[gnc-autoclear.cpp] prune search if remaining amounts are too small

this optimisation will greatly reduce the search size. first, sort
splits by reverse absolute amount. then precompute the positive and
negative sum(amounts) of remaining splits. when exploring any branch,
if the target is falls outside the range remaining negative to
positive sums, then any combination of subsequent splits will not
reach target. bail early.

Author: christopherlam

libgnucash/app-utils/gnc-autoclear.cpp

@@ -69,6 +69,8 @@ struct RuntimeMonitor
 struct SplitInfo
 {
     int64_t m_amount;
+    int64_t m_rem_pos;
+    int64_t m_rem_neg;
     Split* m_split;
 };
 
@@ -111,8 +113,11 @@ subset_sum (SplitInfoVec::const_iterator iter,
             SplitInfoVec& path, Solution& solution,
             int64_t target, RuntimeMonitor& monitor)
 {
-    DEBUG ("this=%" PRId64" target=%" PRId64 " path=%s",
-           iter == end ? 0 : iter->m_amount, target, path_to_str (path));
+    DEBUG ("this=%" PRId64" target=%" PRId64 " rem_pos=%" PRId64
+           " rem_neg=%" PRId64 " path=%s",
+           iter == end ? 0 : iter->m_amount, target,
+           iter == end ? 0 : iter->m_rem_pos,
+           iter == end ? 0 : iter->m_rem_neg, path_to_str (path));
 
     if (target == 0)
     {
@@ -139,6 +144,13 @@ subset_sum (SplitInfoVec::const_iterator iter,
         return;
     }
 
+    if (target < iter->m_rem_neg || target > iter->m_rem_pos)
+    {
+        DEBUG ("PRUNE target=%" PRId64 " rem_pos=%" PRId64" rem_neg=%" PRId64,
+               target, iter->m_rem_pos, iter->m_rem_neg);
+        return;
+    }
+
     auto next = std::next(iter);
 
     // 1st path: use current_num
@@ -177,7 +189,7 @@ gnc_account_get_autoclear_splits (Account *account, gnc_numeric toclear_value,
         else if (amt == 0)
             DEBUG ("skipping zero-amount split %p", split);
         else
-            splits.push_back ({amt, split});
+            splits.push_back ({amt, 0, 0, split});
     }
 
     static GQuark autoclear_quark = g_quark_from_static_string ("autoclear");
@@ -188,6 +200,24 @@ gnc_account_get_autoclear_splits (Account *account, gnc_numeric toclear_value,
         return nullptr;
     }
 
+    // sort splits in descending absolute amount
+    std::sort (splits.begin(), splits.end(),
+               [](const SplitInfo& a, const SplitInfo& b)
+               {
+                   int64_t aa = std::llabs(a.m_amount);
+                   int64_t bb = std::llabs(b.m_amount);
+                   return (aa == bb) ? a.m_amount > b.m_amount : aa > bb;
+               });
+
+    // for each split, precompute sums of remaining pos or neg amounts
+    int64_t rem_pos{0}, rem_neg{0};
+    std::for_each (splits.rbegin(), splits.rend(),
+                   [&rem_pos, &rem_neg](SplitInfo& s)
+                   {
+                       s.m_rem_pos = rem_pos += std::max<int64_t>(s.m_amount, 0);
+                       s.m_rem_neg = rem_neg += std::min<int64_t>(s.m_amount, 0);
+                   });
+
     RuntimeMonitor monitor{max_seconds};
     Solution solution;
     SplitInfoVec path;