index.js

// A core concept in blockchains is the ability to store, update and access the balances of an account. In addition, it allows querying the historical balances of an account as well.
//
// Design a stateful structure allows setting the current balance of an account, as well as retrieving the balance at the current or any previous point in time.
//
// **Interface:**
//
// - `set(account, balance)`
// - `get(account, height)`
// - `increment_height()`
//
// **Example flow:**
//
// - `initialize` // height=0; balances = 0
// - `set(A, 1)`
// - `increment_height()`
// - `set(B, 2)`
// - `increment_height()`
// - `set(A, 3)`
// - `increment_height()`
// - `get(A, 0)` → returns 1
// - `get(B, 0)` → returns 0
// - `get(B, 1)` → returns 2
// - `get(A, 1)` → returns 1
// - `get(B, 2)` -> returns 2
// - `get(A, 2)` → returns  3


class AccountStore {
  constructor () {
    this.balances = {}
    this.height = 0
  }

  set(account, balance) {
    if (!this.balances[account]) {
      this.balances[account] = {}
    }

    this.balances[account][this.height] = balance
  }

  get(account, height) {
    if (!this.balances[account]) {
      return 0
    }

    if (this.balances[account][height]) {
      return this.balances[account][height]
    }

    for (let h = height; h >= 0; h -= 1) {
      if (this.balances[account][h]) {
        return this.balances[account][h]
      }
    }

    return 0
  }

  delete (account) {
    delete !this.balances[account]
  }

  incrementHeight () {
    this.height += 1
  }
}

// TEST
const store = new AccountStore()
store.set('A', 1)
store.incrementHeight()
store.set('B', 2)
store.incrementHeight()
store.set('A', 3)
store.incrementHeight()
store.delete('A')
console.log(store.get('A', 0))
console.log(store.get('B', 0))
console.log(store.get('B', 1))
console.log(store.get('A', 1))
console.log(store.get('B', 2))
console.log(store.get('A', 2))

// console.log(JSON.stringify(store, null, 2))