1 files changed, 16 insertions, 0 deletions
diff --git a/easytool/mifare_classic.c b/easytool/mifare_classic.c
index e516631..5f21fd5 100644
--- a/easytool/mifare_classic.c
+++ b/easytool/mifare_classic.c
@@ -1,6 +1,7 @@
 #include <stdint.h>
 #include <stdio.h>
 #include <string.h>
+#include <errno.h>
 
 #include "mifare_classic.h"
 
@@ -27,3 +28,18 @@ void mfcl_parse_acc_bits(struct acc_bits_parsed *abp, uint8_t *acc_bits)
 			abp->block[block] |= ABP_C3;
 	}
 }
+
+/* apply a delta (positive or negative) to a Mifare Classic VALUE block */
+int mfcl_update_value_block(struct mfcl_value_block *mvb, int32_t delta)
+{
+	int64_t sum = mvb->value + delta;
+
+	if (sum > 0xffffffff || sum < 0)
+		return -ERANGE;
+
+	mvb->value = sum;
+	mvb->value_backup = mvb->value;
+	mvb->value_inv = ~sum;
+
+	return 0;
+}