Count Character Frequencies | Dataford Interview Questions

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Problem

Iru's message tools need a compact character-frequency summary for analyzing user-entered text. Given a message, count each character and return the results ranked from most frequent to least frequent.

Formal Specification

Implement character_frequencies(text, case_sensitive, include_whitespace).

text is a string.
case_sensitive is a Boolean. If False, convert alphabetic characters to lowercase before counting.
include_whitespace is a Boolean. If False, ignore whitespace characters such as spaces, tabs, and newlines.
Return a list of two-element lists, where each element is [character, count].
Sort results by decreasing count. If two characters have the same count, preserve the order in which their normalized characters first appeared in the filtered input.
Return an empty list when no characters remain after filtering.

Character comparisons use Python string characters, so punctuation, digits, and Unicode characters are valid inputs.

Constraints

0 <= len(text) <= 100,000
text may contain ASCII or Unicode characters
Whitespace is identified with character.isspace()
Results are sorted by decreasing frequency
Equal frequencies are ordered by first appearance after filtering and normalization

Function Signature

def character_frequencies(text, case_sensitive, include_whitespace):