Start by creating a HashMap class. It should have at least two variables for load factor and capacity. For a load factor of 0.75, you should have an initial capacity of size 16. Then proceed to create the following methods:

1. #hash(key) takes a string key and produces a hash code with it (in the real world, hash maps can accommodate various data types as keys, such as integers and arrays, but we’ll keep it simple for now). We already implemented a fairly good hash function in the previous lesson. As a reminder:

   def hash(key)
     hash_code = 0
     prime_number = 31
   
     key.each_char { |char| hash_code = prime_number * hash_code + char.ord }
   
     hash_code
   end
   

   You are free to use that, or you can conduct your own research on hashing algorithms. Beware, this is a deep, deep rabbit hole. Remember to modulo (%) the hash code by the current capacity before you return it to ensure the index fits within our buckets (regardless of growth).

   You might find yourself confusing keys with hash codes while accessing key-value pairs later. Keys are the user-provided strings that get passed to the hash function, which will then return a hash code. You will never directly access buckets using keys, only hash codes.

2. #set(key, value) takes two arguments: the first is a key and the second is a value to associate with the key. If the key already exists, then the old value associated with it is overwritten by the new one.

   Note that when two different keys generate the same hash code and get assigned to the same bucket, it is a collision, not an update. For example, say Rama and Sita both get hashed to 3. They’d go in the same bucket without overwriting each other; we know that this is a collision and not an update because the keys are different. Review the hash map lesson on collisions if needed.

   Remember to grow your buckets to double their capacity when your hash map exceeds the load factor. The methods mentioned later in this assignment can help you handle the growth logic, so you may want to leave implementing this particular behavior until later.

3. #get(key) takes one argument as a key and returns the value that is associated with it. If the key is not found, return nil.

4. #has?(key) takes a key as an argument and returns a boolean based on whether or not the key is in the hash map.

5. #remove(key) takes a key as an argument. If the given key is in the hash map, it should remove the entry with that key and return the deleted entry’s value. If the key isn’t in the hash map, it should return nil.

6. #length returns the number of stored keys in the hash map.

7. #clear removes all entries in the hash map.

8. #keys returns an array containing all the keys (not values) inside the hash map.

9. #values returns an array containing all the values (not keys) inside the hash map.

10. #entries returns an array that contains each key-value pair in their own arrays, for example: [[first_key, first_value], [second_key, second_value]].

Remember that our hash map does not preserve insertion order when you are retrieving your hash map’s data. It is normal and expected for keys and values to appear out of the order you inserted them in.

Test your hash map

1. Create a new Ruby file.

2. Create a new instance of your hash map and set the load factor to 0.75.

   test = HashMap.new
   

3. Populate your hash map using the #set(key, value) method by copying the following:

   test.set('apple', 'red')
   test.set('banana', 'yellow')
   test.set('carrot', 'orange')
   test.set('dog', 'brown')
   test.set('elephant', 'gray')
   test.set('frog', 'green')
   test.set('grape', 'purple')
   test.set('hat', 'black')
   test.set('ice cream', 'white')
   test.set('jacket', 'blue')
   test.set('kite', 'pink')
   test.set('lion', 'golden')
   

4. After populating your hash map with the data above, your hash map’s current load levels should now be at 0.75 (full capacity).

5. Now with a full hash map, try overwriting a few nodes using #set(key, value). This should only overwrite the existing values of your nodes and not add new ones, so #length should still return the same value and capacity should remain the same.

6. After that, populate your hash map with the last node below. This will make your load levels exceed your load factor, triggering your hash map’s growth functionality and doubling its capacity:

   test.set('moon', 'silver')
   

7. If you have implemented your hash map correctly, the load levels of your expanded hash map should drop well below your load factor, and the entries should be spread evenly among the expanded buckets.

8. With your new hash map, try overwriting a few nodes using #set(key, value). Again, this should only overwrite existing values of your nodes.

9. Test the other methods of your hash map, such as #get(key), #has?(key), #remove(key), #length, #clear, #keys, #values, and #entries, to check if they are still working as expected after expanding your hash map.

Extra credit

• Create a class HashSet that behaves the same as a HashMap but only contains keys with no values.