// web_fetch — Fetch an HTTP(S) page and render its HTML as Markdown.
package main

import (
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"net/http"
	"net/url"
	"os"
	"strings"
	"time"

	md "github.com/JohannesKaufmann/html-to-markdown"
)

const maxResponseSize = 10 << 20

type arguments struct {
	URL string `json:"url"`
}

func main() {
	var args arguments
	if err := json.NewDecoder(os.Stdin).Decode(&args); err != nil {
		fail(err)
	}
	page, err := fetch(args.URL)
	if err != nil {
		fail(err)
	}
	converter := md.NewConverter(args.URL, true, nil)
	markdown, err := converter.ConvertString(page)
	if err != nil {
		fail(fmt.Errorf("convert HTML: %w", err))
	}
	fmt.Print(markdown)
}

func fetch(rawURL string) (string, error) {
	u, err := url.ParseRequestURI(rawURL)
	if err != nil || u.Scheme == "" || u.Host == "" || (u.Scheme != "http" && u.Scheme != "https") {
		return "", errors.New("url must be an absolute http or https URL")
	}

	client := &http.Client{Timeout: 30 * time.Second}
	request, err := http.NewRequest(http.MethodGet, u.String(), nil)
	if err != nil {
		return "", fmt.Errorf("create request: %w", err)
	}
	request.Header.Set("Accept", "text/html,application/xhtml+xml")
	request.Header.Set("User-Agent", "ollie-web-fetch/1.0")

	response, err := client.Do(request)
	if err != nil {
		return "", fmt.Errorf("fetch URL: %w", err)
	}
	defer response.Body.Close()
	if response.StatusCode < http.StatusOK || response.StatusCode >= http.StatusMultipleChoices {
		return "", fmt.Errorf("fetch URL: HTTP %s", response.Status)
	}

	body, err := io.ReadAll(io.LimitReader(response.Body, maxResponseSize+1))
	if err != nil {
		return "", fmt.Errorf("read response: %w", err)
	}
	if len(body) > maxResponseSize {
		return "", fmt.Errorf("response exceeds %d bytes", maxResponseSize)
	}
	if !strings.Contains(strings.ToLower(response.Header.Get("Content-Type")), "text/html") {
		return "", fmt.Errorf("response is not HTML: %s", response.Header.Get("Content-Type"))
	}
	return string(body), nil
}

func fail(err error) {
	fmt.Fprintf(os.Stderr, "error: %v\n", err)
	os.Exit(1)
}