008.go

package p008

/**
Implement atoi to convert a string to an integer.

Hint: 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.

Notes: 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.

Update (2015-02-10):
The signature of the C++ function had been updated.
If you still see your function signature accepts a const char * argument,
please click the reload button  to reset your code definition.


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 (2147483647) or INT_MIN (-2147483648) is returned.
*/

func myAtoi(str string) int {
	//trim leading whitespace
	bs := trimLeadingSpace([]byte(str))
	flag := 1
	if len(bs) > 0 {
		if bs[0] == '-' || bs[0] == '+' {
			if bs[0] == '-' {
				flag = -1
			}
			bs = bs[1:]
		}
	}

	ans := 0
	for i := 0; i < len(bs); i++ {
		if bs[i] >= '0' && bs[i] <= '9' {
			ans = ans*10 + int(bs[i]-'0')*flag
		} else {
			break
		}
		if ans > 2147483647 {
			ans = 2147483647
			break
		} else if ans < -2147483648 {
			ans = -2147483648
			break
		} else if ans != 0 && ((ans > 0) != (flag > 0)) {
			ans = 2147483647 * flag
			if flag == -1 {
				ans -= 1
			}
		}
	}

	return ans
}

func trimLeadingSpace(bs []byte) []byte {
	for i := 0; i < len(bs); i++ {
		if bs[i] != ' ' {
			return bs[i:]
		}
	}
	return []byte{}
}