Process a JSON file, in Go

process_json.go

package main

import (
	"encoding/json"
	"errors"
	"flag"
	"fmt"
	"io/ioutil"
	"os"
)

type cmdlineArgs struct {
	sourcePath string
	destPath string
	userID string
}

type transaction struct {
	From string `json:"from"`
	To string `json:"to"`
	Amount json.Number `json:"amount"`
}

func main() {
	args := parse_args()

	if err := processJSON(args); err != nil {
		fmt.Println("error filtering transactions:", err.Error())
		os.Exit(1)
	}

	fmt.Println("filtered transactions written to", args.destPath)
}

func processJSON(args *cmdlineArgs) error {
	if args.userID == "" {
		return errors.New("-user-id argument is required")
	}

	filecontent, err := ioutil.ReadFile(args.sourcePath)
	if err != nil {
		return err
	}

	var transactions []transaction
	if err := json.Unmarshal(filecontent, &transactions); err != nil {
		return err
	}

	filteredTransactions := filterTransactions(transactions, args.userID)
	filteredTransData, err := json.Marshal(filteredTransactions)
	if err != nil {
		return err
	}

	return ioutil.WriteFile(args.destPath, filteredTransData, 0644)
}

func filterTransactions(transactions []transaction, userID string) []transaction {
	var filteredTransactions []transaction
	for i := range transactions {
		if transactions[i].From == userID {
			filteredTransactions = append(filteredTransactions, transactions[i])
		}
	}
	return filteredTransactions
}

func parse_args() *cmdlineArgs {
	sourcePath := flag.String("source-path", "transactions.json", "path to source JSON file")
	destPath := flag.String("dest-path", "filtered_transactions.json", "path to output JSON file")
	userID := flag.String("user-id", "", "user ID to filter on")

	flag.Parse()

	return &cmdlineArgs{
		sourcePath: *sourcePath,
		destPath: *destPath,
		userID: *userID,
	}
}