[笔记]一道C语言面试题：IPv4字符串转为UInt整数

#include "stdio.h"
#include "stdlib.h"
#include "string.h"

bool ConvertIPv4ToUInt(const char *strIP, unsigned int *ip)
{
    if (!strIP) {
        return false;
    }

    int Len = strlen(strIP);

    // min len is 7, e.g. 1.2.3.4; max len is 15, e.g. 123.234.121.254
    if ((Len < 7) || (Len > 15)) {
        return false;
    }

    int num[4] = { 0 };     // 4 parts of number
    int partNum = 0;        // 1 part of number 
    int base = 1;           // 10^base
    int dotCount = 0;       // dot count

    // from right to left
    for (int i = Len - 1; i >= 0; --i) {
        char ch = strIP[i];
        if (ch == ‘.‘) {
            // if the first char of last char is ".", e.g. ".1.2.3.4" or "1.2.", exit
            if ((i == 0) || (i == Len - 1)){
                return false;
            }
            dotCount++;

            // if more than 3 dot found, e.g. "1.2.3.4.5", exit
            if (dotCount > 3) {
                return false;
            }

            // save partNum to num[]
            num[dotCount - 1] = partNum;
            partNum = 0;
            base = 1;
        }
        else if ((ch < ‘0‘) || (ch > ‘9‘)) {
            // if illeagal char inside, exit
            return false;
        }
        else {
            // handle digit char
            partNum += (ch - ‘0‘) * base;
            base *= 10;

            if (partNum > 255) {
                return false;
            }

            // handle first part num
            if (i == 0) {
                // if count of "." is not enough, exit
                if (dotCount != 3) {
                    return false;
                }
                num[dotCount] = partNum;
            }
        }
    }

    // output ip
    *ip = 0;
    for (int i = 0; i < 4; ++i) {
        *ip += num[i] << (i * 8);
    }

    return true;
}

int main(int argc, char* argv[])
{
    char* strIP[] = {
        "1",
        "1.2",
        "1.2.3",
        "111.222.113", 
        "1.2.3.",
        ".1.2.3",
        "256.1.2.3",
        "1.2.3.4",
        "1.2.3.4.5",
        "12.234.45.6",
        "12.2345.45.6",
        "1.a.2.3",
        "1.2.3.4 ",
        "1.2. 3.4",
        "1,2,3,4",
    };

    for (int i = 0; i < sizeof(strIP) / sizeof(char *); ++i){
        unsigned int ip = 0;
        if (ConvertIPv4ToUInt(strIP[i], &ip)){
            printf("%s -> %08X\n", strIP[i], ip);
        }
        else {
            printf("%s is not valid\n", strIP[i]);
        }
    }

    getchar();
    return 0;
}

1 is not valid
1.2 is not valid
1.2.3 is not valid
111.222.113 is not valid
1.2.3. is not valid
.1.2.3 is not valid
256.1.2.3 is not valid
1.2.3.4 -> 01020304
1.2.3.4.5 is not valid
12.234.45.6 -> 0CEA2D06
12.2345.45.6 is not valid
1.a.2.3 is not valid
1.2.3.4  is not valid
1.2. 3.4 is not valid
1,2,3,4 is not valid