From 2e80d5e38ed92f67767d87b544894cdc254f9459 Mon Sep 17 00:00:00 2001
From: SPRESENSE <41312067+SPRESENSE@users.noreply.github.com>
Date: Sat, 10 Jul 2021 21:14:17 +0900
Subject: [PATCH] libs/libc/math: Fix calculation error of log function

Fix a problem where the log function would loop infinitely by
calculation error when a specific value was passed. For example,
in the following case, the log function loops infinitely and
never returns. So, this commit fixes to return the right value.

  double a = 7883961.5;
  double b = log(a);
---
 libs/libc/math/lib_log.c  | 6 +++---
 libs/libc/math/lib_logf.c | 6 +++---
 2 files changed, 6 insertions(+), 6 deletions(-)

diff --git a/libs/libc/math/lib_log.c b/libs/libc/math/lib_log.c
index ba7aae3e7c..638ed62b0a 100644
--- a/libs/libc/math/lib_log.c
+++ b/libs/libc/math/lib_log.c
@@ -60,7 +60,7 @@ double log(double x)
 {
   double y;
   double y_old;
-  double ney;
+  double ey;
   double epsilon;
   int    relax_factor;
   int    iter;
@@ -75,8 +75,8 @@ double log(double x)
   while (y > y_old + epsilon || y < y_old - epsilon)
     {
       y_old = y;
-      ney   = exp(-y);
-      y    -= 1.0 - x * ney;
+      ey    = exp(y);
+      y    -= (ey - x) / ey;
 
       if (y > 700.0)
         {
diff --git a/libs/libc/math/lib_logf.c b/libs/libc/math/lib_logf.c
index e813568bf3..b53805da84 100644
--- a/libs/libc/math/lib_logf.c
+++ b/libs/libc/math/lib_logf.c
@@ -58,7 +58,7 @@ float logf(float x)
 {
   float y;
   float y_old;
-  float ney;
+  float ey;
   float epsilon;
   int   relax_factor;
   int   iter;
@@ -73,8 +73,8 @@ float logf(float x)
   while (y > y_old + epsilon || y < y_old - epsilon)
     {
       y_old = y;
-      ney   = expf(-y);
-      y    -= 1.0F - x * ney;
+      ey    = expf(y);
+      y    -= (ey - x) / ey;
 
       if (y > FLT_MAX_EXP_X)
         {