package module

import (
	"encoding/xml"
	"fmt"
	"io"
	"net/http"
	"net/url"
	"strings"
)

// HttpGetter is a Getter implementation that will download a module from
// an HTTP endpoint. The protocol for downloading a module from an HTTP
// endpoing is as follows:
//
// An HTTP GET request is made to the URL with the additional GET parameter
// "terraform-get=1". This lets you handle that scenario specially if you
// wish. The response must be a 2xx.
//
// First, a header is looked for "X-Terraform-Get" which should contain
// a source URL to download.
//
// If the header is not present, then a meta tag is searched for named
// "terraform-get" and the content should be a source URL.
//
// The source URL, whether from the header or meta tag, must be a fully
// formed URL. The shorthand syntax of "github.com/foo/bar" or relative
// paths are not allowed.
type HttpGetter struct{}

func (g *HttpGetter) Get(dst string, u *url.URL) error {
	// Copy the URL so we can modify it
	var newU url.URL = *u
	u = &newU

	// Add terraform-get to the parameter.
	q := u.Query()
	q.Add("terraform-get", "1")
	u.RawQuery = q.Encode()

	// Get the URL
	resp, err := http.Get(u.String())
	if err != nil {
		return err
	}
	defer resp.Body.Close()
	if resp.StatusCode < 200 || resp.StatusCode >= 300 {
		return fmt.Errorf("bad response code: %d", resp.StatusCode)
	}

	// Extract the source URL
	var source string
	if v := resp.Header.Get("X-Terraform-Get"); v != "" {
		source = v
	} else {
		source, err = g.parseMeta(resp.Body)
		if err != nil {
			return err
		}
	}
	if source == "" {
		return fmt.Errorf("no source URL was returned")
	}

	// Get it!
	return Get(dst, source)
}

// parseMeta looks for the first meta tag in the given reader that
// will give us the source URL.
func (g *HttpGetter) parseMeta(r io.Reader) (string, error) {
	d := xml.NewDecoder(r)
	d.CharsetReader = charsetReader
	d.Strict = false
	var err error
	var t xml.Token
	for {
		t, err = d.Token()
		if err != nil {
			if err == io.EOF {
				err = nil
			}
			return "", err
		}
		if e, ok := t.(xml.StartElement); ok && strings.EqualFold(e.Name.Local, "body") {
			return "", nil
		}
		if e, ok := t.(xml.EndElement); ok && strings.EqualFold(e.Name.Local, "head") {
			return "", nil
		}
		e, ok := t.(xml.StartElement)
		if !ok || !strings.EqualFold(e.Name.Local, "meta") {
			continue
		}
		if attrValue(e.Attr, "name") != "terraform-get" {
			continue
		}
		if f := attrValue(e.Attr, "content"); f != "" {
			return f, nil
		}
	}
}

// attrValue returns the attribute value for the case-insensitive key
// `name', or the empty string if nothing is found.
func attrValue(attrs []xml.Attr, name string) string {
	for _, a := range attrs {
		if strings.EqualFold(a.Name.Local, name) {
			return a.Value
		}
	}
	return ""
}

// charsetReader returns a reader for the given charset. Currently
// it only supports UTF-8 and ASCII. Otherwise, it returns a meaningful
// error which is printed by go get, so the user can find why the package
// wasn't downloaded if the encoding is not supported. Note that, in
// order to reduce potential errors, ASCII is treated as UTF-8 (i.e. characters
// greater than 0x7f are not rejected).
func charsetReader(charset string, input io.Reader) (io.Reader, error) {
	switch strings.ToLower(charset) {
	case "ascii":
		return input, nil
	default:
		return nil, fmt.Errorf("can't decode XML document using charset %q", charset)
	}
}