Started wiriting the background on Git

article.tex

@@ -2,22 +2,18 @@
 
 \pdfminorversion=4
 
-\usepackage{fullpage}
 \usepackage{tikz}
-\usetikzlibrary{fit}
+\usetikzlibrary{fit,patterns,decorations.pathreplacing}
 
-\def\DeflateHeader{\begingroup
-  \pdfcompresslevel=0\relax
-  \pdfobjcompresslevel=0\relax
-    \immediate\pdfobj stream
-        file {.DEFLATEDummyHeader}
-\endgroup}
+\usepackage[T1,T2A]{fontenc}
+\usepackage[utf8]{inputenc}
+\usepackage[russian,english]{babel} %Last language is default and active.
+\usepackage{graphicx}
 
-\def\DeflateHeaderObj{\begingroup
-  \pdfcompresslevel=0\relax
-  \pdfobjcompresslevel=0\relax
-    {\catcode`\%=12\immediate\pdfliteral{%RRRRR
-}}\endgroup}
+\def\thindivider{\centerline{\tiny%%
+-- --- --- -- ~~~ --- --- --- ~~~ --- -- --- -- ~~~~~~
+--- --- --- ~~~  -- --- -- ~~~~~~
+--- --- -- ~~~ --- ~~~ -- -- --- -- ~~~ --- --- ---}}
 
 \begin{document}
 
@@ -34,21 +30,129 @@
 repo directly from it. Its purpose is to easily sneakernet pushes or
 even whole repositories across air gaps.
 
+\thindivider
+
+Neighbors, it's possible to create a PDF that is also a Git
+repository.
+
+\begin{center}
+\begingroup
+\setbox9=\hbox{\footnotesize\verb|Receiving objects: 100% (174/174), 103.48 KiB | 0 bytes/s, done.|}
+\begin{minipage}{\wd9}
+\footnotesize\begin{verbatim}
+$ git clone article.pdf foo
+Cloning into 'foo'...
+Receiving objects: 100% (174/174), 103.48 KiB | 0 bytes/s, done.
+Resolving deltas: 100% (100/100), done.
+$ cd foo
+$ ls
+article.tex
+article.pdf
+\end{verbatim}
+\end{minipage}
+\endgroup
+\end{center}
+
+\section{The Git Bundle File Format}
+
 The file format for Git bundles doesn't appear to be formally
 specified anywhere, however, inspecting \texttt{bundle.c} reveals that
 it's relatively straightforward:
+\def\returnkey{\hspace*{1pt}{\setbox9=\hbox{$\hookleftarrow$}\tikz\node[text width=\wd9,fill=lightgray,rounded corners=1pt] at (0,0){$\hookleftarrow$};}\hspace*{1pt}}
 \begin{center}
 \begin{tikzpicture}
-  \node[text width=0.8\hsize] at (0,0) (sig) {\footnotesize\texttt{\# v2 git bundle}};
-  \node[inner sep=0mm,fit=(sig),draw,rounded corners,dashed,minimum width=0.8\hsize] (sigbox) {};
+  \node[text width=0.9\hsize] at (0,0) (sig) {\footnotesize\tt\# v2 git bundle{\tiny\returnkey}};
+  \node[inner sep=0mm,fit=(sig),draw,rounded corners,dashed,minimum width=0.9\hsize] (sigbox) {};
   \node[draw,rounded corners,fill=white] at (sigbox.north) (siglabel) {\footnotesize Git Bundle Signature};
-  \node[text width=0.8\hsize,anchor=north west,yshift=-1mm] at (sig.south west) (b1) {\scriptsize\tt 3aa340a2e3d125ab6703e5c9bdfede2054a9c0c5 refs/heads/master};
-  \node[text width=0.8\hsize,anchor=north west,yshift=-1mm] at (b1.south west) (b2) {\tt 3aa340a2e3d125ab6703e5c9bdfede2054a9c0c5 refs/remotes/origin/master};
-  \node[text width=0.8\hsize,anchor=north west,yshift=-1mm] at (b2.south west) (b3) {\tt 4146cfe2fe9249fc14623f832587efe197ef5d2d refs/stash};
-  \node[text width=0.8\hsize,anchor=north west,yshift=-1mm] at (b3.south west) (b4) {\tt babdda4735ef164b7023be3545860d8b0bae250a HEAD};
-  \node[inner sep=0mm,fit=(b1) (b4),draw,rounded corners,dashed,minimum width=0.8\hsize] (digestbox) {};
+  \node[text width=0.9\hsize,anchor=north west,yshift=-0.5mm] at (sig.south west) (b1) {\footnotesize\tt 3aa340a2e3d125ab6703e5c9bdfede2054a9c0c5 refs/heads/master{\tiny\returnkey}};
+  \node[text width=0.9\hsize,anchor=north west,yshift=-0.5mm] at (b1.south west) (b2) {\footnotesize\tt 3aa340a2e3d125ab6703e5c9bdfede2054a9c0c5 refs/remotes/origin/master{\tiny\returnkey}};
+  \node[text width=0.9\hsize,anchor=north west,yshift=-0.5mm] at (b2.south west) (b3) {\footnotesize\tt 4146cfe2fe9249fc14623f832587efe197ef5d2d refs/stash{\tiny\returnkey}};
+  \node[text width=0.9\hsize,anchor=north west,yshift=-0.5mm] at (b3.south west) (b4) {\footnotesize\tt babdda4735ef164b7023be3545860d8b0bae250a HEAD{\tiny\returnkey}};
+  \node[inner sep=0mm,fit=(b1) (b4),draw,rounded corners,dashed,minimum width=0.9\hsize] (digestbox) {};
   \node[rotate=-90,draw,rounded corners,fill=white] at (digestbox.east) {\footnotesize Digest};
+  \node[text width=0.9\hsize,anchor=north west,yshift=-0.5mm] at (b4.south west) (empty) {{\tiny\returnkey}\footnotesize\ };
+  \node[text width=0.9\hsize,anchor=north west,yshift=-0.5mm] at (empty.south west) (pack) {{\footnotesize\tt PACK}$\ldots$};
+  \node[inner sep=0mm,fit=(pack),draw,rounded corners,dashed,minimum width=0.9\hsize] (packbox) {};
+  \node[draw,rounded corners,fill=white] at (pack.south) {\footnotesize Git Packfile};
 \end{tikzpicture}
 \end{center}
+Git uses its Packfile format to compress the objects in its database,
+as well as to reduce network bandwidth when pushing and pulling, which
+makes it an obvious choice for use inside bundles. This of course
+raises the question: What is the format for a Git Packfile?
+
+Git does have some internal documentation in
+\begin{center}
+  \texttt{Documentation/technical/pack-format.txt}
+\end{center}
+however, it is rather sparse, and does not provide enough detail to
+fully parse the format. The documentation also has some
+``observations'' that suggest it wasn't even written by the file
+format's creator and instead was written by a developer who was later
+trying to make sense of the code.
+
+Luckily, Aditya Mukerjee already had to reverse engineer the file
+format for his GitGo clean-room implementation of Git, and he wrote an
+excellent blog entry about
+it\footnote{\texttt{https://codewords.recurse.com/issues/three/unpacking-git-packfiles}}.
+
+\begin{center}
+{\small
+\def\char#1{\,`\texttt{#1}'\,}
+\setbox9=\hbox{\char{K}}
+\newdimen\bytewidth\bytewidth=\wd9
+\def\byte#1{\hbox to \bytewidth{\hfil\texttt{#1}\hfil}}
+\def\desc#1#2{\hbox to #1\bytewidth{\hfil #2\hfil}}
+\def\underbrace#1{\draw [
+    thick,
+    decoration={
+        brace,
+        mirror,
+        raise=2pt
+    },
+    decorate
+] ([xshift=1pt]#1.base west) -- ([xshift=-1pt]#1.base east) 
+node (#1label) [pos=0.5,anchor=north,yshift=-2pt]}
+\begin{tikzpicture}
+\node[inner sep=0pt,minimum width=4\bytewidth] at (0,0) (pack) {\char{P}\char{A}\char{C}\char{K}};
+\node[inner sep=0pt,anchor=base west] at (pack.base east) (version) {\byte{00}\byte{00}\byte{00}\byte{02}};
+\node[inner sep=0pt,anchor=base west] at (version.base east) (numobj) {\desc{4}{\# objects}};
+\underbrace{pack}{\tiny magic};
+\underbrace{version}{\tiny version};
+\underbrace{numobj}{\tiny big-endian 4 byte int};
+\node[inner sep=0pt,anchor=north west] at (packlabel.south -| pack.west) (chunks) {one data chunk for each object};
+\node[inner sep=0pt,anchor=north west] at ([yshift=-0.5\baselineskip]chunks.south west) (sha1) {20-byte SHA-1 of all the previous data in the pack};
+\end{tikzpicture}}
+\end{center}
+
+I hope you like Soviet holes, dear neighbor, because chasing this
+rabbit might remind you of \foreignlanguage{russian}{Кольская}. If you
+grew bored reading Tristram Shandy, it's safe to skip to the next
+section, 'cause it's turtles all the way down.
+
+\begin{center}
+\includegraphics[width=0.5\hsize]{RazvodityeKrolikov}
+\end{center}
+
+Okay, the next step is to figure out the ``chunk'' format.  The chunk
+header is variable length, and can be as small as one byte. It encodes
+the type of object and the \emph{uncompressed} size of the object. If
+the object is a \textit{delta} (\textit{i.e.}, a diff, as opposed to a
+complete object), the header is followed by either the SHA-1 hash of
+the base object to which the delta should be applied, or a byte
+reference within the packfile for the start of the base object. The
+remainder of the chunk consists of the object data, zlib-compressed.
+\begin{center}
+{\newcount\bitnum\bitnum=0
+\begin{tikzpicture}
+\node[coordinate] at (0,0) (b0) {};
+\foreach \i in {1,0,1,1,0,1,0,0,0,1,0,1,0,1,1,0} {
+  {\newcount\prevbit\prevbit=\bitnum
+  \global\advance\bitnum by 1
+  \node[anchor=west] at (b\the\prevbit.east) (b\the\bitnum) {\i};}
+  \draw ([xshift=1pt]b\the\bitnum.south west) -- ([xshift=-1pt]b\the\bitnum.south east);
+}
+\end{tikzpicture}}
+\end{center}
 
 \end{document}