A scientist has been recording observations of the temperature for the past while. On hot days, they write "H", and for cold days, they write "C". Instead of using a list, they kept all of these observations in a single long string (e.g., "HCH" would be a hot day, followed by a cold day, and then another hot day).

Write a function add_hot_cold that consumes a string of observations and returns the number of hot days minus the number of cold days. To accomplish this, you should process each letter in turn and add 1 if it is hot, and -1 if it is cold. For example, the string "HCH" would result in 1, while the string "CHCC" would result in -2.

You cannot use the built-in count method or the len function. Unit test your function.