main.go

package main

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

	"github.com/ethereum/go-ethereum"
	"github.com/ethereum/go-ethereum/common"
	"github.com/ethereum/go-ethereum/common/hexutil"
	"github.com/ethereum/go-ethereum/core/types"
	"github.com/ethereum/go-ethereum/crypto"
	"github.com/ethereum/go-ethereum/crypto/sha3"
	"github.com/ethereum/go-ethereum/ethclient"
)

func main() {
	client, err := ethclient.Dial("http://127.0.0.1:9545")
	if err != nil {
		log.Fatal(err)
	}

	privateKey, err := crypto.HexToECDSA("...")
	if err != nil {
		log.Fatal(err)
	}

	publicKey := privateKey.Public()
	publicKeyECDSA, ok := publicKey.(*ecdsa.PublicKey)
	if !ok {
		log.Fatal("error casting public key to ECDSA")
	}

	fromAddress := crypto.PubkeyToAddress(*publicKeyECDSA)
	nonce, err := client.PendingNonceAt(context.Background(), fromAddress)
	if err != nil {
		log.Fatal(err)
	}

	value := big.NewInt(0) // in wei (0 eth)
	gasPrice, err := client.SuggestGasPrice(context.Background())
	if err != nil {
		log.Fatal(err)
	}

	toAddress := common.HexToAddress("...")
	tokenAddress := common.HexToAddress("...")

	transferFnSignature := []byte("transfer(address,uint256)")
	hash := sha3.NewKeccak256()
	hash.Write(transferFnSignature)
	methodID := hash.Sum(nil)[:4]
	fmt.Printf("Method ID: %s\n", hexutil.Encode(methodID))

	paddedAddress := common.LeftPadBytes(toAddress.Bytes(), 32)
	fmt.Printf("To address: %s\n", hexutil.Encode(paddedAddress))

	amount := new(big.Int)
	amount.SetString("1000000000000000000000", 10) // 1000 tokens
	paddedAmount := common.LeftPadBytes(amount.Bytes(), 32)
	fmt.Printf("Token amount: %s", hexutil.Encode(paddedAmount))

	var data []byte
	data = append(data, methodID...)
	data = append(data, paddedAddress...)
	data = append(data, paddedAmount...)

	gasLimit, err := client.EstimateGas(context.Background(), ethereum.CallMsg{
		To:   &toAddress,
		Data: data,
	})
	if err != nil {
		log.Fatal(err)
	}
	fmt.Printf("Gas limit: %d", gasLimit)

	tx := types.NewTransaction(nonce, tokenAddress, value, gasLimit, gasPrice, data)
	signedTx, err := types.SignTx(tx, types.HomesteadSigner{}, privateKey)
	if err != nil {
		log.Fatal(err)
	}

	err = client.SendTransaction(context.Background(), signedTx)
	if err != nil {
		log.Fatal(err)
	}

	fmt.Printf("Tokens sent at TX: %s", signedTx.Hash().Hex())
}