1 files changed, 121 insertions, 0 deletions
diff --git a/checksum.c b/checksum.c
new file mode 100644
index 0000000..c728887
--- /dev/null
+++ b/checksum.c
@@ -0,0 +1,121 @@
+#include <assert.h>
+#include <errno.h>
+#include <stdio.h>
+#include <string.h>
+#include <stdint.h>
+#include <stdlib.h>
+
+#define DATA_START_OFFSET (0xa0)
+#define DATA_LEN (29)
+#define CHECKSUM_OFFSET (0xbd)
+
+static void print_usage(FILE *const stream, const char *const argv0)
+{
+    fprintf(stream, "This program calculates Game Boy Advance ROM ");
+    fprintf(stream, "checksum and places it into the header.\n");
+    fprintf(stream, "Usage: %s rom.bin\n", argv0);
+}
+
+static int fseek_set_wrapper(FILE *const stream, const long offset)
+{
+    const int ret = fseek(stream, offset, SEEK_SET);
+    if (ret) {
+        const int err = errno;
+        if (feof(stream)) {
+            fprintf(stderr, "fseek(%zu): EOF, file is too small, exiting\n", offset);
+        } else if (ferror(stream)) {
+            fprintf(stderr, "fseek(%zu): Error (%d): \"%s\"\n", offset, err, strerror(err));
+        } else {
+            assert(0);
+        }
+    }
+    return ret;
+}
+
+static size_t fread_wrapper(void *const ptr, const size_t size, FILE *const stream)
+{
+    const size_t ret = fread(ptr, 1, size, stream);
+    if (ret >= size) {
+        assert(ret == size);
+    } else {
+        const int err = errno;
+        if (feof(stream)) {
+            fprintf(stderr, "fread(%zu): EOF, file is too small, exiting\n", size);
+        } else if (ferror(stream)) {
+            fprintf(stderr, "fread(%zu): Error (%d): \"%s\"\n", size, err, strerror(err));
+        } else {
+            assert(0);
+        }
+    }
+    return ret;
+}
+
+static size_t fwrite_wrapper(void *const ptr, const size_t size, FILE *const stream)
+{
+    const size_t ret = fwrite(ptr, 1, size, stream);
+    if (ret >= size) {
+        assert(ret == size);
+    } else {
+        const int err = errno;
+        if (feof(stream)) {
+            fprintf(stderr, "fwrite(%zu): EOF, file is too small, exiting\n", size);
+        } else if (ferror(stream)) {
+            fprintf(stderr, "fwrite(%zu): Error (%d): \"%s\"\n", size, err, strerror(err));
+        } else {
+            assert(0);
+        }
+    }
+    return ret;
+}
+
+static int run_checksum(FILE *const stream)
+{
+    if (fseek_set_wrapper(stream, DATA_START_OFFSET)) {
+        return EXIT_FAILURE;
+    }
+    uint8_t data_raw[DATA_LEN];
+    if (sizeof(data_raw) > fread_wrapper(&data_raw, sizeof(data_raw), stream)) {
+        return EXIT_FAILURE;
+    }
+    uint8_t checksum;
+    if (sizeof(checksum) > fread_wrapper(&checksum, sizeof(checksum), stream)) {
+        return EXIT_FAILURE;
+    }
+    if (fseek_set_wrapper(stream, DATA_START_OFFSET)) {
+        return EXIT_FAILURE;
+    }
+    uint8_t checksum_calculated = 0;
+	// http://www.problemkaputt.de/gbatek-gba-cartridge-header.htm
+	// chk=0:for i=0A0h to 0BCh:chk=chk-[i]:next:chk=(chk-19h) and 0FFh
+    for (size_t i = 0; i < DATA_LEN; i++) {
+        const uint8_t val = data_raw[i];
+        checksum_calculated -= val;
+    }
+	checksum_calculated -= 0x19;
+    printf("checksum = 0x%02x, checksum_calculated = 0x%02x\n", checksum, checksum_calculated);
+    if (fseek_set_wrapper(stream, CHECKSUM_OFFSET)) {
+        return EXIT_FAILURE;
+    }
+    if (sizeof(checksum) > fwrite_wrapper(&checksum, sizeof(checksum), stream)) {
+        return EXIT_FAILURE;
+    }
+    return EXIT_SUCCESS;
+}
+
+int main(const int argc, char *argv[])
+{
+    if (argc < 2) {
+        print_usage(stderr, argv[0]);
+        return EXIT_FAILURE;
+    }
+    const char* input_file_name = argv[1];
+    FILE *const input_stream = fopen(input_file_name, "rb+");
+    if (input_stream == NULL) {
+        const int err = errno;
+        fprintf(stderr, "main: fopen(\"%s\", \"r\"): Error (%d): \"%s\"\n", input_file_name, err, strerror(err));
+        return EXIT_FAILURE;
+    }
+    const int ret = run_checksum(input_stream);
+    fclose(input_stream);
+    return ret;
+}