package findlinks

import (
	"fmt"
	"io"
	"strings"

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

// Parse collects the unmarshalled [Link] data from the HTML document
// represented by r. The data is returned as a slice, along with an
// error.
func Parse(r io.Reader) ([]Link, error) {
	doc, err := html.Parse(r)
	if err != nil {
		return nil, fmt.Errorf("can't parse html reader: %w", err)
	}

	return parseLinks(doc), nil
}

// parseLinks returns a [Link] slice from doc. Each element is an
// "unmarshalled" version of an anchor tag element inside doc.
func parseLinks(doc *html.Node) []Link {
	linkNodes := harvestLinkNodes(doc)

	var links []Link
	for _, linkNode := range linkNodes {
		var link Link

		// Get the link's inner text.
		link.Text = harvestText(linkNode)

		// Get the href attribute.
		for _, a := range linkNode.Attr {
			if a.Key == "href" {
				link.Href = a.Val
				break
			}
		}

		links = append(links, link)
	}

	return links
}

// harvestText returns the harvestText contained inside n.
//
// Note that the harvestText could be under many layers of HTML
// nesting. Hence the [html.ElementNode] case calls harvestText recursively.
//
// For the current project, harvestText's argument is always an
// anchor-tag element.
func harvestText(n *html.Node) string {
	switch n.Type {
	// The text of an [html.TextNode] is its [html.Node.Data]
	// field.
	case html.TextNode:
		return n.Data

	// The text of an [html.ElementNode] is the aggregate of the
	// text of its children.
	case html.ElementNode:
		var builder strings.Builder
		for c := n.FirstChild; c != nil; c = c.NextSibling {
			fmt.Fprintf(&builder, "%s ", harvestText(c))
		}

		rawResult := builder.String()
		fields := strings.Fields(rawResult)
		return strings.Join(fields, " ")

	// Any other kind of node (e.g. [html.CommentNode]) doesn't
	// have text.
	default:
		return ""
	}
}

// harvestLinkNodes harvests all of the link nodes contained inside n.
//
// For the current project, harvestLinkNodes' argument is always the
// top-level document node.
func harvestLinkNodes(node *html.Node) []*html.Node {
	var links []*html.Node

	for child := range node.Descendants() {
		if child.Type == html.ElementNode && child.DataAtom == atom.A {
			links = append(links, child)
		}
	}

	return links
}