// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
 
using System.Numerics;
using System.Security.Cryptography;
 
namespace Microsoft.AspNetCore.StaticWebAssets.Tasks.Utils;
 
internal static class FileHasher
{
    internal static string GetFileHash(string filePath)
    {
#if !NET9_0_OR_GREATER
        using var hash = SHA256.Create();
        var bytes = Encoding.UTF8.GetBytes(filePath);
        var hashBytes = hash.ComputeHash(bytes);
#else
        var bytes = Encoding.UTF8.GetBytes(filePath);
        var hashBytes = SHA256.HashData(bytes);
#endif
        return ToBase36(hashBytes);
    }
 
    internal static string HashString(string relativePath)
    {
        var bytes = Encoding.UTF8.GetBytes(relativePath);
#if NET9_0_OR_GREATER
        var hashBytes = SHA256.HashData(bytes);
#else
        using var hash = SHA256.Create();
        var hashBytes = hash.ComputeHash(bytes);
#endif
        return ToBase36(hashBytes);
    }
 
    internal static string ToBase36(byte[] hash)
    {
        const string chars = "0123456789abcdefghijklmnopqrstuvwxyz";
 
        var result = new char[10];
#if NET9_0_OR_GREATER
        var dividend = BigInteger.Abs(new BigInteger(hash.AsSpan()[..9].ToArray()));
#else
        var dividend = BigInteger.Abs(new BigInteger(hash.AsSpan().Slice(0, 9).ToArray()));
#endif
        for (var i = 0; i < 10; i++)
        {
            dividend = BigInteger.DivRem(dividend, 36, out var remainder);
            result[i] = chars[(int)remainder];
        }
 
        return new string(result);
    }
}