## GEKPLS Surrogate Tutorial

Gradient Enhanced Kriging with Partial Least Squares Method (GEKPLS) is a surrogate modelling technique that brings down computation time and returns improved accuracy for high-dimensional problems. The Julia implementation of GEKPLS is adapted from the Python version by SMT which is based on this paper.

The following are the inputs when building a GEKPLS surrogate:

1. X - The matrix containing the training points
2. y - The vector containing the training outputs associated with each of the training points
3. grads - The gradients at each of the input X training points
4. n_comp - Number of components to retain for the partial least squares regression (PLS)
5. delta_x - The step size to use for the first order Taylor approximation
6. xlimits - The lower and upper bounds for the training points
7. extra_points - The number of additional points to use for the PLS
8. theta - The hyperparameter to use for the correlation model

The following example illustrates how to use GEKPLS:


using Surrogates
using Zygote

function vector_of_tuples_to_matrix(v)
#helper function to convert training data generated by surrogate sampling into a matrix suitable for GEKPLS
num_rows = length(v)
num_cols = length(first(v))
K = zeros(num_rows, num_cols)
for row in 1:num_rows
for col in 1:num_cols
K[row, col]=v[row][col]
end
end
return K
end

function vector_of_tuples_to_matrix2(v)
#helper function to convert gradients into matrix form
num_rows = length(v)
num_cols = length(first(first(v)))
K = zeros(num_rows, num_cols)
for row in 1:num_rows
for col in 1:num_cols
K[row, col] = v[row][1][col]
end
end
return K
end

function water_flow(x)
r_w = x[1]
r = x[2]
T_u = x[3]
H_u = x[4]
T_l = x[5]
H_l = x[6]
L = x[7]
K_w = x[8]
log_val = log(r/r_w)
return (2*pi*T_u*(H_u - H_l))/ ( log_val*(1 + (2*L*T_u/(log_val*r_w^2*K_w)) + T_u/T_l))
end

n = 1000
d = 8
lb = [0.05,100,63070,990,63.1,700,1120,9855]
ub = [0.15,50000,115600,1110,116,820,1680,12045]
x = sample(n,lb,ub,SobolSample())
X = vector_of_tuples_to_matrix(x)
y = reshape(water_flow.(x),(size(x,1),1))
xlimits = hcat(lb, ub)
n_test = 100
x_test = sample(n_test,lb,ub,GoldenSample())
X_test = vector_of_tuples_to_matrix(x_test)
y_true = water_flow.(x_test)
n_comp = 2
delta_x = 0.0001
extra_points = 2
initial_theta = [0.01 for i in 1:n_comp]
g = GEKPLS(X, y, grads, n_comp, delta_x, xlimits, extra_points, initial_theta)
y_pred = g(X_test)
rmse = sqrt(sum(((y_pred - y_true).^2)/n_test)) #root mean squared error
println(rmse) #0.0347