Implementation of the "atoi" function in Swift (just an exercise, not for use in production)

atoi.swift

/**
 Implementation of the "atoi" function in Swift.
 This implementation is an exercise and should not be used in production,
 Swift has built-in types and functions that can do this sort of conversion.
 */

/// Parses the input string as a 32-bit integer.
/// Returns `nil` if the input contains non-ASCII characters, or is not a valid number.
func myAtoi(_ input: String) -> Int32? {
    /// The base ASCII code, where the numbers begin.
    let base = UInt8(48)
    
    /// This function will be applied to the number before the function returns.
    /// It's used to change the sign according to the input.
    var signfn: ((Int32) -> Int32) = { $0 }
    
    /// This will be updated as the input is parsed and returned from the function. 
    /// It is an `Int` instead of an `Int32` because the value can extrapolate the maximum value
    /// that `Int32` can hold depending upon the input. 
    /// We'll clamp this value before returning it as an `Int32`.
    var output: Int = 0
    
    for (index, char) in input.enumerated() {
        /// We only work with ASCII characters, an input
        /// that contains non-ASCII characters is invalid.
        guard let ascii = char.asciiValue else { return nil }
        /// If the character is not a number.
        guard ascii >= base else {
            /// The only position where a non-number character
            /// is acceptable is in the first position (for the sign).
            guard index == 0 else { return nil }
            /// The only acceptable non-number character in the first
            /// position is ASCII code 45 (the minus sign).
            guard ascii == 45 else { return nil }
            /// Found the minus sign in front of the number string,
            /// set the sign function to multiply the number by -1.
            signfn = { $0 * -1 }
            continue
        }
        
        /// Compute the current digit by subtracting the base
        /// ASCII code from the current digit's ASCII code.
        let digit = Int(ascii - base)
        
        /// Since we're working with the decimal system, each time
        /// we find a new digit to the right, we multiply the
        /// current number by 10, then add the new digit to the result. 
        output = (output * 10) + digit
    }
    /// The nested max/min ensures the final value
    /// lies between Int32.min and Int32.max.
    let clamped = Int32(max(min(output, Int(Int32.max)), Int(Int32.min)))
    
    /// Run signfn, which will multiply the number by -1
    /// if a minus sign was found in the first position.
    return signfn(clamped)
}

myAtoi("0") == 0
myAtoi("123") == 123
myAtoi("2048") == 2048
myAtoi("-999") == -999
myAtoi("99999999999999") == Int32.max
myAtoi("-🤡123") == nil