fix rolling groupby with negative windows

polars/polars-time/src/groupby/dynamic.rs

@@ -653,4 +653,26 @@ mod test {
             },
         );
     }
+
+    #[test]
+    fn test_foo() {
+        let s = Series::new("a", (0..20i32).collect::<Vec<_>>());
+        let df = df![
+            "ints"=> s
+        ]
+        .unwrap();
+        let (_, _, groups) = df
+            .groupby_rolling(
+                vec![],
+                &RollingGroupOptions {
+                    index_column: "ints".into(),
+                    period: Duration::parse("2i"),
+                    offset: Duration::parse("-5i"),
+                    closed_window: ClosedWindow::Both,
+                },
+            )
+            .unwrap();
+
+        dbg!(groups);
+    }
 }

polars/polars-time/src/windows/groupby.rs

@@ -132,6 +132,7 @@ pub fn groupby_windows(
     (groups, lower_bound, upper_bound)
 }
 
+// this assumes that the starting point is alwa
 pub(crate) fn groupby_values_iter_full_lookbehind(
     period: Duration,
     offset: Duration,
@@ -193,6 +194,60 @@ pub(crate) fn groupby_values_iter_full_lookbehind(
         })
 }
 
+// this one is correct for all lookbehind/lookaheads, but is slower
+pub(crate) fn groupby_values_iter_window_behind_t(
+    period: Duration,
+    offset: Duration,
+    time: &[i64],
+    closed_window: ClosedWindow,
+    tu: TimeUnit,
+) -> impl Iterator<Item = (IdxSize, IdxSize)> + TrustedLen + '_ {
+    let add = match tu {
+        TimeUnit::Nanoseconds => Duration::add_ns,
+        TimeUnit::Microseconds => Duration::add_us,
+        TimeUnit::Milliseconds => Duration::add_ms,
+    };
+
+    let mut lagging_offset = 0;
+    let mut last = i64::MIN;
+    time.iter().enumerate().map(move |(i, lower)| {
+        if *lower < last {
+            panic!("index column of 'groupby_rolling' must be sorted!")
+        }
+        last = *lower;
+        let lower = add(&offset, *lower);
+        let upper = add(&period, lower);
+
+        let b = Bounds::new(lower, upper);
+        if b.is_future(time[0], closed_window) {
+            (0, 0)
+        } else {
+            // find starting point of window
+            // we can start searching from lagging offset as that is the minimum boundary because data is sorted
+            // and every iteration this boundary shifts right
+            // we cannot use binary search as a window is not binary,
+            // it is false left from the window, true inside, and false right of the window
+            let mut count = 0;
+            for &t in &time[lagging_offset..] {
+                if b.is_member(t, closed_window) || lagging_offset + count == i {
+                    break;
+                }
+                count += 1
+            }
+            if lagging_offset + count != i {
+                lagging_offset += count;
+            }
+
+            // Safety
+            // we just iterated over value i.
+            let slice = unsafe { time.get_unchecked(lagging_offset..) };
+            let len = slice.partition_point(|v| b.is_member(*v, closed_window));
+
+            (lagging_offset as IdxSize, len as IdxSize)
+        }
+    })
+}
+
 // this one is correct for all lookbehind/lookaheads, but is slower
 pub(crate) fn groupby_values_iter_partial_lookbehind(
     period: Duration,
@@ -333,30 +388,46 @@ pub fn groupby_values(
 
     // we have a (partial) lookbehind window
     if offset.negative {
-        // only lookbehind
         if offset.nanoseconds() >= period.nanoseconds() {
-            let vals = thread_offsets
-                .par_iter()
-                .copied()
-                .map(|(base_offset, len)| {
-                    let upper_bound = base_offset + len;
-                    let iter = groupby_values_iter_full_lookbehind(
-                        period,
-                        offset,
-                        &time[..upper_bound],
-                        closed_window,
-                        tu,
-                        base_offset,
-                    );
-                    iter.map(|(offset, len)| [offset as IdxSize, len])
-                        .collect_trusted::<Vec<_>>()
-                })
-                .collect::<Vec<_>>();
-            flatten(&vals, Some(time.len()))
+            // lookbehind
+            // window is within 2 periods length of t
+            // ------t---
+            // [------]
+            if offset.nanoseconds() < period.nanoseconds() * 2 {
+                let vals = thread_offsets
+                    .par_iter()
+                    .copied()
+                    .map(|(base_offset, len)| {
+                        let upper_bound = base_offset + len;
+                        let iter = groupby_values_iter_full_lookbehind(
+                            period,
+                            offset,
+                            &time[..upper_bound],
+                            closed_window,
+                            tu,
+                            base_offset,
+                        );
+                        iter.map(|(offset, len)| [offset as IdxSize, len])
+                            .collect_trusted::<Vec<_>>()
+                    })
+                    .collect::<Vec<_>>();
+                flatten(&vals, Some(time.len()))
+            }
+            // window is completely behind t and t itself is not a member
+            // ---------------t---
+            //  [---]
+            else {
+                let iter =
+                    groupby_values_iter_window_behind_t(period, offset, time, closed_window, tu);
+                iter.map(|(offset, len)| [offset, len]).collect_trusted()
+            }
         }
         // partial lookbehind
         // this one is still single threaded
         // can make it parallel later, its a bit more complicated because the boundaries are unknown
+        // window is with -1 periods of t
+        // ----t---
+        //  [---]
         else {
             let iter =
                 groupby_values_iter_partial_lookbehind(period, offset, time, closed_window, tu);

py-polars/tests/test_groupby.py

@@ -65,4 +65,35 @@ def test_groupby_rolling_negative_offset_3914() -> None:
     )
     assert df.groupby_rolling(index_column="datetime", period="2d", offset="-4d").agg(
         pl.count().alias("count")
-    )["count"].to_list() == [1, 1, 1, 1, 1]
+    )["count"].to_list() == [0, 0, 1, 2, 2]
+
+    df = pl.DataFrame(
+        {
+            "ints": range(0, 20),
+        }
+    )
+
+    assert df.groupby_rolling(index_column="ints", period="2i", offset="-5i",).agg(
+        [pl.col("ints").alias("matches")]
+    )["matches"].to_list() == [
+        [],
+        [],
+        [],
+        [0],
+        [0, 1],
+        [1, 2],
+        [2, 3],
+        [3, 4],
+        [4, 5],
+        [5, 6],
+        [6, 7],
+        [7, 8],
+        [8, 9],
+        [9, 10],
+        [10, 11],
+        [11, 12],
+        [12, 13],
+        [13, 14],
+        [14, 15],
+        [15, 16],
+    ]