From 607e53fcee20c1c42013ba8c564cec6c56501f4d Mon Sep 17 00:00:00 2001
From: Matt Dainty <matt@bodgit-n-scarper.com>
Date: Wed, 19 May 2021 10:02:49 +0100
Subject: [PATCH] Recalculate checksum if the link order is updated

Fixes #2
---
 cmd/psx/main.go | 4 +++-
 memcard.go      | 6 ++++++
 2 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/cmd/psx/main.go b/cmd/psx/main.go
index 7fc0f2f..01e2b82 100644
--- a/cmd/psx/main.go
+++ b/cmd/psx/main.go
@@ -51,8 +51,10 @@ func splitMemoryCard(base string, smc *psx.MemoryCard) error {
 			for {
 				// Copy the directory frame and data block
 				tmc.HeaderBlock.DirectoryFrame[j] = df
-				if df.LinkOrder != psx.LastLink {
+				if df.LinkOrder != psx.LastLink && tmc.HeaderBlock.DirectoryFrame[j].LinkOrder != uint16(j+1) {
+					// Block has moved during the copy
 					tmc.HeaderBlock.DirectoryFrame[j].LinkOrder = uint16(j + 1)
+					tmc.HeaderBlock.DirectoryFrame[j].UpdateChecksum()
 				}
 				tmc.DataBlock[j] = smc.DataBlock[i]
 
diff --git a/memcard.go b/memcard.go
index 7420ae7..569077e 100644
--- a/memcard.go
+++ b/memcard.go
@@ -68,6 +68,12 @@ type directoryFrame struct {
 	Checksum byte
 }
 
+func (d *directoryFrame) UpdateChecksum() {
+	h := XOR()
+	binary.Write(h, binary.LittleEndian, d.directoryFrameFields)
+	d.Checksum = h.Sum(nil)[0]
+}
+
 type unusedFrame struct {
 	AvailableBlocks byte
 	Reserved        [3]byte