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If you can’t find a strategy that fits your the needs of your space you can create a new custom one. Follow the steps below to learn how to do that:

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1. Fork the score-api repository

Create a fork of the score-api repository: https://github.com/snapshot-labs/score-api

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2. Duplicate the erc20-balance-of strategy folder

Navigate to src/strategies/strategies directory, duplicate the erc20-balance-of directory and rename it to the chosen name for your new strategy. 
└── src
    └── strategies
        └── strategies
            └── erc20-balance-of

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3. Write the logic for your strategy

There are several files you need to edit:

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a. index.ts

This file defines the logic for calculation of the voting power. As an example, the erc20-balance-of is taking as parameters space, network, provider, addresses, options and snapshot in order to be able to retrieve the balances of the token specified in the options parameter for the provided addresses: 

import { BigNumberish } from '@ethersproject/bignumber';
import { formatUnits } from '@ethersproject/units';
import { Multicaller } from '../../utils';

export const author = 'bonustrack';
export const version = '0.1.1';

const abi = [
  'function balanceOf(address account) external view returns (uint256)'
];

export async function strategy(
  space,
  network,
  provider,
  addresses,
  options,
  snapshot
): Promise<Record<string, number>> {
  const blockTag = typeof snapshot === 'number' ? snapshot : 'latest';

  const multi = new Multicaller(network, provider, abi, { blockTag });
  addresses.forEach((address) =>
    multi.call(address, options.address, 'balanceOf', [address])
  );
  const result: Record<string, BigNumberish> = await multi.execute();

  return Object.fromEntries(
    Object.entries(result).map(([address, balance]) => [
      address,
      parseFloat(formatUnits(balance, options.decimals))
    ])
  );
}

​
b. schema.json

Describe the structure of your strategy by editing the properties, required and additionalProperties key-value pairs according to the logic from index.ts file: 
{
  "$schema": "http://json-schema.org/draft-07/schema#",
  "$ref": "#/definitions/Strategy",
  "definitions": {
    "Strategy": {
      "title": "Strategy",
      "type": "object",
      "properties": {
        "symbol": {
          "type": "string",
          "title": "Symbol",
          "examples": ["e.g. UNI"],
          "maxLength": 16
        },
        "address": {
          "type": "string",
          "title": "Contract address",
          "examples": ["e.g. 0x1f9840a85d5aF5bf1D1762F925BDADdC4201F984"],
          "pattern": "^0x[a-fA-F0-9]{40}$",
          "minLength": 42,
          "maxLength": 42
        },
        "decimals": {
          "type": "number",
          "title": "Decimals",
          "examples": ["e.g. 18"]
        }
      },
      "required": ["address", "decimals"],
      "additionalProperties": false
    }
  }
}

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d. ./score-api/src/strategies/strategies/index.ts

Import and declare you new strategy in the index.ts file: 
import * as yourStrategyName from './your-strategy-name';

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d. examples.json

Provide an example for the custom strategy setup which will be displayed on https://snapshot.box on the strategy’s details page. Make sure to include all the parameters you defined above and a list of addresses to test against: 
[
  {
    "name": "Example query",
    "strategy": {
      "name": "erc20-balance-of",
      "params": {
        "address": "0x6b175474e89094c44da98b954eedeac495271d0f",
        "symbol": "DAI",
        "decimals": 18
      }
    },
    "network": "1",
    "addresses": [
      "0xA478c2975Ab1Ea89e8196811F51A7B7Ade33eB11",
      "0xeF8305E140ac520225DAf050e2f71d5fBcC543e7",
      "0x1E1A51E25f2816335cA436D65e9Af7694BE232ad",
      "0x1F717Ce8ff07597ee7c408b5623dF40AaAf1787C",
      "0x1c7a9275F2BD5a260A9c31069F77d53473b8ae2e",
      "0x1d5E65a087eBc3d03a294412E46CE5D6882969f4",
      "0x1f254336E5c46639A851b9CfC165697150a6c327",
      "0x2ec3F80BeDA63Ede96BA20375032CDD3aAfb3030",
      "0x4AcBcA6BE2f8D2540bBF4CA77E45dA0A4a095Fa2",
      "0x4F3D348a6D09837Ae7961B1E0cEe2cc118cec777",
      "0x6D7f23A509E212Ba7773EC1b2505d1A134f54fbe",
      "0x07a1f6fc89223c5ebD4e4ddaE89Ac97629856A0f",
      "0x8d5F05270da470e015b67Ab5042BDbE2D2FEFB48",
      "0x8d07D225a769b7Af3A923481E1FdF49180e6A265",
      "0x8f60501dE5b9b01F9EAf1214dbE1924aA97F7fd0",
      "0x9B8e8dD9151260c21CB6D7cc59067cd8DF306D58",
      "0x17ea92D6FfbAA1c7F6B117c1E9D0c88ABdc8b84C",
      "0x38C0039247A31F3939baE65e953612125cB88268"
    ],
    "snapshot": 11437846
  }
]

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e. README.md

Write the description of how the strategy works and provide an example of the setup. It will be displayed on the strategy’s details page. 
This is the most common strategy, it returns the balances of the voters for a specific ERC20 token.

Here is an example of parameters:

{
  "address": "0x6b175474e89094c44da98b954eedeac495271d0f",
  "symbol": "DAI",
  "decimals": 18
}

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4. Test the strategy locally

Once you saved all the files run the below command with the name of your new strategy: 
npm run test:strategy --strategy=<STRATEGY NAME> // replace <STRATEGY NAME>
It will trigger the tests which you can find in this file. If you get any errors read them carefully as they should point directly to the problem.

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5. Review the checklist

Ensure you meet the requirements for adding a new strategy by reviewing the checklist for adding a new strategy which can be found on this documentation

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6. Create a pull request

Create a Pull Request with the above changes on the original score-api repo. The review can take the team up to 72 hours, so please be patient 🙏 After the PR has been merged, you will need to wait for the release of a new version of Snapshot which can take a couple of days. Once it’s deployed you can move on to the next step.

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7. Try it out!

Head to the strategy’s details page and click Playground button. Follow the instructions from #testing-a-voting-strategy to see if it works as you intended. Congrats, you’ve just added a new custom voting strategy.