Remember and use previous iteration value in DataWeave

I have an array of object.

 [
  {
    "b": 0,
    "a": 5
  },
  {
    "b": 0,
    "a": 10
  },
  {
    "b": 0,
    "a": 15
  }
]

Now I want that current value of b is calculated based on sum of previous object’s "a" and "b" values.
For example, when I use map function, I will reach to first object. It will not change because it is first object. Now I will reach next object which is :{"a":10,"b":0}
So here b will be updated to 5(because for previous object "a" is 5 and "b" is 0. So sum is 5)

Now for the 3rd object which is `{"a":15,"b":0}, b should be updated to 15. Because in the previous iteration, I have updated the value of b.

My problem is DataWeave map function is taking the unupdated value of b instead of updated one. So it is producing below output:

[
  {
    "b": 0,
    "a": 5
  },
  {
    "b": 5,
    "a": 10
  },
  {
    "b": 10,
    "a": 15
  }
]

Instead this I want this output:

[
  {
    "b": 0,
    "a": 5
  },
  {
    "b": 5,
    "a": 10
  },
  {
    "b": 15,
    "a": 15
  }
]

Here is my script

%dw 2.0
output application/json
---
payload map ((item, index) -> {
    b: if (index == 0) 0 else (payload[index - 1].a + payload[index - 1].b),
    a: item.a
})

I don’t know how to get the updated value in every iteration. Any help would be appreciated.

>Solution :

Try the below solution

Input

[
  {
    "b": 0,
    "a": 5
  },
  {
    "b": 0,
    "a": 10
  },
  {
    "b": 0,
    "a": 15
  }
]

Dataweave Script

%dw 2.0
output application/json
---
(payload reduce ((item, acc={b: 0, arr: []}) -> {
    b: if (isEmpty(acc.arr)) 0 else acc.b + acc.arr[-1].a,
    a: item.a,
    arr: acc.arr ++ [{ b: if (isEmpty(acc.arr)) 0 else acc.b + acc.arr[-1].a, a: item.a }]
})).arr

Output

[
  {
    "b": 0,
    "a": 5
  },
  {
    "b": 5,
    "a": 10
  },
  {
    "b": 15,
    "a": 15
  }
]