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minimize.py
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minimize.py
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#This file contains a Python version of Carl Rasmussen's Matlab-function
#minimize.m
#
#minimize.m is copyright (C) 1999 - 2006, Carl Edward Rasmussen.
#Python adaptation by Roland Memisevic 2008.
#
#
#The following is the original copyright notice that comes with the
#function minimize.m
#(from http://www.kyb.tuebingen.mpg.de/bs/people/carl/code/minimize/Copyright):
#
#
#"(C) Copyright 1999 - 2006, Carl Edward Rasmussen
#
#Permission is granted for anyone to copy, use, or modify these
#programs and accompanying documents for purposes of research or
#education, provided this copyright notice is retained, and note is
#made of any changes that have been made.
#
# CHANGES MADE TO THIS FILE, 12/4/2015, Mitch McIntire:
# Removed required argument 'grad' for evaluating function gradient
# Replaced calls to 'grad' under assumption that 'f' returns the gradient
#
#These programs and documents are distributed without any warranty,
#express or implied. As the programs were written for research
#purposes only, they have not been tested to the degree that would be
#advisable in any important application. All use of these programs is
#entirely at the user's own risk."
"""minimize.py
This module contains a function 'minimize' that performs unconstrained
gradient based optimization using nonlinear conjugate gradients.
The function is a straightforward Python-translation of Carl Rasmussen's
Matlab-function minimize.m
"""
from numpy import dot, isinf, isnan, any, sqrt, isreal, real, inf
def minimize(X, f, args, maxnumlinesearch=None, maxnumfuneval=None, red=1.0, verbose=False):
INT = 0.1;# don't reevaluate within 0.1 of the limit of the current bracket
EXT = 3.0; # extrapolate maximum 3 times the current step-size
MAX = 20; # max 20 function evaluations per line search
RATIO = 10; # maximum allowed slope ratio
SIG = 0.1;RHO = SIG/2;# SIG and RHO are the constants controlling the Wolfe-
#Powell conditions. SIG is the maximum allowed absolute ratio between
#previous and new slopes (derivatives in the search direction), thus setting
#SIG to low (positive) values forces higher precision in the line-searches.
#RHO is the minimum allowed fraction of the expected (from the slope at the
#initial point in the linesearch). Constants must satisfy 0 < RHO < SIG < 1.
#Tuning of SIG (depending on the nature of the function to be optimized) may
#speed up the minimization; it is probably not worth playing much with RHO.
SMALL = 10.**-16 #minimize.m uses matlab's realmin
if maxnumlinesearch == None:
if maxnumfuneval == None:
raise "Specify maxnumlinesearch or maxnumfuneval"
else:
S = 'Function evaluation'
length = maxnumfuneval
else:
if maxnumfuneval != None:
raise "Specify either maxnumlinesearch or maxnumfuneval (not both)"
else:
S = 'Linesearch'
length = maxnumlinesearch
i = 0 # zero the run length counter
ls_failed = 0 # no previous line search has failed
(f0,df0) = f(X, *args) # get function value and gradient
#df0 = grad(X, *args)
fX = [f0]
i = i + (length<0) # count epochs?!
s = -df0; d0 = -dot(s,s) # initial search direction (steepest) and slope
x3 = red/(1.0-d0) # initial step is red/(|s|+1)
while i < abs(length): # while not finished
i = i + (length>0) # count iterations?!
X0 = X; F0 = f0; dF0 = df0 # make a copy of current values
if length>0:
M = MAX
else:
M = min(MAX, -length-i)
while 1: # keep extrapolating as long as necessary
x2 = 0; f2 = f0; d2 = d0; f3 = f0; df3 = df0
success = 0
while (not success) and (M > 0):
try:
M = M - 1; i = i + (length<0) # count epochs?!
(f3,df3) = f(X+x3*s, *args)
#df3 = grad(X+x3*s, *args)
if isnan(f3) or isinf(f3) or any(isnan(df3)+isinf(df3)):
print("error")
return
success = 1
except: # catch any error which occured in f
x3 = (x2+x3)/2 # bisect and try again
if f3 < F0:
X0 = X+x3*s; F0 = f3; dF0 = df3 # keep best values
d3 = dot(df3,s) # new slope
if d3 > SIG*d0 or f3 > f0+x3*RHO*d0 or M == 0:
# are we done extrapolating?
break
x1 = x2; f1 = f2; d1 = d2 # move point 2 to point 1
x2 = x3; f2 = f3; d2 = d3 # move point 3 to point 2
A = 6*(f1-f2)+3*(d2+d1)*(x2-x1) # make cubic extrapolation
B = 3*(f2-f1)-(2*d1+d2)*(x2-x1)
Z = B+sqrt(complex(B*B-A*d1*(x2-x1)))
if Z != 0.0:
x3 = x1-d1*(x2-x1)**2/Z # num. error possible, ok!
else:
x3 = inf
if (not isreal(x3)) or isnan(x3) or isinf(x3) or (x3 < 0):
# num prob | wrong sign?
x3 = x2*EXT # extrapolate maximum amount
elif x3 > x2*EXT: # new point beyond extrapolation limit?
x3 = x2*EXT # extrapolate maximum amount
elif x3 < x2+INT*(x2-x1): # new point too close to previous point?
x3 = x2+INT*(x2-x1)
x3 = real(x3)
while (abs(d3) > -SIG*d0 or f3 > f0+x3*RHO*d0) and M > 0:
# keep interpolating
if (d3 > 0) or (f3 > f0+x3*RHO*d0): # choose subinterval
x4 = x3; f4 = f3; d4 = d3 # move point 3 to point 4
else:
x2 = x3; f2 = f3; d2 = d3 # move point 3 to point 2
if f4 > f0:
x3 = x2-(0.5*d2*(x4-x2)**2)/(f4-f2-d2*(x4-x2))
# quadratic interpolation
else:
A = 6*(f2-f4)/(x4-x2)+3*(d4+d2) # cubic interpolation
B = 3*(f4-f2)-(2*d2+d4)*(x4-x2)
if A != 0:
x3=x2+(sqrt(B*B-A*d2*(x4-x2)**2)-B)/A
# num. error possible, ok!
else:
x3 = inf
if isnan(x3) or isinf(x3):
x3 = (x2+x4)/2 # if we had a numerical problem then bisect
x3 = max(min(x3, x4-INT*(x4-x2)),x2+INT*(x4-x2))
# don't accept too close
(f3,df3) = f(X+x3*s, *args)
#df3 = grad(X+x3*s, *args)
if f3 < F0:
X0 = X+x3*s; F0 = f3; dF0 = df3 # keep best values
M = M - 1; i = i + (length<0) # count epochs?!
d3 = dot(df3,s) # new slope
if abs(d3) < -SIG*d0 and f3 < f0+x3*RHO*d0: # if line search succeeded
X = X+x3*s; f0 = f3; fX.append(f0) # update variables
if verbose: print('%s %6i; Value %4.6e\r' % (S, i, f0))
s = (dot(df3,df3)-dot(df0,df3))/dot(df0,df0)*s - df3
# Polack-Ribiere CG direction
df0 = df3 # swap derivatives
d3 = d0; d0 = dot(df0,s)
if d0 > 0: # new slope must be negative
s = -df0; d0 = -dot(s,s) # otherwise use steepest direction
x3 = x3 * min(RATIO, d3/(d0-SMALL)) # slope ratio but max RATIO
ls_failed = 0 # this line search did not fail
else:
X = X0; f0 = F0; df0 = dF0 # restore best point so far
if ls_failed or (i>abs(length)):# line search failed twice in a row
break # or we ran out of time, so we give up
s = -df0; d0 = -dot(s,s) # try steepest
x3 = 1/(1-d0)
ls_failed = 1 # this line search failed
if verbose: print("\n")
return X, fX, i