Count frequency of each character in the input string

Ask whether uppercase and lowercase should be distinct, whether spaces count, and whether input is ASCII, Unicode code points, or user-visible grapheme clusters. For general characters, scan once and increment a hash map keyed by the chosen unit. If the contract is lowercase English letters only, a fixed array of length 26 is simpler and has O(1) auxiliary space. If output order matters, either retain first-seen order separately or sort keys after counting; do not rely on incidental map iteration order. Test an empty string, repeated whitespace, mixed case, and non-ASCII characters if supported. Counting is O(n) expected time and O(u) space for u distinct characters, with an additional O(u log u) only when sorted output is requested.