%%% Thumb indices consist of white text on a rectangular colored background. The
%%% font-size is 75% of the size of thumb height.
\newif\ifthumb
\newlength\thumbheight
\setlength\thumbheight{24pt}
\newlength\thumbedge
\setlength\thumbedge{4pt}
\newlength\thumbhspace
\setlength\thumbhspace{36pt}
\newlength\thumbvspace
\setlength\thumbvspace{2\thumbheight}

\newlength\thumbwidth
\setlength\thumbwidth{36pt}
\newlength\thumbspacing
\setlength\thumbspacing{2\thumbheight}

%% We need the TikZ library calc to calculate the coordinates of the thumb
%% indices.
\usetikzlibrary{calc}

%% The lthumb command prints the current chapter number in a thumb index on the
%% left (even) page.
\newcommand*\lthumb{%
    \ifthumb%
        \begin{tikzpicture}[remember picture,overlay]
            \coordinate (top margin) at (0pt,1in+\topmargin+\headheight+\headsep);
            \coordinate (left margin) at (1in+\evensidemargin,0pt);
            %% Calculate the corners of the thumb index based on the current
            %% chapter number.
            \coordinate (top left) at ($(current page.north west)-(top margin)-(0pt,\value{chapter}\thumbvspace-\thumbvspace)$);
            \coordinate (bottom right) at ($(top left)+(left margin)-(\thumbhspace,\thumbheight)$);
            %% Shift the left edge to prevent the rounded corner from showing.
            \coordinate (top left) at ($(top left)-(\thumbedge,0pt)$);
            %% Draw the thumb index.
            \fill[fill=thumb,rounded corners=\thumbedge](top left) rectangle (bottom right);
            %% Print the chapter number at the center right in the thumb index.
            \coordinate (center right) at ($(bottom right)+(0pt,0.5\thumbheight)$);
            \node at (center right)[anchor=east,inner sep=2\thumbedge]{
                \titlefont\bfseries\color{tudelft-white}
                \fontsize{0.75\thumbheight}{0.75\thumbheight}\selectfont
                \thechapter
            };
        \end{tikzpicture}%
    \fi%
}

%% rthumb draws a thumb index on the right (odd) page.
\newcommand*\rthumb{%
    \ifthumb%
        \begin{tikzpicture}[remember picture,overlay]
            \coordinate (top margin) at (0pt,1in+\topmargin+\headheight+\headsep);
            \coordinate (right margin) at (1in+\evensidemargin,0pt);
            %% Calculate the corners of the thumb index based on the current
            %% chapter number.
            \coordinate (top right) at ($(current page.north east)-(top margin)-(0pt,\value{chapter}\thumbvspace-\thumbvspace)$);
            \coordinate (bottom left) at ($(top right)-(right margin)-(-\thumbhspace,\thumbheight)$);
            %% Shift the left right to prevent the rounded corner from showing.
            \coordinate (top right) at ($(top right)+(\thumbedge,0pt)$);
            %% Draw the thumb index.
            \fill[fill=thumb,rounded corners=\thumbedge](top right) rectangle (bottom left);
            %% Print the chapter number at the center right in the thumb index.
            \coordinate (center left) at ($(bottom left)+(0pt,0.5\thumbheight)$);
            \node at (center left)[anchor=west,inner sep=2\thumbedge]{
                \titlefont\bfseries\color{tudelft-white}
                \fontsize{0.75\thumbheight}{0.75\thumbheight}\selectfont
                \thechapter
            };
        \end{tikzpicture}%
    \fi%
}