: v1: value_mesh: new type ValueOverlay (#1476)

Summary:

introduces `ValueOverlay`, a sparse representation of `(Range<usize>, T) `runs for assembling or patching `ValueMesh` instances without materializing per-rank data. includes structural validation (`EmptyRange`, `OverlappingRanges`), coalescing of equal-adjacent runs, and construction helpers (`push_run`, `try_from_runs`, `normalize`). adds unit tests covering append, coalescing, overlap detection, unsorted inserts, and empty overlays. no behavior change to `ValueMesh`; merge support will follow in the next diff.

Differential Revision: D84266477

Author: shayne-fletcher

hyperactor_mesh/src/v1/value_mesh.rs

@@ -20,6 +20,10 @@ use ndslice::view::Region;
 use serde::Deserialize;
 use serde::Serialize;
 
+mod value_overlay;
+pub use value_overlay::BuildError;
+pub use value_overlay::ValueOverlay;
+
 /// A mesh of values, one per rank in `region`.
 ///
 /// The internal representation (`rep`) may be dense or compressed,
@@ -92,7 +96,7 @@ impl TryFrom<Run> for (Range<usize>, u32) {
     ///
     /// Performs checked conversion of the 64-bit wire fields back
     /// into `usize` indices, returning an error if either bound
-    /// exceeds the platform’s addressable range. This ensures safe
+    /// exceeds the platform's addressable range. This ensures safe
     /// round-tripping between the serialized wire format and native
     /// representation.
     fn try_from(r: Run) -> Result<Self, Self::Error> {
@@ -331,7 +335,7 @@ impl<T: Clone + PartialEq> ValueMesh<T> {
     /// store it efficiently.
     ///
     /// # Parameters
-    /// - `region`: The logical region describing the mesh’s shape and
+    /// - `region`: The logical region describing the mesh's shape and
     ///   rank order.
     /// - `values`: A dense vector of values, one per rank in
     ///   `region`.

hyperactor_mesh/src/v1/value_mesh/value_overlay.rs

@@ -0,0 +1,268 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+use std::ops::Range;
+
+/// Builder error for overlays (structure only; region bounds are
+/// checked at merge time).
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum BuildError {
+    /// A run with an empty range (`start == end`) was provided.
+    EmptyRange,
+
+    /// Two runs overlap or are unsorted: `prev.end > next.start`. The
+    /// offending ranges are returned for debugging.
+    OverlappingRanges {
+        prev: Range<usize>,
+        next: Range<usize>,
+    },
+}
+
+/// A sparse overlay of rank ranges and values, used to assemble or
+/// patch a [`ValueMesh`] without materializing per-rank data.
+///
+/// Unlike `ValueMesh`, which always represents a complete, gap-free
+/// mapping over a [`Region`], a `ValueOverlay` is intentionally
+/// partial: it may describe only the ranks that have changed. This
+/// allows callers to build and merge small, incremental updates
+/// efficiently, while preserving the `ValueMesh` invariants after
+/// merge.
+///
+/// Invariants:
+/// - Runs are sorted by `(start, end)`.
+/// - Runs are non-empty and non-overlapping.
+/// - Adjacent runs with equal values are coalesced.
+/// - Region bounds are validated when the overlay is merged, not on
+///   insert.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct ValueOverlay<T> {
+    runs: Vec<(Range<usize>, T)>,
+}
+
+impl<T> ValueOverlay<T> {
+    /// Creates an empty overlay.
+    pub fn new() -> Self {
+        Self { runs: Vec::new() }
+    }
+
+    /// Returns an iterator over the internal runs.
+    pub fn runs(&self) -> impl Iterator<Item = &(Range<usize>, T)> {
+        self.runs.iter()
+    }
+
+    /// Current number of runs.
+    pub fn len(&self) -> usize {
+        self.runs.len()
+    }
+
+    /// Returns `true` if the overlay contains no runs. This indicates
+    /// that no ranges have been added — i.e., the overlay represents
+    /// an empty or no-op patch.
+    pub fn is_empty(&self) -> bool {
+        self.runs.is_empty()
+    }
+}
+
+impl<T: PartialEq> ValueOverlay<T> {
+    /// Adds a run, maintaining invariants (sorted, non-overlapping,
+    /// coalesced).
+    ///
+    /// This does not check region bounds; that's validated by
+    /// `merge_from_overlay`.
+    pub fn push_run(&mut self, range: Range<usize>, value: T) -> Result<(), BuildError> {
+        // Reject empty ranges.
+        if range.is_empty() {
+            return Err(BuildError::EmptyRange);
+        }
+
+        // Look at the last run.
+        match self.runs.last_mut() {
+            // The common case is appending in sorted order. Fast-path
+            // append if new run is after the last and
+            // non-overlapping.
+            Some((last_r, last_v)) if last_r.end <= range.start => {
+                if last_r.end == range.start && *last_v == value {
+                    // Coalesce equal-adjacent.
+                    last_r.end = range.end;
+                    return Ok(());
+                }
+                self.runs.push((range, value));
+                Ok(())
+            }
+            // The overlay was previously empty or, the caller
+            // inserted out of order (unsorted input).
+            _ => {
+                // Slow path. Re-sort, merge and validate the full
+                // runs vector.
+                self.runs.push((range, value));
+                Self::normalize(&mut self.runs)
+            }
+        }
+    }
+
+    /// Sorts, checks for overlaps, and coalesces equal-adjacent runs
+    /// in-place.
+    fn normalize(v: &mut Vec<(Range<usize>, T)>) -> Result<(), BuildError> {
+        // Early exit for empty overlays.
+        if v.is_empty() {
+            return Ok(());
+        }
+
+        // After this, ever later range has start >= prev.start. If
+        // any later start < prev.end it's an overlap.
+        v.sort_by_key(|(r, _)| (r.start, r.end));
+
+        // Build a fresh vector to collect cleaned using drain(..) on
+        // the input avoiding clone().
+        let mut out: Vec<(Range<usize>, T)> = Vec::with_capacity(v.len());
+        for (r, val) in v.drain(..) {
+            if let Some((prev_r, prev_v)) = out.last_mut() {
+                // If the next run's start is before the previous run's
+                // end we have an overlapping interval.
+                if r.start < prev_r.end {
+                    return Err(BuildError::OverlappingRanges {
+                        prev: prev_r.clone(),
+                        next: r,
+                    });
+                }
+                // If the previous run touches the new run and has the
+                // same value, merge them by extending the end
+                // boundary.
+                if prev_r.end == r.start && *prev_v == val {
+                    // Coalesce equal-adjacent.
+                    prev_r.end = r.end;
+                    continue;
+                }
+            }
+            // Otherwise, push as a new independent run.
+            out.push((r, val));
+        }
+
+        // Replace the old vector.
+        *v = out;
+
+        // Invariant: Runs is sorted, non-overlapping and coalesced.
+        Ok(())
+    }
+
+    /// Builds an overlay from arbitrary runs, validating structure
+    /// and coalescing equal-adjacent.
+    pub fn try_from_runs<I>(runs: I) -> Result<Self, BuildError>
+    where
+        I: IntoIterator<Item = (Range<usize>, T)>,
+    {
+        // We need a modifiable buffer to sort and normalize so we
+        // eagerly collect the iterator.
+        let mut v: Vec<(Range<usize>, T)> = runs.into_iter().collect();
+
+        // Reject empties up-front. Empty intervals are structurally
+        // invalid for an overlay. Fail fast.
+        for (r, _) in &v {
+            if r.is_empty() {
+                return Err(BuildError::EmptyRange);
+            }
+        }
+
+        // Sort by (start, end).
+        v.sort_by_key(|(r, _)| (r.start, r.end));
+
+        // Normalize (validate + coalesce).
+        Self::normalize(&mut v)?;
+
+        // Invariant: Runs is sorted, non-overlapping and coalesced.
+        Ok(Self { runs: v })
+    }
+}
+
+#[cfg(test)]
+mod tests {
+
+    use super::*;
+
+    #[test]
+    fn push_run_appends_and_coalesces() {
+        let mut ov = ValueOverlay::new();
+
+        // First insert.
+        ov.push_run(0..3, 1).unwrap();
+        assert_eq!(ov.runs, vec![(0..3, 1)]);
+
+        // Non-overlapping append.
+        ov.push_run(5..7, 2).unwrap();
+        assert_eq!(ov.runs, vec![(0..3, 1), (5..7, 2)]);
+
+        // Coalesce equal-adjacent (touching with same value).
+        ov.push_run(7..10, 2).unwrap();
+        assert_eq!(ov.runs, vec![(0..3, 1), (5..10, 2)]);
+    }
+
+    #[test]
+    fn push_run_detects_overlap() {
+        let mut ov = ValueOverlay::new();
+        ov.push_run(0..3, 1).unwrap();
+
+        // Overlaps 2..4 with existing 0..3.
+        let err = ov.push_run(2..4, 9).unwrap_err();
+        assert!(matches!(err, BuildError::OverlappingRanges { .. }));
+    }
+
+    #[test]
+    fn push_run_handles_unsorted_inserts() {
+        let mut ov = ValueOverlay::new();
+        // Insert out of order; normalize should sort and coalesce.
+        ov.push_run(10..12, 3).unwrap();
+        ov.push_run(5..8, 2).unwrap(); // Unsorted relative to last.
+        ov.push_run(8..10, 2).unwrap(); // Coalesce with previous.
+
+        assert_eq!(ov.runs, vec![(5..10, 2), (10..12, 3)]);
+    }
+
+    #[test]
+    fn try_from_runs_builds_and_coalesces() {
+        use super::ValueOverlay;
+
+        // Unsorted, with adjacent equal-value ranges that should
+        // coalesce.
+        let ov = ValueOverlay::try_from_runs(vec![(8..10, 2), (5..8, 2), (12..14, 3)]).unwrap();
+
+        assert_eq!(ov.runs, vec![(5..10, 2), (12..14, 3)]);
+    }
+
+    #[test]
+    fn try_from_runs_rejects_overlap_and_empty() {
+        // Overlap should error.
+        let err = ValueOverlay::try_from_runs(vec![(0..3, 1), (2..5, 2)]).unwrap_err();
+        assert!(matches!(err, BuildError::OverlappingRanges { .. }));
+
+        // Empty range should error.
+        let err = ValueOverlay::try_from_runs(vec![(0..0, 1)]).unwrap_err();
+        assert!(matches!(err, BuildError::EmptyRange));
+    }
+
+    #[test]
+    fn is_empty_reflects_state() {
+        let mut ov = ValueOverlay::<i32>::new();
+        assert!(ov.is_empty());
+
+        ov.push_run(0..1, 7).unwrap();
+        assert!(!ov.is_empty());
+    }
+
+    #[test]
+    fn normalize_sorts_coalesces_and_detects_overlap() {
+        // 1) Sort + coalesce equal-adjacent.
+        let mut v = vec![(5..7, 2), (3..5, 2), (7..9, 2)]; // unsorted, all value=2
+        ValueOverlay::<i32>::normalize(&mut v).unwrap();
+        assert_eq!(v, vec![(3..9, 2)]);
+
+        // 2) Overlap triggers error.
+        let mut v = vec![(3..6, 1), (5..8, 2)];
+        let err = ValueOverlay::<i32>::normalize(&mut v).unwrap_err();
+        assert!(matches!(err, BuildError::OverlappingRanges { .. }));
+    }
+}