Use standard FILE I/O as parameters

While stdin/stdout is all nice and dandy for piping, we want to show useful
information about compression, so remove this feature.

huffman.c

+#include <stdbool.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
@@ -83,10 +84,45 @@ int decode_file(FILE *in, FILE *out)
 
 int main(int argc, char *argv[])
 {
-	if (argc == 2 && !strcmp(argv[1], "-d")) {
-		decode_file(stdin, stdout);
+	const char *infile, *outfile;
+	bool decompress = false;
+
+	if (argc > 1)
+		decompress = !strcmp(argv[1], "-d");
+
+	if ((decompress && argc < 4) || (!decompress && argc != 3)) {
+		printf("Usage: huffman [-d] <input file> <output file>\n");
+		return -1;
+	}
+
+	/* Gotta get that decompression going on. */
+	if (decompress) {
+		infile = argv[2];
+		outfile = argv[3];
 	} else {
-		encode_file(stdin, stdout);
+		infile = argv[1];
+		outfile = argv[2];
+	}
+
+	FILE *input = fopen(infile, "r");
+	if (!input) {
+		fprintf(stderr, "Failed to open input file\n");
+		return -1;
+	}
+
+	FILE *output = fopen(outfile, "w");
+	if (!output) {
+		fprintf(stderr, "Failed to open output file\n");
+		fclose(input);
+		return -1;
 	}
+
+	if (decompress)
+		decode_file(input, output);
+	else
+		encode_file(input, output);
+
+	fclose(input);
+	fclose(output);
 	return 0;
 }