How do I query the Balance of a KT1 account Cycle by Cycle?

Question

Is there a RPC in Tezos-client, that allows to query the Balance of a KT1 account cycle by cycle?

I've tried it with this (TzScan Public-Node RPC):

https://mainnet-node.tzscan.io/<block_id>/context/contracts/KT1XPU4EYP12ubNPjZwPDa8ydcPxUQ9JtpNC/balance

but I am not sure what <block_id> means and also not sure if this RPC leads me to my Result.

Answer 1

<block_id> - is a hash or a level of the block at which you ask for balance.

If you want to get a simple balance at the specific point in the cycle:

(this is pseudocode, I hope you figure it out)

var cycle = 60;

var p1 = (cycle * 4096 + 1) + 0; //beginning of the cycle
var p2 = (cycle * 4096 + 1) + 4095; //end of the cycle

var balance_at_p1 = get('/chains/main/blocks/{p1}/context/contracts/KT1.../balance');
var balance_at_p2 = get('/chains/main/blocks/{p2}/context/contracts/KT1.../balance');

But if you are trying to get a balance for calculating delegation rewards:

At first you should find the snapshot block that was used to count the rolls. Then use this block to get the balance like above.

var cycle = 60;
var cycle_lvl = cycle * 4096 + 1;

var snapshot = get('/chains/main/blocks/{cycle_lvl}/context/raw/json/cycle/{cycle}');

var snapshot_block = ((cycle - 7) * 4096 + 1) + (snapshot.roll_snapshot + 1) * 256 - 1;

var balance = get('/chains/main/blocks/{snapshot_block}/context/contracts/KT1.../balance');

Answer 2

You can use TzScan API for this: /v3/balance_history/KT1..., the documentation is here: balance history doc. For example https://api6.tzscan.io/v3/balance_history/KT1GgUJwMQoFayRYNwamRAYCvHBLzgorLoGo