From 7dfcc480ba1e19bd3232349fc733caef94034292 Mon Sep 17 00:00:00 2001
From: stainer_t <thomas.stainer@oecd-nea.org>
Date: Mon, 8 Sep 2025 13:48:49 +0200
Subject: Initial commit from Polytechnique Montreal

---
 Dragon/src/LIBLAG.f | 107 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 107 insertions(+)
 create mode 100644 Dragon/src/LIBLAG.f

(limited to 'Dragon/src/LIBLAG.f')

diff --git a/Dragon/src/LIBLAG.f b/Dragon/src/LIBLAG.f
new file mode 100644
index 0000000..9aadef3
--- /dev/null
+++ b/Dragon/src/LIBLAG.f
@@ -0,0 +1,107 @@
+*DECK LIBLAG
+      SUBROUTINE LIBLAG(NEF,XE,GE,XI,GS)
+*
+*-----------------------------------------------------------------------
+*
+*Purpose:
+* Lagrange interpolation in a table of points using the APOLLO2 recipe.
+*
+*Copyright:
+* Copyright (C) 2002 Ecole Polytechnique de Montreal
+* This library is free software; you can redistribute it and/or
+* modify it under the terms of the GNU Lesser General Public
+* License as published by the Free Software Foundation; either
+* version 2.1 of the License, or (at your option) any later version
+*
+*Author(s): A. Hebert
+*
+*Parameters: input
+* NEF     number of points.
+* XE      x-values.
+* GE      f(x)-values.
+* XI      interpolating x-value.
+*
+*Parameters: output
+* GS      interpolated value f(XI).
+*
+*-----------------------------------------------------------------------
+*
+*----
+*  SUBROUTINE ARGUMENTS
+*----
+      INTEGER NEF
+      REAL XE(NEF),GE(NEF),XI,GS
+*----
+*  LOCAL VARIABLES
+*----
+      PARAMETER(NINT=3)
+      REAL WEIGHT(NINT)
+*----
+*  CONSTANT FUNCTION DEFINED BY A SINGLE POINT
+*----
+      IF(NEF.EQ.1) THEN
+         GS=GE(1)
+         RETURN
+      ENDIF
+*
+      IORD=MIN(NINT,NEF)
+*----
+*  LOCATE FIRST POINT TO THE RIGHT OF XI (PT IXP)
+*----
+      DO 30 IXP=1,NEF
+       IF(ABS(XE(IXP)-XI).LE.1.0E-5*ABS(XI)) THEN
+          GS=GE(IXP)
+          RETURN
+       ELSE IF(XE(IXP).GT.XI) THEN
+          IMIN=IXP-(IORD+1)/2
+          IMAX=IMIN+IORD-1
+          GO TO 40
+       ENDIF
+   30 CONTINUE
+*----
+*  XI IS TO THE RIGHT OF EVERY POINT XE
+*----
+      IMAX=NEF
+      IMIN=NEF-IORD+1
+      GO TO 70
+*
+   40 IF(IMIN.LT.1) THEN
+         IMIN=1
+         IMAX=IORD
+      ELSE IF(IMAX.GT.NEF) THEN
+         IMIN=NEF-IORD+1
+         IMAX=NEF
+      ENDIF
+*
+   70 I0=IMIN-1
+      DO 90 I=IMIN,IMAX
+      PP=1.0
+      DO 80 J=IMIN,IMAX
+       IF(I.NE.J) PP=PP*((XI-XE(J))/(XE(I)-XE(J)))
+   80 CONTINUE
+      WEIGHT(I+1-IMIN)=PP
+   90 CONTINUE
+*
+      GS=0.0
+      DO 110 I=1,IORD
+      GS=GS+WEIGHT(I)*GE(I+I0)
+  110 CONTINUE
+      DO 120 I=1,IORD
+      I1=I+I0
+      IF(XE(I1).GT.XI) THEN
+         IF(I1-1.GT.0) THEN
+            YMIN=MIN(GE(I1-1),GE(I1))
+            YMAX=MAX(GE(I1-1),GE(I1))
+            IF((GS.GT.YMAX).OR.(GS.LT.YMIN)) THEN
+               GS=GE(I1-1)+(GE(I1)-GE(I1-1))*
+     1         (XI-XE(I1-1))/(XE(I1)-XE(I1-1))
+            ENDIF
+         ELSE
+            GS=GE(1)+(GE(2)-GE(1))*(XI-XE(1))/(XE(2)-XE(1))
+            IF(GS.LE.0.) GS=GE(1)
+         ENDIF
+         RETURN
+      ENDIF
+  120 CONTINUE
+      RETURN
+      END
-- 
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