From c8c735009615a30940c0378eab8f2936b33445c3 Mon Sep 17 00:00:00 2001
From: Aleksandr Kazachkov <akazachk@users.noreply.github.com>
Date: Mon, 26 Jul 2021 18:32:09 -0400
Subject: [PATCH] Added fix_vars to src/model/formulations/Gar1962/status.jl

---
 src/model/formulations/Gar1962/status.jl | 98 ++++++++++++++++++++++--
 1 file changed, 91 insertions(+), 7 deletions(-)

diff --git a/src/model/formulations/Gar1962/status.jl b/src/model/formulations/Gar1962/status.jl
index 14c055f..4678bb0 100644
--- a/src/model/formulations/Gar1962/status.jl
+++ b/src/model/formulations/Gar1962/status.jl
@@ -2,6 +2,12 @@
 # Copyright (C) 2020, UChicago Argonne, LLC. All rights reserved.
 # Released under the modified BSD license. See COPYING.md for more details.
 
+"""
+    _add_status_vars!
+
+Adds symbols identified by `Gar1962.StatusVars` to `model`.
+Fix variables if a certain generator _must_ run or based on initial conditions.
+"""
 function _add_status_vars!(
     model::JuMP.Model,
     g::Unit,
@@ -10,15 +16,93 @@ function _add_status_vars!(
     is_on = _init(model, :is_on)
     switch_on = _init(model, :switch_on)
     switch_off = _init(model, :switch_off)
+    FIX_VARS = !formulation_status_vars.fix_vars_via_constraint
+    is_initially_on = _is_initially_on(g) > 0
     for t in 1:model[:instance].time
-        if g.must_run[t]
-            is_on[g.name, t] = 1.0
-            switch_on[g.name, t] = (t == 1 ? 1.0 - _is_initially_on(g) : 0.0)
-            switch_off[g.name, t] = 0.0
+        is_on[g.name, t] = @variable(model, binary = true)
+        switch_on[g.name, t] = @variable(model, binary = true)
+        switch_off[g.name, t] = @variable(model, binary = true)
+
+        # Use initial conditions and whether a unit must run to fix variables
+        if FIX_VARS
+            # Fix variables using fix function
+            if g.must_run[t]
+                # If the generator _must_ run, then it is obviously on and cannot be switched off
+                # In the first time period, force unit to switch on if was off before
+                # Otherwise, unit is on, and will never turn off, so will never need to turn on
+                fix(is_on[g.name, t], 1.0; force = true)
+                fix(
+                    switch_on[g.name, t],
+                    (t == 1 ? 1.0 - _is_initially_on(g) : 0.0);
+                    force = true,
+                )
+                fix(switch_off[g.name, t], 0.0; force = true)
+            elseif t == 1
+                if is_initially_on
+                    # Generator was on (for g.initial_status time periods),
+                    # so cannot be more switched on until the period after the first time it can be turned off
+                    fix(switch_on[g.name, 1], 0.0; force = true)
+                else
+                    # Generator is initially off (for -g.initial_status time periods)
+                    # Cannot be switched off more
+                    fix(switch_off[g.name, 1], 0.0; force = true)
+                end
+            end
         else
-            is_on[g.name, t] = @variable(model, binary = true)
-            switch_on[g.name, t] = @variable(model, binary = true)
-            switch_off[g.name, t] = @variable(model, binary = true)
+            # Add explicit constraint if !FIX_VARS
+            if g.must_run[t]
+                is_on[g.name, t] = 1.0
+                switch_on[g.name, t] =
+                    (t == 1 ? 1.0 - _is_initially_on(g) : 0.0)
+                switch_off[g.name, t] = 0.0
+            elseif t == 1
+                if is_initially_on
+                    switch_on[g.name, t] = 0.0
+                else
+                    switch_off[g.name, t] = 0.0
+                end
+            end
+        end
+
+        # Use initial conditions and whether a unit must run to fix variables
+        if FIX_VARS
+            # Fix variables using fix function
+            if g.must_run[t]
+                # If the generator _must_ run, then it is obviously on and cannot be switched off
+                # In the first time period, force unit to switch on if was off before
+                # Otherwise, unit is on, and will never turn off, so will never need to turn on
+                fix(is_on[g.name, t], 1.0; force = true)
+                fix(
+                    switch_on[g.name, t],
+                    (t == 1 ? 1.0 - _is_initially_on(g) : 0.0);
+                    force = true,
+                )
+                fix(switch_off[g.name, t], 0.0; force = true)
+            elseif t == 1
+                if is_initially_on
+                    # Generator was on (for g.initial_status time periods),
+                    # so cannot be more switched on until the period after the first time it can be turned off
+                    fix(switch_on[g.name, 1], 0.0; force = true)
+                else
+                    # Generator is initially off (for -g.initial_status time periods)
+                    # Cannot be switched off more
+                    fix(switch_off[g.name, 1], 0.0; force = true)
+                end
+            end
+        else
+            # Add explicit constraint if !FIX_VARS
+            if g.must_run[t]
+                is_on[g.name, t] = 1.0
+                switch_on[g.name, t] =
+                    (t == 1 ? 1.0 - _is_initially_on(g) : 0.0)
+                switch_off[g.name, t] = 0.0
+            elseif t == 1
+                if is_initially_on
+                    switch_on[g.name, t] = 0.0
+                else
+                    switch_off[g.name, t] = 0.0
+                end
+            end
         end
     end
     return