Initial commit

2021-03-31 15:48:06 +01:00 · 2021-03-31 15:48:06 +01:00 · d613db392a
commit d613db392a
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--- a/index.html
+++ b/index.html
@ -0,0 +1,704 @@
+<html>
+  <head>
+<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
+<title></title>
+<style>
+body { width: 63rem; font-size: 1.2rem; text-align:justify; }
+textarea { display:block; width: 63rem; height: 18rem; font-size: 1.2rem; }
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+.space { text-decoration: underline; color: brown; opacity: 0.5; }
+.deflated { color: red; }
+.directory { color: darkcyan; }
+.error { color: orangered; }
+</style>
+</head>
+<body>
+<a href="https://git-scm.com/book/en/v2/Git-Internals-Git-Objects">git-book</a>
+
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+           + "</span>";
+    } else {
+      return ___specialchars_and_colour(x);
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+  }
+  function ___format_entry(x) {
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+      + ___format_filepath(x[0])
+      + "</code></td><td>"
+      + (x[1] === null
+          ? '<span class="directory">Directory</span>'
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+  function ___filesystem_to_string(fs) {
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+      .map(___format_entry);
+    var id = global_element_id++;
+    return "Filesystem contents: " + entries.length + " files and directories. "
+      + '<a href="javascript: ___copyprintf_click(\'elem-'+id+'\');">'
+      + "Copy commands to recreate in *nix terminal"
+      + "</a>."
+      + "<br />"
+      + '<textarea id="elem-'+id+'" disabled="disabled" style="display:none">'
+      + ___specialchars(___filesystem_to_printf(fs))
+      + '</textarea>'
+      + "<table>" + entries.join('') + "</table>";
+  }
+  function ___copyprintf_click(id) {
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+    if (elem.style.display != "none") {
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+      elem.style.display = "inherit";
+      elem.focus();
+      elem.disabled = false;
+      elem.select();
+      elem.setSelectionRange(0, elem.value.length * 10); // for mobile devices?
+      document.execCommand('copy');
+      elem.disabled = true;
+    }
+  }
+  var global_filesystem=false;
+  function ___git_eval(current) {
+    var script = '';
+    for (i = 0; i <= current; i++) {
+      script += document.getElementById('in' + i).value;
+    }
+    script += "document.getElementById('out' + current).innerHTML = ___filesystem_to_string(filesystem); filesystem;";
+    try {
+      global_filesystem = eval(script);
+    } catch (e) {
+      var error = ___specialchars("" + e + "\n\n" + e.stack);
+      document.getElementById('out' + current).innerHTML = '<pre class="error">' + error + '</pre>';
+    }
+  }
+</script>
+
+<div id="lines"></div>
+
+<h2>Introduction</h2>
+<p>
+GIT is based on a simple model, with a lot of shorthands for common
+use cases. This model is sometimes hard to guess just from the
+everyday commands. To illustrate how GIT works, we'll implement a
+stripped down clone of GIT in a few lines of JavaScript.
+</p>
+
+<h2>The Operating System's filesystem</h2>
+<p>We will simulate the Operating System's filesystem with a very
+simple key-value store. In this very simple filesystem, directories
+are entries mapped to <code>null</code> and files are entries mapped
+to strings.</p>
+<textarea id="in0">
+var filesystem = {};
+</textarea>
+<input type="button" value="eval" onClick="___git_eval(0)">
+<div id="out0"></div>
+
+<p>The filesystem exposes functions to read an entire file, create or
+replace an entire file, and create a directory.</p>
+<textarea id="in1">
+function read(filename) {
+  return filesystem[filename];
+}
+
+function write(filename, data) {
+  return filesystem[filename] = ""+data;
+}
+
+function mkdir(dirname) {
+  return filesystem[dirname] = null;
+}
+</textarea>
+<input type="button" value="eval" onClick="___git_eval(1)">
+<div id="out1"></div>
+
+<p>It will be handy for some operations to list the contents of a
+directory.</p>
+<textarea id="in2">
+function listdir(dirname) {
+  var depth = dirname.split('/').length + 1;
+  var descendents = filesystem
+      .filter(filename => filename.startsWith(dirname + '/'));
+  var children = descendents
+      .map(filename => filename.split('/')[depth]);
+  // remove duplicates:
+  return Array.from(new Set(children));
+}
+</textarea>
+<input type="button" value="eval" onClick="___git_eval(2)">
+<div id="out2"></div>
+
+<h2>Example working directory</h2>
+<p>Our imaginary user will create a <code>proj</code> directory,
+and start filling in some files.</p>
+<textarea id="in3">
+var workdir='proj';
+mkdir('proj');
+write('proj/README', 'This is my Scheme project.\n');
+mkdir('proj/src');
+write('proj/src/main.scm', '(map (lambda (x) (+ x 1)) (list 1 2 3))\n');
+</textarea>
+<input type="button" value="eval" onClick="___git_eval(3)">
+<div id="out3"></div>
+
+<h2><code>git init</code> (creating <code>.git</code>)</h2>
+<p>The first thing to do is to initialize the GIT directory.
+For now, only the <code>.git</code> folder is needed, The rest
+of the function implementing <code>git init</code> will be
+implemented later.</p>
+<textarea id="in4">
+function join_paths(a, b) {
+  return (a == "") ? b : (a + "/" + b);
+}
+
+function git_init_mkdir() {
+  mkdir(join_paths(workdir, '.git'));
+}
+
+git_init_mkdir();
+</textarea>
+<input type="button" value="eval" onClick="___git_eval(4)">
+<div id="out4"></div>
+
+<h2><code>git hash-object</code> (storing a copy of a file in <code>.git</code>)</h2>
+<p>The most basic element of a GIT repository is an object. It is a
+copy of a file that is stored in GIT&apos;s database. That copy is
+stored under a unique name. The unique name is obtained by hashing the
+contents of the file. <!-- or have a hash oracle that always returns a
+new number. --></p>
+<textarea id="in5">
+function hash_object(must_write, type, is_data, path_or_data) {
+  var data = is_data ? path_or_data : read(workdir + "/" + path_or_data);
+
+  object_contents = type + ' ' + data.length + '\0' + data;
+
+  var hash = sha1(object_contents)
+
+  if (must_write) {
+    mkdir(workdir + '/.git/objects');
+    mkdir(workdir + '/.git/objects/' + hash.slice(0,2));
+    var object_path = workdir + '/.git/objects/' + hash.slice(0,2) + '/' + hash.slice(2);
+    write(object_path, deflate(object_contents));
+  }
+
+  return hash;
+}
+</textarea>
+<input type="button" value="eval" onClick="___git_eval(5)">
+<div id="out5"></div>
+
+<p>So far, our GIT database does not know about any of the user&apos;s
+files. In order to add the contents of the <code>README</code> file in
+the database, we use <code>git hash-object -w -t blob README</code>,
+where <code>-w</code> tells GIT to <em>write</em> the object in its
+database, and <code>-t blob</code> indicates that we want to create
+a <em>blob</em> object, i.e. the contents of a file.</p>
+<textarea id="in6">
+// git hash-object -w -t blob README
+hash_object(true, 'blob', false, 'README');
+</textarea>
+<input type="button" value="eval" onClick="___git_eval(6)">
+<p>The objects stored in the GIT database are compressed with zlib
+(using the "deflate" compression method). The filesystem view shows
+the <span class="deflated">deflated:</span> followed by the uncompressed
+data. Click on the file contents to toggle between this pretty-printed
+view and the raw compressed data.
+</p>
+<div id="out6"></div>
+
+<p>You will notice that the database does not contain the name of the
+file, only its contents, stored under a unique identifier which is
+derived by hashing its contents. Let&apos;s add the second user file
+to the database.</p>
+<textarea id="in7">
+// git hash-object -w -t blob src/main.scm
+hash_object(true, 'blob', false, 'src/main.scm');
+</textarea>
+<input type="button" value="eval" onClick="___git_eval(7)">
+<div id="out7"></div>
+
+<h2><code>zlib</code> compression</h2>
+<p>The real implementation of GIT compresses objects with zlib. To
+view a zlib-compressed object in your terminal, simply write this
+declaration in your shell, and then call e.g. <code>unzlib
+.git/objects/95/d318ae78cee607a77c453ead4db344fc1221b7</code></p>
+
+<pre>
+unzlib() {
+  python -c \
+    "import sys,zlib; \
+     sys.stdout.buffer.write(zlib.decompress(open(sys.argv[1], 'rb').read()));" \
+    "$1"
+}
+</pre>
+
+<h2>Storing trees (list of hashed files and subtrees)</h2>
+<p>Now GIT knows about the contents of both of the user's
+files, but it would be nice to also store the filenames.
+This is done by creating a <em>tree</em> object</p>
+
+<p>A tree object can contain files (by associating the file's blob to its name), or directories (by associating the hash of other subtrees to their name).
+  The mode (<code>100644</code> for the file and <code>40000</code>) incidates the permissions, and is given in octal using <a href="https://unix.stackexchange.com/a/145118/19059">the values used by *nix</a></p>
+
+<textarea id="in8">
+// base_directory is a string
+// filenames is a list of strings
+// subtrees is a list of {name, hash} objects.
+function store_tree(base_directory, filenames, subtrees) {
+  var get_file_hash = filename =>
+    from_hex(hash_object(true, 'blob', false, join_paths(base_directory, filename)));
+
+  var blobs = filenames.map(filename =>
+    "100644 " + filename + "\0" + get_file_hash(filename));
+
+  var trees = subtrees.map(subtree =>
+    "40000 " + subtree.name + "\0" + from_hex(subtree.hash));
+  
+  tree_contents = blobs.join('') + trees.join('');
+
+  // cat tree_contents | git hash-object -w -t tree --stdin
+  return hash_object(true, 'tree', true, tree_contents);
+}
+</textarea>
+<input type="button" value="eval" onClick="___git_eval(8)">
+<div id="out8"></div>
+
+This function needs a small utility to convert hashes encoded in hexadecimal to a binary form.
+<textarea id="in9">
+  function from_hex(hex) {
+    var hex = String(hex);
+    var str = ""
+    for (var i = 0; i < hex.length; i+=2) {
+      str += String.fromCharCode(parseInt(hex.substr(i, 2), 16));
+    }
+    return str;
+  }
+</textarea>
+<input type="button" value="eval" onClick="___git_eval(9)">
+<div id="out9"></div>
+
+<textarea id="in10">
+//hash_src_tree = store_tree("src/", ["main.scm"], []);
+//                store_tree("",     ["README"],   [{name:"src", hash:hash_src_tree}]);
+</textarea>
+<input type="button" value="eval" onClick="___git_eval(10)">
+<div id="out10"></div>
+
+Making trees out of the subfolders one by one is cumbersome. Here's a utility function which takes a list of paths, and builds a tree from those.
+
+<textarea id="in11">
+function store_tree_from_paths(paths) {
+  var hierarchy = { subfolders: {}, files: [] };
+  for (var i = 0; i < paths.length; i++) {
+    var path_components = paths[i].split('/');
+    var h = hierarchy;
+    for (var j = 0; j < path_components.length - 1; j++) {
+      if (! h.subfolders.hasOwnProperty(path_components[j])) {
+        h.subfolders[path_components[j]] = { subfolders: {}, files: [] };
+      }
+      h = h.subfolders[path_components[j]];
+    }
+    h.files.push(path_components[i]);
+  }
+
+  var to_tree = function(base_directory, hierarchy) {
+    var subtrees = [];
+    for (var i in hierarchy.subfolders) {
+      if (hierarchy.subfolders.hasOwnProperty(i)) {
+        subtrees.push({ name: i, hash: to_tree(join_paths(base_directory, i), hierarchy.subfolders[i]) });
+      }
+    }
+    return store_tree(base_directory, hierarchy.files, subtrees);
+  }
+  return to_tree("", hierarchy);
+}
+
+// git add README src/main.scm
+store_tree_from_paths(["README", "src/main.scm"]);
+</textarea>
+<input type="button" value="eval" id="initial-focus" onClick="___git_eval(11)">
+<div id="out11"></div>
+  
+<p>Now that the GIT database contains the entire tree for the current version,
+a commit can be created. A commit contains
+<ul>
+  <li>a pointer to the tree</li>
+  <li>a pointer to the previous ("parent") commit (or to multiple parent commits merging them)</li>
+  <li>information about the author (the person who initially wrote the code)</li>
+  <li>information about the committer (the person who adds the code to the GIT
+    database, often the same person as the author, but it can be a different person
+    e.g. when someone else makes changes to the history or applies a patch recieved
+    by e-mail)</li>
+  <li>a description</li>
+</ul> 
+</p>
+<textarea id="in12">
+function store_commit(tree, parents, author, committer, message) {
+  var commit_contents = '';
+  commit_contents += 'tree ' + tree + '\n';
+  for (var i = 0; i < parents.length; i++) {
+    commit_contents += 'parent ' + parents[i] + '\n';
+  }
+  commit_contents += 'author ' + author.name
+                  + ' <' + author.email + '> '
+                  + format_date(author.date) + ' '
+                  + format_timezone(author.timezoneMinutes) + '\n';
+  commit_contents += 'committer ' + committer.name
+                  + ' <' + committer.email + '> '
+                  + format_date(committer.date) + ' '
+                  + format_timezone(committer.timezoneMinutes) + '\n';
+  commit_contents += '\n';
+  commit_contents += '' + message + (message[message.length-1] == '\n' ? '' : '\n');
+  // cat commit_contents | git hash-object -w -t commit --stdin
+  return hash_object(true, 'commit', true, commit_contents);
+}
+
+function format_date(d) {
+  return Math.floor((+d) / 1000);
+}
+function left_pad(s, char, len) {
+  while ((''+s).length < len) { s = '' + char + s; }
+  return s;
+}
+function format_timezone(tm) {
+  var h = Math.floor(Math.abs(+tm)/60);
+  var m = Math.abs(+tm)%60;
+  return (tm >= 0 ? '+' : '-') + left_pad(h, '0', 2) + left_pad(m, '0', 2);
+}
+</textarea>
+<input type="button" value="eval" onClick="___git_eval(12)">
+<div id="out12"></div>
+
+<p>It is now possible to store a commit in the database. This saves
+  a copy of the tree along with some metadata about this version.
+  The first commit has no parent, which is represented by passing
+  the empty list.</p>
+<textarea id="in13">
+store_commit(
+  store_tree_from_paths(["README", "src/main.scm"]),
+  [],
+  {name:'Example User', email:'user@example.com', date:new Date(1617120803000), timezoneMinutes: +60},
+  {name:'Example User', email:'user@example.com', date:new Date(1617120803000), timezoneMinutes: +60},
+  'Initial commit');
+</textarea>
+<input type="button" value="eval" onClick="___git_eval(13)">
+<div id="out13"></div>
+
+<h2>Branches</h2>
+<textarea id="in14">
+</textarea>
+<input type="button" value="eval" onClick="___git_eval(14)">
+<div id="out14"></div>
+
+<h2><code>HEAD</code></h2>
+<p>
+  write here
+</p>
+<textarea id="in15">
+  write(join_paths(workdir, '.git/HEAD'), 'ref: refs/heads/main');
+</textarea>
+<input type="button" value="eval" onClick="___git_eval(15)">
+<div id="out15"></div>
+
+<h2>Tags</h2>
+<textarea id="in16">
+  gitconfig = {
+    user: {
+      name: 'Example User',
+      email: 'user@example.com',
+    }
+  }
+</textarea>
+<input type="button" value="eval" onClick="___git_eval(16)">
+<div id="out16"></div>
+
+<h2><code>git commit</code></h2>
+<p></p>
+<textarea id="in17">
+gitconfig = {
+  user: {
+    name: 'Example User',
+    email: 'user@example.com',
+  }
+}
+function git_commit(file_paths) {
+  var now = Date.now();
+  var timezoneMinutes = -(now.getTimezoneOffset());
+  store_commit(
+  store_tree_from_paths(file_paths),
+  [parse_head(join_paths(workdir, '.git/HEAD'))],
+  {name:gitconfig.user.name, email:gitconfig.user.email, date:now, timezoneMinutes:timezoneMinutes },
+  {name:gitconfig.user.name, email:gitconfig.user.email, date:now, timezoneMinutes:timezoneMinutes },
+  'Initial commit');
+
+}
+</textarea>
+<input type="button" value="eval" onClick="___git_eval(17)">
+<div id="out17"></div>
+
+
+END OF DOCUMENT
+
+<script>
+document.getElementById("initial-focus").focus();
+</script>
+
+<p></p>
+<textarea id="in000">
+</textarea>
+<input type="button" value="eval" onClick="___git_eval(000)">
+<div id="out000"></div>
+
+<!--
+/*
+function partition(array, filter) {
+  var pass = [];
+  var fail = [];
+  for (i = 0; i < array.length; i++) {
+    if (filter(array[i])) {
+      pass.push(array[i]);
+    } else {
+      fail.push(array[i]);
+    }
+  }
+  return { "pass" : pass, "fail" : fail }
+}
+function filenames_to_hierarchy(filenames) {
+  for (var i = 0; i < filenames.length; i++) {
+    filenames[i];
+  }
+}*/
+-->
+</body>
+</html>
--- a/pako.min.js
+++ b/pako.min.js
--- a/sha1.js
+++ b/sha1.js
@ -0,0 +1,160 @@
+/* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -  */
+/* SHA-1 (FIPS 180-4) implementation in JavaScript                    (c) Chris Veness 2002-2019  */
+/*                                                                                   MIT Licence  */
+/* www.movable-type.co.uk/scripts/sha1.html                                                       */
+/* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -  */
+
+
+/**
+ * SHA-1 hash function reference implementation.
+ *
+ * This is an annotated direct implementation of FIPS 180-4, without any optimisations. It is
+ * intended to aid understanding of the algorithm rather than for production use.
+ *
+ * While it could be used where performance is not critical, I would recommend using the ‘Web
+ * Cryptography API’ (developer.mozilla.org/en-US/docs/Web/API/SubtleCrypto/digest) for the browser,
+ * or the ‘crypto’ library (nodejs.org/api/crypto.html#crypto_class_hash) in Node.js.
+ *
+ * See csrc.nist.gov/groups/ST/toolkit/secure_hashing.html
+ *     csrc.nist.gov/groups/ST/toolkit/examples.html
+ */
+class Sha1 {
+
+    /**
+     * Generates SHA-1 hash of string.
+     *
+     * @param   {string} msg - (Unicode) string to be hashed.
+     * @param   {Object} [options]
+     * @param   {string} [options.msgFormat=string] - Message format: 'string' for JavaScript string
+     *   (gets converted to UTF-8 for hashing); 'hex-bytes' for string of hex bytes ('616263' ≡ 'abc') .
+     * @param   {string} [options.outFormat=hex] - Output format: 'hex' for string of contiguous
+     *   hex bytes; 'hex-w' for grouping hex bytes into groups of (4 byte / 8 character) words.
+     * @returns {string} Hash of msg as hex character string.
+     *
+     * @example
+     *   import Sha1 from './sha1.js';
+     *   const hash = Sha1.hash('abc'); // 'a9993e364706816aba3e25717850c26c9cd0d89d'
+     */
+    static hash(msg, options) {
+        const defaults = { msgFormat: 'string', outFormat: 'hex' };
+        const opt = Object.assign(defaults, options);
+
+        switch (opt.msgFormat) {
+            default: // default is to convert string to UTF-8, as SHA only deals with byte-streams
+            case 'string':   msg = msg;                   break; // utf8Encode(msg);       break;
+            case 'hex-bytes':msg = hexBytesToString(msg); break; // mostly for running tests
+        }
+
+        // constants [§4.2.1]
+        const K = [ 0x5a827999, 0x6ed9eba1, 0x8f1bbcdc, 0xca62c1d6 ];
+
+        // initial hash value [§5.3.1]
+        const H = [ 0x67452301, 0xefcdab89, 0x98badcfe, 0x10325476, 0xc3d2e1f0 ];
+
+        // PREPROCESSING [§6.1.1]
+
+        msg += String.fromCharCode(0x80);  // add trailing '1' bit (+ 0's padding) to string [§5.1.1]
+
+        // convert string msg into 512-bit/16-integer blocks arrays of ints [§5.2.1]
+        const l = msg.length/4 + 2; // length (in 32-bit integers) of msg + ‘1’ + appended length
+        const N = Math.ceil(l/16);  // number of 16-integer-blocks required to hold 'l' ints
+        const M = new Array(N);
+
+        for (let i=0; i<N; i++) {
+            M[i] = new Array(16);
+            for (let j=0; j<16; j++) {  // encode 4 chars per integer, big-endian encoding
+                M[i][j] = (msg.charCodeAt(i*64+j*4+0)<<24) | (msg.charCodeAt(i*64+j*4+1)<<16)
+                        | (msg.charCodeAt(i*64+j*4+2)<< 8) | (msg.charCodeAt(i*64+j*4+3)<< 0);
+            } // note running off the end of msg is ok 'cos bitwise ops on NaN return 0
+        }
+        // add length (in bits) into final pair of 32-bit integers (big-endian) [§5.1.1]
+        // note: most significant word would be (len-1)*8 >>> 32, but since JS converts
+        // bitwise-op args to 32 bits, we need to simulate this by arithmetic operators
+        M[N-1][14] = ((msg.length-1)*8) / Math.pow(2, 32); M[N-1][14] = Math.floor(M[N-1][14]);
+        M[N-1][15] = ((msg.length-1)*8) & 0xffffffff;
+
+        // HASH COMPUTATION [§6.1.2]
+
+        for (let i=0; i<N; i++) {
+            const W = new Array(80);
+
+            // 1 - prepare message schedule 'W'
+            for (let t=0;  t<16; t++) W[t] = M[i][t];
+            for (let t=16; t<80; t++) W[t] = Sha1.ROTL(W[t-3] ^ W[t-8] ^ W[t-14] ^ W[t-16], 1);
+
+            // 2 - initialise five working variables a, b, c, d, e with previous hash value
+            let a = H[0], b = H[1], c = H[2], d = H[3], e = H[4];
+
+            // 3 - main loop (use JavaScript '>>> 0' to emulate UInt32 variables)
+            for (let t=0; t<80; t++) {
+                const s = Math.floor(t/20); // seq for blocks of 'f' functions and 'K' constants
+                const T = (Sha1.ROTL(a, 5) + Sha1.f(s, b, c, d) + e + K[s] + W[t]) >>> 0;
+                e = d;
+                d = c;
+                c = Sha1.ROTL(b, 30) >>> 0;
+                b = a;
+                a = T;
+            }
+
+            // 4 - compute the new intermediate hash value (note 'addition modulo 2^32' – JavaScript
+            // '>>> 0' coerces to unsigned UInt32 which achieves modulo 2^32 addition)
+            H[0] = (H[0]+a) >>> 0;
+            H[1] = (H[1]+b) >>> 0;
+            H[2] = (H[2]+c) >>> 0;
+            H[3] = (H[3]+d) >>> 0;
+            H[4] = (H[4]+e) >>> 0;
+        }
+
+        // convert H0..H4 to hex strings (with leading zeros)
+        for (let h=0; h<H.length; h++) H[h] = ('00000000'+H[h].toString(16)).slice(-8);
+
+        // concatenate H0..H4, with separator if required
+        const separator = opt.outFormat=='hex-w' ? ' ' : '';
+
+        return H.join(separator);
+
+        /* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -  */
+
+        function utf8Encode(str) {
+            try {
+                return new TextEncoder().encode(str, 'utf-8').reduce((prev, curr) => prev + String.fromCharCode(curr), '');
+            } catch (e) { // no TextEncoder available?
+                return unescape(encodeURIComponent(str)); // monsur.hossa.in/2012/07/20/utf-8-in-javascript.html
+            }
+        }
+
+        function hexBytesToString(hexStr) { // convert string of hex numbers to a string of chars (eg '616263' -> 'abc').
+            const str = hexStr.replace(' ', ''); // allow space-separated groups
+            return str=='' ? '' : str.match(/.{2}/g).map(byte => String.fromCharCode(parseInt(byte, 16))).join('');
+        }
+    }
+
+
+    /**
+     * Function 'f' [§4.1.1].
+     * @private
+     */
+    static f(s, x, y, z)  {
+        switch (s) {
+            case 0: return (x & y) ^ (~x & z);          // Ch()
+            case 1: return  x ^ y  ^  z;                // Parity()
+            case 2: return (x & y) ^ (x & z) ^ (y & z); // Maj()
+            case 3: return  x ^ y  ^  z;                // Parity()
+        }
+    }
+
+
+    /**
+     * Rotates left (circular left shift) value x by n positions [§3.2.5].
+     * @private
+     */
+    static ROTL(x, n) {
+        return (x<<n) | (x>>>(32-n));
+    }
+
+}
+
+
+/* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -  */
+
+//export default Sha1;