#include <stdio.h>
#include <stdlib.h>

int string_length(char *string)
{
    int i, length = 0;
    for(i = 0; string[i] != '\0'; i++)
    {
        length++;
    }
    return length;
}

void string_copy(char string_destination[], char string_source[])
{
    int i = 0;
    do
    {
        string_destination[i] = string_source[i];
    }while(string_destination[i++] != '\0');
}

char* string_find(char string_total[], char string_part[])
{
    int i, j, aux = 0, position = 999, find = 0, boolean;
    for(i = 0; i<string_length(string_total) && !find; i++)
    {
        boolean = 1;
        aux = 0;
        j = i;
        for(; string_part[aux] != '\0' && string_total[j] != '\0' && boolean; j++, aux++)
        {
            //printf("%c = %c\n",string_part[aux], string_total[j]);
            if(string_part[aux] == string_total[j])
            {
                //printf("YES\n");
                boolean = 1;
            }
            else
            {
                //printf("NOT\n");
                boolean = 0;
            }

            if(boolean && string_part[aux+1] == '\0')
            {
                //printf("FINALY\n");
                find = 1;
                position = i;
            }
        }
    }
    if(position != 999)
    {
        return string_total+position;
    }
    else return NULL;
}

char* insert_txt(char *text, char *to_change, char *to_insert)
{
    int i, cont = 0, sprintf_cont = 0;
    char *start = string_find(text, to_change), *temp;
    char *inserted = (char*) malloc((string_length(text) - string_length(to_change) + string_length(to_insert))*sizeof(char));
    if(start != NULL)
    {
        temp = text;

        //printf("!= NULL\n(%s)\n",text,text[0],text[1],text[2]);
        i = 0;
        while(text != start)
        {
            sprintf_cont += sprintf(inserted+sprintf_cont,"%c",text[i]);
            //printf("%c",text[i]);
            text++;
            cont++;
        }
        i = 0;
        while(to_insert[i] != '\0')
        {
            sprintf_cont += sprintf(inserted+sprintf_cont,"%c",to_insert[i]);
            //printf("%c",to_insert[i]);
            i++;
        }
        i = 0;
        temp += cont+string_length(to_change);
        while(temp[i] != '\0')
        {
            sprintf_cont += sprintf(inserted+sprintf_cont,"%c",temp[i]);
            //printf("%c",temp[i]);
            i++;
        }

        return inserted;
    }
    else
    {
        return NULL;
    }
}

int main()
{
    char *text = (char*) malloc(100*sizeof(char));
    char *to_change = (char*) malloc(100*sizeof(char));
    char *to_insert = (char*) malloc(100*sizeof(char));
    char *inserted;

    printf("Set your string: ");
    scanf("%[^\n]",text);
    fflush(stdin);
    printf("Select a part of this string to change: ");
    scanf("%[^\n]",to_change);
    fflush(stdin);
    printf("Set a string to replace the part: ");
    scanf("%[^\n]",to_insert);
    fflush(stdin);

    inserted = insert_txt(text, to_change, to_insert);
    if(inserted != NULL)
    {
        printf("\n%s",inserted);
    }
    else printf("\nTHE REQUESTED STRING DOES NOT EXISTS!");

    return 0;
}