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+## Copyright (C) 2008  David Bateman
+##
+## This file is part of Octave.
+##
+## Octave is free software; you can redistribute it and/or modify it
+## under the terms of the GNU General Public License as published by
+## the Free Software Foundation; either version 3 of the License, or (at
+## your option) any later version.
+##
+## Octave is distributed in the hope that it will be useful, but
+## WITHOUT ANY WARRANTY; without even the implied warranty of
+## MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+## General Public License for more details.
+##
+## You should have received a copy of the GNU General Public License
+## along with Octave; see the file COPYING.  If not, see
+## <http://www.gnu.org/licenses/>.
+
+## -*- texinfo -*-
+## @deftypefn {Function File} {@var{q} =} quadv (@var{f}, @var{a}, @var{b})
+## @deftypefnx {Function File} {@var{q} =} quadl (@var{f}, @var{a}, @var{b}, @var{tol})
+## @deftypefnx {Function File} {@var{q} =} quadl (@var{f}, @var{a}, @var{b}, @var{tol}, @var{trace})
+## @deftypefnx {Function File} {@var{q} =} quadl (@var{f}, @var{a}, @var{b}, @var{tol}, @var{trace}, @var{p1}, @var{p2}, @dots{})
+## @deftypefnx {Function File} {[@var{q}, @var{fcnt}] =} quadl (@dots{})
+##
+## Numerically evaluate integral using adaptive Simpson's rule.
+## @code{quadv (@var{f}, @var{a}, @var{b})} approximates the integral of
+## @code{@var{f}(@var{x})} to the default absolute tolerance of @code{1e-6}. 
+## @var{f} is either a function handle, inline function or string
+## containing the name of the function to evaluate. The function @var{f}
+## must accept a string, and can return a vector representing the
+## approximation to @var{n} different sub-functions.
+##
+## If defined, @var{tol} defines the absolute tolerance to which to
+## which to integrate each sub-interval of @code{@var{f}(@var{x})}.
+## While if @var{trace} is defined, displays the left end point of the
+## current interval, the interval length, and the partial integral.
+##
+## Additional arguments @var{p1}, etc, are passed directly to @var{f}.
+## To use default values for @var{tol} and @var{trace}, one may pass
+## empty matrices.
+## @seealso{triplequad, dblquad, quad, quadl, quadgk, trapz}
+## @end deftypefn
+
+function [Q, fcnt] = quadv (f, a, b, tol, trace, varargin)
+  if (nargin < 3)
+    print_usage ();
+  endif
+  if (nargin < 4)
+    tol = []; 
+  endif
+  if (nargin < 5)
+    trace = []; 
+  endif
+  if (isempty (tol))
+    tol = 1e-6; 
+  endif
+  if (isempty (trace))
+    trace = 0; 
+  endif
+
+  ## Split the interval into 3 abscissa, and apply a 3 point Simpson's rule
+  c = (a + b) / 2;
+  fa = feval (f, a, varargin{:});
+  fc = feval (f, c, varargin{:});
+  fb = feval (f, b, varargin{:});
+  fcnt = 3;
+
+  ## If have edge singularities, move edge point by eps*(b-a) as
+  ## discussed in Shampine paper used to implement quadgk
+  if (isinf (fa))
+    fa = feval (f, a + eps * (b-a), varargin{:});
+  endif
+  if (isinf (fb))
+    fb = feval (f, b - eps * (b-a), varargin{:});
+  endif
+
+  h = (b - a) / 2;
+  Q = (b - a) / 6 * (fa + 4 * fc + fb);
+ 
+  [Q, fcnt, hmin] = simpsonstp (f, a, b, c, fa, fb, fc, Q, fcnt, abs (b - a), 
+				tol, trace, varargin{:});
+
+  if (fcnt > 10000)
+    warning("Maximum iteration count reached");
+  elseif (isnan(Q) || isinf (Q))
+    warning ("Infinite or NaN function evaluations were returned");
+  elseif (hmin < (b - a) * eps)
+    warning ("Minimum step size reached. Possibly singular integral");
+  endif
+endfunction
+
+function [Q, fcnt, hmin] = simpsonstp (f, a, b, c, fa, fb, fc, Q0, 
+				       fcnt, hmin, tol, trace, varargin)
+  if (fcnt > 10000)
+    Q = Q0;
+  else
+    d = (a + c) / 2;
+    e = (c + b) / 2;
+    fd = feval (f, d, varargin{:});
+    fe = feval (f, e, varargin{:});
+    fcnt += 2;
+    Q1 = (c - a) / 6 * (fa + 4 * fd + fc);
+    Q2 = (b - c) / 6 * (fc + 4 * fe + fb);
+    Q = Q1 + Q2;
+
+    if (abs(a -  c) < hmin)
+      hmin = abs (a - c);
+    endif
+
+    if (trace)
+      disp ([fcnt, a, b-a, Q]);
+    endif
+
+    ## Force at least one adpative step
+    if (fcnt == 5 || abs (Q - Q0) > tol)
+      [Q1, fcnt, hmin] = simpsonstp (f, a, c, d, fa, fc, fd, Q1, fcnt, hmin,
+				    tol, trace, varargin{:});
+      [Q2, fcnt, hmin] = simpsonstp (f, c, b, e, fc, fb, fe, Q2, fcnt, hmin,
+				     tol, trace, varargin{:});
+      Q = Q1 + Q2;
+    endif
+  endif
+endfunction
+
+%!assert (quadv (@sin, 0, 2 * pi), 0, 1e-5)
+%!assert (quadv (@sin, 0, pi), 2, 1e-5)
+
+%% Handles weak singularities at the edge
+%!assert (quadv (@(x) 1 ./ sqrt(x), 0, 1), 2, 1e-5)
+