Login light
// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
// License: https://creativecommons.org/licenses/by-nc-sa/4.0/

// See page 133.

// Outline prints the outline of an HTML document tree.
package main

import (
    "fmt"
    "net/http"
    "os"

    "golang.org/x/net/html"
)

func main() {
    for _, url := range os.Args[1:] {
        outline(url)
    }
}

func outline(url string) error {
    resp, err := http.Get(url)
    if err != nil {
        return err
    }
    defer resp.Body.Close()

    doc, err := html.Parse(resp.Body)
    if err != nil {
        return err
    }

    //!+call
    forEachNode(doc, startElement, endElement)
    //!-call

    return nil
}

//!+forEachNode
// forEachNode calls the functions pre(x) and post(x) for each node
// x in the tree rooted at n. Both functions are optional.
// pre is called before the children are visited (preorder) and
// post is called after (postorder).
func forEachNode(n *html.Node, pre, post func(n *html.Node)) {
    if pre != nil {
        pre(n)
    }

    for c := n.FirstChild; c != nil; c = c.NextSibling {
        forEachNode(c, pre, post)
    }

    if post != nil {
        post(n)
    }
}

//!-forEachNode

//!+startend
var depth int

func startElement(n *html.Node) {
    if n.Type == html.ElementNode {
        fmt.Printf("%*s<%s>\n", depth*2, "", n.Data)
        depth++
    }
}

func endElement(n *html.Node) {
    if n.Type == html.ElementNode {
        depth--
        fmt.Printf("%*s</%s>\n", depth*2, "", n.Data)
    }
}

//!-startend