package email

import (
	"crypto/sha1"
	"hash"
	"io"
	"net/mail"
	"os"
	"sort"

	"github.com/golang/glog"
)

// Hash will parse r as an email, and return a hash.Hash that has been applied
// to the values of the specified headers.
func Hash(r io.Reader, headers []string) (hash.Hash, error) {
	// Add deterministic behavior regardless of the order the users specified.
	sort.Strings(headers)
	var name string
	if f, ok := r.(*os.File); ok {
		name = f.Name()
	}

	h := sha1.New()
	msg, err := mail.ReadMessage(r)
	if err != nil {
		return nil, err
	}

	for _, header := range headers {
		v := msg.Header.Get(header)
		if v == "" {
			glog.V(2).Infoln(name, "Empty", header, "header")
		}
		io.WriteString(h, v)
	}
	return h, nil
}