String Capitalization Without Built-ins

Barracuda Networks security tooling may need to normalize or toggle text without relying on convenience APIs. Implement a function that changes the capitalization of ASCII letters in a string without using native Python functions or string methods such as upper(), lower(), swapcase(), ord(), chr(), len(), range(), or str.translate().

The mode argument is one of "upper", "lower", or "toggle". Convert only the 52 English alphabetic characters. Preserve digits, whitespace, punctuation, symbols, and all character ordering. The input is guaranteed to contain only ASCII characters.

Formal Specification

• Input: text, a string of ASCII characters, and mode, one of the three permitted mode strings.
• Output: A new string with the requested capitalization transformation.
• Do not modify the input string. Do not use native functions or string case-conversion methods.