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 estimate of what the engine still holds is only an estimate. An
    /// engine that says it has stopped transmitting has an empty buffer, and
    /// the next character goes to it at once rather than a character time
    /// later.
    [Fact]
    public async Task AnIdleEngineIsFedWithoutWaiting()
    {
        using TypeAhead buffer = Buffer(baud: Slow);
        buffer.Append("CQ TEST");
        await WaitForAsync(() => Sent.Length >= 2);

        buffer.EngineIdle();

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

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

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