Integrity Score

A data packet is represented as a string dataPacket of lowercase English letters. Each character has the following value:
a and b have value 1.
c, d, and e have value 2.
f, g, and h have value 3.
i, j, and k have value 4.
l, m, and n have value 5.
o, p, and q have value 6.
r, s, and t have value 7.
u, v, and w have value 8.
x, y, and z have value 9.
The integrity score is the number of substrings whose character-value sum is divisible by the substring length. Return the integrity score of dataPacket as a long.

Function
integrityScore(dataPacket: String) → long

Examples
Example 1
dataPacket = "cat"
return = 4
The character values are [2, 1, 7]. The valid substrings are "c", "a", "t", and "at". The substring "at" has sum 8 and length 2, so its sum is divisible by its length.

Constraints
1 <= dataPacket.length <= 10^5
dataPacket consists of lowercase English letters.

Rewrite the condition. Sum divisible by length means sum/length is an integer k, so the average value is an integer. Values only run 1 to 9, so k is between 1 and 9. For each k, subtract k from every character value and build prefix sums. A substring has sum = k * length exactly when its shifted sum is zero, meaning two equal prefix sums. Count equal pairs with a hash map or an array offset. That's 9 passes of O(n), so about 9n total. The pitfall is brute forcing with prefix sums, which is still O(n^2) and times out. Another trap is returning an int. The answer can reach roughly n^2/2, so use a long. Single characters always count, since any value is divisible by 1. If the trick slips away mid-assessment, StealthCoder is the hedge that surfaces the k loop and the prefix-equality count.

The honest play: practice the pattern, and have StealthCoder ready for the one you didn't see coming.