package main

import (
	"bytes"
	"encoding/json"
	"fmt"
	"io"
	"os"
	"strings"
	"testing"
)

// PrettyJSON validates src as JSON and returns it formatted with two-space indentation.
func PrettyJSON(src []byte) ([]byte, error) {
	var buf bytes.Buffer

	dec := json.NewDecoder(bytes.NewReader(src))
	dec.UseNumber()

	var value any
	if err := dec.Decode(&value); err != nil {
		return nil, err
	}

	// Reject trailing non-whitespace data after the first JSON value.
	if err := dec.Decode(&struct{}{}); err != io.EOF {
		return nil, fmt.Errorf("invalid trailing JSON")
	}

	enc := json.NewEncoder(&buf)
	enc.SetIndent("", "  ")
	enc.SetEscapeHTML(false)

	if err := enc.Encode(value); err != nil {
		return nil, err
	}

	return buf.Bytes(), nil
}

func main() {
	input, err := io.ReadAll(os.Stdin)
	if err != nil {
		fmt.Fprintln(os.Stderr, "read stdin:", err)
		os.Exit(1)
	}

	output, err := PrettyJSON(input)
	if err != nil {
		fmt.Fprintln(os.Stderr, "pretty json:", err)
		os.Exit(1)
	}

	if _, err := os.Stdout.Write(output); err != nil {
		fmt.Fprintln(os.Stderr, "write stdout:", err)
		os.Exit(1)
	}
}

func TestPrettyJSONFormatsObjects(t *testing.T) {
	got, err := PrettyJSON([]byte(`{"name":"Ada","langs":["go","json"],"active":true}`))
	if err != nil {
		t.Fatal(err)
	}

	want := strings.Join([]string{
		"{",
		`  "active": true,`,
		`  "langs": [`,
		`    "go",`,
		`    "json"`,
		"  ],",
		`  "name": "Ada"`,
		"}",
		"",
	}, "\n")

	if string(got) != want {
		t.Fatalf("unexpected output:\n%s", got)
	}
}

func TestPrettyJSONRejectsTrailingData(t *testing.T) {
	if _, err := PrettyJSON([]byte(`{"ok":true} false`)); err == nil {
		t.Fatal("expected trailing JSON error")
	}
}