function out=convert(in,varargin)
%OPTS Set gradient limits of the MR system.
%   out=convert(in,from,to) Convert the numerical data in, given in
%   specificed units 'from' to units specified in 'to'.
%
%   Valid unit strings are:
%    'Hz/m' 'mT/m' 'rad/ms/mm' 'Hz/m/s' 'mT/m/ms' 'T/m/s' 'rad/ms/mm/ms'

persistent parser
validB1Units={'Hz','T','mT','uT'}; % todo: gauss?
validGradUnits={'Hz/m','mT/m','rad/ms/mm'};
validSlewUnits={'Hz/m/s','mT/m/ms','T/m/s','rad/ms/mm/ms'};
validUnits=cat(2,validB1Units,validGradUnits,validSlewUnits);
if isempty(parser)
    parser = mr.aux.InputParserCompat;
    parser.FunctionName = 'convert';
    parser.addRequired('in',@isnumeric);
    parser.addRequired('fromUnit',...
        @(x) any(validatestring(x,validUnits)));
    parser.addOptional('toUnit',[],...
        @(x) any(validatestring(x,validUnits)));
    parser.addParamValue('gamma',42.576e6,@isnumeric); % Hz/T
end
parse(parser,in,varargin{:});
opt = parser.Results;

% Set default output unit if not given
if isempty(opt.toUnit)
    if ismember(opt.fromUnit,validGradUnits)
        opt.toUnit = validGradUnits{1};
    elseif ismember(opt.fromUnit,validSlewUnits)
        opt.toUnit = validSlewUnits{1};
    elseif ismember(opt.fromUnit,validB1Units)
        opt.toUnit = validB1Units{1};
    end
end

% Verify fromUnit and toUnit are in the same category.
if ismember(opt.fromUnit,validB1Units),       fromCat = 'B1';
elseif ismember(opt.fromUnit,validGradUnits), fromCat = 'gradient';
elseif ismember(opt.fromUnit,validSlewUnits), fromCat = 'slew rate';
end

if ismember(opt.toUnit,validB1Units),         toCat = 'B1';
elseif ismember(opt.toUnit,validGradUnits),   toCat = 'gradient';
elseif ismember(opt.toUnit,validSlewUnits),   toCat = 'slew rate';
end

if ~strcmp(fromCat,toCat)
    error('mr.convert: fromUnit ''%s'' (%s) and toUnit ''%s'' (%s) are in different unit categories.', ...
        opt.fromUnit, fromCat, opt.toUnit, toCat);
end

% Convert to standard units
switch opt.fromUnit
    case {'Hz','Hz/m','Hz/m/s'}
        standard = in;
    case {'mT','mT/m'};
        standard = in*1e-3*opt.gamma;
    case {'uT'};
        standard = in*1e-6*opt.gamma;
    case 'rad/ms/mm'
        standard = in*1e6/(2*pi);
    case {'T','mT/m/ms','T/m/s'}
        standard = in*opt.gamma;
    case 'rad/ms/mm/ms'
        standard = in*1e9/(2*pi);
end

% Convert from standard units
switch opt.toUnit
    case {'Hz','Hz/m','Hz/m/s'}
        out = standard;
    case {'mT','mT/m'}
        out = 1e3*standard/opt.gamma;
    case 'uT'
        out = 1e6*standard/opt.gamma;
    case 'rad/ms/mm'
        out = standard*2*pi*1e-6;
    case {'T','mT/m/ms','T/m/s'}
        out = standard/opt.gamma;
    case 'rad/ms/mm/ms'
        out = standard*2*pi*1e-9;
end