package main

import (
	"net/http"
	"fmt"
	"encoding/json"
	"os"
	"context"
	"github.com/jackc/pgx/v5"
)
//mixed opinions on global variables online, need production example to verify this practice
var conn = ConnectPostgres()

//----------------------------------------------------------------------------------------------------
//root that returns a static string
func RootHandler(w http.ResponseWriter, r *http.Request) {
	fmt.Fprintln(w, "Hello World!")
}

//----------------------------------------------------------------------------------------------------
//json struct for incoming requests
type IncomingJson struct {
	Message string `json:"message"`
}

//take note that for structs, Capitalised fields indicates exported fields, and vice versa
//therefore "name string" will not be decoded/encoded, but "Name string" will
//the `json:name`
//json struct for api response
type OutboundJson struct {
	Status string `json:"status"`
	RequestData IncomingJson `json:"request_data"`
}

//API that accepts and returns a json defined in the struct above
func JsonHandler(w http.ResponseWriter, r *http.Request){
	//checks if the method is Post, returns error if not
	if r.Method != http.MethodPost {
		http.Error(w, "Method not allowed,Only Post Requests", http.StatusMethodNotAllowed)
		return
	}

	//decode Json from request body, returns error if doesnt fit struct
	var incoming IncomingJson
	err := json.NewDecoder(r.Body).Decode(&incoming)
	if err != nil {
		http.Error(w,"Bad Request, Failed to parse body({message:str})", http.StatusBadRequest)
		return
	}

	//set response header to json
	w.Header().Set("Content-Type", "application/json")

	//encode and set response
	resp := OutboundJson{
		Status: "received",
		RequestData: incoming,
	}

	err = json.NewEncoder(w).Encode(resp)
	if err != nil {
		fmt.Println("JSON encoding Error:",err)
		http.Error(w, "Error encoding JSON", http.StatusInternalServerError)
		return
	}
}
//----------------------------------------------------------------------------------------------------
func ConnectPostgres()(*pgx.Conn){
	//yes i need to put this in a .env, im just testing for now and this database has nothing important
	conn, err := pgx.Connect(context.Background(), "postgres://postgres:12345678@localhost:5432/postgres")
	if err != nil {
		fmt.Println("Unable to connect to database", err)
		os.Exit(1)
	}

	fmt.Println("Connected to database")
	return conn
}

//operation dictates which CRUD operation to perform (c,r,u,d)
//identifier dictates which row the operation is performed on (int)
//value dictates the actual change of operation
type IncomingPostgres struct{
	Operation string `json:"operation"`
	Identifier int `json:identifier",omitempty`
	Value string `json:"value",omitempty`
}

type OutgoingPostgres struct{
	Identifier int `json:"identifier"`
	Value string `json:"value"`
	Status string `json:"status"`
}

//the pointed table only has two columns: id(int) and value(str)
//the following handler provides a simple example on handling CRUD operations via an API using pgx
func PostgresHandler(w http.ResponseWriter, r *http.Request){
	//checks if the method is Post, returns error if not
	if r.Method != http.MethodPost {
		http.Error(w, "Method not allowed,Only Post Requests", http.StatusMethodNotAllowed)
		return
	}

	//decode Json from request body, returns error if doesnt fit struct
	var incoming IncomingPostgres
	err := json.NewDecoder(r.Body).Decode(&incoming)
	if err != nil {
		http.Error(w,"Bad Request, Failed to parse body({operation:str,identifier[optional]:int,value[optional]:string})", http.StatusBadRequest)
		return
	}

	//map of allowed inputs for the operation field, returns error if doesnt fit
	allowed_operations := map[string]bool {
		"c":true,
		"r":true,
		"u":true,
		"d":true,
	}

	if !allowed_operations[incoming.Operation] {
		http.Error(w, "Bad Request, operation field only allows the following inputs: {c,r,u,d}", http.StatusBadRequest)
		return
	}
	//switch case to determine the operation
	//this method directly checks for zero values, which might not be feasible if zero values are allowed inputs
	//consider transforming the struct into a map then checking if it exists, that would be a more robust method
	var id int
	var value string
	switch incoming.Operation{
	case "c":
		if incoming.Value != ""{
			fmt.Println("create",incoming.Value)
			//SQL query for the db
			rows, err := conn.Query(context.Background(),"INSERT INTO test_table (value) VALUES ($1) RETURNING(id)",incoming.Value)
			if err != nil{
				fmt.Println(err)
				http.Error(w,"Server Error, something is wrong", http.StatusInternalServerError)
				return
			}
			//close the connection when the function is finished
			defer rows.Close()

			if rows.Next(){
				err = rows.Scan(&id)
				if err != nil{
					fmt.Println(err)
					http.Error(w,"Server Error, Scanning Failed",http.StatusInternalServerError)
					return
				}
			}
			value = incoming.Value
		} else {
			http.Error(w,"Bad Request, Value Key is needed for create operation",http.StatusBadRequest)
			return
		}
	case "r":
		if incoming.Identifier != 0{
			fmt.Println("read",incoming.Identifier)
			//SQL query for the db
			rows, err := conn.Query(context.Background(),"SELECT id,value FROM test_table WHERE id=$1",incoming.Identifier)
			if err != nil{
				fmt.Println(err)
				http.Error(w, "Server Error, something is wrong", http.StatusInternalServerError)
				return 
			}
			//close the connection when the function is finished
			defer rows.Close()
			//this is a readone() method, only the first item queried will be considered and returned
			if rows.Next(){
				err = rows.Scan(&id,&value)
				if err != nil{
					http.Error(w,"Server Error, Scanning Failed",http.StatusInternalServerError)
					return
				}
			} else {
				http.Error(w,"Not Found, identifier does not exist",http.StatusNotFound)
				return
			}
		} else {
			http.Error(w,"Bad Request, Identifier Key is needed for read operation",http.StatusBadRequest)
			return
		}

	case "u":
		if incoming.Identifier != 0 && incoming.Value != ""{
			fmt.Println("update",incoming.Identifier,incoming.Value)
			_, err := conn.Query(context.Background(),"UPDATE test_table SET value=$1 WHERE id=$2",incoming.Value,incoming.Identifier)
			if err != nil{
				fmt.Println(err)
				http.Error(w,"Update Failed, double check if identifier exists",http.StatusBadRequest)
				return
			}
			id = incoming.Identifier
			value = incoming.Value
		} else {
			http.Error(w,"Bad Request, Identifier Key and Value Key is needed for update operation",http.StatusBadRequest)
			return
		}

	case "d":
		if incoming.Identifier != 0{
			fmt.Println("delete",incoming.Identifier)
			_,err := conn.Query(context.Background(),"DELETE FROM test_table WHERE id=$1",incoming.Identifier)
			if err != nil{
				fmt.Println(err)
				http.Error(w,"Delete Failed, double check if identifier exists",http.StatusBadRequest)
				return
			}
			value = incoming.Value
			id = incoming.Identifier
		} else {
			http.Error(w,"Bad Request, Value Key is needed for create operation",http.StatusBadRequest)
			return
		}
	}

	w.Header().Set("Content-Type","application/json")
	resp := OutgoingPostgres{
		Status: "Successful",
		Identifier: id,
		Value: value,
	}
	json.NewEncoder(w).Encode(resp)
}


//----------------------------------------------------------------------------------------------------
//main entry function to register routes and serve
func main() {
	defer conn.Close(context.Background())
	http.HandleFunc("/pgx",PostgresHandler)
	http.HandleFunc("/json", JsonHandler)
	http.HandleFunc("/", RootHandler)
	fmt.Println("Starting server at http://localhost:8080")
	err := http.ListenAndServe(":8080", nil)
	if err != nil {
		fmt.Println("Error starting server:", err)
	}
}