How to Access (Historic) $VET Price on VeRocket Using the Call.API

VeRocket, a Uniswap v2 compatible exchange, provides on-chain swaps for different tokens. This allows you to access price information on-chain and deliver it through a public API without connecting to centralized services like Coingecko. In this example, we'll use the Call.API (opens in a new tab) to load the current VET price (measured in VeUSD) and the price from a specific point in the past.

ABI

Uniswap defines getAmountOut (opens in a new tab) that can be used to get the VeUSD value from a swap of a single vVET. The returned value will be the USD value of VET.

The function is defined as this:

function getAmountsOut(uint amountIn, address[] memory path) public view returns (uint[] memory amounts);

The input is the contract's amount, respecting its decimals.
The address list represents the routing of the swap; for this example, the path will be VET to VeUSD.
The output is the result for each step of the path.

Parameters

The input parameters are defined as:

amountIn = 1000000000000000000
This equals 1 VET, as the contract has 18 decimals 1 + 18 x 0 (opens in a new tab).
path[0] = 0x45429A2255e7248e57fce99E7239aED3f84B7a53
Input: the Wrapped VET contract.
path[1] = 0x4E17357053dA4b473e2daa2c65C2c949545724b8
Output: the VeUSD contract.

Output

The result will be a list of values, currently:

[
   "1000000000000000000",
   "24135"
]

The decimals of the VeUSD contract are 6 (opens in a new tab) which requires the value 24135 to be divided by a 1 + 6 x 0:

24135 / 1000000 = 0.024135

That's the current value of VET in USD as of today (2023-04-20).

API Access

GET

The GET variant is easy to share and can be easily implemented into frontend web applications. It is harder to read:

Readable version:

https://api.vechain.energy/v1/call/main/0x576da7124c7bb65a692d95848276367e5a844d95/getAmountsOut (uint256 1000000000000000000, address[] 0x45429A2255e7248e57fce99E7239aED3f84B7a53|0x4E17357053dA4b473e2daa2c65C2c949545724b8) returns (uint256[] amounts)

URL encoded:
https://api.vechain.energy/v1/call/main/0x576da7124c7bb65a692d95848276367e5a844d95/getAmountsOut%20(uint256%201000000000000000000,%20address[]%200x45429A2255e7248e57fce99E7239aED3f84B7a53%7C0x4E17357053dA4b473e2daa2c65C2c949545724b8)%20returns%20(uint256[]%20amounts) (opens in a new tab)

POST

The POST variant is more verbose and easier to read. It should be used in backends:

curl -XPOST https://api.vechain.energy/v1/call/main -d '
{
  "clauses": [ 
  {
    "to": "0x576da7124c7bb65a692d95848276367e5a844d95",
    "abi": {
      "name": "getAmountsOut",
      "inputs": [
          {
              "internalType": "uint256",
              "name": "amountIn",
              "type": "uint256"
          },
          {
              "internalType": "address[]",
              "name": "path",
              "type": "address[]"
          }
      ],
      "outputs": [
          {
              "internalType": "uint256[]",
              "name": "amounts",
              "type": "uint256[]"
          }
      ],
      "type": "function"
    },
    "args": [
      "1000000000000000000",
      [
        "0x45429A2255e7248e57fce99E7239aED3f84B7a53",
        "0x4E17357053dA4b473e2daa2c65C2c949545724b8"
      ]
    ]
  }
  ]
}
'

Historical Data

revision can be passed as a parameter to retrieve the information for a specific block number (or ID) in the past.

For example, Block 11,824,314 was forged at 2022-04-01 and will reference the information from that time: (opens in a new tab)

[
  "1000000000000000000",
  "78358"
]

That's $0,078358 for $VET at that time.

Conclusion

Using the Call.API (opens in a new tab) to access information from contracts can remove the need for custom backends. It opens the possibility to build a public API based on any kind of contract.

In this article's example, an API to access current and historical price information for tokens was demonstrated.

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