diff --git a/include/misc/optim_options.hpp b/include/misc/optim_options.hpp
index 50b4bf8..7f1fe62 100644
--- a/include/misc/optim_options.hpp
+++ b/include/misc/optim_options.hpp
@@ -41,12 +41,6 @@
 
 //
 
-#ifdef _MSC_VER
-    #error OptimLib: MSVC is not supported
-#endif
-
-//
-
 #if defined(_OPENMP) && !defined(OPTIM_DONT_USE_OPENMP)
     #undef OPTIM_USE_OPENMP
     #define OPTIM_USE_OPENMP
@@ -76,15 +70,20 @@
 #endif
 
 // floating point number type
-
+#ifndef FLOAT_TYPE_KEY
+    #define FLOAT_TYPE_KEY 0
+#endif
+#ifndef DOUBLE_TYPE_KEY
+    #define DOUBLE_TYPE_KEY 1
+#endif
 #ifndef OPTIM_FPN_TYPE
-    #define OPTIM_FPN_TYPE double
+    #define OPTIM_FPN_TYPE DOUBLE_TYPE_KEY
 #endif
 
-#if OPTIM_FPN_TYPE == float
+#if OPTIM_FPN_TYPE == FLOAT_TYPE_KEY
     #undef OPTIM_FPN_SMALL_NUMBER
     #define OPTIM_FPN_SMALL_NUMBER fp_t(1e-05)
-#elif OPTIM_FPN_TYPE == double
+#elif OPTIM_FPN_TYPE == DOUBLE_TYPE_KEY
     #undef OPTIM_FPN_SMALL_NUMBER
     #define OPTIM_FPN_SMALL_NUMBER fp_t(1e-08)
 #else
@@ -96,12 +95,22 @@
 namespace optim
 {
     using uint_t = unsigned int;
-    using fp_t = OPTIM_FPN_TYPE;
+#if OPTIM_FPN_TYPE == FLOAT_TYPE_KEY
+    using fp_t = float;
+#else
+    using fp_t = double;
+#endif
 
     using rand_engine_t = std::mt19937_64;
 
     static const double eps_dbl = std::numeric_limits<fp_t>::epsilon();
     static const double inf = std::numeric_limits<fp_t>::infinity();
+
+#ifdef _MSC_VER
+    using ompint_t = int64_t;
+#else
+    using ompint_t = size_t;
+#endif
 }
 
 //
diff --git a/src/unconstrained/de.cpp b/src/unconstrained/de.cpp
index 083e6b4..db57541 100644
--- a/src/unconstrained/de.cpp
+++ b/src/unconstrained/de.cpp
@@ -117,7 +117,7 @@ optim::internal::de_impl(
 #ifdef OPTIM_USE_OPENMP
     #pragma omp parallel for num_threads(omp_n_threads) firstprivate(rand_vec)
 #endif
-    for (size_t i = 0; i < n_pop; ++i) {
+    for (ompint_t i = 0; i < n_pop; ++i) {
         size_t thread_num = 0;
 
 #ifdef OPTIM_USE_OPENMP
@@ -164,7 +164,7 @@ optim::internal::de_impl(
 #ifdef OPTIM_USE_OPENMP
         #pragma omp parallel for num_threads(omp_n_threads) firstprivate(rand_vec)
 #endif
-        for (size_t i = 0; i < n_pop; ++i) {
+    for (ompint_t i = 0; i < n_pop; ++i) {
             size_t thread_num = 0;
 
 #ifdef OPTIM_USE_OPENMP
@@ -258,7 +258,7 @@ optim::internal::de_impl(
 #ifdef OPTIM_USE_OPENMP
         #pragma omp parallel for num_threads(omp_n_threads)
 #endif
-            for (size_t i = 0; i < n_pop; ++i) {
+            for (ompint_t i = 0; i < n_pop; ++i) {
                 X_next.row(i) = inv_transform<RowVec_t>(X_next.row(i), bounds_type, lower_bounds, upper_bounds);
             }
         }
diff --git a/src/unconstrained/de_prmm.cpp b/src/unconstrained/de_prmm.cpp
index 70e3457..2c040ec 100644
--- a/src/unconstrained/de_prmm.cpp
+++ b/src/unconstrained/de_prmm.cpp
@@ -134,7 +134,7 @@ optim::internal::de_prmm_impl(
 #ifdef OPTIM_USE_OPENMP
     #pragma omp parallel for num_threads(omp_n_threads) firstprivate(rand_vec)
 #endif
-    for (size_t i = 0; i < n_pop; ++i) {
+    for (ompint_t i = 0; i < n_pop; ++i) {
         size_t thread_num = 0;
 
 #ifdef OPTIM_USE_OPENMP
@@ -195,7 +195,7 @@ optim::internal::de_prmm_impl(
 #ifdef OPTIM_USE_OPENMP
             #pragma omp parallel for num_threads(omp_n_threads)
 #endif
-            for (size_t j = 0; j < n_pop_temp; ++j) {
+            for (ompint_t j = 0; j < n_pop_temp; ++j) {
                 if (objfn_vals(j) < objfn_vals(j + n_pop_temp)) {
                     X_reset.row(j) = X_next.row(j);
                     objfn_vals_reset(j) = objfn_vals(j);
@@ -223,7 +223,7 @@ optim::internal::de_prmm_impl(
 #ifdef OPTIM_USE_OPENMP
         #pragma omp parallel for num_threads(omp_n_threads) firstprivate(rand_vec,rand_pars)
 #endif
-        for (size_t i = 0; i < n_pop - n_pop_best; ++i) {
+        for (ompint_t i = 0; i < n_pop - n_pop_best; ++i) {
             size_t thread_num = 0;
 
 #ifdef OPTIM_USE_OPENMP
@@ -306,7 +306,7 @@ optim::internal::de_prmm_impl(
 #ifdef OPTIM_USE_OPENMP
         #pragma omp parallel for num_threads(omp_n_threads) firstprivate(rand_vec,rand_pars)
 #endif
-        for (size_t i = n_pop - n_pop_best; i < n_pop; ++i) {
+        for (ompint_t i = n_pop - n_pop_best; i < n_pop; ++i) {
             size_t thread_num = 0;
 
 #ifdef OPTIM_USE_OPENMP
@@ -417,7 +417,7 @@ optim::internal::de_prmm_impl(
 #ifdef OPTIM_USE_OPENMP
         #pragma omp parallel for num_threads(omp_n_threads)
 #endif
-            for (size_t i = 0; i < n_pop; ++i) {
+            for (ompint_t i = 0; i < n_pop; ++i) {
                 X_next.row(i) = inv_transform<RowVec_t>(X_next.row(i), bounds_type, lower_bounds, upper_bounds);
             }
         }
diff --git a/src/unconstrained/nm.cpp b/src/unconstrained/nm.cpp
index 00a22c6..46fffe6 100644
--- a/src/unconstrained/nm.cpp
+++ b/src/unconstrained/nm.cpp
@@ -251,7 +251,7 @@ optim::internal::nm_impl(
 #ifdef OPTIM_USE_OPENMP
             #pragma omp parallel for num_threads(omp_n_threads)
 #endif
-            for (size_t i = 1; i < n_vals + 1; i++) {
+            for (ompint_t i = 1; i < n_vals + 1; i++) {
                 simplex_fn_vals(i) = box_objfn( BMO_MATOPS_TRANSPOSE(simplex_points.row(i)), nullptr, opt_data);
             }
         }
diff --git a/src/unconstrained/pso.cpp b/src/unconstrained/pso.cpp
index 5361f03..57121c6 100644
--- a/src/unconstrained/pso.cpp
+++ b/src/unconstrained/pso.cpp
@@ -133,7 +133,7 @@ optim::internal::pso_impl(
 #ifdef OPTIM_USE_OPENMP
     #pragma omp parallel for num_threads(omp_n_threads) firstprivate(rand_vec)
 #endif
-    for (size_t i = 0; i < n_pop; ++i) {
+    for (ompint_t i = 0; i < n_pop; ++i) {
         size_t thread_num = 0;
 
 #ifdef OPTIM_USE_OPENMP
@@ -202,7 +202,7 @@ optim::internal::pso_impl(
 #ifdef OPTIM_USE_OPENMP
         #pragma omp parallel for num_threads(omp_n_threads) firstprivate(rand_vec_1,rand_vec_2)
 #endif
-        for (size_t i=0; i < n_pop; ++i) {
+        for (ompint_t i=0; i < n_pop; ++i) {
             size_t thread_num = 0;
 
 #ifdef OPTIM_USE_OPENMP
@@ -270,7 +270,7 @@ optim::internal::pso_impl(
 #ifdef OPTIM_USE_OPENMP
         #pragma omp parallel for num_threads(omp_n_threads)
 #endif
-            for (size_t i = 0; i < n_pop; ++i) {
+            for (ompint_t i = 0; i < n_pop; ++i) {
                 P.row(i) = inv_transform<RowVec_t>(P.row(i), bounds_type, lower_bounds, upper_bounds);
             }
         }
diff --git a/src/unconstrained/pso_dv.cpp b/src/unconstrained/pso_dv.cpp
index 9b44239..7ef0b79 100644
--- a/src/unconstrained/pso_dv.cpp
+++ b/src/unconstrained/pso_dv.cpp
@@ -125,7 +125,7 @@ optim::internal::pso_dv_impl(
 #ifdef OPTIM_USE_OPENMP
     #pragma omp parallel for num_threads(omp_n_threads) firstprivate(rand_vec)
 #endif
-    for (size_t i = 0; i < n_pop; ++i) {
+    for (ompint_t i = 0; i < n_pop; ++i) {
         size_t thread_num = 0;
 
 #ifdef OPTIM_USE_OPENMP
@@ -178,7 +178,7 @@ optim::internal::pso_dv_impl(
 #ifdef OPTIM_USE_OPENMP
         #pragma omp parallel for num_threads(omp_n_threads) firstprivate(rand_vec,rand_CR)
 #endif
-        for (size_t i = 0; i < n_pop; ++i) {
+        for (ompint_t i = 0; i < n_pop; ++i) {
             size_t thread_num = 0;
 
 #ifdef OPTIM_USE_OPENMP