From a4c27d029a84ba079e25f3bf58ab1184901c4e0d Mon Sep 17 00:00:00 2001
From: Christian Emmer <10749361+emmercm@users.noreply.github.com>
Date: Wed, 15 Nov 2023 21:18:42 -0800
Subject: [PATCH] Fix BPS patch TARGET_COPY reading

---
 src/types/patches/bpsPatch.ts | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/src/types/patches/bpsPatch.ts b/src/types/patches/bpsPatch.ts
index bde039f89..6758479cd 100644
--- a/src/types/patches/bpsPatch.ts
+++ b/src/types/patches/bpsPatch.ts
@@ -102,11 +102,18 @@ export default class BPSPatch extends Patch {
         sourceRelativeOffset += (offset & 1 ? -1 : +1) * (offset >> 1);
         await targetFile.write(await sourceFile.readAt(sourceRelativeOffset, length));
         sourceRelativeOffset += length;
-      } else {
+      } else if (action === BPSAction.TARGET_COPY) {
         const offset = await Patch.readUpsUint(patchFile);
         targetRelativeOffset += (offset & 1 ? -1 : +1) * (offset >> 1);
-        await targetFile.write(await targetFile.readAt(targetRelativeOffset, length));
-        targetRelativeOffset += length;
+        // WARN: you explicitly can't read the target file all at once, you have to read byte by
+        // byte, because later iterations of the loop may need to read data that was changed by
+        // earlier iterations of the loop.
+        for (let i = 0; i < length; i += 1) {
+          await targetFile.write(await targetFile.readAt(targetRelativeOffset, 1));
+          targetRelativeOffset += 1;
+        }
+      } else {
+        throw new Error(`BPS action ${action} isn't supported`);
       }
     }
   }