add step4 pullrequests/string_to_integer_atoi

Author: rihib

pullrequests/string_to_integer_atoi/step4.go

@@ -0,0 +1,41 @@
+//lint:file-ignore U1000 Ignore all unused code
+package stringtointegeratoi
+
+import "math"
+
+/*
+Step3のオーバーフローしてしまうバグを直しました。
+*/
+func myAtoi_step4(s string) int {
+	const (
+		intMax = int(math.MaxInt32)
+		intMin = int(math.MinInt32)
+	)
+
+	i := 0
+	for i < len(s) && s[i] == ' ' {
+		i++
+	}
+
+	sign := 1
+	if i < len(s) && (s[i] == '-' || s[i] == '+') {
+		if s[i] == '-' {
+			sign = -1
+		}
+		i++
+	}
+
+	num := 0
+	for i < len(s) && '0' <= s[i] && s[i] <= '9' {
+		digit := int(s[i] - '0')
+		if sign == 1 && (num > intMax/10 || num == intMax/10 && digit >= intMax%10) {
+			return intMax
+		}
+		if sign == -1 && (-num < intMin/10 || -num == intMin/10 && -digit <= intMin%10) {
+			return intMin
+		}
+		num = num*10 + digit
+		i++
+	}
+	return sign * num
+}