Sent, StringComparison.Ordinal);
        Assert.DoesNotContain("001", Sent, StringComparison.Ordinal);
    }

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

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

        await WaitForAsync(() => Sent.Length == 2);
        await Task.Delay(50);
        Assert.Equal("CQ", Sent);
        Assert.Equal(" TEST", buffer.Pending);
    }

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

        buffer.Cursor = TypeAhead.NoCursor;

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

    [Fact]
    public async Task TextAddedBehindTheCursorStillGoesOut()
    {
        using TypeAhead buffer = Buffer();
        buffer.Rewrite("CQ", 2);
        await WaitForAsync(() => Sent == "CQ");

        buffer.Append(" TEST");

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

    [Fact]
    public async Task TheBufferSaysWhenTheMessageHasGoneOut()
    {
        using TypeAhead buffer = Buffer();
        int drained = 0;
        buffer.Drained += (_, _) => Interlocked.Increment(ref drained);

        buffer.Append("TU");

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

    [Fact]
    public async Task AMessageAddedWhileOneIsGoingOutFollowsIt()
    {
        using TypeAhead buffer = Buffer(baud: 400);
        buffer.Append("CQ ");

        buffer.Append("DE OM5M");

        await WaitForAsync(() => buffer.Pending.Length == 0);
        Assert.Equal("CQ DE OM5M", Sent);
    }

    [Fact]
    public async Task DroppingLeavesWhatHasAlreadyGone()
    {
        using TypeAhead buffer = Buffer(baud: 60);
        buffer.Append("CQ TEST DE OM5M");
        await WaitForAsync(() => buffer.Sent.Length >= 3);

        buffer.Drop();

        string gone = Sent;
        await Task.Delay(60);
        Assert.Equal(gone, Sent);
        Assert.Equal("", buffer.Pending);
        Assert.Equal(gone, buffer.Sent);
    }

    [Fact]
    public void ClearingEmptiesBothHalves()
    {
        using TypeAhead buffer = Buffer(baud: 1);
        buffer.Append("CQ TEST");

        buffer.Clear();

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

    /// The engine is given the second character before it has transmitted the
    /// first, so it has one in hand when the first is done. An engine left with
    /// an empty buffer transmits idle instead, and that idle is added to how
    /// long the message takes.
    [Fact]
    public async Task TheEngineIsKeptOneCharacterAhead()
    {
        using TypeAhead buffer = Buffer(baud: Slow);

        buffer.Append("CQ TEST");

        await WaitForAsync(() => Sent.Length >= 2);
        Assert.Equal("CQ", Sent);
    }

    /// Only the lead goes out ahead. The rest waits, which is what leaves it
    /// where the operator can still change it.
    [Fact]
    public async Task NoMoreThanTheLeadGoesToTheEngineAtOnce()
    {
        using TypeAhead buffer = Buffer(baud: Slow);

        buffer.Append("CQ TEST");

        await WaitForAsync(() => Sent.Length >= 2);
        await Task.Delay(20);
        Assert.Equal(2, Sent.Length);
    }

    /// The engine's count of what it has left runs behind what it is really
    /// doing: the probe read 0 fifty milliseconds after twenty-one characters
    /// had been pushed and were already going out. A pump that believed that
    /// would hand over the whole message at once, so the count only ever stops
    /// it, and the engine is never given more than `Lead` plus `Slack`.
    [Fact]
    public async Task AnEngineBehindTheClockIsNotFedPastTheCap()
    {
        FakeEngine engine = new();
        using TypeAhead buffer = new(engine, baud: Slow);
        // half the speed of the baud rate the pump is paced by
        using Timer transmitting = new(_ => engine.Transmit(1), null, 0, 200);

        buffer.Append("CQ TEST DE OM5M");

        int most = 0;
        for (int look = 0; look < 50; look++)
        {
            most = Math.Max(most, engine.Waiting.Length);
            await Task.Delay(20);
        }
        Assert.True(most <= TypeAhead.DefaultLead + TypeAhead.Slack, $"the engine was given {most}");
        Assert.True(engine.Transmitted.Length > 0, "nothing was transmitted at all");
    }

    /// The clock is the fallback for an engine that will not answer.
    [Fact]
    public async Task AnEngineThatWillNotCountIsPacedByTheClock()
    {
        FakeEngine engine = new() { Counts = false };
        using TypeAhead buffer = new(engine, baud: Fast);

        buffer.Append("CQ TEST");

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

    [Fact]
    public void ACharacterTakesAsLongAsTheBaudRateSays()
    {
        using TypeAhead buffer = Buffer(baud: TypeAhead.DefaultBaud);

        Assert.Equal(165, buffer.CharacterTime.TotalMilliseconds, 0.5);
    }

    /// An engine with a buffer of its own: it takes characters, holds them
    /// until the test says they have been transmitted, and says how many it
    /// has. `Counts` false is the engine that will not answer.
    private sealed class FakeEngine : EngineBuffer
    {
        private readonly Lock gate = new();
        private readonly StringBuilder waiting = new();
        private readonly StringBuilder transmitted = new();

        public bool Counts { get; init; } = true;

        public event EventHandler<int>? Buffered;

        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)
            {
                waiting.Append(character);
            }
            return Task.CompletedTask;
        }

        public Task AskBufferedAsync(string property = "", CancellationToken cancellation = default)
        {
            int left;
            lock (gate)
            {
                left = Counts ? waiting.Length : -1;
            }
            Buffered?.Invoke(this, left);
            return Task.CompletedTask;
        }

        public void Transmit(int count)
        {
            lock (gate)
            {
                int going = Math.Min(count, waiting.Length);
                transmitted.Append(waiting.ToString(0, going));
                waiting.Remove(0, going);
            }
        }
    }
}