DEV Community: Nirmal Krishna

Finding element that appears once in an array where other elements appear twice : Leetcode

Nirmal Krishna — Thu, 01 Jul 2021 03:56:16 +0000

This is an example implementation using hashmap. The input array nums is considered to have only one unique number found once, other numbers occurs > once.

// ts
function singleNumber(nums: number[]): number {

    const hash = {};

    for(let i = 0; i< nums.length; i++){   
        hash[nums[i]] = hash[nums[i]] ? hash[nums[i]] + 1 : 1
    }

    return Object.keys(hash).filter(k=> hash[k] === 1).map(k=> parseInt(k))[0];

};

ASP.NET Core Identity User Locked out

Nirmal Krishna — Thu, 24 Jun 2021 15:20:39 +0000

The user lockout feature is the way to improve application security by locking out a user that enters a password incorrectly several times. This technique can help us in protecting against brute force attacks, where an attacker repeatedly tries to guess a password. ⛳

Quite a basic feature for an authentication service, but adding it in my Web API app was quite a head scratcher.

1. Adding to configuration service

In your startup.cs or container configuration file, the config for locking out can be set.

services.AddIdentity<User, IdentityRole>(opt =>
{
    // previous code removed for clarity reasons
    opt.Lockout.AllowedForNewUsers = true;
    opt.Lockout.DefaultLockoutTimeSpan = TimeSpan.FromMinutes(2);
    opt.Lockout.MaxFailedAccessAttempts = 3;
})

The property names are self explanatory here.

The above code will by default setup locking out feature and if a person is trying to login with wrong password for a given username the user account will be locked out for 5 minutes updated accordingly in LockoutEnd column

2. How to check if this user is Locked Out?

//  AuthService.cs
..
    var result = await signInManager.CheckPasswordSignInAsync(user, model.Password, lockoutOnFailure: true);
..

The properties of result : SignInResult we are concerned here are Succeeded, IsLockedOut.

✅ Succeeded == true if the username and password match

❌ Succeeded == false if the username and password do not match

✅ IsLockedOut == true if this user has been locked out after x number of trials

3. 🤔 What is here to scratch your hear for

I was expecting LockOutEnabled will become true (1) in the identity user table. It took me few hours to get to the documentation but it was stated clearly in the Library class.

// Microsoft.AspNetCore.Identity.IdentityUser

// Gets or sets a flag indicating if the user could be locked out.

public virtual bool LockoutEnabled { get; set; }

I missed the could be and it costed me some hours.😪

Updating this column to true for necessary users then locks out the user for particular a time limit set in the config

The proper logging and an error can be thrown with
result.IsLockedOut flag from the service layer 😅

📚 References

Importance of a Praise folder

Nirmal Krishna — Fri, 04 Dec 2020 07:12:50 +0000

Create a folder called Praise in your inbox.
Remember to move appreciation, job well done emails to that folder.

Keeping a praise folder in your email inbox is essential, it serves a number of purposes:

It acknowledge your successes and keep track of them
Gives a morale boost when you read it in future
Comes handy during a performance review to talk about accomplishments

Querying JSON and its nested objects with TSQL

Nirmal Krishna — Wed, 18 Nov 2020 17:42:00 +0000

When we were faced to store lot of dynamically modelled data in a relational DB, we decided on storing them in a JSON column of SQL server.

The well documented recourses came in handy during initial development.

Although with some workaround

Runtime JSON deserialization
Getting around with Store procedures while whole application runs with entity framework

Until today I never got to query nested arrays of a JSON object with SQL. That is when I stumbled upon another method in T-SQL called OPENJSON

OPENJSON is a table-valued function that parses JSON text and returns objects and properties from the JSON input as rows and columns.

In other words, OPENJSON provides a rowset view over a JSON document. You can explicitly specify the columns in the rowset and the JSON property paths used to populate the columns. Since OPENJSON returns a set of rows, you can use OPENJSON in the FROM clause of a T-SQL statement just as you can use any other table, view, or table-valued function.

Few samples

Consider this sample json data



DECLARE @json NVARCHAR(MAX) = '{
    "firstName":"James",
    "lastName":"Doe",
    "ChildrenDetails":[
        {
            "Id":1,
            "Name":"John Doe",
            "Languages":["English", "German"]
        },
        {
            "Id":2,
            "Name":"Jane Doe",
            "Languages":["English"]
        },
        {
            "Id":3,
            "Name":"June Doe",
            "Languages":["German", "Tamil"]
        }]
}'

1. Querying into arrays (ChildrenDetails)



SELECT 
    JSON_VALUE(d.value,'$.Id') AS Id,
    JSON_VALUE(d.value,'$.Name') AS Name
FROM OPENJSON(@json,'$.ChildrenDetails') AS d

2. Flatten Nested arrays (ChildrenDetails + each language)



SELECT 
JSON_VALUE(d.value,'$.Id') AS Id,
JSON_VALUE(d.value,'$.Name') AS Languages,
l.value AS Name
  FROM OPENJSON(@json,'$.ChildrenDetails') AS d CROSS APPLY OPENJSON (d.value,'$.Languages') AS l

Hopefully you can try this with complex JSON structures. The use cases are many, but today this came in handy for a feature to us 😃

Before you leave, a meme:

EOF

I learnt to shuffle an array and an important lesson.

Nirmal Krishna — Tue, 04 Aug 2020 17:14:28 +0000

😏All these years I have faced the constant problem of sorting an array of int, strings, by property value.

😳I have never thought how can I shuffle this sorted array (even unsorted array).

I came across this problem while I was building a Duolingo style Math quiz app. I had to randomize the position of the correct answer in the array of options.

🤔 I pondered over writing some pseudo code for it, I knew it has to do with randomly selecting an index between the boundaries of the array.

😴 Yet as a lazy engineer I googled for it immediately, to see how people do it. I came across Fisher–Yates shuffle algorithm.

for i from n−1 downto 1 do
     j = random integer such that 0 ≤ j ≤ i
     exchange a[j] and a[i]

🤯 It was quite intriguing with its simplicity. I was able to shuffle my options array.
But then I thought,

Would I have come up with similar algo. without google? I do not know but it would have taken some time

😇 I will not be able to solve all problems without taking help. But I should be comfortable to try and solve it, before getting answers from google.

I had to practice on solving problems that make me uncomfortable.

🚩 I need to take on new challenges outside of my fulltime work(codewars, Leetcode, Freecodecamp, #100daysofreact)

The experience of reinventing the wheel is different for every person

The implementation

shuffleOptions = (optionsArr) => {
        var m = optionsArr.length, t, i;
        while (m) {
            i = Math.floor(Math.random() * m--);
            t = optionsArr[m];
            optionsArr[m] = optionsArr[i];
            optionsArr[i] = t;
        }
        return optionsArr;
    }

Amazing visualization + complexity comparison here.

Randomness mentioned here are pseudo randomness