Add routine for computing file sum

1 files changed, 30 insertions, 0 deletions

diff --git a/src/main.c b/src/main.c
index 7bae17c..0dd5633 100644
--- a/src/main.c
+++ b/src/main.c
@@ -95,6 +95,36 @@ static void sanitize_sql_str(char **str)
 	*str = new_str;
 }
 
+/* Compute something that identifies the content of FILE and write it to
+   OUT. Algorithm from http://www.cse.yorku.ca/~oz/hash.html via
+   https://stackoverflow.com/a/7666577/20138083.  Return 0 on success, -1 on
+   error. */
+static int sum(uint64_t *out, const char *file)
+{
+	FILE *stream = fopen(file, "r");
+	uint8_t buf[4096];
+	size_t buf_len;
+	uint64_t sum = 5381;
+	if (!file) {
+		perror("fopen");
+		return -1;
+	}
+	while ((buf_len = fread(&buf, 1, sizeof(buf), stream)) != 0) {
+		for (int i = 0; i < buf_len; i++) {
+			uint8_t next = buf[i];
+			sum = sum*33 + next;
+		}
+	}
+	if (ferror(stream)) {
+		perror("fread");
+		fclose(stream);
+		return -1;
+	}
+	fclose(stream);
+	*out = sum;
+	return 0;
+}
+
 static time_t time_from_str(const char *str)
 {
 	int rc;