Fix supersede checksum calculation

Author: simolus3

crates/core/src/sync/checksum.rs

@@ -1,7 +1,7 @@
 use core::{
     fmt::Display,
     num::Wrapping,
-    ops::{Add, AddAssign},
+    ops::{Add, AddAssign, Sub, SubAssign},
 };
 
 use num_traits::float::FloatCore;
@@ -60,6 +60,20 @@ impl AddAssign for Checksum {
     }
 }
 
+impl Sub for Checksum {
+    type Output = Self;
+
+    fn sub(self, rhs: Self) -> Self::Output {
+        Self(self.0 - rhs.0)
+    }
+}
+
+impl SubAssign for Checksum {
+    fn sub_assign(&mut self, rhs: Self) {
+        self.0 -= rhs.0;
+    }
+}
+
 impl From<u32> for Checksum {
     fn from(value: u32) -> Self {
         Self::from_value(value)

crates/core/src/sync/operations.rs

@@ -81,7 +81,7 @@ INSERT OR IGNORE INTO ps_updated_rows(row_type, row_id) VALUES(?1, ?2)",
                     // Superseded (deleted) a previous operation, add the checksum
                     let supersede_checksum = Checksum::from_i32(supersede_statement.column_int(1));
                     add_checksum += supersede_checksum;
-                    op_checksum += supersede_checksum;
+                    op_checksum -= supersede_checksum;
 
                     // Superseded an operation, only skip if the bucket was empty
                     // Previously this checked "superseded_op <= last_applied_op".