From 5d221fa17630caf8ab4f7237314ba84e998bf793 Mon Sep 17 00:00:00 2001
From: Samuel Williams <samuel.williams@shopify.com>
Date: Mon, 15 Jun 2026 09:24:14 +1200
Subject: [PATCH] Resize coverage count arrays in one step

---
 ext/ruby/coverage/tracer.c   |  4 ++--
 test/ruby/coverage/tracer.rb | 17 +++++++++++++++++
 2 files changed, 19 insertions(+), 2 deletions(-)

diff --git a/ext/ruby/coverage/tracer.c b/ext/ruby/coverage/tracer.c
index 46a29ab..ad39177 100644
--- a/ext/ruby/coverage/tracer.c
+++ b/ext/ruby/coverage/tracer.c
@@ -273,8 +273,8 @@ static void Ruby_Coverage_Tracer_on_line(rb_event_flag_t event, VALUE data, VALU
 
 	// Counts are 1-indexed: index 0 is unused (nil), index N is the hit count
 	// for source line N. Grow the array if necessary.
-	while (RARRAY_LEN(tracer->last_counts) <= line) {
-		rb_ary_push(tracer->last_counts, Qnil);
+	if (RARRAY_LEN(tracer->last_counts) <= line) {
+		rb_ary_resize(tracer->last_counts, line + 1);
 	}
 
 	VALUE current = rb_ary_entry(tracer->last_counts, line);
diff --git a/test/ruby/coverage/tracer.rb b/test/ruby/coverage/tracer.rb
index b7228ad..fb4ed72 100644
--- a/test/ruby/coverage/tracer.rb
+++ b/test/ruby/coverage/tracer.rb
@@ -112,6 +112,23 @@ def around
 		expect(counts[1]).to be == 2
 	end
 	
+	it "resizes count arrays for sparse high line numbers" do
+		files = {}
+		tracer = Ruby::Coverage::Tracer.new{|path, iseq| files[path] ||= []}
+		
+		tracer.start
+		
+		path = "/tmp/ruby_coverage_tracer_sparse.rb"
+		line = 100
+		Module.new.module_eval("x = 1", path, line)
+		
+		tracer.stop
+		
+		counts = files[path]
+		expect(counts).not.to be_nil
+		expect(counts[line]).to be == 1
+	end
+	
 	it "returns nil from the callback to skip tracking a file" do
 		files = {}
 		tracer = Ruby::Coverage::Tracer.new{|path, iseq| nil}