main.ts

export interface Position<T> {
  value: T
}

interface Node<T> extends Position<T> {
  index: number
}

interface PositionalList<T> {
  first(): Position<T>
  last(): Position<T>
  elementAt(index: number): Position<T>

  before(p: Position<T>): Position<T>
  after(p: Position<T>): Position<T>

  append(value:T): void
  prepend(value: T): void
  set(value: T, p: Position<T>): void
  
  remove(p: Position<T>): void
}

const list= <T extends any>(): PositionalList<T> => {
  const data : T[] = []

  return {
    first: () => ({ value: data[0], index: 0 }),

    last: () => ({ value: data[data.length-1], index: data.length - 1 }),

    elementAt: i => ({ value: data[0], index: i}),

    before: p => {
      const index = (p as Node<T>).index - 1
      return { value: data[index], index }
    },
    
    after: p => {
      const index = (p as Node<T>).index + 1
      return { value: data[index], index }
    },

    prepend: v => data.unshift(v),

    append: v => data.push(v),
    
    set: (v, p) => data[(p as Node<T>).index] = v,

    remove: p => {
      const index = (p as Node<T>).index + 1
      data.splice(index, 1)
    },
  }
}