package main

// The mail signal's own words (novox/hq ADR 0461 §3, §4; issue 1429): a pure function, no store, no bus.
// Sending it — messenger_check, the safe form, the cap, marking mails told — is 1522.

import (
	"fmt"
	"sort"
	"time"
)

// SignalMail is one mail the mail signal may name (ADR 0461 §3): just enough of its entry, never a body.
// Date is the mail's own date, the "oldest first" tiebreak among mails with no due date (1429/AC1); it is
// not itself said.
type SignalMail struct {
	SenderName string // "" answers "a sender without a name" (1429/AC2), never the address
	Summary    string
	Date       time.Time
	DueDate    time.Time // zero means none
}

// SignalWords is what one mail signal says (ADR 0461 §1): a headline, and at most two lines.
type SignalWords struct {
	Headline string
	Lines    []string
}

// orderSignalMails is 1429/AC1, AC4: nearest due date first (one without a due date after all those with
// one), then oldest first among mails sharing that place — "mails carried from earlier (untold)" are simply
// part of the same list handed in, ordered exactly as any other mail that matters is.
func orderSignalMails(mails []SignalMail) []SignalMail {
	sorted := append([]SignalMail(nil), mails...)
	sort.SliceStable(sorted, func(i, j int) bool {
		a, b := sorted[i], sorted[j]
		aNone, bNone := a.DueDate.IsZero(), b.DueDate.IsZero()
		if aNone != bNone {
			return bNone // a has a due date, b does not: a first
		}
		if !aNone {
			return a.DueDate.Before(b.DueDate)
		}
		return a.Date.Before(b.Date) // oldest first, among mails with no due date
	})
	return sorted
}

// headline is "1 mail matters" or "N mails matter" (1429/AC1, AC3: the same headline whether the words are
// said in full or the safe form).
func headline(n int) string {
	if n == 1 {
		return "1 mail matters"
	}
	return fmt.Sprintf("%d mails matter", n)
}

// mailLine is one mail's own line (1429/AC1): "<sender's name> — <summary> — due <date>", the due part left
// out entirely when there is none; a sender with no display name is "a sender without a name" (AC2), never
// its address.
func mailLine(m SignalMail) string {
	sender := m.SenderName
	if sender == "" {
		sender = "a sender without a name"
	}
	line := sender + " — " + m.Summary
	if !m.DueDate.IsZero() {
		line += " — due " + m.DueDate.Format("2006-01-02")
	}
	return line
}

// BuildSignal is the mail signal's words in full (1429/AC1, AC2, AC4): the headline, the nearest-due mail's
// own line, and — with more than one — the next one's, ending ", and N-2 more on the mail page" once there
// are more than two in all.
func BuildSignal(mails []SignalMail) SignalWords {
	ordered := orderSignalMails(mails)
	n := len(ordered)
	w := SignalWords{Headline: headline(n)}
	if n == 0 {
		return w
	}
	w.Lines = append(w.Lines, mailLine(ordered[0]))
	if n >= 2 {
		second := mailLine(ordered[1])
		if n > 2 {
			second += fmt.Sprintf(", and %d more on the mail page", n-2)
		}
		w.Lines = append(w.Lines, second)
	}
	return w
}

// SafeSignal is 1429/AC3: the safe form a refused set of words falls back to — the same headline, one line
// naming nothing of any mail.
func SafeSignal(n int) SignalWords {
	return SignalWords{Headline: headline(n), Lines: []string{"open the mail page"}}
}