working

1 files changed, 45 insertions, 17 deletions

diff --git a/cksum.c b/cksum.c
index 18f0f1a..7eacb6e 100644
--- a/cksum.c
+++ b/cksum.c
@@ -17,7 +17,7 @@
  *
  */
 
-#define _POSIX_C_SOURCE 2
+#define _POSIX_C_SOURCE 200809L
 #include <errno.h>
 #include <inttypes.h>
 #include <limits.h>
@@ -26,16 +26,51 @@
 #include <string.h>
 #include <unistd.h>
 
-/* TODO: study http://ross.net/crc/download/crc_v3.txt */
+#define MAX_SUM_WIDTH	(32)
+#define UINT32_BIT	(32)
 
-int cksum(const char *path)
+struct sum {
+	uintmax_t size;
+	char sum[MAX_SUM_WIDTH];
+};
+
+static uint32_t reverse(uint32_t n, int width)
+{
+	uint32_t r = 0;
+	for (int i = 0; i < width; i++) {
+		r |= (n & 0x1) << ((width - 1) - i);
+		n >>= 1;
+	}
+	return r;
+}
+
+static struct sum crc32(FILE *f)
 {
-	const uint_least32_t polynomial =
-	/* 0x04c11db7; */	/* x^32 is implicit */
-	0x82608edb;		/* +1 is implicit */
+	const uint32_t polynomial = 0x04c11db7;
+	uint32_t crc = UINT32_MAX;
+	struct sum sum = { 0 };
+
+	int c;
+	while ((c = fgetc(f)) != EOF) {
+		sum.size++;
+		crc ^= reverse((uint8_t)c, CHAR_BIT) << (UINT32_BIT - CHAR_BIT);
+		for (int i = 0; i < CHAR_BIT; i++) {
+			if (crc & (1 << (UINT32_BIT - 1))) {
+				crc = (crc << 1) ^ polynomial;
+			} else {
+				crc = (crc << 1);
+			}
+		}
+	}
+	crc = reverse(crc, CHAR_BIT * sizeof(crc)) ^ UINT32_MAX;
 
-	uint_least32_t crc = UINT_LEAST32_MAX;
-	intmax_t octets = 0;
+	snprintf(sum.sum, sizeof(sum.sum), "%"PRIu32, crc);
+	return sum;
+}
+
+int cksum(const char *path)
+{
+	uintmax_t octets = 0;
 
 	FILE *f = stdin;
 	if (path && strcmp(path, "-")) {
@@ -47,16 +82,9 @@ int cksum(const char *path)
 		return 1;
 	}
 
-	int c;
-	while ((c = fgetc(f)) != EOF) {
-		octets++;
-		crc ^= (unsigned char)c;
-		for (int k = 0; k < CHAR_BIT; k++) {
-			crc = crc & 1 ? (crc >> 1) ^ polynomial : crc >> 1;
-		}
-	}
+	struct sum sum = crc32(f);
+	printf("%s %"PRIuMAX"", sum.sum, sum.size);
 
-	printf("%"PRIuLEAST32" %"PRIdMAX"", crc, octets);
 	if (f != stdin) {
 		printf(" %s", path);
 		fclose(f);