feat: Fix logic in filter_with_previous_loop and add comprehensive …

…tests for `filter_with_previous` function

src/gs.gleam

@@ -984,13 +984,14 @@ fn filter_with_previous_loop(
         case pred(previous, value) {
           True ->
             Some(#(value, filter_with_previous_loop(next, Some(value), pred)))
-          False -> filter_with_previous_loop(next, previous, pred).pull()
+          False -> filter_with_previous_loop(next, Some(value), pred).pull()
         }
       }
       None -> None
     }
   })
 }
+
 /// Finds the first element in a stream that satisfies a predicate and returns a stream containing only that element.
 /// 
 /// ## Example

test/gs_test.gleam

@@ -665,3 +665,66 @@ pub fn bracket_test() {
     },
   )
 }
+
+pub fn filter_with_previous_test() {
+  // Test basic filtering (keep only increasing values)
+  gs.from_list([1, 2, 2, 3, 2, 4])
+  |> gs.filter_with_previous(fn(prev, current) {
+    case prev {
+      Some(p) -> current > p
+      None -> True
+    }
+  })
+  |> gs.to_list
+  |> should.equal([1, 2, 3, 4])
+
+  // Test empty stream
+  gs.from_empty()
+  |> gs.filter_with_previous(fn(_, _) { True })
+  |> gs.to_list
+  |> should.equal([])
+  // Test single element (first element always passes with None)
+  gs.from_pure(42)
+  |> gs.filter_with_previous(fn(prev, _) {
+    case prev {
+      Some(_) -> False
+      None -> True
+    }
+  })
+  |> gs.to_list
+  |> should.equal([42])
+  // Test filtering all elements except first
+  gs.from_list([1, 2, 3])
+  |> gs.filter_with_previous(fn(prev, _) {
+    case prev {
+      Some(_) -> False
+      None -> True
+    }
+  })
+  |> gs.to_list
+  |> should.equal([1])
+  // Test keeping only duplicates
+  gs.from_list([1, 1, 2, 2, 2, 3, 4, 4])
+  |> gs.filter_with_previous(fn(prev, current) {
+    case prev {
+      Some(p) -> {
+        current == p
+      }
+
+      None -> False
+    }
+  })
+  |> gs.to_list
+  |> should.equal([1, 2, 2, 4])
+  // Test with take
+  gs.from_counter(1)
+  |> gs.filter_with_previous(fn(prev, current) {
+    case prev {
+      Some(p) -> current == p + 1
+      None -> True
+    }
+  })
+  |> gs.take(3)
+  |> gs.to_list
+  |> should.equal([1, 2, 3])
+}