From 93a76d8e216014905bf4944dcd504cfe7c7abbaa Mon Sep 17 00:00:00 2001
From: Holger Vogt <holger.vogt@uni-due.de>
Date: Mon, 13 Apr 2020 11:05:03 +0200
Subject: [PATCH] add a limit exp(arg) to the exp function. If arg is larger
 than 14 (arbitrarily selected), continue with linear response.

---
 src/include/ngspice/inpptree.h |  4 ++++
 src/spicelib/parser/inpptree.c | 14 ++++++++++++--
 src/spicelib/parser/ptfuncs.c  |  6 +++++-
 3 files changed, 21 insertions(+), 3 deletions(-)

diff --git a/src/include/ngspice/inpptree.h b/src/include/ngspice/inpptree.h
index 2aaffbb46..81686a492 100644
--- a/src/include/ngspice/inpptree.h
+++ b/src/include/ngspice/inpptree.h
@@ -166,6 +166,10 @@ typedef struct PTelement {
 
 #define PT_STACKSIZE 200
 
+/* limits for exp function */
+#define EXPARGMAX 14.
+#define EXPMAX 1202604.284
+
 /* And in IFeval.c */
 
 extern int IFeval(IFparseTree *tree, double gmin, double *result, double *vals, double *derivs);
diff --git a/src/spicelib/parser/inpptree.c b/src/spicelib/parser/inpptree.c
index 92e1d8766..0d8bef123 100644
--- a/src/spicelib/parser/inpptree.c
+++ b/src/spicelib/parser/inpptree.c
@@ -434,8 +434,18 @@ static INPparseNode *PTdifferentiate(INPparseNode * p, int varnum)
             arg1 = mkf(PTF_SINH, p->left);
             break;
 
-        case PTF_EXP:                /* exp(u) */
-            arg1 = mkf(PTF_EXP, p->left);
+        case PTF_EXP:                /* u > EXPARGMAX -> EXPMAX, that is exp(EXPARGMAX), else exp(u) */
+            arg1 = mkb(PT_TERN,
+                mkf(PTF_GT0, mkb(PT_MINUS, p->left, mkcon(EXPARGMAX))),
+                mkb(PT_COMMA,
+                    mkcon(EXPMAX),
+                    mkf(PTF_EXP, p->left)));
+
+#ifdef TRACE
+            printf("debug, %s, returns; ", __func__);
+            printTree(arg1);
+            printf("\n");
+#endif
             break;
 
         case PTF_LOG:               /* 1 / u */
diff --git a/src/spicelib/parser/ptfuncs.c b/src/spicelib/parser/ptfuncs.c
index d3f945756..9e0011057 100644
--- a/src/spicelib/parser/ptfuncs.c
+++ b/src/spicelib/parser/ptfuncs.c
@@ -217,10 +217,14 @@ PTcosh(double arg)
     return (cosh(arg));
 }
 
+/* Limit the exp: If arg > EXPARGMAX (arbitrarily selected to 14), continue with linear output */
 double
 PTexp(double arg)
 {
-    return (exp(arg));
+    if (arg > EXPARGMAX)
+        return EXPMAX * (arg - EXPARGMAX + 1.);
+    else
+        return (exp(arg));
 }
 
 double