From 6d8c673efb10b6b6c16e44017d4325f825d88408 Mon Sep 17 00:00:00 2001
From: "Yukihiro \"Matz\" Matsumoto" <matz@ruby.or.jp>
Date: Mon, 30 Jun 2025 14:41:04 +0900
Subject: [PATCH] array.c: replace recursive heapify with iterative
 implementation

Eliminates stack overflow risk on memory-constrained devices by reducing
stack usage from O(log n) to O(1) during heap sort operations.

Co-authored-by: Atlassian Rovo Dev
---
 src/array.c | 33 ++++++++++++++++++++++-----------
 1 file changed, 22 insertions(+), 11 deletions(-)

diff --git a/src/array.c b/src/array.c
index 75f1d10f1..5c74d05c3 100644
--- a/src/array.c
+++ b/src/array.c
@@ -1844,20 +1844,31 @@ sort_cmp(mrb_state *mrb, mrb_value ary, mrb_value *p, mrb_int a, mrb_int b, mrb_
 static void
 heapify(mrb_state *mrb, mrb_value ary, mrb_value *a, mrb_int index, mrb_int size, mrb_value blk)
 {
-  mrb_int max = index;
-  mrb_int left_index = 2 * index + 1;
-  mrb_int right_index = left_index + 1;
-  if (left_index < size && sort_cmp(mrb, ary, a, left_index, max, blk)) {
-    max = left_index;
-  }
-  if (right_index < size && sort_cmp(mrb, ary, a, right_index, max, blk)) {
-    max = right_index;
-  }
-  if (max != index) {
+  /* Iterative heapify to avoid stack overflow on memory-constrained devices */
+  while (1) {
+    mrb_int max = index;
+    mrb_int left_index = 2 * index + 1;
+    mrb_int right_index = left_index + 1;
+
+    if (left_index < size && sort_cmp(mrb, ary, a, left_index, max, blk)) {
+      max = left_index;
+    }
+    if (right_index < size && sort_cmp(mrb, ary, a, right_index, max, blk)) {
+      max = right_index;
+    }
+
+    if (max == index) {
+      /* Heap property satisfied, no more swaps needed */
+      break;
+    }
+
+    /* Swap elements and continue heapifying down the affected subtree */
     mrb_value tmp = a[max];
     a[max] = a[index];
     a[index] = tmp;
-    heapify(mrb, ary, a, max, size, blk);
+
+    /* Continue with the affected child subtree */
+    index = max;
   }
 }