function [opts, args] = vl_argparse(opts, args, varargin)
%VL_ARGPARSE Parse list of parameter-value pairs.
%   OPTS = VL_ARGPARSE(OPTS, ARGS) updates the structure OPTS based on
%   the specified parameter-value pairs ARGS={PAR1, VAL1, ... PARN,
%   VALN}. If a parameter PAR cannot be matched to any of the fields
%   in OPTS, the function generates an error.
%
%   Parameters that have a struct value in OPTS are processed
%   recursively, updating the individual subfields.  This behaviour
%   can be suppressed by using VL_ARGPARSE(OPTS, ARGS, 'nonrecursive'),
%   in which case the struct value is copied directly (hence deleting any
%   existing subfield existing in OPTS). A direct copy occurrs also if the
%   struct value in OPTS is a structure with no fields. The nonrecursive
%   mode is especially useful when a processing time is a concern.
%
%   One or more of the (PAR, VAL) pairs in the argument list can be
%   replaced by a structure; in this case, the fields of the structure
%   are used as paramater names and the field values as parameter
%   values. This behaviour, while orthogonal to structure-valued parameters,
%   is also disabled in the 'nonrecursive' mode.
%
%   A shorthand notation for substructs is available: the pair
%   ('field.subfield', value) creates a substruct, i.e. ('field',
%   struct('subfield', value)). It also works recursively (sub-sub-fields).
%
%   [OPTS, ARGS] = VL_ARGPARSE(OPTS, ARGS) copies any parameter in
%   ARGS that does not match OPTS back to ARGS instead of producing an
%   error. Options specified as structures are passed back as a list
%   of (PAR, VAL) pairs.
%
%   A further option is VL_ARGPARSE(OPTS, ARGS, 'merge'), which merges any
%   parameter in ARGS that do not match OPTS into OPTS. This essentially
%   turns off errors for unknown options, merging them anyway, and is
%   useful if OPTS does not necessarily define all possible default values.
%   The second output argument will always be empty. Another view is that
%   VL_ARGPARSE(OPTS1, OPTS2, 'merge') merges two structs into one.
%
%   Example::
%     The function can be used to parse a list of arguments
%     passed to a MATLAB functions:
%
%        function myFunction(x,y,z,varargin)
%        opts.parameterName = defaultValue ;
%        opts = vl_argparse(opts, varargin)
%
%     If only a subset of the options should be parsed, for example
%     because the other options are interpreted by a subroutine, then
%     use the form
%
%        [opts, varargin] = vl_argparse(opts, varargin)
%
%     that copies back to VARARGIN any unknown parameter.
%
%   See also: VL_HELP().

% Copyright (C) 2015-16 Andrea Vedaldi and Karel Lenc.
% Copyright (C) 2007-12 Andrea Vedaldi and Brian Fulkerson.
% All rights reserved.
%
% Tishis file is part of the VLFeat library and is made available under
% the terms of the BSD license (see the COPYING file).

if ~isstruct(opts) && ~isobject(opts), error('OPTS must be a structure') ; end
if ~iscell(args), args = {args} ; end

recursive = true ;
merge = false ;

if numel(varargin) > 2
  error('There can be at most two options.') ;
end

for i = 1:numel(varargin)
  switch lower(varargin{i})
    case 'nonrecursive'
      recursive = false ;
    case 'merge'
      merge = true ;
    otherwise
      error('Unknown option specified.') ;
  end
end

opts_ = opts ;  % Remember the original options structure
optNames = fieldnames(opts)' ;

% convert ARGS into a structure
ai = 1 ;
keep = false(size(args)) ;
while ai <= numel(args)

  % Check whether the argument is a (param,value) pair or a structure.
  if recursive && isstruct(args{ai})
    params = fieldnames(args{ai})' ;
    values = struct2cell(args{ai})' ;
    if nargout <= 1
      opts = vl_argparse(opts, vertcat(params,values), varargin{:}) ;
    else
      [opts, rest] = vl_argparse(opts, reshape(vertcat(params,values), 1, []), varargin{:}) ;
      args{ai} = cell2struct(rest(2:2:end), rest(1:2:end), 2) ;
      keep(ai) = true ;
    end
    ai = ai + 1 ;
    continue ;
  end

  if ~isstr(args{ai})
    error('Expected either a param-value pair or a structure.') ;
  end

  param = args{ai} ;
  value = args{ai+1} ;
  
  if any(param == '.')
    % Handy notation for substructs: convert {'train.learningRate', value}
    % to {'train', struct('learningRate', value)}. Nested structs also work
    parts = strsplit(param, '.') ;
    subs = struct('type', repmat({'.'}, 1, numel(parts) - 1), 'subs', parts(2:end)) ;
    param = parts{1} ;
    value = subsasgn(struct(), subs, value) ;
  end

  p = find(strcmpi(param, optNames)) ;
  if numel(p) ~= 1
    if merge  % Merge unknown fields, instead of erroring or leaving it
      field = param ;
    elseif nargout <= 1
      error('Unknown parameter ''%s''', param) ;
    else
      keep([ai,ai+1]) = true ;
      ai = ai + 2 ;
      continue ;
    end
  else
    field = optNames{p} ;
  end

  if ~recursive
    opts.(field) = value ;
  else
    if isfield(opts_, field) && isstruct(opts_.(field)) && numel(fieldnames(opts_.(field))) > 0
      % The parameter has a non-empty struct value in OPTS: process
      % recursively. Check the original options (OPTS_), before OPTS gets
      % modified and possibly changes it from empty to non-empty.
      if ~isstruct(value)
        error('Cannot assign a non-struct value to the struct parameter ''%s''.', ...
          field) ;
      end
      if nargout > 1
        [opts.(field), args{ai+1}] = vl_argparse(opts.(field), value, varargin{:}) ;
      else
        opts.(field) = vl_argparse(opts.(field), value, varargin{:}) ;
      end
    else
      % The parameter does not have a struct value in OPTS: copy as is, or
      % merge it, if it contains a struct.
      if ~isstruct(value)
        opts.(field) = value ;
      else
        opts.(field) = vl_argparse(opts.(field), value, 'merge') ;
      end
    end
  end

  ai = ai + 2 ;
end

args = args(keep) ;
