package main

import (
    "context"
    "crypto/ecdsa"
    "errors"
    "fmt"
    "math/big"

    "github.com/ethereum/go-ethereum/common"
    "github.com/ethereum/go-ethereum/core/types"
    "github.com/ethereum/go-ethereum/crypto"
    "github.com/ethereum/go-ethereum/ethclient"
    "golang.org/x/crypto/sha3"
)

// After imports....

const (
    // replace <API-KEY> with alchemy api key provided.
    AlchemyPolygonRPCEndpoint = "https://polygon-mainnet.g.alchemy.com/v2/<API-KEY>"

    // USDTTokenAddress is USDT contract address for the USDT token on Polygon
    // network. Can be checked in the following polygonscan link:
    // https://polygonscan.com/token/0xc2132d05d31c914a87c6611c10748aeb04b58e8f
    USDTTokenAddress = "0xc2132D05D31c914a87C6611C10748AEb04B58e8F"

    DefaultGasLimit uint64 = 100000
)

// Client for making transaction.
type Client struct {    
    client      *ethclient.Client
    publickKey  common.Address
    privateKey  *ecdsa.PrivateKey
}

// NewWithPrivateKey creates a new Client with the private key
// provided.
func NewWithPrivateKey(pKeyStr string) (*Client, error) {
    client, err := ethclient.Dial(AlchemyPolygonRPCEndpoint)
    if err != nil {
        return nil, err
    }

    privateKey, err := crypto.HexToECDSA(pKeyStr)
    if err != nil {
        return nil, err
    }

    publicKey := privateKey.Public()
    publicKeyECDSA, ok := publicKey.(*ecdsa.PublicKey)
    if !ok {
        return nil, errors.New("unable to convert publicKey to *ecdsa.PublicKey type")
    }

    // extracting public address of the wallet with the supplied private key.
    pubAddrs := crypto.PubkeyToAddress(*publicKeyECDSA)

    return &Client{     
        client:      client,
        publickKey:  pubAddrs,
        privateKey:  privateKey,
    }, nil
}

// TransferUSDT make transaction of usdts to the specified address.
// The amount should be provided in 6 decimals.
// Meaning, 1 USDT should be represented as 1e6.
// ctx: context
// toAddressStrHex: hexadecimal representation of receiver address(Public Address)
// amount: usdt amount to be sent.
func (c *Client) TransferUSDT(
    ctx context.Context,
    toAddressStrHex string,
    amount uint64,
) (string, error) {
    // Retrieving pending nonce. The nonce, according to
    // ethereum glossary is a:
    // "An account nonce is a transaction counter in each account,
    // which is used to prevent replay attacks."
    nonce, err := c.client.PendingNonceAt(ctx, c.publickKey)
    if err != nil {
        return "", err
    }

    // given that we are going to transfer
    // usdts we don't need eths wei (0 eth).
    value := big.NewInt(0)

    // receiver address.
    toAddress := common.HexToAddress(toAddressStrHex)
    // usdt token address.
    tokenAddress := common.HexToAddress(USDTTokenAddress)

    // we will use the transfer method
    // on the smart contract associated with usdt token
    // in order to use this method, we need to provide the method id
    // this is how we get that number.
    // You could also check it here in this link
    // https://polygonscan.com/token/0xc2132d05d31c914a87c6611c10748aeb04b58e8f#writeProxyContract#F11
    transferFnSignature := []byte("transfer(address,uint256)")
    hash := sha3.NewLegacyKeccak256()
    hash.Write(transferFnSignature)
    methodID := hash.Sum(nil)[:4]

    // we need to add 32 bytes of zeros to our address.
    paddedAddress := common.LeftPadBytes(toAddress.Bytes(), 32)

    // we need to add 32 bytes of zeros to our amount of tokens.
    // we are assuming this amount of tokens is expressed in 6 decimals.
    // which are the decimals for usdt.
    amountBigInt := new(big.Int)
    amountBigInt.SetUint64(amount)
    paddedAmount := common.LeftPadBytes(amountBigInt.Bytes(), 32)

    // now let's put this three parts into
    // the data we are going to pass in the transaction
    // part one: methodID
    // part two: receiver address padded 32 bytes
    // part three: padded amount to be sent
    var data []byte
    data = append(data, methodID...)
    data = append(data, paddedAddress...)
    data = append(data, paddedAmount...)

    // retrieving suggested gas fees and gas price.
    tipCap, err := c.client.SuggestGasTipCap(ctx)
    if err != nil {
        return "", err
    }

    feeCap, err := c.client.SuggestGasPrice(ctx)
    if err != nil {
        return "", err
    }

    //  network ID for this client.
    chainID, err := c.client.NetworkID(ctx)
    if err != nil {
        return "", err
    }

    // creating our transaction, in this case we are going to use
    // dynamic fees txns instead of the legacy system.
    tx := types.NewTx(&types.DynamicFeeTx{
        ChainID:   chainID,
        Nonce:     nonce,
        GasTipCap: tipCap,
        GasFeeCap: feeCap,
        Gas:       DefaultGasLimit,
        To:        &tokenAddress,
        Value:     value,
        Data:      data,
    })

    // sign the transaction with our private key.
    signedTx, err := types.SignTx(tx, types.LatestSignerForChainID(chainID), c.privateKey)
    if err != nil {
        return "", err
    }

    // send the transaction.
    err = c.client.SendTransaction(ctx, signedTx)
    if err != nil {
        return "", err
    }

    // return the hexadecimal representation of the txnHash.
    return signedTx.Hash().Hex(), nil
}

func main() {
    ctx := context.Background()
    client, err := NewWithPrivateKey("<PRIVATE-KEY>")
    if err != nil {
        panic(err)
    }

    txnHash, err := client.TransferUSDT(ctx, "<RECEIVER ADDRESS>", 1e6) // Sending 1 usdt.
    if err != nil {
        panic(err)
    }

    fmt.Println("TXN HASH: ", txnHash)
}