SPDX-FileCopyrightText: 2021 Bastian Bittorf <bb@npl.de>
SPDX-FileCopyrightText: 2021 fosslinux <fosslinux@aussies.space>

SPDX-License-Identifier: Unlicense

This adds a main() function to this tool, giving a frontend so we can call
it from the command line.

--- sha-256.c	2019-08-04 22:20:53.000000000 +1000
+++ sha-256.c	2021-02-15 17:21:27.478945543 +1100
@@ -1,10 +1,16 @@
 #include <stdint.h>
 #include <string.h>
-
-#include "sha-256.h"
+#include <stdio.h>
 
 #define CHUNK_SIZE 64
 #define TOTAL_LEN_LEN 8
+#define MAX_FILE_SIZE 2*1024*1024
+
+void show_usage(void)
+{
+	printf("Usage: sha256sum <file>\n");
+	printf("       sha256sum <file> -c <known_SHA256_hash>\n");
+}
 
 /*
  * ABOUT bool: this file does not use bool in order to be as pre-C99 compatible as possible.
@@ -217,3 +223,121 @@
 		hash[j++] = (uint8_t) h[i];
 	}
 }
+
+static void hash_to_string(char hash_string[66], uint8_t hash[32])
+{
+	for (size_t i = 0; i < 32; i++) {
+		hash_string += sprintf(hash_string, "%02x", hash[i]);
+	}
+   hash_string[65] = '\0';
+}
+
+static void hash_as_string(char filename[128], char hash_string[66])
+{
+	char buffer[MAX_FILE_SIZE];
+
+	FILE* fp = fopen(filename, "r");
+	if (fp == NULL)
+	{
+		printf("Opening file %s failed!", filename);
+		hash_string = NULL;
+		return;
+	}
+
+	size_t i;
+	for (i = 0; i < MAX_FILE_SIZE; i++) {
+		int c = fgetc(fp);
+		if (c == EOF) {
+			break;
+		}
+		buffer[i] = c;
+	}
+
+	uint8_t hash[32];
+
+	calc_sha_256(hash, buffer, i);
+	hash_to_string(hash_string, hash);
+}
+
+int main(int argc, char **argv)
+{
+	char buffer[MAX_FILE_SIZE];
+	size_t i;
+	FILE *fp;
+	FILE *csum_fp;
+
+	if (argc == 1) {
+		show_usage();
+		return 2;
+	}
+
+	if ((strcmp(argv[1], "-c") == 0) || (strcmp(argv[1], "--check") == 0)) {
+		/* Operate in check mode */
+		int failed = 0;
+
+		/* Get and open checksum file */
+		char *csum_filename = argv[2];
+		csum_fp = fopen(csum_filename, "r");
+		if (csum_fp == NULL) {
+			printf("Opening file %s failed!", csum_filename);
+		};
+		
+		/* We break out of this loop at EOF */
+		while (1) {
+			/* Get the hash we need to check against */
+			char hash_true[256];
+			char filename[512];
+			int c = fgetc(csum_fp);
+			i = 0;
+			while (c != ' ' && c != EOF) {
+				hash_true[i] = c;
+				c = fgetc(csum_fp);
+				i++;
+			}
+			if(c == EOF) break;
+			/* Skip the next space(s) */
+			while (c == ' ') {
+				c = fgetc(csum_fp);
+			}
+			if(c == EOF) break;
+			/* Get the filename */
+			i = 0;
+			while (c != '\n' && c != EOF) {
+				filename[i] = c;
+				c = fgetc(csum_fp);
+				i++;
+			}
+           filename[i] = '\0';
+
+			/* Break out @ EOF */
+			if (c == EOF) break;
+
+			/* Get the hash */
+			char hash_string[66];
+			hash_as_string(filename, hash_string);
+			if (hash_string == NULL) return 127;
+
+			/* Output results */
+			printf("%s: ", filename);
+			if (strcmp(hash_true, hash_string)) {
+				printf("FAILED\n");
+				failed = 1;
+			} else {
+				printf("OK\n");
+			}
+		}
+
+		if (failed) return 1;
+	} else {
+		/* All the arguments are files to generate checksums for */
+
+		for (i = 1; i < argc; i++) {
+			char hash[66];
+			hash_as_string(argv[i], hash);
+			if (hash == NULL) return 127;
+			printf("%s  %s\n", hash, argv[i]);
+		}
+	}
+
+	return 0;
+}