function [] = boxfib(n)
% boxfib(n): animated trace of fib(n) with $fib$ renamed to $f$ --
%     user must type a key to advance from one movie frame to the next

if nargin < 1
    n = 3;
end

if n>3
    warning('boxfib does not work well for n>3')
end

big.command = []; 
fib(n, big);

function x = fib(p,big)

code = strvcat(...
'if n < 2', ...
'  x = n;', ...
'else', ...
'                      ', ...
'  x = f(n-1) + f(n-2);', ...
'end');

code = [repmat(' ',6,4) code];
mename = fibname(p);

me.act_ = 'draw activation record';
me.header_ = strvcat(' ', 'function x = f(n)');
me.code_ = [];
big = setfield(big, mename, me); boxdisp(big);

me.act_ = strvcat('draw parameter;', 'fill with argument');
me.n = p(1);
big = setfield(big, mename, me); boxdisp(big);

me.act_ = 'run function body';

me.code_ = code;
me.code_ = code; me.code_(1,1:3) = '-->';
big = setfield(big, mename, me); boxdisp(big);

if p(1)<2
    me.code_ = code; me.code_(2,1:3) = '-->';
    me.x = me.n;
    big = setfield(big, mename, me); boxdisp(big);
else
    me.code_ = code; me.code_(3,1:3) = '-->';
    big = setfield(big, mename, me); boxdisp(big);

    me.code_ = code; me.code_(4,1:3) = '-->';
    me.code_(4, 11:16) = '?';
    big = setfield(big, mename, me); boxdisp(big);

    left = fib([p(1)-1 p], big);
    sleft = num2str(left);
    me.code_(4, :) = ' ';  me.code_(4,1:3) = '-->';
    me.code_(4, 10+(1:length(sleft))) = sleft;
    me.code_(4, 20:25) = '?';
    big = setfield(big, mename, me); boxdisp(big);

    right = fib([p(1)-2 p], big);
    sright = num2str(right);
    me.code_(4, :) = ' ';  me.code_(4,1:3) = '-->';
    me.code_(4, 10+(1:length(sleft))) = sleft;
    me.code_(4, 19+(1:length(sright))) = sright;
    big = setfield(big, mename, me); boxdisp(big);

    me.code_ = code; me.code_(5,1:3) = '-->';
    me.code_(4, 10+(1:length(sleft))) = sleft;
    me.code_(4, 19+(1:length(sright))) = sright;
    me.x = left + right;
    big = setfield(big, mename, me); boxdisp(big);
end

x = me.x;

me.code_ = code; me.code_(6,1:3) = '-->';
big = setfield(big, mename, me); boxdisp(big);

me.code_ = [];
me.act_ = strvcat('copy out return value;', 'kill activation record');

if length(p) > 1
    codename = [fibname(p(2:end)) '.code_'];
    hicode = getfield(big, codename);
    s = num2str(x);
    where = 10;
    if p(2) - p(1) == 2, where = 19; end
    hicode(4,where+(1:6)) = ' ';
    hicode(4,where+(1:length(s))) = s;
    big = setfield(big, codename, hicode);
end

me = rmfield(me,'n');
big = setfield(big, mename, me); boxdisp(big);

if length(p) == 1
    big = rmfield(big, mename);
    big.command.ans = x;
    boxdisp(big);
end

function s = fibname(p)
    s = ['f' num2str(fliplr(p), '_%1d')];

function [] = boxdisp(big)
    s = boxscope(big,1); % pre-compute to avoid delay between $clc$ and $disp$
    clc; disp(s); pause