From d0b40bd4bc64719bae98e1400ba475fe1ab4652b Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Steffan=20S=C3=B8lvsten?= <soelvsten@cs.au.dk>
Date: Tue, 4 Jul 2023 11:10:24 +0200
Subject: [PATCH] Add Run-Time Heuristics for Non-canonical Reduce in Nested
 Sweep

If a level in the Outer Sweep is not decreased by more than 5%, then we will
switch over to use the fast reduce on all levels above this one until the next
Inner Sweep.
---
 .../internal/algorithms/nested_sweeping.h     | 26 ++++++++++++++++---
 src/adiar/internal/algorithms/quantify.h      |  3 +--
 src/adiar/internal/algorithms/reduce.h        | 11 +++++---
 3 files changed, 31 insertions(+), 9 deletions(-)

diff --git a/src/adiar/internal/algorithms/nested_sweeping.h b/src/adiar/internal/algorithms/nested_sweeping.h
index f47dd64af..e9d74834c 100644
--- a/src/adiar/internal/algorithms/nested_sweeping.h
+++ b/src/adiar/internal/algorithms/nested_sweeping.h
@@ -1438,6 +1438,8 @@ namespace adiar::internal
     using outer_pq_decorator_t =
       nested_sweeping::outer::up__pq_decorator<outer_pq_t, outer_roots_t>;
 
+    bool auto_fast_reduce = false;
+
     while (outer_levels.can_pull()) {
       adiar_debug(outer_arcs.can_pull_terminal() || !outer_pq.empty(),
                   "If there is a level, then there should also be something for it.");
@@ -1488,8 +1490,8 @@ namespace adiar::internal
         outer_pq_decorator_t outer_pq_decorator(outer_pq, outer_roots, next_inner);
 
         if (/*constexpr*/ nesting_policy::reduce_strategy == nested_sweeping::NEVER_CANONICAL ||
-            /*constexpr*/ nesting_policy::reduce_strategy == nested_sweeping::FINAL_CANONICAL
-            /*TODO: AUTO (probably with a different threshold than for the inner up-sweep)*/) {
+            /*constexpr*/ nesting_policy::reduce_strategy == nested_sweeping::FINAL_CANONICAL ||
+            (nesting_policy::reduce_strategy == nested_sweeping::AUTO && auto_fast_reduce)) {
 #ifdef ADIAR_STATS
           // TODO
 #endif
@@ -1500,15 +1502,28 @@ namespace adiar::internal
           // TODO
 #endif
           const size_t unreduced_width = outer_level.width();
+          size_t reduced_width;
+
           if(unreduced_width <= outer_internal_sorter_can_fit) {
-            __reduce_level<nesting_policy, internal_sorter>
+            reduced_width = __reduce_level<nesting_policy, internal_sorter>
               (outer_arcs, outer_level.level(), outer_pq_decorator, outer_writer,
                outer_sorters_memory, unreduced_width);
           } else {
-            __reduce_level<nesting_policy, external_sorter>
+            reduced_width = __reduce_level<nesting_policy, external_sorter>
               (outer_arcs, outer_level.level(), outer_pq_decorator, outer_writer,
                outer_sorters_memory, unreduced_width);
           }
+
+          // AUTO Strategy: Use the fast reduce from the next level (until next
+          // inner sweep) if this level did not change considerably in size.
+          if constexpr (nesting_policy::reduce_strategy == nested_sweeping::AUTO) {
+            const double threshold = 0.05;
+
+            const double prior = static_cast<double>(unreduced_width);
+            const double delta = static_cast<double>(unreduced_width - reduced_width);
+
+            auto_fast_reduce |= (delta / prior) < threshold;
+          }
         }
 
         continue;
@@ -1520,6 +1535,9 @@ namespace adiar::internal
       adiar_debug(outer_level.level() == next_inner,
                   "'next_inner' level is not skipped");
 
+      // Reset fast reduce for AUTO strategy.
+      auto_fast_reduce = false;
+
       // -----------------------------------------------------------------------
       // Collect all recursions for this level
 
diff --git a/src/adiar/internal/algorithms/quantify.h b/src/adiar/internal/algorithms/quantify.h
index f5b7a2aa8..5a1bd8811 100644
--- a/src/adiar/internal/algorithms/quantify.h
+++ b/src/adiar/internal/algorithms/quantify.h
@@ -511,9 +511,8 @@ namespace adiar::internal
     }
 
     ////////////////////////////////////////////////////////////////////////////
-    // TODO: set to AUTO and add heuristic hooks
     static constexpr internal::nested_sweeping::reduce_strategy reduce_strategy =
-      internal::nested_sweeping::ALWAYS_CANONICAL;
+      internal::nested_sweeping::AUTO;
   };
 
   //////////////////////////////////////////////////////////////////////////////
diff --git a/src/adiar/internal/algorithms/reduce.h b/src/adiar/internal/algorithms/reduce.h
index 66232a868..c6b06432c 100644
--- a/src/adiar/internal/algorithms/reduce.h
+++ b/src/adiar/internal/algorithms/reduce.h
@@ -340,9 +340,9 @@ namespace adiar::internal
     }
 
     // Add number of nodes to level information, if any nodes were pushed to the output.
-    if (out_id != dd_policy::MAX_ID) {
-      const size_t width = dd_policy::MAX_ID - out_id;
-      out_writer.unsafe_push(level_info(label, width));
+    const size_t reduced_width = dd_policy::MAX_ID - out_id;
+    if (reduced_width > 0) {
+      out_writer.unsafe_push(level_info(label, reduced_width));
     }
 
     // Sort mappings for Reduction rule 2 back in order of arcs.internal
@@ -413,6 +413,11 @@ namespace adiar::internal
 
     const bool terminal_value = next_red1.new_uid.is_terminal() && next_red1.new_uid.value();
     __reduce_level__epilogue<>(arcs, label, reduce_pq, out_writer, terminal_value);
+
+    adiar_debug(reduced_width <= unreduced_width,
+                "Reduction should only ever remove nodes");
+
+    return reduced_width;
   }
 
   //////////////////////////////////////////////////////////////////////////////