function [ c, fc ] = bisection_integer ( f, a, b )

%*****************************************************************************80
%
%% BISECTION_INTEGER seeks an integer root using bisection.
%
%  Discussion:
%
%    A function F(X) confined to integer arguments is given, with an
%    interval [A,B] over which F changes sign.  An integer C is sought
%    such that A <= C <= B and F(C) = 0.
%
%    Because we are restricted to integer arguments, it may the case that
%    there is no such C.
%
%    This routine proceeds by a form of bisection, in which the enclosing
%    interval is restricted to be defined by integer values.
%
%    If the user has given a true change of sign interval [A,B], and if,
%    in the interval, there is a single integer value C for which F(C) = 0,
%    with the additional restrictions that F(C-1) and F(C+1) are of opposite
%    signs, then this procedure should locate and return C.
%
%    In particular, if the function F is monotone, and there is an integer
%    solution C in the interval, then this procedure will find it.
%
%    However, in general, even if there is an integer C in the interval,
%    such that F(C) = 0, this procedure may be unable to find it, particularly
%    if there are also nonintegral solutions within the same interval.
%
%    While any integer function can be used with this program, the bisection
%    approach is most useful if the integer function is monotone, or
%    varies slowly, or can be regarded as the restriction to integer arguments
%    of a continuous (and smoothly varying) function of a real argument.
%    In such cases, knowing that F is negative at A and positive at B
%    suggests that F generally increases from A to B, and might attain 
%    the value 0 at some intermediate argument C.
%
%  Licensing:
%
%    This code is distributed under the GNU LGPL license.
%
%  Modified:
%
%    26 May 2012
%
%  Author:
%
%    John Burkardt
%
%  Parameters:
%
%    Input, function V = F ( C ), the name of a user-supplied 
%    procedure that evaluates the function.
%
%   Input, integer A, B, two arguments that define a change of
%   sign interval for F.  In other words, F(A) and F(B) must be of opposite
%   sign.
%
%   Output, integer C, FC, the candidate for the root, as 
%   determined by the program, and its function value.  If FC is not zero,
%   then the procedure did not find a root in the interval, and C is only
%   an "approximate" root.
%

%
%  Ensure that F(A) < 0 < F(B).
%
  fa = f ( a );
  fb = f ( b );

  if ( fa == 0 )
    c = a;
    fc = fa;
  elseif ( fb == 0 )
    c = b;
    fc = fb;
  elseif ( fa < 0 && 0 < fb )

  elseif ( fb < 0 && 0 < fa )
    t = a;
    a = b;
    b = t;
    t = fa;
    fa = fb;
    fb = t;
  else
    fprintf ( 1, '\n' );
    fprintf ( 1, 'BISECTION_INTEGER - Fatal error!\n' );
    fprintf ( 1, '  No change of sign interval supplied.\n' );
    fprintf ( 1, '  F(%d) = %d\n', a, fa );
    fprintf ( 1, '  F(%d) = %d\n', b, fb );
    error ( 'BISECTION_INTEGER - Fatal error!' );
  end
%
%  Bisection.
%
  while ( 1 < abs ( b - a ) )

    c = floor ( ( a + b ) / 2 );
    fc = f ( c );

    if ( fc == 0 )
      return
    elseif ( fc < 0 )
      a = c;
      fa = fc;
    elseif ( 0 < fc )
      b = c;
      fb = fc;
    end
    
  end
%
%  Interval is empty, with FA < 0 and 0 < FB.
%  Bisection did not produce an integer solution.
%  Return the argument with smallest function norm.
%
  if ( - fa < fb )
    c = a;
    fc = fa;
  else
    c = b;
    fc = fb;
  end

  return
end

