ARPA Network

Example - Pick a Rarity

The PickRarityExample contract, by extending GeneralRandcastConsumerBase and calling functions from RandcastSDK, demonstrates a system for drawing a rarity level.
// SPDX-License-Identifier: MIT
pragma solidity ^0.8.18;
import {
} from "randcast-user-contract/user/GeneralRandcastConsumerBase.sol";
// solhint-disable-next-line no-global-import
import "randcast-user-contract/user/RandcastSDK.sol" as RandcastSDK;
contract PickRarityExample is GeneralRandcastConsumerBase {
/* requestId -> randomness */
mapping(bytes32 => uint256) public randomResults;
mapping(uint256 => string) public rarityValue;
uint256 public indexResult;
uint256[] public rarityWeights = new uint256[](5);
event RarityValueResult(string);
constructor(address controller) BasicRandcastConsumerBase(controller) {
rarityValue[0] = "SSSR";
rarityValue[1] = "SSR";
rarityValue[2] = "SR";
rarityValue[3] = "R";
rarityValue[4] = "C";
rarityWeights[0] = 1; // SSSR(1%)
rarityWeights[1] = 4; // SSR(4%)
rarityWeights[2] = 10; // SR(10%)
rarityWeights[3] = 20; // R(20%)
rarityWeights[4] = 65; // C(65%)
* Requests randomness
function getRarity() external returns (bytes32) {
bytes memory params;
return _requestRandomness(RequestType.Randomness, params);
* Callback function used by Randcast Adapter
// solhint-disable-next-line
function _fulfillRandomness(bytes32 requestId, uint256 randomness) internal override {
randomResults[requestId] = randomness;
uint256 rarityIndex = RandcastSDK.pickByWeights(randomness, rarityWeights);
indexResult = rarityIndex;
emit RarityValueResult(rarityValue[rarityIndex]);
This contract also has three important parts: the constructor function, getRarity function, and _fulfillRandomWords function.
In the constructor , the rarityValue is being initialized with five string elements. These values correspond to the rarity levels of certain items or characters: 'SSSR,' 'SSR,' 'SR,' 'R,' and 'C,' each with an index from 0 to 4. The rarityWeights array is initialized with five integer elements representing the weights associated with the rarity levels defined earlier.
The function getRarity is used to request a random number.
_fulfillRandomness calls the RandcastSDK.pickByWeights function which returns an index based on a predefined rarityWeights to determine the rarity of the card drawn in this round. The lower weight corresponds to a lower probability and, therefore, a higher rarity. In this example, 'SSR' is the rarest, with the lowest probability(weight), while 'C' is the most common, with the highest probability(weight).