// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
 
#nullable enable
 
using System.Diagnostics;
using System.Globalization;
using Microsoft.AspNetCore.Http;
using Microsoft.Extensions.Logging;
 
namespace Microsoft.AspNetCore.Mvc.ModelBinding.Binders;
 
/// <summary>
/// An <see cref="IModelBinder"/> which binds models from the request headers when a model
/// has the binding source <see cref="BindingSource.Header"/>.
/// </summary>
public class HeaderModelBinder : IModelBinder
{
    private readonly ILogger _logger;
 
    /// <summary>
    /// Initializes a new instance of <see cref="HeaderModelBinder"/>.
    /// </summary>
    /// <param name="loggerFactory">The <see cref="ILoggerFactory"/>.</param>
    public HeaderModelBinder(ILoggerFactory loggerFactory)
    {
        _logger = loggerFactory.CreateLogger(typeof(HeaderModelBinder));
    }
 
    /// <summary>
    /// Initializes a new instance of <see cref="HeaderModelBinder"/>.
    /// </summary>
    /// <param name="loggerFactory">The <see cref="ILoggerFactory"/>.</param>
    /// <param name="innerModelBinder">The <see cref="IModelBinder"/> which does the actual
    /// binding of values.</param>
    public HeaderModelBinder(ILoggerFactory loggerFactory, IModelBinder innerModelBinder)
    {
        ArgumentNullException.ThrowIfNull(loggerFactory);
        ArgumentNullException.ThrowIfNull(innerModelBinder);
 
        _logger = loggerFactory.CreateLogger(typeof(HeaderModelBinder));
        InnerModelBinder = innerModelBinder;
    }
 
    // to enable unit testing
    internal IModelBinder? InnerModelBinder { get; }
 
    /// <inheritdoc />
    public async Task BindModelAsync(ModelBindingContext bindingContext)
    {
        ArgumentNullException.ThrowIfNull(bindingContext);
 
        _logger.AttemptingToBindModel(bindingContext);
 
        // Property name can be null if the model metadata represents a type (rather than a property or parameter).
        var headerName = bindingContext.FieldName;
 
        // Do not set ModelBindingResult to Failed on not finding the value in the header as we want the inner
        // ModelBinder to do that. This would give a chance to the inner binder to add more useful information.
        // For example, SimpleTypeModelBinder adds a model error when binding to let's say an integer and the
        // model is null.
        var request = bindingContext.HttpContext.Request;
        if (!request.Headers.ContainsKey(headerName))
        {
            _logger.FoundNoValueInRequest(bindingContext);
        }
 
        if (InnerModelBinder == null)
        {
            BindWithoutInnerBinder(bindingContext);
            return;
        }
 
        var headerValueProvider = GetHeaderValueProvider(headerName, bindingContext);
 
        // Capture the top level object here as entering nested scope would make it 'false'.
        var isTopLevelObject = bindingContext.IsTopLevelObject;
 
        // Create a new binding scope in order to supply the HeaderValueProvider so that the binders like
        // SimpleTypeModelBinder can find values from header.
        ModelBindingResult result;
        using (bindingContext.EnterNestedScope(
                bindingContext.ModelMetadata,
                fieldName: bindingContext.FieldName,
                modelName: bindingContext.ModelName,
                model: bindingContext.Model))
        {
            bindingContext.IsTopLevelObject = isTopLevelObject;
            bindingContext.ValueProvider = headerValueProvider;
 
            await InnerModelBinder.BindModelAsync(bindingContext);
            result = bindingContext.Result;
        }
 
        bindingContext.Result = result;
 
        _logger.DoneAttemptingToBindModel(bindingContext);
    }
 
    private static HeaderValueProvider GetHeaderValueProvider(string headerName, ModelBindingContext bindingContext)
    {
        var request = bindingContext.HttpContext.Request;
 
        // Prevent breaking existing users in scenarios where they are binding to a 'string' property
        // and expect the whole comma separated string, if any, as a single string and not as a string array.
        var values = Array.Empty<string>();
        if (request.Headers.TryGetValue(headerName, out var header))
        {
            if (bindingContext.ModelMetadata.IsEnumerableType)
            {
                values = request.Headers.GetCommaSeparatedValues(headerName);
            }
            else
            {
                values = new[] { header.ToString() };
            }
        }
 
        return new HeaderValueProvider(values);
    }
 
    private void BindWithoutInnerBinder(ModelBindingContext bindingContext)
    {
        var headerName = bindingContext.FieldName;
        var request = bindingContext.HttpContext.Request;
 
        object? model;
        if (bindingContext.ModelType == typeof(string))
        {
            var value = request.Headers[headerName];
            model = value.ToString();
        }
        else if (ModelBindingHelper.CanGetCompatibleCollection<string>(bindingContext))
        {
            var values = request.Headers.GetCommaSeparatedValues(headerName);
            model = GetCompatibleCollection(bindingContext, values);
        }
        else
        {
            // An unsupported datatype or a new collection is needed (perhaps because target type is an array) but
            // can't assign it to the property.
            model = null;
        }
 
        if (model == null)
        {
            // Silently fail if unable to create an instance or use the current instance. Also reach here in the
            // typeof(string) case if the header does not exist in the request and in the
            // typeof(IEnumerable<string>) case if the header does not exist and this is not a top-level object.
            bindingContext.Result = ModelBindingResult.Failed();
        }
        else
        {
            bindingContext.ModelState.SetModelValue(
                bindingContext.ModelName,
                request.Headers.GetCommaSeparatedValues(headerName),
                request.Headers[headerName]);
 
            bindingContext.Result = ModelBindingResult.Success(model);
        }
 
        _logger.DoneAttemptingToBindModel(bindingContext);
    }
 
    private static object? GetCompatibleCollection(ModelBindingContext bindingContext, string[] values)
    {
        // Almost-always success if IsTopLevelObject.
        if (!bindingContext.IsTopLevelObject && values.Length == 0)
        {
            return null;
        }
 
        if (bindingContext.ModelType.IsAssignableFrom(typeof(string[])))
        {
            // Array we already have is compatible.
            return values;
        }
 
        var collection = ModelBindingHelper.GetCompatibleCollection<string>(bindingContext, values.Length);
        for (var i = 0; i < values.Length; i++)
        {
            collection.Add(values[i]);
        }
 
        return collection;
    }
 
    private sealed class HeaderValueProvider : IValueProvider
    {
        private readonly string[] _values;
 
        public HeaderValueProvider(string[] values)
        {
            Debug.Assert(values != null);
 
            _values = values;
        }
 
        public bool ContainsPrefix(string prefix)
        {
            return _values.Length != 0;
        }
 
        public ValueProviderResult GetValue(string key)
        {
            if (_values.Length == 0)
            {
                return ValueProviderResult.None;
            }
            else
            {
                return new ValueProviderResult(_values, CultureInfo.InvariantCulture);
            }
        }
    }
}