100 lines
2.1 KiB
Go
100 lines
2.1 KiB
Go
// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
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// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
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// See page 153.
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// Title3 prints the title of an HTML document specified by a URL.
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package main
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import (
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"fmt"
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"net/http"
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"os"
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"strings"
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"golang.org/x/net/html"
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)
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// Copied from gopl.io/ch5/outline2.
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func forEachNode(n *html.Node, pre, post func(n *html.Node)) {
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if pre != nil {
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pre(n)
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}
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for c := n.FirstChild; c != nil; c = c.NextSibling {
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forEachNode(c, pre, post)
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}
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if post != nil {
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post(n)
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}
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}
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//!+
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// soleTitle returns the text of the first non-empty title element
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// in doc, and an error if there was not exactly one.
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func soleTitle(doc *html.Node) (title string, err error) {
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type bailout struct{}
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defer func() {
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switch p := recover(); p {
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case nil:
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// no panic
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case bailout{}:
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// "expected" panic
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err = fmt.Errorf("multiple title elements")
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default:
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panic(p) // unexpected panic; carry on panicking
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}
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}()
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// Bail out of recursion if we find more than one non-empty title.
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forEachNode(doc, func(n *html.Node) {
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if n.Type == html.ElementNode && n.Data == "title" &&
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n.FirstChild != nil {
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if title != "" {
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panic(bailout{}) // multiple title elements
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}
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title = n.FirstChild.Data
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}
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}, nil)
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if title == "" {
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return "", fmt.Errorf("no title element")
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}
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return title, nil
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}
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//!-
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func title(url string) error {
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resp, err := http.Get(url)
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if err != nil {
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return err
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}
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// Check Content-Type is HTML (e.g., "text/html; charset=utf-8").
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ct := resp.Header.Get("Content-Type")
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if ct != "text/html" && !strings.HasPrefix(ct, "text/html;") {
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resp.Body.Close()
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return fmt.Errorf("%s has type %s, not text/html", url, ct)
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}
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doc, err := html.Parse(resp.Body)
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resp.Body.Close()
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if err != nil {
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return fmt.Errorf("parsing %s as HTML: %v", url, err)
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}
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title, err := soleTitle(doc)
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if err != nil {
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return err
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}
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fmt.Println(title)
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return nil
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}
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func main() {
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for _, arg := range os.Args[1:] {
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if err := title(arg); err != nil {
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fmt.Fprintf(os.Stderr, "title: %v\n", err)
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}
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}
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}
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