From 9b3ac793bca14dc5efd424ba28324937b7bed40d Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=E3=83=95=E3=82=A3=E3=83=AB=E3=82=BF=E3=83=BC=E3=83=9A?=
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 <76888457+filterpaper@users.noreply.github.com>
Date: Fri, 7 Jul 2023 22:50:21 +0800
Subject: Refactor times inverse of sqrt 2 calculation (#21293)

---
 quantum/mousekey.c | 15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)

(limited to 'quantum/mousekey.c')

diff --git a/quantum/mousekey.c b/quantum/mousekey.c
index aaaf7fff22..c982a2f40b 100644
--- a/quantum/mousekey.c
+++ b/quantum/mousekey.c
@@ -25,11 +25,16 @@
 #include "mousekey.h"
 
 static inline int8_t times_inv_sqrt2(int8_t x) {
-    // 181/256 is pretty close to 1/sqrt(2)
-    // 0.70703125                 0.707106781
-    // 1 too small for x=99 and x=198
-    // This ends up being a mult and discard lower 8 bits
-    return (x * 181) >> 8;
+    // 181/256 (0.70703125) is used as an approximation for 1/sqrt(2)
+    // because it is close to the exact value which is 0.707106781
+    const int16_t  n = x * 181;
+    const uint16_t d = 256;
+
+    // To ensure that the integer result is rounded accurately after
+    // division, check the sign of the numerator:
+    // If negative, subtract half of the denominator before dividing
+    // Otherwise, add half of the denominator before dividing
+    return n < 0 ? (n - d / 2) / d : (n + d / 2) / d;
 }
 
 static report_mouse_t mouse_report = {0};
-- 
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