// Copyright (c) .NET Foundation. All rights reserved.
// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.

using System;
using System.Collections.Generic;
using System.Text;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Mvc.Core;
using Microsoft.AspNetCore.Mvc.ModelBinding;

namespace Microsoft.AspNetCore.Mvc.Formatters
{
    /// <summary>
    /// Reads an object from a request body with a text format.
    /// </summary>
    public abstract class TextInputFormatter : InputFormatter
    {
        /// <summary>
        /// Returns UTF8 Encoding without BOM and throws on invalid bytes.
        /// </summary>
        protected static readonly Encoding UTF8EncodingWithoutBOM
            = new UTF8Encoding(encoderShouldEmitUTF8Identifier: false, throwOnInvalidBytes: true);

        /// <summary>
        /// Returns UTF16 Encoding which uses littleEndian byte order with BOM and throws on invalid bytes.
        /// </summary>
        protected static readonly Encoding UTF16EncodingLittleEndian
            = new UnicodeEncoding(bigEndian: false, byteOrderMark: true, throwOnInvalidBytes: true);

        /// <summary>
        /// Gets the mutable collection of character encodings supported by
        /// this <see cref="TextInputFormatter"/>. The encodings are
        /// used when reading the data.
        /// </summary>
        public IList<Encoding> SupportedEncodings { get; } = new List<Encoding>();

        /// <inheritdoc />
        public override Task<InputFormatterResult> ReadRequestBodyAsync(InputFormatterContext context)
        {
            if (context == null)
            {
                throw new ArgumentNullException(nameof(context));
            }

            var selectedEncoding = SelectCharacterEncoding(context);
            if (selectedEncoding == null)
            {
                var message = Resources.FormatUnsupportedContentType(
                    context.HttpContext.Request.ContentType);

                var exception = new UnsupportedContentTypeException(message);
                context.ModelState.AddModelError(context.ModelName, exception, context.Metadata);

                return InputFormatterResult.FailureAsync();
            }

            return ReadRequestBodyAsync(context, selectedEncoding);
        }

        /// <summary>
        /// Reads an object from the request body.
        /// </summary>
        /// <param name="context">The <see cref="InputFormatterContext"/>.</param>
        /// <param name="encoding">The <see cref="Encoding"/> used to read the request body.</param>
        /// <returns>A <see cref="Task"/> that on completion deserializes the request body.</returns>
        public abstract Task<InputFormatterResult> ReadRequestBodyAsync(
            InputFormatterContext context,
            Encoding encoding);

        /// <summary>
        /// Returns an <see cref="Encoding"/> based on <paramref name="context"/>'s
        /// character set.
        /// </summary>
        /// <param name="context">The <see cref="InputFormatterContext"/>.</param>
        /// <returns>
        /// An <see cref="Encoding"/> based on <paramref name="context"/>'s
        /// character set. <c>null</c> if no supported encoding was found.
        /// </returns>
        protected Encoding SelectCharacterEncoding(InputFormatterContext context)
        {
            if (context == null)
            {
                throw new ArgumentNullException(nameof(context));
            }

            if (SupportedEncodings.Count == 0)
            {
                var message = Resources.FormatTextInputFormatter_SupportedEncodingsMustNotBeEmpty(
                    nameof(SupportedEncodings));
                throw new InvalidOperationException(message);
            }

            var requestContentType = context.HttpContext.Request.ContentType;
            var requestMediaType = string.IsNullOrEmpty(requestContentType) ? default : new MediaType(requestContentType);
            if (requestMediaType.Charset.HasValue)
            {
                // Create Encoding based on requestMediaType.Charset to support charset aliases and custom Encoding
                // providers. Charset -> Encoding -> encoding.WebName chain canonicalizes the charset name.
                var requestEncoding = requestMediaType.Encoding;
                if (requestEncoding != null)
                {
                    for (int i = 0; i < SupportedEncodings.Count; i++)
                    {
                        if (string.Equals(
                            requestEncoding.WebName,
                            SupportedEncodings[i].WebName,
                            StringComparison.OrdinalIgnoreCase))
                        {
                            return SupportedEncodings[i];
                        }
                    }
                }

                // The client specified an encoding in the content type header of the request
                // but we don't understand it. In this situation we don't try to pick any other encoding
                // from the list of supported encodings and read the body with that encoding.
                // Instead, we return null and that will translate later on into a 415 Unsupported Media Type
                // response.
                return null;
            }

            // We want to do our best effort to read the body of the request even in the
            // cases where the client doesn't send a content type header or sends a content
            // type header without encoding. For that reason we pick the first encoding of the
            // list of supported encodings and try to use that to read the body. This encoding
            // is UTF-8 by default in our formatters, which generally is a safe choice for the
            // encoding.
            return SupportedEncodings[0];
        }
    }
}