From d85a5ae57e20959f4dbb9b0d468e017711c8a8f7 Mon Sep 17 00:00:00 2001
From: Matthew Gretton-Dann <matt@gretton-dann.org.uk>
Date: Sun, 31 Aug 2025 17:40:30 +0100
Subject: [PATCH] Change the iteration method used.

We stop using function calls to move down an index but instead use a for
 loop.

This actually provides a slight performance improvement as we are not
repeated making and destroying function frames.
---
 main.cc | 33 +++++++++++++++++----------------
 1 file changed, 17 insertions(+), 16 deletions(-)

diff --git a/main.cc b/main.cc
index 189ac8d..0fbde4f 100644
--- a/main.cc
+++ b/main.cc
@@ -130,22 +130,23 @@ auto triangle_num(int n) { return (n * (n + 1)) / 2; }
 
 using Avail = std::vector<int>;
 
-auto find_solution_impl(Grid& grid, int n, Pos const& pos, int idx, Avail& avail_sqs)-> bool {
-  auto const sq = Square(pos, idx);
+auto find_solution_impl(Grid& grid, int n, Pos const& pos, Avail& avail_sqs)-> bool {
   if (x(pos) == 0 && y(pos) == grid.length()) { return true; }
-  if (idx == 0) { return false; }
-  if (avail_sqs[idx] == 0) { return find_solution_impl(grid, n, pos, idx - 1, avail_sqs); }
-  if (grid.fits(sq)) {
-    --avail_sqs[idx];
-    grid.add(sq);
-    auto next_pos = grid.next_pos(pos);
-    auto const result = find_solution_impl(grid, n, next_pos, n, avail_sqs);
-    ++avail_sqs[idx];
-    if (result) { return true; }
-    grid.clear(sq);
-    return find_solution_impl(grid, n, pos, idx - 1, avail_sqs);
+
+  for (auto idx = n; idx != 0; --idx) {
+    if (avail_sqs[idx] == 0) { continue; }
+
+    auto const sq = Square(pos, idx);
+    if (grid.fits(sq)) {
+      --avail_sqs[idx];
+      grid.add(sq);
+      if (find_solution_impl(grid, n, grid.next_pos(pos), avail_sqs)) { return true; }
+      ++avail_sqs[idx];
+      grid.clear(sq);
+    }
   }
-    return find_solution_impl(grid, n, pos, idx - 1, avail_sqs);
+
+  return false;
 }
 
 auto find_solution(int n) -> Grid{
@@ -153,13 +154,13 @@ auto find_solution(int n) -> Grid{
   Grid grid(length);
   Avail avail_sqs;
   for (auto i = 0; i <= n; ++i) { avail_sqs.push_back(i); }
-  find_solution_impl(grid, n, std::make_pair(0, 0), n, avail_sqs);
+  find_solution_impl(grid, n, std::make_pair(0, 0), avail_sqs);
   return grid;
 }
 
 int main(int argc, char** argv)
 {
-  auto n = (argc == 1) ? 9 : atoi(argv[1]);
+  auto n = (argc == 1) ? 8 : atoi(argv[1]);
   auto grid = find_solution(n);
   std::cout << "Partridge problem " << n << " side length " << grid.length() << '\n';
   grid.output();