using System;
using System.Collections.Generic;
using System.IO;
using System.Runtime.InteropServices.JavaScript;
using System.Runtime.Versioning;
using System.Threading.Tasks;

// Required for the [JSImport]/[JSExport] source generator to compile.
[assembly: SupportedOSPlatform("browser")]

namespace Glass.WebGL2
{
    /// <summary>
    /// The browser's local storage, which keeps text for the site across visits and holds only a few megabytes.  Only
    /// keys that start with a given prefix are visible, so one page can keep several separate stores.
    /// </summary>
    public sealed partial class WebGL2LocalStorage : IKeyValueStore
    {
        private static Task loadTask;

        private static partial class Interop
        {
            [JSImport("getKeys", "glass-webgl2")]
            [return: JSMarshalAs<JSType.Array<JSType.String>>]
            public static partial string[] GetKeys();

            [JSImport("get", "glass-webgl2")]
            public static partial string Get(string key);

            [JSImport("set", "glass-webgl2")]
            public static partial bool Set(string key, string value);

            [JSImport("remove", "glass-webgl2")]
            public static partial void Remove(string key);
        }

        private readonly string keyPrefix;

        /// <summary>
        /// Must be awaited exactly once, from the browser host's own startup code, before any store is used.
        /// JSHost.ImportAsync can't be awaited synchronously, since nothing can pump the browser's microtask queue
        /// while the single WASM thread is blocked on it.
        /// </summary>
        public static Task EnsureLoadedAsync()
        {
            return loadTask ??= JSHost.ImportAsync("glass-webgl2", "../Glass.WebGL2.js");
        }

        /// <summary>
        /// Makes a store over the local storage keys that start with <paramref name="keyPrefix"/>.  An empty prefix
        /// covers all of them.
        /// </summary>
        public WebGL2LocalStorage(string keyPrefix)
        {
            this.keyPrefix = keyPrefix ?? throw new ArgumentNullException(nameof(keyPrefix));
        }

        public IReadOnlyList<string> GetKeys()
        {
            var returnMe = new List<string>();

            foreach (string key in Interop.GetKeys())
            {
                if (key.StartsWith(keyPrefix, StringComparison.Ordinal))
                {
                    returnMe.Add(key.Substring(keyPrefix.Length));
                }
            }

            return returnMe;
        }

        public string Get(string key)
        {
            if (key == null)
            {
                throw new ArgumentNullException(nameof(key));
            }

            return Interop.Get(keyPrefix + key);
        }

        public void Set(string key, string value)
        {
            if (key == null)
            {
                throw new ArgumentNullException(nameof(key));
            }

            if (value == null)
            {
                throw new ArgumentNullException(nameof(value));
            }

            if (!Interop.Set(keyPrefix + key, value))
            {
                throw new IOException("The browser's storage is full or turned off.");
            }
        }

        public void Remove(string key)
        {
            if (key == null)
            {
                throw new ArgumentNullException(nameof(key));
            }

            Interop.Remove(keyPrefix + key);
        }
    }
}