package email

import (
	"net/mail"
	"strings"
)

// The junk filter — the cheapest and most important half of reading mail.
//
// A mailbox is mostly machine-generated: newsletters, receipts nobody acts on,
// social notifications, marketing. Sending all of it to a 1.7B and asking "is
// there a task here" produces confident nonsense at a rate proportional to the
// volume, so junk is decided by HEADERS, before any model sees the message.
//
// The rules are all bulk-mail markers that senders set on themselves, never
// guesses about content:
//
//   - List-Unsubscribe / List-Id — by definition a mailing list. If he can
//     unsubscribe from it, it is not asking him to do anything.
//   - Precedence: bulk|junk|list — the sender declaring itself bulk.
//   - Auto-Submitted other than "no" (RFC 3834) — generated by a machine.
//   - X-Spam-Flag: YES, X-Spam-Status: Yes — the spam filter upstream already
//     decided; we do not second-guess it in the other direction.
//
// There is deliberately NO Gmail-category rule. One was written and removed:
// it matched X-GM-LABELS and X-Gmail-Labels against the parsed header block,
// and neither is a header. X-GM-LABELS is a Gmail FETCH data item, requested as
// "UID FETCH n (X-GM-LABELS)" and never present in the message source;
// X-Gmail-Labels only exists in a Takeout mbox export. This client asks for
// BODY.PEEK[] and nothing else, so the rule could not fire against a real
// mailbox while its doc comment promised a Promotions filter. Gmail's promotion
// mail carries List-Unsubscribe in practice and is caught by the rule above.
// Bringing the category rule back means adding the FETCH item and carrying the
// labels into classifyJunk out of band, not matching a header that never
// arrives.
//
// Deliberately NOT here: sender allow/deny lists and subject keyword matching.
// Both are configuration that ages badly and both would be a place for his
// contacts to end up in a config file. If a real correspondent's mail is being
// dropped, the fix is a rule about a header, not a list of names.
//
// A junk verdict never deletes anything and never touches a flag on the server.
// It means "do not spend the model on this", nothing more.

// junkHeaders — headers whose mere presence marks bulk mail.
var junkPresence = []string{"List-Unsubscribe", "List-Id", "List-Post"}

// classifyJunk returns whether the message is bulk/automated and why. The
// reason is a short header name, safe to log — it names the marker, never the
// sender or the subject.
func classifyJunk(h mail.Header) (bool, string) {
	for _, name := range junkPresence {
		if strings.TrimSpace(h.Get(name)) != "" {
			return true, strings.ToLower(name)
		}
	}
	switch strings.ToLower(strings.TrimSpace(h.Get("Precedence"))) {
	case "bulk", "junk", "list":
		return true, "precedence"
	}
	if v := strings.ToLower(strings.TrimSpace(h.Get("Auto-Submitted"))); v != "" && v != "no" {
		return true, "auto-submitted"
	}
	if strings.EqualFold(strings.TrimSpace(h.Get("X-Spam-Flag")), "yes") {
		return true, "x-spam-flag"
	}
	if v := strings.ToLower(strings.TrimSpace(h.Get("X-Spam-Status"))); strings.HasPrefix(v, "yes") {
		return true, "x-spam-status"
	}
	return false, ""
}