/* 
 * Program to copy text from input to output, changing case of letters.
 *
 * Input:  names of input and output text files.
 *
 * Output:  text file with name as specified, contents same as input file,
 *   but with uppercase letters have been changed to lowercase, and vice versa.
 */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>

#define FILENAME_LENGTH 100

/* safer way to get string from standard input:
 *
 * reads a whole line into "str" (maximum length array_length, including
 *   '\0')
 * replaces '\n' (if found) with '\0'
 *
 * prints a warning message if no '\n' was found -- this indicates that
 *   there was more input than would fit in str
 */
void my_get_string(char * str, int array_length) {
	char * newline_pos;
	fgets(str, array_length, stdin);
	newline_pos = strchr(str, '\n');
	if (newline_pos != NULL) {
		*newline_pos = '\0';
        }
        else {
            printf("warning:  input truncated to %d characters\n",
                    array_length-1);
        }
}

/* main program */
int main(void) {

	FILE * in;
	FILE * out;

        char in_name[FILENAME_LENGTH+1];
        char out_name[FILENAME_LENGTH+1];

	int input_char;

        /* get input, output file names */
        printf("enter name of input file:\n");
	my_get_string(in_name, sizeof(in_name));
        printf("enter name of output file:\n");
	my_get_string(out_name, sizeof(out_name));

	/* open input file and check result */
	in = fopen(in_name, "r");
	if (in == NULL) {
		printf("unable to open input file %s\n", in_name);
		exit(EXIT_FAILURE);
	}

	/* open output file and check result */
	out = fopen(out_name, "w");
	if (out == NULL) {
		printf("unable to open output file %s\n", out_name);
		exit(EXIT_FAILURE);
	}

	/* read characters and copy to output, changing case */
	while ((input_char = fgetc(in)) != EOF) {
		if(isupper(input_char)) {
			fputc(tolower(input_char), out);
		}
		else if (islower(input_char)) {
			fputc(toupper(input_char), out);
		}
		else {
			fputc(input_char, out);
		}
	}

	/* close files */
	fclose(in);
	fclose(out);

	printf("All done!\n");
	return EXIT_SUCCESS;
}