using System;
using System.Collections.Generic;
using System.IO;
using System.Text;
using System.Threading;

using MfGames.Gallium;
using MfGames.Nitride.Contents;

using YamlDotNet.Serialization;
using YamlDotNet.Serialization.NamingConventions;

namespace MfGames.Nitride.Yaml;

/// <summary>
/// An operation that parses the header of a text file and pulls out the
/// YAML header with a specific type.
/// </summary>
/// <typeparam name="TModel">The model that represents the header.</typeparam>
public class ParseYamlHeader<TModel> : OperationBase
    where TModel : class, new()
{
    private bool ignoreUnmatchedProperties;

    private INamingConvention namingConvention;

    public ParseYamlHeader()
    {
        this.namingConvention = CamelCaseNamingConvention.Instance;
        this.ignoreUnmatchedProperties = true;
        this.RemoveHeader = true;
    }

    public Func<Entity, string, Exception, ParseYamlHeaderErrorHandling>?
        EntityError
    {
        get;
        set;
    }

    /// <summary>
    /// Gets or sets a value indicating whether the header should be removed
    /// and the text content be removed.
    /// </summary>
    private bool RemoveHeader { get; set; }

    /// <inheritdoc />
    public override IEnumerable<Entity> Run(
        IEnumerable<Entity> input,
        CancellationToken cancellationToken = default)
    {
        // Set up the YAML parsing.
        DeserializerBuilder builder =
            new DeserializerBuilder().WithNamingConvention(
                this.namingConvention);

        if (this.ignoreUnmatchedProperties)
        {
            builder = builder.IgnoreUnmatchedProperties();
        }

        IDeserializer deserializer = builder.Build();

        // Process through the files. We only care about the text ones
        // and we'll put a default TModel in for those that don't have a
        // header.
        return input
            .SelectEntity<ITextContent>(
                (
                    entity,
                    content) => this.Parse(entity, content, deserializer));
    }

    public ParseYamlHeader<TModel> WithEntityError(
        Func<Entity, string, Exception, ParseYamlHeaderErrorHandling>? value)
    {
        this.EntityError = value;

        return this;
    }

    /// <summary>
    /// Sets the flag if the module should ignore unmatched properties
    /// which defaults to true (ignore).
    /// </summary>
    /// <param name="value">The new value.</param>
    /// <returns>The module for chaining.</returns>
    public ParseYamlHeader<TModel> WithIgnoreUnmatchedProperties(bool value)
    {
        this.ignoreUnmatchedProperties = value;

        return this;
    }

    /// <summary>
    /// Sets the naming convention to something other than the default
    /// camel case.
    /// </summary>
    /// <param name="value">The new naming convention.</param>
    /// <returns>The module for chaining.</returns>
    public ParseYamlHeader<TModel> WithNamingConvention(INamingConvention value)
    {
        this.namingConvention = value;

        return this;
    }

    public ParseYamlHeader<TModel> WithRemoveHeader(bool value)
    {
        this.RemoveHeader = value;

        return this;
    }

    private Entity Parse(
        Entity entity,
        ITextContent content,
        IDeserializer deserializer)
    {
        // Get the textual input from the stream.
        using TextReader reader = content.GetReader();

        // See if the first line is one that indicates a YAML header.
        string? line = reader.ReadLine();

        if (line != "---")
        {
            // This file doesn't have a YAML header, so add the default
            // version of our model and move on.
            return entity.Set(new TModel());
        }

        // Read the rest of the header until we get to the end of the
        // header. If we get to the end of the file first, then we don't
        // have a valid file and just return the default.
        StringBuilder buffer = new();

        buffer.AppendLine(line);

        while ((line = reader.ReadLine()) != null)
        {
            // If have a separator, then we're done processing.
            if (line == "---")
            {
                break;
            }

            // Read the next line into memory.
            buffer.AppendLine(line);
        }

        // If the line is null, then we got to the end of the file without
        // finding a proper header, so use a default and have no other
        // changes.
        if (line == null)
        {
            return entity.Set(new TModel());
        }

        // Pull out the model so we can append it later.
        string yaml = buffer.ToString();
        TModel? model;

        try
        {
            model = deserializer.Deserialize<TModel>(yaml);
        }
        catch (Exception exception)
        {
            ParseYamlHeaderErrorHandling disposition =
                this.EntityError?.Invoke(entity, yaml, exception)
                ?? ParseYamlHeaderErrorHandling.Throw;

            if (disposition == ParseYamlHeaderErrorHandling.Ignore)
            {
                return entity;
            }

            throw;
        }

        // If we are not removing the header, then we're done.
        if (!this.RemoveHeader)
        {
            return entity.Set(model);
        }

        // Read the rest of the reader into a new (reused) buffer so we can
        // set the new text content to the text without the header.
        buffer.Length = 0;

        while ((line = reader.ReadLine()) != null)
        {
            buffer.AppendLine(line);
        }

        // Set the model and return it.
        return entity.Set(model)
            .SetTextContent(new StringTextContent(buffer.ToString()))
            .Set(HasYamlModel.Instance);
    }
}