package main

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

// PrettyJSON formats JSON with stable indentation and a trailing newline.
func PrettyJSON(input []byte, indent string) ([]byte, error) {
	var buf bytes.Buffer
	if err := json.Indent(&buf, input, "", indent); err != nil {
		return nil, err
	}
	buf.WriteByte('\n')
	return buf.Bytes(), nil
}

// prettyFromReader reads all input and returns formatted JSON text.
func prettyFromReader(r io.Reader) ([]byte, error) {
	input, err := io.ReadAll(r)
	if err != nil {
		return nil, err
	}
	return PrettyJSON(input, "  ")
}

func main() {
	out, err := prettyFromReader(os.Stdin)
	if err != nil {
		fmt.Fprintln(os.Stderr, "prettyjson:", err)
		os.Exit(1)
	}
	if _, err := os.Stdout.Write(out); err != nil {
		fmt.Fprintln(os.Stderr, "prettyjson:", err)
		os.Exit(1)
	}
}

func TestPrettyJSON(t *testing.T) {
	got, err := PrettyJSON([]byte(`{"b":[true,null],"a":1}`), "  ")
	if err != nil {
		t.Fatal(err)
	}

	want := "{\n  \"b\": [\n    true,\n    null\n  ],\n  \"a\": 1\n}\n"
	if string(got) != want {
		t.Fatalf("unexpected output:\n%s", got)
	}
}

func TestPrettyJSONRejectsInvalidInput(t *testing.T) {
	_, err := PrettyJSON([]byte(`{"broken":`), "  ")
	if err == nil || !strings.Contains(err.Error(), "unexpected end") {
		t.Fatalf("expected parse error, got %v", err)
	}
}