Simple yet expressive format to describe documents' contents and changes 🗃

Deltas are a simple, yet expressive format that can be used to describe contents and changes. The format is a strict subset of JSON, is human readable, and easily parsible by machines. Deltas can describe any rich-text document, includes all text and formatting information, without the ambiguity and complexity of HTML.

The Delta format is suitable for Operational Transform and can be used in real-time, collaborative document editors (e.g. Slab, Google Docs). A walkthough of the motivation and design thinking behind Deltas are on Designing the Delta Format.

See the Documentation.

Add delta to your project dependencies in mix.exs:

def deps do
  [{:delta, "~> 0.4.0"}]
end

A Delta is made up of a list of operations, which describe changes to a document. These can be insert, delete or retain. These operations do not take an index, but instead describe the change at the current index. Retains are used to "keep" parts of the document.

alias Delta.Op

# Document with text "Gandalf the Grey", with "Gandalf" bolded
# and "Grey" in grey
delta = [
  Op.insert("Gandalf", %{"bold" => true}),
  Op.insert(" the "),
  Op.insert("Grey", %{"color" => "#ccc"}),
]

# Define change intended to be applied to above:
# Keep the first 12 characters, delete the next 4,
# and insert a white "White"
death = [
  Op.retain(12),
  Op.delete(4),
  Op.insert("White", %{"color" => "#fff"}),
]


# Applying the change:
Delta.compose(delta, death)
# => [
#   %{"insert" => "Gandalf", "attributes" => %{"bold" => true}},
#   %{"insert" => " the "},
#   %{"insert" => "White", "attributes" => %{"color" => "#fff"}},
# ]

Insert operations have an insert key defined. A String value represents inserting text. Any other type represents inserting an embed (however only one level of object comparison will be performed for equality).

In both cases of text and embeds, an optional attributes key can be defined with a map to describe additonal formatting information. Formats can be changed by the retain operation.

# Insert a text
Op.insert("Some Text")

# Insert a bolded text
Op.insert("Bolded Text", %{"bold" => true})

# Insert a link
Op.insert("Google", %{"link" => "https://google.com"})

# Insert an embed
Op.insert(%{"image" => "https://app.com/logo.png"}, %{"alt" => "App Logo"})

# Insert another embed
Op.insert(%{"video" => "https://www.youtube.com/watch?v=dQw4w9WgXcQ"}, %{"width" => 420, "height" => 315})

Delete operations have a positive integer delete key defined representing the number of characters to delete. All embeds have a length of 1.

# Delete the next 10 characters
Op.delete(10)

Retain operations have a positive integer retain key defined representing the number of characters to keep (other libraries might use the name keep or skip). An optional attributes key can be defined with a map to describe formatting changes to the character range. A value of nil in the attributes map represents removal of that key.

Note: It is not necessary to retain the last characters of a document as this is implied.

# Keep the next 5 characters
Op.retain(5)

# Keep and bold the next 5 characters
Op.retain(5, %{"bold" => true})

# Keep and unbold the next 5 characters
Op.retain(5, %{"bold" => nil})

Operational Transform (OT) is a technology for building collabortive experiences, and is especially useful in application sharing and building real-time document editors that support multi-user collaboration (e.g. Google Docs, Slab).

Delta supports OT out of the box and can be very useful in employing Operational Transform techniques in Elixir. It supports the following properties:

Returns a new Delta that is equivalent to applying the operations of one Delta, followed by another Delta:

a = [Op.insert("abc")]
b = [Op.retain(1), Op.delete(1)]

Delta.compose(a, b)
# => [%{"insert" => "ac"}]

Transforms given delta against another's operations. This accepts an optional priority argument (default: false), used to break ties. If true, the first delta takes priority over other, that is, its actions are considered to happen "first."

a = [Op.insert("a")]
b = [Op.insert("b"), Op.retain(5), Op.insert("c")]

Delta.transform(a, b, true)
# => [
#  %{"retain" => 1},
#  %{"insert" => "b"},
#  %{"retain" => 5},
#  %{"insert" => "c"},
# ]

Delta.transform(a, b)
# => [
#  %{"insert" => "b"},
#  %{"retain" => 6},
#  %{"insert" => "c"},
# ]

Note that even though delete operations support attributes, it is only safe to use transform with deletes that don't have them.

Returns an inverted delta that has the opposite effect of against a base document delta. That is base |> Delta.compose(change) |> Delta.compose(inverted) == base.

base = [Op.insert("Hello\nWorld")]

change = [
  Op.retain(6, %{"bold" => true}),
  Op.delete(5),
  Op.insert("!"),
]

inverted = Delta.invert(change, base)
# => [
#   %{"retain" => 6, "attributes" => %{"bold" => nil}},
#   %{"insert" => "World"},
#   %{"delete" => 1},
# ]

base |> Delta.compose(change) |> Delta.compose(inverted) == base
# => true

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