using System;
using System.IO;
using System.Security.Cryptography;
using System.Threading.Tasks;
using Elsa;
using Elsa.ActivityResults;
using Elsa.Attributes;
using Elsa.Expressions;
using Elsa.Providers.WorkflowStorage;
using Elsa.Services;
using Elsa.Services.Models;

namespace DocumentManagement.Workflows.Activities
{
    [Activity(
        Category = "Document Management",
        Description = "Saves a hash of the specified file onto the blockchain to prevent tampering."
    )]
    public class UpdateBlockchain : Activity
    {
        [ActivityInput(
            Label = "File",
            Hint = "The file to store its hash of onto the blockchain. Can be byte[] or Stream.",
            SupportedSyntaxes = new[] {SyntaxNames.JavaScript, SyntaxNames.Liquid},
            DefaultWorkflowStorageProvider = TransientWorkflowStorageProvider.ProviderName
        )]
        public object File { get; set; } = default!;
        
        [ActivityOutput(Hint = "The computed file signature as stored on the blockchain.")]
        public string Output { get; set; } = default!;

        protected override async ValueTask<IActivityExecutionResult> OnExecuteAsync(ActivityExecutionContext context)
        {
            // Determine the type of File object: is it a Stream or a byte array?
            var bytes = File is Stream stream 
                ? await stream.ReadBytesToEndAsync() 
                : File is byte[] buffer 
                    ? buffer 
                    : throw new NotSupportedException();
            
            // Compute hash.
            var fileSignature = ComputeSignature(bytes);

            // TODO: Schedule background work using Hangfire that will fake-update an imaginary blockchain with the file signature.

            // Suspend the workflow.
            return Suspend();
        }

        protected override IActivityExecutionResult OnResume(ActivityExecutionContext context)
        {
            // When we resume, read the simply complete this activity.
            // TODO: Receive the computed file signature and set it as Output.
            return Done();
        }

        private static string ComputeSignature(byte[] bytes)
        {
            using var algorithm = SHA256.Create();
            var hashValue = algorithm.ComputeHash(bytes);
            return Convert.ToBase64String(hashValue);
        }
    }
}