detect the client's platform and check the appropriate input in the log parser

src/SMAPI.Web/Framework/UserAgentParsing/ClientPlatformMiddleware.cs

@@ -0,0 +1,52 @@
+using System.Threading.Tasks;
+using Microsoft.AspNetCore.Http;
+
+namespace StardewModdingAPI.Web.Framework.UserAgentParsing
+{
+    /// <summary>Middleware that detects the client's platform.</summary>
+    public class ClientPlatformMiddleware
+    {
+        /// <summary>The key used to retrieve the client's platform from <see cref="HttpContext.Items"/>.</summary>
+        public const string ClientPlatformKey = "ClientPlatformKey";
+
+        /// <summary>The next delegate in the middleware pipeline.</summary>
+        private readonly RequestDelegate Next;
+
+        /// <summary>Construct an instance.</summary>
+        /// <param name="next">The next delegate in the middleware pipeline.</param>
+        public ClientPlatformMiddleware(RequestDelegate next)
+        {
+            this.Next = next;
+        }
+
+        /// <summary>Invoke the middleware.</summary>
+        /// <param name="context">The HTTP request context.</param>
+        public async Task InvokeAsync(HttpContext context)
+        {
+            context.Items[ClientPlatformMiddleware.ClientPlatformKey] = this.DetectClientPlatform(context.Request.Headers["User-Agent"]);
+
+            await this.Next(context);
+        }
+
+        /// <summary>Detect the platform that the client is on.</summary>
+        /// <param name="userAgent">The client's user agent.</param>
+        /// <returns>The client's platform, or null if no platforms could be detected.</returns>
+        private Platform? DetectClientPlatform(string userAgent)
+        {
+            switch (userAgent)
+            {
+                case string ua when ua.Contains("Windows"):
+                    return Platform.Windows;
+                // check for Android before Linux because Android user agents also contain Linux
+                case string ua when ua.Contains("Android"):
+                    return Platform.Android;
+                case string ua when ua.Contains("Linux"):
+                    return Platform.Linux;
+                case string ua when ua.Contains("Mac"):
+                    return Platform.Mac;
+                default:
+                    return null;
+            }
+        }
+    }
+}

src/SMAPI.Web/Framework/UserAgentParsing/ClientPlatformMiddlewareExtensions.cs

@@ -0,0 +1,16 @@
+using Microsoft.AspNetCore.Builder;
+
+namespace StardewModdingAPI.Web.Framework.UserAgentParsing
+{
+    /// <summary>Extension methods for the client platform middleware.</summary>
+    internal static class ClientPlatformMiddlewareExtensions
+    {
+        /// <summary>Adds client platform detection to the request pipeline.</summary>
+        /// <param name="builder">The application builder.</param>
+        /// <returns>The application builder with the client platform middleware enabled.</returns>
+        public static IApplicationBuilder UseClientPlatform(this IApplicationBuilder builder)
+        {
+            return builder.UseMiddleware<ClientPlatformMiddleware>();
+        }
+    }
+}

src/SMAPI.Web/Framework/UserAgentParsing/Platform.cs

@@ -0,0 +1,18 @@
+namespace StardewModdingAPI.Web.Framework.UserAgentParsing
+{
+    /// <summary>A software platform.</summary>
+    public enum Platform
+    {
+        /// <summary>The Android platform.</summary>
+        Android,
+
+        /// <summary>The Linux platform.</summary>
+        Linux,
+
+        /// <summary>The Mac platform.</summary>
+        Mac,
+
+        /// <summary>The Windows platform.</summary>
+        Windows
+    }
+}

src/SMAPI.Web/Startup.cs

@@ -23,6 +23,7 @@ using StardewModdingAPI.Web.Framework.Clients.Pastebin;
 using StardewModdingAPI.Web.Framework.Compression;
 using StardewModdingAPI.Web.Framework.ConfigModels;
 using StardewModdingAPI.Web.Framework.RewriteRules;
+using StardewModdingAPI.Web.Framework.UserAgentParsing;
 
 namespace StardewModdingAPI.Web
 {
@@ -172,6 +173,7 @@ namespace StardewModdingAPI.Web
                 )
                 .UseRewriter(this.GetRedirectRules())
                 .UseStaticFiles() // wwwroot folder
+                .UseClientPlatform()
                 .UseMvc();
 
             // enable Hangfire dashboard

src/SMAPI.Web/Views/LogParser/Index.cshtml

@@ -1,5 +1,6 @@
 @using Newtonsoft.Json
 @using StardewModdingAPI.Web.Framework.LogParsing.Models
+@using StardewModdingAPI.Web.Framework.UserAgentParsing
 @model StardewModdingAPI.Web.ViewModels.LogParserModel
 
 @{
@@ -67,10 +68,13 @@ else if (Model.ParsedLog?.IsValid == true)
     <h2>Where do I find my SMAPI log?</h2>
     <div>What system do you use?</div>
     <ul id="os-list">
-        <li><input type="radio" name="os" value="android" id="os-android" /> <label for="os-android">Android</label></li>
-        <li><input type="radio" name="os" value="linux" id="os-linux" /> <label for="os-linux">Linux</label></li>
-        <li><input type="radio" name="os" value="mac" id="os-mac" /> <label for="os-mac">Mac</label></li>
-        <li><input type="radio" name="os" value="windows" id="os-windows" /> <label for="os-windows">Windows</label></li>
+        @{
+            Platform? clientPlatform = Context.Items[ClientPlatformMiddleware.ClientPlatformKey] as Platform?;
+        }
+        <li><input type="radio" name="os" value="android" id="os-android" @(clientPlatform == Platform.Android ? "checked" : "")/> <label for="os-android">Android</label></li>
+        <li><input type="radio" name="os" value="linux" id="os-linux" @(clientPlatform == Platform.Linux ? "checked" : "")/> <label for="os-linux">Linux</label></li>
+        <li><input type="radio" name="os" value="mac" id="os-mac" @(clientPlatform == Platform.Mac ? "checked" : "")/> <label for="os-mac">Mac</label></li>
+        <li><input type="radio" name="os" value="windows" id="os-windows" @(clientPlatform == Platform.Windows ? "checked" : "")/> <label for="os-windows">Windows</label></li>
     </ul>
     <div data-os="android">
         On Android: