Implementation of a piece table in Swift

Question

I have implemented a piece table in Swift, based on a Javascript implementation. I don't have much experience with Swift yet, so I'm especially interested if I'm doing things "the Swift way". Any feedback is highly appreciated.

import Foundation

public class PieceTable {
    private let original: String
    private var buffer: String
    private var bufferLength: Int
    private var pieces: [Piece]

    public convenience init() {
        self.init(from: "")
    }

    public init(from original: String) {
        self.original = original
        self.buffer = ""
        self.bufferLength = 0
        self.pieces = [Piece(buffer: false, offset: 0, length: original.characters.count)]
    }

    public func insert(string: String, offset: Int) {
        if string.isEmpty {
            return
        }

        let addBufferOffset = bufferLength
        self.buffer += string
        self.bufferLength += string.characters.count

        let (piece, index, offsetInPiece) = self.pieceIndexAndOffset(fromSequenceOffset: offset)

        // If we are inserting at the end of the piece and at the end of the buffer, we can just increase its length
        if piece.buffer && offsetInPiece == piece.length && (piece.offset + piece.length) == addBufferOffset {
            self.pieces[index].length += string.characters.count
            return
        }

        self.pieces.replaceSubrange(index...index, with: [
            Piece(buffer: piece.buffer, offset: piece.offset, length: offsetInPiece),
            Piece(buffer: true, offset: addBufferOffset, length: string.characters.count),
            Piece(buffer: piece.buffer, offset: piece.offset + offsetInPiece, length: piece.length - offsetInPiece)
        ].filter({$0.length > 0}))
    }

    public func delete(offset: Int, length: Int) {
        if length == 0 {
            return
        }

        if length < 0 {
            self.delete(offset: offset + length, length: -length)
            return
        }

        let (firstPiece, firstPieceIndex, offsetInFirstPiece) = self.pieceIndexAndOffset(fromSequenceOffset: offset)
        let (lastPiece, lastPieceIndex, offsetInLastPiece) = self.pieceIndexAndOffset(fromSequenceOffset: offset + length)

        // If the delete spans only one piece and is at the very start or end of the piece, we can just modify it
        if firstPieceIndex == lastPieceIndex {
            if offsetInFirstPiece == 0 {
                self.pieces[firstPieceIndex].offset += length
                self.pieces[firstPieceIndex].length -= length
                return
            } else if offsetInLastPiece == lastPiece.length {
                self.pieces[firstPieceIndex].length -= length
                return
            }
        }

        self.pieces.replaceSubrange(firstPieceIndex...lastPieceIndex, with: [
            Piece(buffer: firstPiece.buffer, offset: firstPiece.offset, length: offsetInFirstPiece),
            Piece(buffer: lastPiece.buffer, offset: lastPiece.offset + offsetInLastPiece, length: lastPiece.length - offsetInLastPiece)
        ].filter({$0.length > 0}))
    }

    public func get() -> String {
        var string = ""

        for piece in self.pieces {
            string += self.substring(for: piece)
        }

        return string
    }

    public func get(offset: Int, length: Int) -> String {
        if length < 0 {
            return self.get(offset: offset + length, length: -length)
        }

        let (firstPiece, firstPieceIndex, offsetInFirstPiece) = self.pieceIndexAndOffset(fromSequenceOffset: offset)
        let (lastPiece, lastPieceIndex, offsetInLastPiece) = self.pieceIndexAndOffset(fromSequenceOffset: offset + length)

        if offsetInFirstPiece + length < firstPiece.length {
            return self.substring(for: firstPiece, offset: offsetInFirstPiece, length: length)
        }

        var string = self.substring(for: firstPiece, offset: offsetInFirstPiece, length: firstPiece.length - offsetInFirstPiece)

        for index in (firstPieceIndex + 1)..<lastPieceIndex {
            string += self.substring(for: self.pieces[index])
        }

        string += self.substring(for: lastPiece, offset: 0, length: offsetInLastPiece)

        return string
    }

    private func pieceIndexAndOffset(fromSequenceOffset offset: Int) -> (piece: Piece, index: Int, offset: Int) {
        precondition(offset >= 0, "Offset out of bounds")

        var remainingOffset = offset

        for (index, piece) in self.pieces.enumerated() {
            if remainingOffset <= piece.length {
                return (piece, index, remainingOffset)
            }

            remainingOffset -= piece.length
        }

        precondition(false, "Offset out of bounds")
    }

    private func substring(for piece: Piece) -> String {
        let buffer = piece.buffer ? self.buffer : self.original
        let startIndex = buffer.index(buffer.startIndex, offsetBy: piece.offset)
        let endIndex = buffer.index(startIndex, offsetBy: piece.length)
        return buffer[startIndex..<endIndex]
    }

    private func substring(for piece: Piece, offset: Int, length: Int) -> String {
        let buffer = piece.buffer ? self.buffer : self.original
        let startIndex = buffer.index(buffer.startIndex, offsetBy: piece.offset + offset)
        let endIndex = buffer.index(startIndex, offsetBy: length)
        return buffer[startIndex..<endIndex]
    }
}

private struct Piece {
    public var buffer: Bool
    public var offset: Int
    public var length: Int
}

Usage example:

var buffer = PieceTable(from: "Hello, world!")
buffer.insert(string: "crazy ", offset: 7)
buffer.get() // "Hello, crazy world!"

Answer 1

Generally your Swift code looks very good to me, and I could not detect something totally "unswifty". There are some things which can improved or simplified.

struct vs class

You have defined PieceTable as class which is OK if you intend to pass references to a "Piece Table instances" around. In that case you can define instances as a constant (let):

let buffer = PieceTable(from: "Hello, world!")
buffer.insert(string: "crazy ", offset: 7)

If you don't need the reference semantics then it is preferable to define a struct (and mark methods as mutating where necessary).

Access modifiers

You added public/private access modifiers, which is good. But I don't see why the properties of Piece are declared private if the type itself is public. It should be changed to

private struct Piece {
    var buffer: Bool
    var offset: Int
    var length: Int
}

let vs var

You declared properties a constants where possible, which is good. The buffer property of Piece is also never modified after initialization, so let's make this

private struct Piece {
    let buffer: Bool
    var offset: Int
    var length: Int
}

Using self

You used self for all accesses to type properties or when invoking methods, which is not necessary in Swift. From the documentation:

In practice, you don’t need to write self in your code very often. If you don’t explicitly write self, Swift assumes that you are referring to a property or method of the current instance whenever you use a known property or method name within a method. ...

The main exception to this rule occurs when a parameter name for an instance method has the same name as a property of that instance. In this situation, the parameter name takes precedence, and it becomes necessary to refer to the property in a more qualified way. You use the self property to distinguish between the parameter name and the property name.

There are different opinions about this issue (see e.g. When should I access properties with self in swift? on Stack Overflow). The The Official raywenderlich.com Swift Style Guide recommends:

For conciseness, avoid using self since Swift does not require it to access an object's properties or invoke its methods.

Use self only when required by the compiler (in @escaping closures, or in initializers to disambiguate properties from arguments). In other words, if it compiles without self then omit it.

So if you used self intentionally: that's fine (since you are consequent about that). Otherwise note that you don't have to.

Method names

These method names do not provide any information about what they do:

public func get() -> String
public func get(offset: Int, length: Int) -> String

For the first one I would suggest

public var stringValue -> String

as e.g. in NSNumber.stringValue, and for the second one something like

public func substring(offset: Int, length: Int) -> String
// or
public func substring(range: Range<Int>) -> String

A simplification

The get()/stringValue method can be simplified using reduce:

public var stringValue: String {
    return pieces.reduce("") { $0 + substring(for: $1) }
}

Possible improvements

Both the get(offset: Int, length: Int) and the delete(offset: Int, length: Int) method call pieceIndexAndOffset() twice: first with offset and then with offset + length. A possible improvement would be to start the second search at the piece and offset found in the first search, instead of starting at the very beginning again.

Useful protocols to adopt

If you adopt the CustomStringConvertible protocol

extension PieceTable: CustomStringConvertible {
    public var description: String {
        return stringValue
    }
}

then a piece table can be printed or used in string interpolation without explicit conversion to a string:

let buffer = PieceTable(from: "Hello, world!")
buffer.insert(string: "crazy ", offset: 7)
print("The text is: \(buffer)")

And CustomDebugStringConvertible is useful for debugging, here is a possible implementation:

extension PieceTable: CustomDebugStringConvertible {
    public var debugDescription: String {
        var s = ""
        print("Original:", original, to: &s)
        print("Buffer:", buffer, to: &s)
        for (i, piece) in pieces.enumerated() {
            print("Piece#\(i):", piece.buffer ? "add " : "orig", piece.offset, piece.length, to: &s)
        }
        return s
    }
}

In Swift 3 you'll have to declare the properties original, buffer, and pieces as fileprivate in order to make them accessible in an extension in the same file. This has been fixed in Swift 4.

Example:

let buffer = PieceTable(from: "Hello, world!")
buffer.insert(string: "crazy ", offset: 7)
debugPrint(buffer)

Output:

Original: Hello, world!
Buffer: crazy 
Piece#0: orig 0 7
Piece#1: add  0 6
Piece#2: orig 7 6