/*Program for seeking a substring in a given string and replacing it's occorance with another given string*/
/*Program strenths: replaces substring with same, smaller or bigger size string, pre-determines overflow*/
/*Program weaknesses: replaces only first occorance: but modular enough to be modified to replace all of the occorances or any targetted one*/

#include<stdio.h>
#include<malloc.h>

/*define a size limit for the string lengths to be used*/
#define SIZ 100

void chksiz(int );
void remnl(char *);
char * seekstr(char *,char *);
int chk_ov(char *,char *,char *);
void repstr(char *,char *,char *);

int main()
{
	char string[SIZ],from[SIZ],to[SIZ],*p;
	puts("String substitution program\n");
	printf("Enter the string: ");
	fgets(string,SIZ,stdin);
	printf("Enter the sub-string: ");
	fgets(from,SIZ,stdin);
	remnl(string);	/*remove any stored newlines - as fgets stores newline from istream*/
	remnl(from);	/*remove any stored newlines - as fgets stores newline from istream*/
	p=seekstr(from,string);	/*seek occorance of from in string*/
	if(p==NULL)	/*NULL returned - string not found*/
	{
		printf("\nString not found!\n");
		exit(1);	/*exit with error code 1 on not found*/
	}
	/*control reaches here means string found*/
	printf("Enter the substitution sub-string: ");
	fgets(to,SIZ,stdin);
	remnl(to);	/*remove any stored newlines - as fgets stores newline from istream*/
	if(chk_ov(string,from,to))	/*pre-determine if string overflows due to substitution: 1 for ov and 0 otherwise*/
	{
		printf("\nString overflow!\n");
		exit(2);	/*exit with error code 2 on overflow*/
	}
	/*control reaches here means no overflow pre-determined*/
	repstr(p,from,to);	/*replace occorance of from with to*/
	puts("------------");
	printf("\nResult: ");
	puts(string);	/*print result*/
	return 0;	/*exit with success: error code 0*/
}

void repstr(char *str,char *from,char *to)
{
	char *tmp=(char *)malloc(sizeof(char)*strlen(str)+1);	/*allocate only required amount of memory for temporary array*/
	strcpy(tmp,(char *)(str+strlen(from)));/*copy remainder of sting str to tmp starting from ahead of end of occorance of from in it*/
	strcpy(str,to);	/*overwrite str with null-terminated to*/
	strcat(str,tmp);	/*concatenate temporarily stored remainder of str with str*/
	return;
}

char * seekstr(char *substr,char *str)
{
	int iter;
	char *tmp=(char *)malloc(sizeof(char)*strlen(str)-strlen(substr)+1);	/*allocate only required amount of memory for temporary array*/
	char *p;
	for(iter=strlen(str)-strlen(substr),p=str;iter>=0;iter--,p++)	/*iterations continue till iter becomes negative*/
	{
		strncpy(tmp,p,strlen(substr));	/*copy strlen(substr) characters from position p to tmp*/
		*(char *)(tmp+strlen(substr))='\0';	/*as strncpy does not null terminate the string*/
		if(strcmp(substr,tmp)==0)
			return p;	/*return point of occorance of substr in str*/
	}
	return NULL;
}

void remnl(char *str)
{
	if(*(char *)(str+strlen(str)-1)=='\n')	/*if newline is encountered before null character, substitute it with null*/
		*(char *)(str+strlen(str)-1)='\0';
	return;
}

int chk_ov(char *str,char *from,char *to)
{
	if(strlen(str)-strlen(from)+strlen(to)>SIZ-1||strlen(from)>strlen(str))	/*pre-determine if overflow occors after substitution*/
		return 1;	/*return 1 if ov occors*/
	return 0;	/*return 0 if no ov occors*/
}