Implement your own hashtable from scratch, you are not allowed to use existing hash table libraries. (2) To be useful, your output should include the coded data as well as the decoder (code -> token) mapping file.

Using C++  Please add the specific explanation. Thanks.

Huffman coding is used to compress data. The idea is straightforward: represent more common
longer strings with shorter ones via a basic translation matrix. The translation matrix is easily
computed from the data itself by counting and sorting by frequency.
For example, in a well-known corpus used in Natural Language Processing called the "Brown"
corpus (see nltk.org), the top-20 most frequent tokens, which are words or punctuation marks
are listed below associated with frequency and code. The word "and" for example requires
writing three characters. However, if I encoded it differently, say, using the word "5" (yes, I
called "5" a word on purpose), then I save having to write two extra characters! Note, the word
"and" is so frequent, I save those two extra characters many times over!
Token Frequency Code
the 62713 1
, 58334 2
. 49346 3
of 36080 4
and 27932 5
to 25732 6
a 21881 7
in 19536 8
that 10237 9
is 10011 10
was 9777 11
for 8841 12
`` 8837 13
'' 8789 14
The 7258 15
with 7012 16
it 6723 17
as 6706 18
he 6566 19
his 6466 20
So the steps of Huffman coding are relatively straightforward:
1. Pass through the data once, collecting a list of token-frequency counts.
2. Sort the token-frequency counts by frequency, in descending order.
3. Assign codes to tokens using a simple counter, for example by incrementing over the
integers; this is just to keep things simple.
4. Store the new mapping (token -> code) in a hashtable called “encoder”.
5. Store the reverse mapping (code -> token) in a hashtable called "decoder".
6. Pass through the data a second time. This time, replace all tokens with their codes.
Now, be amazed at how much you've shrunk your data!
Delivery Notes:
(1) Implement your own hashtable from scratch, you are not allowed to use existing hash
table libraries.
(2) To be useful, your output should include the coded data as well as the decoder (code ->
token) mapping file.
Now GZIP all that and watch it shrink immensely!

Using C++