Undo/Redo helper for immutable state changes.

History.js

export default function History(initialState) {
    this.reset(initialState);
}

History.prototype = {
    reset: function(initialState) {
        this.history = [initialState];
        this.pointer = 0;
        this.tail = 0;
    },
    canUndo: function() {
        return this.pointer > 0;
    },
    canRedo: function() {
        return this.pointer < this.tail;
    },
    undo: function() {
        if (!this.canUndo()) { return null; }

        this.pointer--;

        return this.getState();
    },
    redo: function() {
        if (!this.canRedo()) { return null; }

        this.pointer++;

        return this.getState();
    },
    getState: function() {
        return this.history[this.pointer];
    },
    pushState: function(state) {
        const tail = this.tail;
        const pointer = this.pointer;
        const nextPointer = pointer + 1;

        if (pointer === tail) {
            this.history[nextPointer] = state;
        } else {
            const removeCount = tail - pointer;
            this.history.splice(nextPointer, removeCount, state);
        }

        this.pointer = nextPointer;
        this.tail = nextPointer;
    }
};

export function RingBufferHistory(capacity, initialState) {
    this.capacity = capacity;
    this.reset(initialState);
}

RingBufferHistory.prototype = {

    reset: function(initialState) {
        this.buffer = [initialState];
        this.buffer.length = this.capacity;

        this.head = 0;
        this.tail = 0;
        this.pointer = 0;
    },
    canUndo: function() {
        return this.pointer !== this.head;
    },
    canRedo: function() {
        return this.pointer !== this.tail;
    },
    undo: function() {
        if (!this.canUndo()) { return null; }

        this.pointer = (this.pointer - 1 + this.capacity) % this.capacity;

        return this.getState();
    },
    redo: function() {
        if (!this.canRedo()) { return null; }

        this.pointer = (this.pointer + 1) % this.capacity;

        return this.getState();
    },
    getState: function() {
        return this.buffer[this.pointer];
    },
    pushState: function(state) {
        const pointer = this.pointer;
        const capacity = this.capacity;

        const nextPointer = (pointer + 1) % capacity;

        const head = this.head;

        if (nextPointer === head) {
            this.head = ( head + 1 ) % capacity;
        }

        this.pointer = nextPointer;
        this.tail = nextPointer;

        this.buffer[nextPointer] = state;
    }
};