C程序，把字符串source中出现的pattern替换成dest

replace.c

#include <stdio.h>
#include <string.h>

const int max_length = 100;

int match_result[max_length];
char source[max_length * max_length];
char pattern[max_length];
char dest[max_length];

void find_str(char *src, char *ptrn, int mtb[]);
void replace_pattern(char *src, char *ptrn, char *dst, int mtb[]);
void subtract_newline(char str[]); // 去掉gets结果里的回车符

int main(void)
{
    fgets(source, max_length, stdin);
    fgets(pattern, max_length, stdin);
    fgets(dest, max_length, stdin);
    
    subtract_newline(source);
    subtract_newline(pattern);
    subtract_newline(dest);
    
    match_result[0] = -1;
    find_str(source, pattern, match_result);
    
    replace_pattern(source, pattern, dest, match_result);
    
    puts(source);
    
    return 0;
}

void subtract_newline(char str[])
{
    if (str[strlen(str) - 1] == '\n') {
        str[strlen(str) - 1] = '\0';
    }
}

void find_str(char *src, char *ptrn, int mtb[])
{
    char *src_start = src;
    int mtb_i = 0;
    int ptrn_size = strlen(ptrn);
    int ptrn_i = 0;
    
    while (*src) {
        if (*src == *ptrn) {
            ptrn_i = 0;
            while ((ptrn_i < ptrn_size) && (*src) && (*src++ = ptrn[ptrn_i++]));
            if (ptrn_i == ptrn_size) { // 匹配成功
                mtb[mtb_i++] = src - src_start - ptrn_i;
            }
        } else {
            src++;
        }
    }
    
    mtb[mtb_i] = -1; // 标志结束
}

void replace_pattern(char *src, char *ptrn, char *dst, int mtb[])
{
    int src_size = strlen(src);
    int ptrn_size = strlen(ptrn);
    int dest_size = strlen(dst);
    int offset = 0;
    
    for (int i = 0; mtb[i] != -1; i++) {
        if (ptrn_size < dest_size) { // 往后移
            offset = dest_size - ptrn_size;
            for (int j = src_size; j >= mtb[i]; j--) {
                src[j + offset] = src[j];
            }
            for (int j = i + 1; mtb[j] != -1; ++j) {
                mtb[j] += offset;
            }
            src_size += offset;
        } else if (ptrn_size > dest_size) { // 向前缩
            offset = ptrn_size - dest_size;
            for (int j = mtb[i] - offset; j <= src_size; j++) {
                // 有等号是为了包含最后的0
                src[j] = src[j + offset];
            }
            for (int j = i + 1; mtb[j] != -1; ++j) {
                mtb[j] -= offset;
            }
            src_size -= offset;
        }
        
        for (int j = 0; j < dest_size; ++j) {
            src[mtb[i] + j] = dest[j];
        }
    }
}