Riddler Classic: August 19, 2022

function res = fullDosage(N)
% Returns simulated result of whether a full dosage was taken given N days
% to take N*1.5 pills. Assume N*1.5 is a positive integer.

pillRow = N*1.5; % Assume integer
numPills = pillRow;
pills = ones(numPills, 2); % Each column represents half a pill
for k = 1:N-1
    % Each day one takes equivalent of three half-pills
    pillVec = randperm(numPills);
    pillRows = pillVec(1:3); % First three pills ready to be taken
    if sum(pills(pillRows(1), :)) == 2
        % Full pill
        pills(pillRows(1), :) = 0;
        if sum(pills(pillRows(2), :)) == 2
            % Second is also a full pill
            pills(pillRows(2), 1) = 0; % Take half the second pill
        else
            % Second is a half pill
            pills(pillRows(2), :) = 0;
        end
    else
        % Half pill for the first pill
        pills(pillRows(1), :) = 0;
        if sum(pills(pillRows(2), :)) == 2
            % Second is a full pill, take all
            pills(pillRows(2), :) = 0;
        else
            % Second is also a half pill
            pills(pillRows(2), 2) = 0;
            % We need third pill
            if sum(pills(pillRows(3), :)) == 2
                % Take half of third pill
                pills(pillRows(3), 1) = 0;
            else
                % Take the remaining half of third pill
                pills(pillRows(3), :) = 0;
            end
        end
    end

    % Update pill vec to get rid of all pills with zeros
    pills = pills(any(pills, 2), :);
    [numPills, ~] = size(pills);
end

% On the last day, check whether we have two pills or three (only options)
if numPills == 2
    res = 1;
else
    res = 0;
end

end