You are given a string s of length n and an integer k, where n is a multiple of k. Your goal is to transform the string s into a new compressed string called result that has a length of n / k.

The transformation process works as follows:

1. Divide the string: Split s into n / k consecutive substrings, where each substring has exactly k characters.

2. Process each substring: For each substring (processing them in order from left to right):

   • Calculate the hash value of each character, where the hash value is the character's position in the alphabet ('a' → 0, 'b' → 1, 'c' → 2, ..., 'z' → 25)
   • Sum up all the hash values of the characters in the current substring
   • Take the remainder when this sum is divided by 26 (this gives you hashedChar)
   • Convert hashedChar back to a lowercase letter (0 → 'a', 1 → 'b', ..., 25 → 'z')
   • Append this character to the result string

3. Return the result: After processing all substrings, return the final result string.

For example, if s = "abcd" and k = 2:

• First substring: "ab" → hash values: 0 + 1 = 1 → 1 % 26 = 1 → 'b'
• Second substring: "cd" → hash values: 2 + 3 = 5 → 5 % 26 = 5 → 'f'
• Result: "bf"

The solution iterates through the string in chunks of size k, computes the sum of character positions for each chunk, takes modulo 26, and converts back to a character to build the final result.

The problem is essentially asking us to create a hash function that compresses a string by mapping every k characters to a single character. This is a straightforward simulation problem where we need to follow the given instructions step by step.

The key insight is recognizing that this is a chunking and hashing operation:

• We're dividing the input into equal-sized chunks
• Each chunk gets reduced to a single character through a hashing process
• The hash function is simply the sum of character positions modulo 26

Why does this approach make sense?

1. Fixed chunk size: Since n is guaranteed to be a multiple of k, we know we can cleanly divide the string into chunks of size k without any remainder. This makes the iteration pattern simple - we can jump by k positions each time.

2. Character mapping: The problem defines a clear mapping from characters to numbers ('a' → 0, 'b' → 1, etc.). This is just ord(char) - ord('a'), which gives us the alphabetical position.

3. Modulo operation: Taking the sum modulo 26 ensures our result always maps back to a valid lowercase letter (since there are 26 letters in the alphabet). No matter how large the sum gets, sum % 26 will always be in the range [0, 25].

4. Reverse mapping: Converting back from a number to a character is the inverse operation: chr(ord('a') + hashedChar).

The solution naturally flows from these observations - we iterate through the string in steps of k, calculate the hash for each substring, and build our result character by character. There's no need for complex data structures or algorithms; it's purely a matter of following the hashing recipe provided in the problem statement.

The solution implements a direct simulation of the hashing process described in the problem. Let's walk through the implementation step by step:

1. Initialize the result container:

ans = []

We use a list to collect the hashed characters, which will be joined into a string at the end. This is more efficient than string concatenation in Python.

2. Iterate through the string in chunks of size k:

for i in range(0, len(s), k):

The loop starts at index 0 and jumps by k positions each iteration. This ensures we process each substring of length k exactly once. For example, if len(s) = 6 and k = 2, we'll have i values of 0, 2, and 4.

3. Calculate the hash sum for each substring:

t = 0
for j in range(i, i + k):
    t += ord(s[j]) - ord("a")

For each substring starting at position i:

• We iterate through the next k characters (from index i to i + k - 1)
• Convert each character to its hash value using ord(s[j]) - ord("a")
  • ord("a") gives 97, ord("b") gives 98, etc.
  • Subtracting ord("a") normalizes these to 0, 1, 2, ... 25
• Accumulate the sum in variable t

4. Apply modulo and convert to character:

hashedChar = t % 26
ans.append(chr(ord("a") + hashedChar))

• Take the sum modulo 26 to get a value in range [0, 25]
• Convert this number back to a character:
  • ord("a") gives the ASCII value of 'a' (97)
  • Adding hashedChar gives us the ASCII value of our target character
  • chr() converts the ASCII value back to a character
• Append the resulting character to our answer list

5. Build and return the final string:

return "".join(ans)

Join all the hashed characters into a single string and return it.

Time Complexity: O(n) where n is the length of the input string, as we visit each character exactly once.

Space Complexity: O(n/k) for storing the result string, which has length n/k.

The algorithm is straightforward - no complex data structures or advanced techniques are needed. It's a pure simulation that follows the problem's hashing recipe exactly as specified.