#
# = Multidimensional Minimization
# This chapter describes routines for finding minima of arbitrary
# multidimensional functions. The library provides low level components for a
# variety of iterative minimizers and convergence tests. These can be combined
# by the user to achieve the desired solution, while providing full access to
# the intermediate steps of the algorithms. Each class of methods uses the
# same framework, so that you can switch between minimizers at runtime without
# needing to recompile your program. Each instance of a minimizer keeps track
# of its own state, allowing the minimizers to be used in multi-threaded
# programs.
#
# Contents:
# 1. {Overview}[link:multimin_rdoc.html#label-Overview]
# 1. {Caveats}[link:multimin_rdoc.html#label-Caveats]
# 1. {Initializing the Multidimensional Minimizer}[link:multimin_rdoc.html#label-Initializing+the+Multidimensional+Minimizer]
# 1. {Providing a function to minimize}[link:multimin_rdoc.html#label-Providing+a+function+to+minimize]
# 1. {Iteration}[link:multimin_rdoc.html#label-Iteration]
# 1. {Stopping Criteria}[link:multimin_rdoc.html#label-Stopping+Criteria]
# 1. {Examples}[link:multimin_rdoc.html#label-Examples]
#    1. {FdfMinimizer}[link:multimin_rdoc.html#label-FdfMinimizer]
#    1. {FMinimizer}[link:multimin_rdoc.html#label-FMinimizer]
#
# == Overview
# The problem of multidimensional minimization requires finding a point x such
# that the scalar function, takes a value which is lower than at any neighboring
# point. For smooth functions the gradient g = \nabla f vanishes at the minimum.
# In general there are no bracketing methods available for the minimization of
# n-dimensional functions. The algorithms proceed from an initial guess using a
# search algorithm which attempts to move in a downhill direction.
#
# Algorithms making use of the gradient of the function perform a
# one-dimensional line minimisation along this direction until the lowest point
# is found to a suitable tolerance. The search direction is then updated with
# local information from the function and its derivatives, and the whole process
# repeated until the true n-dimensional minimum is found.
#
# The Nelder-Mead Simplex algorithm applies a different strategy. It maintains
# n+1 trial parameter vectors as the vertices of a n-dimensional simplex.
# In each iteration step it tries to improve the worst vertex by a simple
# geometrical transformation until the size of the simplex falls below a given
# tolerance.
#
# Both types of algorithms use a standard framework. The user provides a
# high-level driver for the algorithms, and the library provides the individual
# functions necessary for each of the steps. There are three main phases of the
# iteration. The steps are,
#
# * initialize minimizer state, s, for algorithm T
# * update s using the iteration T
# * test s for convergence, and repeat iteration if necessary
#
# Each iteration step consists either of an improvement to the line-minimisation
# in the current direction or an update to the search direction itself. The
# state for the minimizers is held in a <tt>GSL::MultiMin::FdfMinimizer</tt> or
# a <tt>GSL::MultiMin::FMinimizer</tt> object.
#
# == Caveats
# Note that the minimization algorithms can only search for one local minimum
# at a time. When there are several local minima in the search area, the first
# minimum to be found will be returned; however it is difficult to predict which
# of the minima this will be. In most cases, no error will be reported if you
# try to find a local minimum in an area where there is more than one.
#
# It is also important to note that the minimization algorithms find local
# minima; there is no way to determine whether a minimum is a global minimum of
# the function in question.
#
#
# == Initializing the Multidimensional Minimizer
# ---
# * GSL::MultiMin::FdfMinimizer.alloc(type, n)
# * GSL::MultiMin::FMinimizer.alloc(type, n)
#
#   These method create a minimizer of type <tt>type</tt> for an <tt>n</tt>-dimension function.
#   The type is given by a string, or by a Ruby constant.
#
#   * <tt>GSL::MultiMin::FdfMinimizer::CONJUGATE_FR</tt> or <tt>"conjugate_fr"</tt>
#   * <tt>GSL::MultiMin::FdfMinimizer::CONJUGATE_PR</tt> or <tt>"conjugate_pr"</tt>
#   * <tt>GSL::MultiMin::FdfMinimizer::VECTOR_BFGS</tt> or <tt>"vector_bfgs"</tt>
#   * <tt>GSL::MultiMin::FdfMinimizer::VECTOR_BFGS2</tt> or <tt>"vector_bfgs2"</tt> (GSL-1.9 or later)
#   * <tt>GSL::MultiMin::FdfMinimizer::STEEPEST_DESCENT</tt> or <tt>"steepest_descent"</tt>
#   * <tt>GSL::MultiMin::FMinimizer::NMSIMPLEX</tt> or <tt>"nmsimplex"</tt>
#   * <tt>GSL::MultiMin::FMinimizer::NMSIMPLEX2RAND</tt> or <tt>"nmsimplex2rand"</tt> (GSL-1.13)
#
#   ex:
#       include GSL::MultiMin
#       m1 = FdfMinimizer.alloc(FdfMinimizer::CONJUGATE_FR, 2)
#       m2 = FdfMinimizer.alloc("steepest_descent", 4)
#       m3 = FMinimizer.alloc(FMinimizer::NMSIMPLEX, 3)
#       m4 = FMinimizer.alloc("nmsimplex", 2)
#
# ---
# * GSL::MultiMin::FdfMinimizer#set(func, x, step_size, tol)
#
#   This method initializes the minimizer <tt>self</tt> to minimize the function
#   <tt>fdf</tt> (the <tt>GSL::MultiMin::Function_fdf</tt> class, see below) starting from
#   the initial point <tt>x</tt> (<tt>GSL::Vector</tt>). The size of the first trial step is
#   given by <tt>step_size</tt> (<tt>Vector</tt>). The accuracy of the line minimization is
#   specified by <tt>tol</tt>.
#
# ---
# * GSL::MultiMin::FMinimizer#set(func, x, step_size)
#
#   This method initializes the minimizer <tt>self</tt> to minimize the function <tt>func</tt>,
#   starting from the initial point <tt>x</tt> (Vector). The size of the initial trial steps
#   is given in vector <tt>step_size</tt>.
#
# ---
# * GSL::MultiMin::FdfMinimizer#name
# * GSL::MultiMin::FMinimizer#name
#
#   These return the name of the minimizer <tt>self</tt>.
#
# == Providing a function to minimize
# You must provide a parametric function of <tt>n</tt> variables for the minimizers to
# operate on. You may also need to provide a routine which calculates the gradient of the
# function. In order to allow for general parameters the functions are defined by the
# classes, <tt>GSL::MultiMin::Function_fdf</tt> and <tt>GSL::MultiMin::Function</tt>.
#
# ---
# * GSL::MultiMin:Function_fdf.alloc(proc_f, proc_df, n)
# * GSL::MultiMin:Function_fdf.alloc(proc_f, proc_df, proc_fdf, n)
# * GSL::MultiMin:Function_fdf#set_procs(proc_f, proc_df)
# * GSL::MultiMin:Function_fdf#set_procs(proc_f, proc_df, n)
# * GSL::MultiMin:Function_fdf#set_procs(proc_f, proc_df, proc_fdf, n)
# * GSL::MultiMin:Function_fdf#set_params(params)
#
#   See example below.
#
#     include GSL::MultiMin
#
#     my_f = Proc.new { |v, params|
#       x = v[0]; y = v[1]
#       p0 = params[0]; p1 = params[1]
#       10.0*(x - p0)*(x - p0) + 20.0*(y - p1)*(y - p1) + 30.0
#     }
#
#     my_df = Proc.new { |v, params, df|
#       x = v[0]; y = v[1]
#       p0 = params[0]; p1 = params[1]
#       df[0] = 20.0*(x-p0)
#       df[1] = 40.0*(y-p1)
#     }
#
#     my_func = Function_fdf.alloc(my_f, my_df, 2)
#     my_func.set_params([1.0, 2.0])      # parameters
#
# ---
# * GSL::MultiMin:Function.alloc(proc_f, n)
# * GSL::MultiMin:Function#set_proc(proc_f)
# * GSL::MultiMin:Function#set_proc(proc_f, n)
# * GSL::MultiMin:Function#set_params(params)
#
#   See example below.
#
#     include GSL::MultiMin
#
#     np = 2
#     my_f = Proc.alloc { |v, params|
#       x = v[0]; y = v[1]
#       p0 = params[0]; p1 = params[1]
#       10.0*(x - p0)*(x - p0) + 20.0*(y - p1)*(y - p1) + 30.0
#     }
#
#     my_func = Function.alloc(my_f, np)
#     my_func.set_params([1.0, 2.0])      # parameters
#
# == Iteration
# ---
# * GSL::MultiMin::FdfMinimizer#iterate
# * GSL::MultiMin::FMinimizer#iterate
#
#   These methods perform a single iteration of the minimizer <tt>self</tt>.
#   If the iteration encounters an unexpected problem then an error code will be returned.
#   The minimizer maintains a current best estimate of the minimum at all times.
#   This information can be accessed with the following methods,
#
# ---
# * GSL::MultiMin::FdfMinimizer#x
# * GSL::MultiMin::FdfMinimizer#minimum
# * GSL::MultiMin::FdfMinimizer#gradient
# * GSL::MultiMin::FMinimizer#x
# * GSL::MultiMin::FMinimizer#minimum
# * GSL::MultiMin::FMinimizer#size
#
#   These method return the current best estimate of the location of the minimum,
#   the value of the function at that point, its gradient, and minimizer specific
#   characteristic size for the minimizer <tt>self</tt>.
#
# ---
# * GSL::MultiMin::FdfMinimizer#restart
#
#   This method resets the minimizer <tt>self</tt> to use the current point as a new
#   starting point.
#
# == Stopping Criteria
# A minimization procedure should stop when one of the following conditions is true:
# * A minimum has been found to within the user-specified precision.
# * A user-specified maximum number of iterations has been reached.
# * An error has occurred.
# The handling of these conditions is under user control. The methods below allow the
# user to test the precision of the current result.
#
# ---
# * GSL::MultiMin::FdfMinimizer#test_gradient(epsabs)
# * GSL::MultiMin::FdfMinimizer.test_gradient(g, epsabs)
#
#   These method test the norm of the gradient <tt>g</tt> against the absolute tolerance
#   <tt>epsabs</tt>. The gradient of a multidimensional function goes to zero at a minimum.
#   The tests return <tt>GSL::SUCCESS</tt> if the following condition is achieved,
#     |g| < epsabs
#   and returns <tt>GSL::CONTINUE</tt> otherwise. A suitable choice of <tt>epsabs</tt> can
#   be made from the desired accuracy in the function for small variations in <tt>x</tt>.
#   The relationship between these quantities is given by <tt>\delta f = g \delta x</tt>.
#
# ---
# * GSL::MultiMin::FdfMinimizer#test_size(epsabs)
# * GSL::MultiMin::FdfMinimizer.test_size(size, epsabs)
#
#   These method test the minimizer specific characteristic <tt>size</tt>
#   (if applicable to the used minimizer) against absolute tolerance <tt>epsabs</tt>.
#   The tests return (<tt>GSL::SUCCESS</tt> if the size is smaller than tolerance,
#   otherwise <tt>GSL::CONTINUE</tt> is returned.
#
# == Examples
#
# === FdfMinimizer
#      #!/usr/bin/env ruby
#      require("gsl")
#      include GSL::MultiMin
#
#      my_f = Proc.new { |v, params|
#        x = v[0]; y = v[1]
#        p0 = params[0]; p1 = params[1]
#        10.0*(x - p0)*(x - p0) + 20.0*(y - p1)*(y - p1) + 30.0
#      }
#
#      my_df = Proc.new { |v, params, df|
#        x = v[0]; y = v[1]
#        p0 = params[0]; p1 = params[1]
#        df[0] = 20.0*(x-p0)
#        df[1] = 40.0*(y-p1)
#      }
#
#      my_func = Function_fdf.alloc(my_f, my_df, 2)
#      my_func.set_params([1.0, 2.0])      # parameters
#
#      x = Vector.alloc(5.0, 7.0)          # starting point
#
#      minimizer = FdfMinimizer.alloc("conjugate_fr", 2)
#      minimizer.set(my_func, x, 0.01, 1e-4)
#
#      iter = 0
#      begin
#        iter += 1
#        status = minimizer.iterate()
#        status = minimizer.test_gradient(1e-3)
#        if status == GSL::SUCCESS
#          puts("Minimum found at")
#        end
#        x = minimizer.x
#        f = minimizer.f
#        printf("%5d %.5f %.5f %10.5f\n", iter, x[0], x[1], f)
#      end while status == GSL::CONTINUE and iter < 100
#
# === FMinimizer
#      #!/usr/bin/env ruby
#      require("gsl")
#      include GSL::MultiMin
#
#      np = 2
#
#      my_f = Proc.new { |v, params|
#        x = v[0]; y = v[1]
#        p0 = params[0]; p1 = params[1]
#        10.0*(x - p0)*(x - p0) + 20.0*(y - p1)*(y - p1) + 30.0
#      }
#
#      my_func = Function.alloc(my_f, np)
#      my_func.set_params([1.0, 2.0])      # parameters
#
#      x = Vector.alloc([5, 7])
#      ss = Vector.alloc(np)
#      ss.set_all(1.0)
#
#      minimizer = FMinimizer.alloc("nmsimplex", np)
#      minimizer.set(my_func, x, ss)
#
#      iter = 0
#      begin
#        iter += 1
#        status = minimizer.iterate()
#        status = minimizer.test_size(1e-2)
#        if status == GSL::SUCCESS
#          puts("converged to minimum at")
#        end
#        x = minimizer.x
#        printf("%5d ", iter);
#        for i in 0...np do
#          printf("%10.3e ", x[i])
#        end
#        printf("f() = %7.3f size = %.3f\n", minimizer.fval, minimizer.size);
#      end while status == GSL::CONTINUE and iter < 100
#
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