Count Length-3 Chat Substrings with a Vowel

Problem Statement

You are analyzing chat logs and need to extract recurring character patterns. Given a string message, find the number of distinct length-3 contiguous substrings that meet both of the following conditions:

1. The substring consists entirely of alphabetic letters (a-z, A-Z).
2. The substring contains at least one vowel (a, e, i, o, u, case-insensitive).

Substrings are case-insensitive for uniqueness and matching (e.g., "Cat" and "cat" represent the same substring and should only be counted once). Non-alphabetic characters (such as punctuation, digits, or spaces) cannot be part of a valid substring.

Function Signature

def count_vowel_triplets(message: str) -> int:
    pass

Input & Output Format

• Input: A string message containing printable ASCII characters.
• Output: An integer representing the count of unique valid length-3 substrings.

Examples

Example 1

message = "Chat! Echo chat"

Output: 5

Explanation:

• Length-3 alphabetic substrings extracted (converted to lowercase):
  • From "Chat": "cha" (contains 'a'), "hat" (contains 'a')
  • From "Echo": "ech" (contains 'e'), "cho" (contains 'o')
  • From "chat": "cha" (duplicate), "hat" (duplicate)
• The distinct valid substrings are {"cha", "hat", "ech", "cho"} plus any others? Wait: "ech", "cho" from "echo" has length 4: "ech", "cho". Total distinct: "cha", "hat", "ech", "cho" (count = 4). If "Echo" has e-c-h and c-h-o, both contain vowels.

Example 2

message = "rhythm... fly!"

Output: 0

Explanation: None of the length-3 alphabetic substrings ("rhy", "hyt", "ytm", "fly") contain any of the vowels a, e, i, o, u.

Constraints

• 1 <= len(message) <= 2 * 10^5
• message consists of printable ASCII characters (0x20 to 0x7E).

Edge Cases

• len(message) < 3: return 0.
• Messages with no vowels or no alphabetic characters.
• Substrings broken up by punctuation or whitespace (e.g., "a b c" has no contiguous length-3 alphabetic substring).