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

---
 Utilib/src/steam_pT.c | 94 +++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 94 insertions(+)
 create mode 100644 Utilib/src/steam_pT.c

(limited to 'Utilib/src/steam_pT.c')

diff --git a/Utilib/src/steam_pT.c b/Utilib/src/steam_pT.c
new file mode 100644
index 0000000..3aa16b2
--- /dev/null
+++ b/Utilib/src/steam_pT.c
@@ -0,0 +1,94 @@
+/*
+freesteam - IAPWS-IF97 steam tables library
+Copyright (C) 2004-2009  John Pye
+
+This program is free software; you can redistribute it and/or
+modify it under the terms of the GNU General Public License
+as published by the Free Software Foundation; either version 2
+of the License, or (at your option) any later version.
+
+This program is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with this program; if not, write to the Free Software
+Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA  02111-1307, USA.
+*/
+#define FREESTEAM_BUILDING_LIB
+#include "steam_pT.h"
+#include "region.h"
+#include "zeroin.h"
+#include "b23.h"
+
+#include <stdlib.h>
+#include <assert.h>
+#include <math.h>
+
+typedef struct{
+	double p, T;
+} SteamPTData;
+
+static double pT_region3_fn(double rho, void *user_data){
+#define D ((SteamPTData *)user_data)
+	return D->p - freesteam_region3_p_rhoT(rho, D->T);
+#undef D
+}
+
+/**
+	This function will never return region 4, because it's not possible
+	to 'sit on the knife' of saturation. If you need to set saturated states,
+	you should use another function such as freesteam_region1_set_Tx.
+*/
+SteamState freesteam_set_pT(double p, double T){
+        double tol, sol, err;
+	SteamState S;
+	if(T < REGION1_TMAX){
+		if(p > freesteam_region4_psat_T(T)){
+			S.region = 1;
+			S.valueR.R1.T = T;
+			S.valueR.R1.p = p;
+		}else{
+			S.region = 2;
+			S.valueR.R2.T = T;
+			S.valueR.R2.p = p;
+		}
+	}else{
+		/* FIXME some optimisation possiblxe here with test for lower pressures */
+		double T23 = freesteam_b23_T_p(p);
+		double p23min = freesteam_b23_p_T(REGION1_TMAX);
+		if(p < p23min || T > T23){
+			S.region = 2;
+			S.valueR.R2.T = T;
+			S.valueR.R2.p = p;
+		}else{
+			/* FIXME the limit values are all wrong here! */
+                        double ub, lb;
+			SteamPTData D = {p,T};
+			ub = 1./freesteam_region1_v_pT(IAPWS97_PMAX,REGION1_TMAX);
+			lb = 1./freesteam_region2_v_pT(freesteam_b23_p_T(T),T);
+			/* if we're in the little wee area around the critical pt... */
+			if(T < IAPWS97_TCRIT){
+				double psat = freesteam_region4_psat_T(T);
+				if(p < psat){
+					ub = freesteam_region4_rhog_T(T);
+					assert(lb<ub);
+				}else{
+					lb = freesteam_region4_rhof_T(T);
+					assert(lb<ub);
+				}
+			}
+			tol = 1e-7;
+			err = 0;
+			if(zeroin_solve(&pT_region3_fn, &D, lb, ub, tol, &sol, &err)){
+				fprintf(stderr,"%s (%s:%d): failed to solve for rho\n",__func__,__FILE__,__LINE__);
+				exit(1);
+			}
+			S.region = 3;
+			S.valueR.R3.T = T;
+			S.valueR.R3.rho = sol;
+		}
+	}
+	return S;
+}		
-- 
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