zackthehuman/fs-tree-diff

fs-tree-diff

FSTree provides the means to calculate a patch (set of operations) between one file system tree and another.

The possible operations are:

• unlink – remove the specified file
• rmdir – remove the specified folder
• mkdir – create the specified folder
• create – create the specified file
• change – update the specified file to reflect changes

The operations chosen aim to minimize the amount of IO required to apply a given patch. For example, a naive rm -rf of a directory tree is actually quite costly, as child directories must be recursively traversed, entries stated.. etc, all to figure out what first must be deleted. Since we patch from tree to tree, discovering new files is both wasteful and un-needed.

The operations will also be provided in a correct order, allowing us to safely replay operations without having to first confirm the FS is as we expect. For example, unlinks for files will occur before a rmdir of those files' parent dir. Although the ordering will be safe, a specific order is not guaranteed.

A simple example:

var FSTree = require('fs-tree-diff');
var current = FSTree.fromPaths([
  'a.js'
]);

var next = FSTree.fromPaths([
  'b.js'
]);

current.calculatePatch(next) === [
  ['unlink', 'a.js'],
  ['create', 'b.js']
];

A slightly more complicated example:

var FSTree = require('fs-tree-diff');
var current = FSTree.fromPaths([
  'a.js',
  'b/',
  'b/f.js'
]);

var next = FSTree.fromPaths([
  'b.js',
  'b/',
  'b/c/',
  'b/c/d.js',
  'b/e.js'
]);

current.calculatePatch(next) === [
  ['unlink', 'a.js', entryA],
  ['create', 'b.js', entryB],
  ['mkdir', 'b/c', entryBC],
  ['create', 'b/c/d.js', entryBCD],
  ['create', 'b/e.js', entryBE]
  ['unlink', 'b/f.js', entryBF],
]

Now, the above examples do not demonstrate update operations. This is because when providing only paths, we do not have sufficient information to check if one entry is merely different from another with the same relativePath.

For this, FSTree supports more complex input structure. To demonstrate, We will use the walk-sync module. (note: walk-sync >= 0.2.7 is required`) Which provides higher fidelity input, allowing FSTree to also detect changes. More on what an entry from walkSync.entries is

var walkSync = require('walk-sync');

// path/to/root/foo.js
// path/to/root/bar.js
var current = new FSTree({
  entries: walkSync.entries('path/to/root')
});

writeFileSync('path/to/root/foo.js', 'new content');
writeFileSync('path/to/root/baz.js', 'new file');

var next = new FSTree({
  entries: walkSync.entries('path/to/root')
});

current.calculatePatch(next) === [
  ['update', 'foo.js', entryFoo], // mtime + size changed, so this input is stale and needs updating.
  ['create', 'baz.js', entryBaz]  // new file, so we should create it
  /* bar stays the same and is left inert*/
];

The entry objects provided depend on the operation. For rmdir and unlink operations, the current entry is provided. For mkdir, change and create operations the new entry is provided.

API

The public API is:

• FSTree.fromPaths initialize a tree from an array of string paths.

• FSTree.fromEntries initialize a tree from an object containing an entries property. Each entry must have the following properties (but may have more):

  • relativePath
  • mode
  • size
  • mtime

• FSTree.prototype.calculatePatch(newTree, isEqual) calculate a patch against newTree. Optionally specify a custom isEqual (see Change Calculation).

Input

FSTree.fromPaths and FSTree.fromEntries both validate their inputs. Inputs must be sorted, path-unique (ie two entries with the same relativePath but different sizes would still be illegal input) and include intermediate directories.

For example, the following input is invaild

FSTree.fromPaths([
  // => missing a/ and a/b/
  'a/b/c.js'
]);

To have FSTree sort and expand (include intermediate directories) for you, add the option sortAndExpand).

FStree.fromPaths([
	'a/b/q/r/bar.js',
	'a/b/c/d/foo.js',
], { sortAndExpand: true });

// The above is equivalent to

FSTree.fromPaths([
	'a/',
	'a/b/',
	'a/b/c/',
	'a/b/c/d/',
	'a/b/c/d/foo.js',
	'a/b/q/',
	'a/b/q/r/',
	'a/b/q/r/bar.js',
]);

Entry

FSTree.fromEntries requires you to supply your own Entry objects. Your entry objects must contain the following properties:

• relativePath
• mode
• size
• mtime

They must also implement the following API:

• isDirectory() true iff this entry is a directory

Change Calculation

When a prior entry has a relativePath that matches that of a current entry, a change operation is included if the new entry is different from the previous entry. This is determined by calling isEqual, the optional second argument to calculatePatch. If no isEqual is provided, a default isEqual is used.

The default isEqual treats directories as always equal and files as different if any of the following properties have changed.

• mode
• size
• mtime

User specified isEqual will often want to use the default isEqual, so it is exported on FSTree.

Example

var defaultIsEqual = FSTtreeDiff.isEqual;

function isEqualCheckingMeta(a, b) {
  return defaultIsEqual(a, b) && isMetaEqual(a, b);
}

function isMetaEqual(a, b) {
  // ...
}