$ mkdir merkle
$ cd merkle
$ echo 'This is the beginning' > README
$ git init
$ git add .
$ git -m 'Initial Commit'
$ find .git/objects/ -type f
.git/objects/1b/9f426a8407ffee551ad2993c5d7d3780296353
.git/objects/09/8e6de29daf4e55f83406b49f5768df9bc7d624
.git/objects/1a/06ce381ac14f7a5baa1670691c2ff8a73aa6da

$ cat .git/objects/1b/9f426a8407ffee551ad2993c5d7d3780296353
xKOR02,V¢T¤̼¼̼t.

$ python2 -c 'import sys,zlib; \
  print zlib.decompress(sys.stdin.read());' \
    < .git/objects/1a/06ce381ac14f7a5baa1670691c2ff8a73aa6da
commit 195tree 098e6de29daf4e55f83406b49f5768df9bc7d624
author Tyler Cipriani <tcipriani@wikimedia.org> 1458604120 -0700
committer Tyler Cipriani <tcipriani@wikimedia.org> 1458604120 -0700

Initial Commit

$ echo 'message' | sha1sum
1133e3acf0a4cbb9d8b3bfd3f227731b8cd2650b  -
$ echo 'message' | sha1sum
1133e3acf0a4cbb9d8b3bfd3f227731b8cd2650b  -
$ echo 'message1' | sha1sum
c133514a60a4641b83b365d3dc7b715dc954e010  -

$  python2 -c 'import sys,zlib; \
print zlib.decompress(sys.stdin.read());' \
< .git/objects/1a/06ce381ac14f7a5baa1670691c2ff8a73aa6da | \
od -c
 0000000   c   o   m   m   i   t       1   9   5  \0   t   r   e   e    
 0000020   0   9   8   e   6   d   e   2   9   d   a   f   4   e   5   5
 0000040   f   8   3   4   0   6   b   4   9   f   5   7   6   8   d   f
 0000060   9   b   c   7   d   6   2   4  \n   a   u   t   h   o   r    
 0000100   T   y   l   e   r       C   i   p   r   i   a   n   i       <
 0000120   t   c   i   p   r   i   a   n   i   @   w   i   k   i   m   e
 0000140   d   i   a   .   o   r   g   >       1   4   5   8   6   0   4
 0000160   1   2   0       -   0   7   0   0  \n   c   o   m   m   i   t
 0000200   t   e   r       T   y   l   e   r       C   i   p   r   i   a
 0000220   n   i       <   t   c   i   p   r   i   a   n   i   @   w   i
 0000240   k   i   m   e   d   i   a   .   o   r   g   >       1   4   5
 0000260   8   6   0   4   1   2   0       -   0   7   0   0  \n  \n   I
 0000300   n   i   t   i   a   l       C   o   m   m   i   t  \n  \n
 0000317

$ printf 'tree 098e6de29daf4e55f83406b49f5768df9bc7d62k4\n' >> commit-msg
$ printf 'author Tyler Cipriani <tcipriani@wikimedia.org> 1458604120 -0700\n' >> commit-msg
$ printf 'committer Tyler Cipriani <tcipriani@wikimedia.org> 1458604120 -0700\n' >> commit-msg
$ printf '\nInitial Commit\n' >> commit-msg

$ sha1sum <(cat \
    <(printf "commit ") \
    <(wc -c < commit-msg | tr -d '\n') \
    <(printf '%b' '\0') commit-msg)
1a06ce381ac14f7a5baa1670691c2ff8a73aa6da  /dev/fd/63

$ export COMMIT_HASH=$(sha1sum <(cat <(printf "commit ") <(wc -c < commit-msg | tr -d '\n') <(printf '%b' '\0') commit-msg) | cut -d' ' -f1)
$ find ".git/objects/${COMMIT_HASH:0:2}" -type f -name "${COMMIT_HASH:(-38)}"
.git/objects/1a/06ce381ac14f7a5baa1670691c2ff8a73aa6da

$ cat .git/HEAD
ref: refs/heads/master

$ cat .git/refs/heads/master
1a06ce381ac14f7a5baa1670691c2ff8a73aa6da

$ git cat-file -p 1a06ce381ac14f7a5baa1670691c2ff8a73aa6da
tree 098e6de29daf4e55f83406b49f5768df9bc7d624
author Tyler Cipriani <tcipriani@wikimedia.org> 1458604120 -0700
committer Tyler Cipriani <tcipriani@wikimedia.org> 1458604120 -0700

$ git cat-file -p 098e6de29daf4e55f83406b49f5768df9bc7d624
100644 blob 1b9f426a8407ffee551ad2993c5d7d3780296353    README

$ git cat-file -p 1b9f426a8407ffee551ad2993c5d7d3780296353
This is the beginning

var gitDag = {
    // blob (add .b for blob)
    '1b9f426a8407ffee551ad2993c5d7d3780296353.b': [],
    // tree (.t == tree) is a hash that includes the hash from blob
    '098e6de29daf4e55f83406b49f5768df9bc7d624.t': ['1b9f426a8407ffee551ad2993c5d7d3780296353.b'],
    // commit (.c == commit) is a hash that includes the hash from tree
    '1a06ce381ac14f7a5baa1670691c2ff8a73aa6da.c': ['098e6de29daf4e55f83406b49f5768df9bc7d624.t'],
};

makeGraph('.merkle-2', forceFormat(gitDag));

#!/usr/bin/env nodejs
/* makeDag - creates a JSON dependency graph from .git/objects */

var glob = require('glob'),
    fs = require('fs'),
    zlib = require('zlib');

var types = ['tree', 'commit', 'blob'],
    treeRegex = {
        // 100644 README\0[20 byte sha1]
        regex: /[0-9]+\s[^\0]+\0((.|\n){20})/gm,
        fn: function(sha) {
            var buf = new Buffer(sha[1], 'binary');
            return buf.toString('hex') + '.b';
        }
    },
    commitRegex = {
        // tree 098e6de29daf4e55f83406b49f5768df9bc7d624
        regex: /(tree|parent)\s([a-f0-9]{40})/gm,
        fn: function(sha) {
            if (sha[1] === 'tree') {
                return sha[2] + '.t';
            }
            return sha[2] + '.c';
        }
    },
    total = 0,
    final = {};

// determine file type, parse out SHA1s
var handleObjects = function(objData, name) {
    types.forEach(function(type) {
        var re, regex, match, key;

        if (!objData.startsWith(type)) { return; }

        key = name + '.' + type[0];
        final[key] = [];
        if (type === 'tree') { objType = treeRegex; }
        if (type === 'commit') { objType = commitRegex; }
        if (type === 'blob') { return; }

        // Remove the object-type and size from file
        objData = objData.split('\0');
        objData.shift();
        objData = objData.join('\0');

        // Recursive regex match remainder
        while ((match = objType.regex.exec(objData)) !== null) {
            final[key].push(objType.fn(match));
        }
    });

    // Don't output until you've got it all
    if (Object.keys(final).length !== total) {
        return;
    }

    // Output what ya got.
    console.log(final);
};

// Readable object names not file names
var getName = function(file) {
    var fileParts = file.split('/'),
        len = fileParts.length;
    return fileParts[len - 2] + fileParts[len - 1];
}

// Inflate the deflated git object file
var handleFile = function(file, out) {
    var name = getName(file);

    fs.readFile(file, function(e, data) {
        zlib.inflate(data, function(e, data) {
            if (e) { console.log(file, e); return; }
            handleObjects(data.toString('binary'), name);
        });
    });
};

// Sort through the gitobjects directory
var handleFiles = function(files) {
    files.forEach(function(file) {
        fs.stat(file, function(e, f) {
            if (e) { return; }
            if (f.isFile()) {
                // Don't worry about pack files for now
                if (file.indexOf('pack') > -1) { return; }
                total++;
                handleFile(file);
            }
        });

    });
};

(function() {
    glob('.git/objects/**/*', function(e, f) {
        if (e) { throw e; }
        handleFiles(f);
    });
})();