improve detection of fake "better" programs (#159)

Author: ckrause

CHANGELOG.md

@@ -5,6 +5,7 @@ To install or update LODA, please follow the [installation instructions](https:/
 ### Enhancements
 
 * Improve handling of loops with constant number of iterations
+* Improve detection of fake "better" programs
 
 # v22.6.19
 

src/finder.cpp

@@ -313,10 +313,12 @@ std::pair<std::string, steps_t> Finder::isOptimizedBetter(
   }
   result.second = optimized_steps;
 
-  // check if the first non-decreasing term is beyond the know sequence terms
-  if (tmp.get_first_non_decreasing_term() >=
-      static_cast<int64_t>(terms.size())) {
-    return result;  // bad
+  // check if the first decreasing/non-increasing term is beyond the known
+  // sequence terms => fake "better" program
+  const int64_t s = terms.size();
+  if (tmp.get_first_delta_lt(Number::ZERO) >= s ||  // decreasing
+      tmp.get_first_delta_lt(Number::ONE) >= s) {   // non-increasing
+    return result;                                  // => fake "better" program
   }
 
   // evaluate existing program for same number of terms

src/include/sequence.hpp

@@ -17,7 +17,7 @@ class Sequence : public std::vector<Number> {
 
   bool is_linear(size_t start) const;
 
-  int64_t get_first_non_decreasing_term() const;
+  int64_t get_first_delta_lt(const Number &d) const;
 
   bool align(const Sequence &s, int64_t max_offset);
 

src/sequence.cpp

@@ -38,9 +38,10 @@ bool Sequence::is_linear(size_t start) const {
   return true;
 }
 
-int64_t Sequence::get_first_non_decreasing_term() const {
+int64_t Sequence::get_first_delta_lt(const Number &d) const {
   for (size_t i = 1; i < size(); i++) {
-    if (!((*this)[i - 1] < (*this)[i])) {
+    const auto delta = Semantics::sub((*this)[i], (*this)[i - 1]);
+    if (delta < d) {
       return i;
     }
   }