\input regression-test.tex
\documentclass{article}
\usepackage{tikzphysics}
\newdimen\Actual
\newdimen\Expected
\newdimen\Difference
\newcommand{\AssertAnchor}[5]{%
  \pgfextractx{\Actual}{\pgfpointanchor{#1}{#2}}\Expected=#3\relax
  \Difference=\dimexpr\Actual-\Expected\relax
  \ifdim\Difference<0pt\Difference=-\Difference\fi
  \ifdim\Difference<0.03pt\TYPE{PASS: #5 x}\else\TYPE{FAIL: #5 x got \the\Actual expected \the\Expected}\fi
  \pgfextracty{\Actual}{\pgfpointanchor{#1}{#2}}\Expected=#4\relax
  \Difference=\dimexpr\Actual-\Expected\relax
  \ifdim\Difference<0pt\Difference=-\Difference\fi
  \ifdim\Difference<0.03pt\TYPE{PASS: #5 y}\else\TYPE{FAIL: #5 y}\fi}
\makeatletter
\newcommand{\AssertShape}[3]{%
  \edef\ActualShape{\csname pgf@sh@ns@#1\endcsname}%
  \def\ExpectedShape{#2}%
  \ifx\ActualShape\ExpectedShape\TYPE{PASS: #3}\else\TYPE{FAIL: #3}\fi}
\begin{document}
\START
\setbox0=\hbox{\begin{tikzpicture}
  \node[spring] (D) {};
  \AssertShape{D}{physicsspring}{spring dispatches to node shape}
  \AssertAnchor{D}{start}{-1.5cm}{0pt}{default start}
  \AssertAnchor{D}{end}{1.5cm}{0pt}{default end}
  \AssertAnchor{D}{north}{0pt}{4.5pt}{amplitude bounds}
  \node[physicsspring,minimum width=4cm,pre length=3mm,post length=5mm,
    amplitude=2mm] (S) {};
  \AssertAnchor{S}{coil-start}{-17mm}{0pt}{pre lead endpoint}
  \AssertAnchor{S}{coil-end}{15mm}{0pt}{post lead endpoint}
  \AssertAnchor{S}{axis-25}{-1cm}{0pt}{axis quarter}
  \AssertAnchor{S}{axis-50}{0pt}{0pt}{axis midpoint}
  \AssertAnchor{S}{axis-100}{2cm}{0pt}{axis end}
  \node[physics spring,spring length=2cm,pre length=0pt,post length=0pt,
    amplitude=1mm] (Short) {};
  \AssertAnchor{Short}{start}{-1cm}{0pt}{spring length alias}
  \AssertAnchor{S}{coil-start}{-17mm}{0pt}{saved lead survives next node}
  \AssertAnchor{S}{north}{0pt}{2mm}{saved amplitude survives next node}
  \node[spring,minimum width=4cm,rotate=90] (Vertical) {};
  \AssertAnchor{Vertical}{start}{0pt}{-2cm}{vertical start}
  \AssertAnchor{Vertical}{end}{0pt}{2cm}{vertical end}
  \node[spring,minimum width=4cm,rotate=30,anchor=start] (Tilt) at (0,0) {};
  \AssertAnchor{Tilt}{start}{0pt}{0pt}{rotated anchored start}
  \AssertAnchor{Tilt}{end}{3.4641cm}{2cm}{rotated anchored end}
  \node[spring,minimum width=4cm,xscale=-1,yscale=2,transform shape] (Scaled) {};
  \AssertAnchor{Scaled}{start}{2cm}{0pt}{mirrored start}
  \AssertAnchor{Scaled}{north}{0pt}{9pt}{scaled amplitude bound}
  \node (Contents) [node contents={},spring,spring length=2cm];
  \AssertShape{Contents}{physicsspring}{node contents before spring dispatches}
  \draw[spring] (0,-1)--node[above] (Label) {$k$} (3,-1);
  \AssertShape{Label}{rectangle}{path label remains ordinary node}
  \path (0,0)--node[midway,spring,spring length=2cm] (Inline) {} (4,0);
  \AssertShape{Inline}{physicsspring}{inline spring node dispatches}
  \node (Outer) {\tikz{\draw[spring] (0,0)--node[above] (Nested) {$k$} (2,0);}};
  \AssertShape{Nested}{rectangle}{nested spring path preserves label shape}
\end{tikzpicture}}
\setbox0=\hbox{\begin{tikzpicture}
  \tikzset{every spring/.style={spring length=5cm,amplitude=3mm}}
  \node[spring] (Hook) {};
  \AssertAnchor{Hook}{end}{2.5cm}{0pt}{spring hook length}
  \node[spring,spring length=2cm,amplitude=1mm] (Override) {};
  \AssertAnchor{Override}{end}{1cm}{0pt}{local length overrides hook}
  \AssertAnchor{Override}{north}{0pt}{1mm}{local amplitude overrides hook}
  \draw[physicsspring,pre length=0,post length=0] (0,0)--(3,0);
  \TYPE{PASS: path alias and zero leads compile after spring nodes}
  \draw[spring,pre length=3mm,post length=5mm,amplitude=2mm,
    segment length=4mm] (0,0)--(3,0)
    \pgfextra{
      \pgfmathsetlength{\Actual}{\pgfkeysvalueof{/pgf/decoration/pre length}}
      \Difference=\dimexpr\Actual-3mm\relax
      \ifdim\Difference<0pt\Difference=-\Difference\fi
      \ifdim\Difference<0.03pt\TYPE{PASS: path pre lead stays independent}\else\TYPE{FAIL: path pre lead}\fi
      \pgfmathsetlength{\Actual}{\pgfkeysvalueof{/pgf/decoration/post length}}
      \Difference=\dimexpr\Actual-5mm\relax
      \ifdim\Difference<0pt\Difference=-\Difference\fi
      \ifdim\Difference<0.03pt\TYPE{PASS: path post lead stays independent}\else\TYPE{FAIL: path post lead}\fi
    };

\end{tikzpicture}}
\setbox0=\hbox{\begin{tikzpicture}
  \node[spring] (Fresh) {};
  \AssertAnchor{Fresh}{end}{1.5cm}{0pt}{spring hook remains scoped}
  \node[spring,minimum height=1cm,outer sep=2pt] (Box) {};
  \AssertAnchor{Box}{north}{0pt}{\dimexpr5mm+2pt\relax}{reserved height and outer sep}
  \AssertAnchor{Box}{end}{1.5cm}{0pt}{attachment excludes outer sep}
\end{tikzpicture}}
\END
\end{document}