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diff --git a/src/armadillo/include/armadillo_bits/fn_interp2.hpp b/src/armadillo/include/armadillo_bits/fn_interp2.hpp
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+// SPDX-License-Identifier: Apache-2.0
+//
+// Copyright 2008-2016 Conrad Sanderson (http://conradsanderson.id.au)
+// Copyright 2008-2016 National ICT Australia (NICTA)
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+// ------------------------------------------------------------------------
+
+
+//! \addtogroup fn_interp2
+//! @{
+
+
+
+template<typename eT>
+inline
+void
+interp2_helper_nearest(const Mat<eT>& XG, const Mat<eT>& ZG, const Mat<eT>& XI, Mat<eT>& ZI, const eT extrap_val, const uword mode)
+ {
+ arma_extra_debug_sigprint();
+
+ const eT XG_min = XG.min();
+ const eT XG_max = XG.max();
+
+ // mode = 0: interpolate across rows (eg. expand in vertical direction)
+ // mode = 1: interpolate across columns (eg. expand in horizontal direction)
+
+ if(mode == 0) { ZI.set_size(XI.n_elem, ZG.n_cols); }
+ if(mode == 1) { ZI.set_size(ZG.n_rows, XI.n_elem); }
+
+ const eT* XG_mem = XG.memptr();
+ const eT* XI_mem = XI.memptr();
+
+ const uword NG = XG.n_elem;
+ const uword NI = XI.n_elem;
+
+ uword best_j = 0;
+
+ for(uword i=0; i<NI; ++i)
+ {
+ eT best_err = Datum<eT>::inf;
+
+ const eT XI_val = XI_mem[i];
+
+ if((XI_val < XG_min) || (XI_val > XG_max))
+ {
+ if(mode == 0) { ZI.row(i).fill(extrap_val); }
+ if(mode == 1) { ZI.col(i).fill(extrap_val); }
+ }
+ else
+ {
+ // XG and XI are guaranteed to be sorted in ascending manner,
+ // so start searching XG from last known optimum position
+
+ for(uword j=best_j; j<NG; ++j)
+ {
+ const eT tmp = XG_mem[j] - XI_val;
+ const eT err = (tmp >= eT(0)) ? tmp : -tmp;
+
+ if(err >= best_err)
+ {
+ // error is going up, so we have found the optimum position
+ break;
+ }
+ else
+ {
+ best_err = err;
+ best_j = j; // remember the optimum position
+ }
+ }
+
+ if(mode == 0) { ZI.row(i) = ZG.row(best_j); }
+ if(mode == 1) { ZI.col(i) = ZG.col(best_j); }
+ }
+ }
+ }
+
+
+
+template<typename eT>
+inline
+void
+interp2_helper_linear(const Mat<eT>& XG, const Mat<eT>& ZG, const Mat<eT>& XI, Mat<eT>& ZI, const eT extrap_val, const uword mode)
+ {
+ arma_extra_debug_sigprint();
+
+ const eT XG_min = XG.min();
+ const eT XG_max = XG.max();
+
+ // mode = 0: interpolate across rows (eg. expand in vertical direction)
+ // mode = 1: interpolate across columns (eg. expand in horizontal direction)
+
+ if(mode == 0) { ZI.set_size(XI.n_elem, ZG.n_cols); }
+ if(mode == 1) { ZI.set_size(ZG.n_rows, XI.n_elem); }
+
+ const eT* XG_mem = XG.memptr();
+ const eT* XI_mem = XI.memptr();
+
+ const uword NG = XG.n_elem;
+ const uword NI = XI.n_elem;
+
+ uword a_best_j = 0;
+ uword b_best_j = 0;
+
+ for(uword i=0; i<NI; ++i)
+ {
+ const eT XI_val = XI_mem[i];
+
+ if((XI_val < XG_min) || (XI_val > XG_max))
+ {
+ if(mode == 0) { ZI.row(i).fill(extrap_val); }
+ if(mode == 1) { ZI.col(i).fill(extrap_val); }
+ }
+ else
+ {
+ // XG and XI are guaranteed to be sorted in ascending manner,
+ // so start searching XG from last known optimum position
+
+ eT a_best_err = Datum<eT>::inf;
+ eT b_best_err = Datum<eT>::inf;
+
+ for(uword j=a_best_j; j<NG; ++j)
+ {
+ const eT tmp = XG_mem[j] - XI_val;
+ const eT err = (tmp >= eT(0)) ? tmp : -tmp;
+
+ if(err >= a_best_err)
+ {
+ break;
+ }
+ else
+ {
+ a_best_err = err;
+ a_best_j = j;
+ }
+ }
+
+ if( (XG_mem[a_best_j] - XI_val) <= eT(0) )
+ {
+ // a_best_j is to the left of the interpolated position
+
+ b_best_j = ( (a_best_j+1) < NG) ? (a_best_j+1) : a_best_j;
+ }
+ else
+ {
+ // a_best_j is to the right of the interpolated position
+
+ b_best_j = (a_best_j >= 1) ? (a_best_j-1) : a_best_j;
+ }
+
+ b_best_err = std::abs( XG_mem[b_best_j] - XI_val );
+
+ if(a_best_j > b_best_j)
+ {
+ std::swap(a_best_j, b_best_j );
+ std::swap(a_best_err, b_best_err);
+ }
+
+ const eT weight = (a_best_err > eT(0)) ? (a_best_err / (a_best_err + b_best_err)) : eT(0);
+
+ if(mode == 0) { ZI.row(i) = (eT(1) - weight)*ZG.row(a_best_j) + (weight)*ZG.row(b_best_j); }
+ if(mode == 1) { ZI.col(i) = (eT(1) - weight)*ZG.col(a_best_j) + (weight)*ZG.col(b_best_j); }
+ }
+ }
+ }
+
+
+
+template<typename T1, typename T2, typename T3, typename T4, typename T5>
+inline
+typename
+enable_if2< is_real<typename T1::elem_type>::value, void >::result
+interp2
+ (
+ const Base<typename T1::elem_type, T1>& X,
+ const Base<typename T1::elem_type, T2>& Y,
+ const Base<typename T1::elem_type, T3>& Z,
+ const Base<typename T1::elem_type, T4>& XI,
+ const Base<typename T1::elem_type, T5>& YI,
+ Mat<typename T1::elem_type>& ZI,
+ const char* method = "linear",
+ const typename T1::elem_type extrap_val = Datum<typename T1::elem_type>::nan
+ )
+ {
+ arma_extra_debug_sigprint();
+
+ typedef typename T1::elem_type eT;
+
+ const char sig = (method != nullptr) ? method[0] : char(0);
+
+ arma_debug_check( ((sig != 'n') && (sig != 'l')), "interp2(): unsupported interpolation type" );
+
+ const quasi_unwrap<T1> UXG( X.get_ref() );
+ const quasi_unwrap<T2> UYG( Y.get_ref() );
+ const quasi_unwrap<T3> UZG( Z.get_ref() );
+ const quasi_unwrap<T4> UXI( XI.get_ref() );
+ const quasi_unwrap<T5> UYI( YI.get_ref() );
+
+ arma_debug_check( (UXG.M.is_vec() == false), "interp2(): X must resolve to a vector" );
+ arma_debug_check( (UYG.M.is_vec() == false), "interp2(): Y must resolve to a vector" );
+
+ arma_debug_check( (UXI.M.is_vec() == false), "interp2(): XI must resolve to a vector" );
+ arma_debug_check( (UYI.M.is_vec() == false), "interp2(): YI must resolve to a vector" );
+
+ arma_debug_check( (UXG.M.n_elem < 2), "interp2(): X must have at least two unique elements" );
+ arma_debug_check( (UYG.M.n_elem < 2), "interp2(): Y must have at least two unique elements" );
+
+ arma_debug_check( (UXG.M.n_elem != UZG.M.n_cols), "interp2(): number of elements in X must equal the number of columns in Z" );
+ arma_debug_check( (UYG.M.n_elem != UZG.M.n_rows), "interp2(): number of elements in Y must equal the number of rows in Z" );
+
+ arma_debug_check( (UXG.M.is_sorted("strictascend") == false), "interp2(): X must be monotonically increasing" );
+ arma_debug_check( (UYG.M.is_sorted("strictascend") == false), "interp2(): Y must be monotonically increasing" );
+
+ arma_debug_check( (UXI.M.is_sorted("strictascend") == false), "interp2(): XI must be monotonically increasing" );
+ arma_debug_check( (UYI.M.is_sorted("strictascend") == false), "interp2(): YI must be monotonically increasing" );
+
+ Mat<eT> tmp;
+
+ if( UXG.is_alias(ZI) || UXI.is_alias(ZI) )
+ {
+ Mat<eT> out;
+
+ if(sig == 'n')
+ {
+ interp2_helper_nearest(UYG.M, UZG.M, UYI.M, tmp, extrap_val, 0);
+ interp2_helper_nearest(UXG.M, tmp, UXI.M, out, extrap_val, 1);
+ }
+ else
+ if(sig == 'l')
+ {
+ interp2_helper_linear(UYG.M, UZG.M, UYI.M, tmp, extrap_val, 0);
+ interp2_helper_linear(UXG.M, tmp, UXI.M, out, extrap_val, 1);
+ }
+
+ ZI.steal_mem(out);
+ }
+ else
+ {
+ if(sig == 'n')
+ {
+ interp2_helper_nearest(UYG.M, UZG.M, UYI.M, tmp, extrap_val, 0);
+ interp2_helper_nearest(UXG.M, tmp, UXI.M, ZI, extrap_val, 1);
+ }
+ else
+ if(sig == 'l')
+ {
+ interp2_helper_linear(UYG.M, UZG.M, UYI.M, tmp, extrap_val, 0);
+ interp2_helper_linear(UXG.M, tmp, UXI.M, ZI, extrap_val, 1);
+ }
+ }
+ }
+
+
+
+//! @}
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