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imperfect_notes/pragy/LLD 3.md

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+LLD 3
+-----
+
+Factory Pattern
+---------------
+
+1. Trying to validate address
+    ```java
+    class Address {
+        String country, state, city, []addressLines, zip;
+    }
+    ```
+1. Algorithm for validating address will differ  from country to country.
+1. Different countries have different breakups.
+    - Some countries don't have states
+    - Japan has provinces
+    - Some European countries have counties
+    - Meaning of state could be different
+
+    - Could have different ways of writing the addresses
+    - US addresses are more structured
+    - Indian addresses have lots of different ways - c/o xyz
+    - Chinese people write addresses in Chinese
+1. So, different classes for each country
+    ```java
+    public class INValidator {
+        public bool validate(Address a) {}
+    }
+
+    public class USValidator {
+        public bool validate(Address a) {}
+    }
+    ```
+
+    Now, let's say that I ship these validators as a library
+    Some client wants to use these validators to validate a random address
+
+    ```java
+    public class ClientValidator {
+        public bool validate(Address a) {
+            if(a.country == 'India')
+                return new INValidator().validate(a);
+            else if(a.country == 'USA')
+                return new USValidator().validate(a);
+            ...
+        }
+    }
+    ```
+1. **Is this good?**
+    - client needs to know the name of the class for each country
+    - difficult to test
+    - unknown country - Indonesia. Does a class exist for it?
+    - IN stands for India or Indonesia?
+        - could lead to correct addresses being invalidated or wrong addresses be accepted
+1. **Pull into interface**
+    - ![24debe26.png](:storage/e65b85d8-61b9-4991-b67d-d0c285e92bb4/24debe26.png =300x)
+    - all these classes are multiple forms of the same thing
+    - interface `Validator`, which asks to implement method `valdiate`
+    - coding your classes against an interface
+1. **Runtime polymorphism**: `Validator v = new INValidator();`
+1. **Who should worry about the mapping from country name to Validator?**
+    - client should not
+    - I should
+    - Keep a map
+1. Need to create a class whose responsibility is to keep this mapping
+    ```java
+    class X {
+        // why private? don't allow client to change this
+        private static Map<String, Validator> validators;
+        
+        public X() {
+            validators.put("USA", USValidator);
+            validators.put("USA", INValidator);
+        }
+        
+        public Validator getValidator(String country) {
+            // return validators.get(country); - bad
+            // could lead to null pointer exception
+            if(validators.has(country))
+                return validators.get(country);
+            
+            throw new InvalidCountryException();
+        }
+    }
+    ```
+1. Too many objects being created in the constructor. Will be used many times. Use Singleton Pattern.
+    - make constructor private
+    - create an instance variable
+    - expose a getinstance function
+1. Client code
+    ```java
+    public class ClientValidator {
+        public bool validate(Address a) {
+            return X.getInstance(
+                    .getValidator(a.country)
+                    .validate(a);
+        }
+    }
+    ```
+1. Client just knows about the interface Validator, and that there's a factory which gives you different implementations of the Validators.
+1. Factory Design Pattern
+1. Rename X to ValidatorFactory
+1. Remember that factories should usually be singleton
+1. Notice that we use a pattern that we've already used
+1. Usecases - 
+
+Gotchas
+- don't use this in any language
+- Python has dispathcing libraries
+- Strategy pattern
+- Don't abstract too much
+- unless you're a library author
+
+-- --
+
+Stackoverflow
+-------------
+
+- Interviewer asks you to implement Stackoverflow
+- First step? Start coding?
+- **Use Cases**
+    - Anyone can search and view questions
+    - Question, answers, comments
+    - Guest can only search. To add/vote you need to be a member
+    - Each question can have an active bounty
+    - Text can contain photos
+    - User can earn badges
+    - Admin can blcok users
+    - Moderator can close questions
+- Identify entities - will become classes
+- Identify attributes and behavior
+- Identify relationships
+- ![5d93c037.png](:storage/e65b85d8-61b9-4991-b67d-d0c285e92bb4/5d93c037.png =500x)
+    ```python
+    class Question:
+        status: Enum
+        title: str
+        description: str
+        votes: int
+
+        def close(): pass
+
+    class Comment:
+        content: stre
+        votes: int
+
+    class Answer:
+        content: str
+        votes: int
+        accepted: bool
+    ```
+-  ![Screenshot 2019-10-09 at 8.48.21 PM.png](:storage/e65b85d8-61b9-4991-b67d-d0c285e92bb4/0b1d9eeb.png =500x)
+    ```python
+    class Bounty:
+        prize: int
+        expiry: DateTime
+
+        def claim(): pass
+
+    class Photo:
+        alt_text: str
+        url: str
+    ```
+- Note where we have composition or not
+    ![f2f50632.png](:storage/e65b85d8-61b9-4991-b67d-d0c285e92bb4/f2f50632.png =600x)
+    ```python
+    class Member:
+        age: int
+        sex: str
+        def create_question(): pass
+    
+    class Moderator(Member):
+        def close_question(): pass
+    
+    class Admin(Member):
+        def block_member(): pass
+    ```  
+- **Direction of arrows.** Member -> Question = Member has instance of Question
+- **Composition vs Association**
+- ![741698f7.png](:storage/e65b85d8-61b9-4991-b67d-d0c285e92bb4/741698f7.png =400x)
+    ```python
+    class Account:
+        password: str
+        creation_date: DateTime
+        
+        def delete_account(): pass
+    ```
+- Badge
+    - badges are static
+    - same badge, multiple people can own
+    - same person can own multiple badges
+    - not composition, because deleting either doesn't delete the other
+    ```python
+    class Badge:
+        text: str
+    ```
+- Search interface
+- Guest account - doesn't extend member
+- Guest uses search
+- ![7b76882f.png](:storage/e65b85d8-61b9-4991-b67d-d0c285e92bb4/7b76882f.png =400x)
+
+
+
+Movie Ticket Booking
+--------------------
+
+- bookmyticket
+- list all the centers of halls - **PVR** (brand), etc
+- each **cinema center** can have multiple **halls / audis**
+- movie vs movie show (cost, etc)
+- hall can run some movie at some time
+- each movie can have multiple shows
+- search by title, language
+- show seating arrangement
+- show list of reserved and available seats
+- difference b/w the physical chair vs a seat
+- pay via cash/card
+- no two customers should have the same seat
+- discount coupons