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

void errormessage(int);
void encrypt_file(char *,char *,char *,char*);
void decrypt_file(char *,char *,char *,char*);

int main(int argc, char *argv[])
{
	char array1[]={'a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z'};
	char array2[]={'u','d','n','q','w','r','x','a','z','s','p','y','e','v','t','f','g','c','k','j','h','m','o','i','b','l'};

	if(argc<4)
		errormessage(1);
	if(argc>4)
		errormessage(2);
	if(!strcmp(argv[1],"-encrypt")&&!strcmp(argv[1],"-decrypt"))
	{
		printf("\n%s\n",argv[1]);
		errormessage(3);
	}
	if(!strcmp(argv[1],"-encrypt"))
		encrypt_file(argv[2],argv[3],array1,array2);
	if(!strcmp(argv[1],"-decrypt"))
		decrypt_file(argv[2],argv[3],array1,array2);
	return 0;
}

void encrypt_file(char *filesrc, char *filedest,char *array1, char *array2)
{
	int i;
	char ch;
	FILE *fp/*ip file*/,*fc/*opfile*/;

	fp=fopen(filesrc,"r");
	if(fp==NULL)
		printf("\nCould not open file %s\n",filesrc);
	else
	{
		fc=fopen(filedest,"r");
		if(fc==NULL)
		{
			fc=fopen(filedest,"w");
			if(fc==NULL)
				printf("\nFailed to write to file %s\n",filedest);
			else
			{
				while(ch!=EOF)
				{
					ch=fgetc(fp);
					for(i=0;i<26;i++)
						if(ch==array1[i]||ch==(array1[i]-32))
							break;
					if(i<26)
					{
						if(ch==array1[i])
							fputc(array2[i],fc);
						if(ch==(array1[i]-32))
							fputc((array2[i]-32),fc);
					}
					else
						fputc(ch,fc);
				}
				fputc(EOF,fc);
				fclose(fc);
			}
		}
		else
		{
			printf("\nFile %s already exists.\n",filedest);
			fclose(fc);
		}
		fclose(fp);
	}
	return;
}

void decrypt_file(char *filesrc, char *filedest,char *array1, char *array2)
{	
	int i;
	char ch;
	FILE *fp/*ip file*/,*fc/*opfile*/;

	fp=fopen(filesrc,"r");
	if(fp==NULL)
		printf("\nCould not open file %s\n",filesrc);
	else
	{
		fc=fopen(filedest,"r");
		if(fc==NULL)
		{
			fc=fopen(filedest,"w");
			if(fc==NULL)
				printf("\nFailed to write to file %s\n",filedest);
			else
			{
				while(ch!=EOF)
				{
					ch=fgetc(fp);
					for(i=0;i<26;i++)
						if(ch==array2[i]||ch==(array2[i]-32))
							break;
					if(i<26)
					{
						if(ch==array2[i])
							fputc(array1[i],fc);
						if(ch==(array2[i]-32))
							fputc((array1[i]-32),fc);
					}
					else
						fputc(ch,fc);
				}
				fputc(EOF,fc);
				fclose(fc);
			}
		}
		else
		{
			printf("\nFile %s already exists.\n",filedest);
			fclose(fc);
		}
		fclose(fp);
	}
	return;
}

void errormessage(int messno)
{
	switch(messno)
	{
		case 1:	printf("\nInsufficient arguments.\n");
				break;
		case 2:	printf("\nToo many arguments.\n");
				break;
		case 3:	printf("\nInvalid call parameter.\n");
				break;
		default: printf("\nInvalid error number.\n");
				break;
	}
	exit(0);
}