# # Chebyshev center
# Boyd & Vandenberghe, "Convex Optimization"
# Joëlle Skaf - 08/16/05
#
# Adapted for Convex.jl by Karanveer Mohan and David Zeng - 26/05/14
#
# The goal is to find the largest Euclidean ball (i.e. its center and
# radius) that lies in a polyhedron described by affine inequalites in this
# fashion: $P = \{x : a_i'*x \leq b_i, i=1,\ldots,m \}$ where $x \in \mathbb{R}^2$.

using Convex, LinearAlgebra, SCS

# Generate the input data
a1 = [ 2;  1];
a2 = [ 2; -1];
a3 = [-1;  2];
a4 = [-1; -2];
b = ones(4, 1);

# Create and solve the model
r = Variable(1)
x_c = Variable(2)
p = maximize(r)
p.constraints += a1' * x_c + r * norm(a1, 2) <= b[1];
p.constraints += a2' * x_c + r * norm(a2, 2) <= b[2];
p.constraints += a3' * x_c + r * norm(a3, 2) <= b[3];
p.constraints += a4' * x_c + r * norm(a4, 2) <= b[4];
solve!(p, () -> SCS.Optimizer(verbose=0))
p.optval

# Generate the figure
x = range(-1.5, stop=1.5, length=100);
theta = 0:pi/100:2*pi;
using Plots
plot(x, x -> -x * a1[1] / a1[2] + b[1] / a1[2])
plot!(x, x -> -x * a2[1]/ a2[2] + b[2] / a2[2])
plot!(x, x -> -x * a3[1]/ a3[2] + b[3] / a3[2])
plot!(x, x -> -x * a4[1]/ a4[2] + b[4] / a4[2])
plot!(evaluate(x_c)[1] .+ evaluate(r) * cos.(theta), evaluate(x_c)[2] .+ evaluate(r) * sin.(theta), linewidth = 2)
plot!(title ="Largest Euclidean ball lying in a 2D polyhedron", legend = nothing)
