String to Integer

Implement atoi to convert a string to an integer.
Carefully consider all possible input cases. If you want a challenge, please do not see below and ask yourself what are the possible input cases.
Example
It is intended for this problem to be specified vaguely (ie, no given input specs). You are responsible to gather all the input requirements up front.
spoilers alert... requirements for atoi:
The function first discards as many whitespace characters
as necessary until the first non-whitespace character is found.
Then, starting from this character, takes an optional initial
plus or minus sign followed by as many numerical digits as possible,
and interprets them as a numerical value.
The string can contain additional characters after those that
form the integral number, which are ignored and have no effect
on the behavior of this function.
If the first sequence of non-whitespace characters in str is
not a valid integral number, or if no such sequence exists
because either str is empty or it contains only whitespace
characters, no conversion is performed.
If no valid conversion could be performed, a zero value is
returned. If the correct value is out of the range of
representable values, INT_MAX or INT_MIN is returned.

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