From b2cb305b1b98c02ce476fecea553e01109a81637 Mon Sep 17 00:00:00 2001
From: Antoine Pitrou <antoine@python.org>
Date: Mon, 9 Dec 2024 09:35:41 +0100
Subject: [PATCH] Add explanatory comment for kOneUlp

---
 cpp/src/arrow/testing/math.cc | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/cpp/src/arrow/testing/math.cc b/cpp/src/arrow/testing/math.cc
index 2f81207951b6f..05d08c2c52a48 100644
--- a/cpp/src/arrow/testing/math.cc
+++ b/cpp/src/arrow/testing/math.cc
@@ -27,6 +27,9 @@
 namespace arrow {
 namespace {
 
+// `kOneUlp` is equal to 1.0 - (1.0 - nextafter(1.0, -inf)).
+// That is, it is the delta between 1.0 and the FP value immediately before 1.0.
+// We're using this value because `frexp` returns a mantissa between 0.5 and 1.0.
 template <typename Float>
 constexpr Float kOneUlp;
 template <>