package main

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

// Pretty validates and formats JSON using two-space indentation.
func Pretty(src []byte) ([]byte, error) {
	src = bytes.TrimSpace(src)

	var out bytes.Buffer
	if err := json.Indent(&out, src, "", "  "); err != nil {
		return nil, fmt.Errorf("invalid JSON: %w", err)
	}
	out.WriteByte('\n')
	return out.Bytes(), nil
}

func main() {
	if len(os.Args) == 2 && os.Args[1] == "--test" {
		runTests()
		return
	}

	src, err := io.ReadAll(os.Stdin)
	if err != nil {
		fail(err)
	}

	pretty, err := Pretty(src)
	if err != nil {
		fail(err)
	}

	if _, err := os.Stdout.Write(pretty); err != nil {
		fail(err)
	}
}

func fail(err error) {
	fmt.Fprintln(os.Stderr, err)
	os.Exit(1)
}

// runTests provides a couple of lightweight tests without a separate test file.
func runTests() {
	tests := []struct {
		name, input, want string
	}{
		{
			name:  "object",
			input: `{"name":"Ada","active":true}`,
			want:  "{\n  \"name\": \"Ada\",\n  \"active\": true\n}\n",
		},
		{
			name:  "nested array",
			input: `[1,{"items":["a","b"]}]`,
			want:  "[\n  1,\n  {\n    \"items\": [\n      \"a\",\n      \"b\"\n    ]\n  }\n]\n",
		},
	}

	for _, test := range tests {
		got, err := Pretty([]byte(test.input))
		if err != nil || string(got) != test.want {
			fmt.Fprintf(os.Stderr, "%s failed\nwant:\n%s\ngot:\n%s\nerror: %v\n",
				test.name, test.want, got, err)
			os.Exit(1)
		}
	}

	if _, err := Pretty([]byte(`{"broken":}`)); err == nil {
		fmt.Fprintln(os.Stderr, "invalid JSON test failed")
		os.Exit(1)
	}

	fmt.Println("all tests passed")
}