// Package tasks ranks captured work (Vikunja #129).
//
// The ordering is COMPUTED, not generated. Asking a 1.7B model which of his
// tasks matters most would produce a fluent opinion about his life with no
// basis in anything, and a confidently wrong priority is worse than no
// priority at all — the same reasoning as the behaviour profile in
// internal/memory, which counts instead of summarising.
//
// So: four signals, all of them things he told her, and a reason string naming
// the one that decided each row. Nothing here invents urgency. A task with no
// due date and no weight scores nothing and sits where its age puts it, which
// is the honest answer to "which of these matters?" when he never said.
//
// Ranking is a READ. It sorts and renders; it never writes, schedules or
// announces. Maven is not a nag: a task rising to the top of this list is not a
// reason to speak, only the order she recites in when asked.
package tasks

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

// Status values, mirroring internal/store so a caller can rank ipc.Task rows
// without importing the store.
const (
	StatusCandidate = "candidate"
	StatusOpen      = "open"
)

// Item — one task to rank. The subset of a task that ranking depends on;
// callers map their own row type onto it.
type Item struct {
	ID      int64
	Text    string
	Status  string
	Created time.Time
	Due     *time.Time
	Weight  int
}

// Ranked — one task with its score and the reason that decided it.
type Ranked struct {
	Item
	Score float64
	// Reason — the dominant signal, in Russian, for the page and the spoken
	// list. Empty when nothing distinguished this task: no due date, no
	// weight, not old. Saying "потому что" about a task he never prioritised
	// would be making something up.
	Reason string
}

// Scoring weights. Deliberately coarse round numbers: this is a knob, not
// math, and the only property that has to hold is the ordering between classes
// (overdue beats today beats this week beats undated).
const (
	scoreOverdue     = 100 // he already missed it
	scoreOverduePer  = 5   // per further day late, capped
	scoreOverdueCap  = 40
	scoreDueToday    = 60
	scoreDueTomorrow = 40
	scoreDueWeek     = 20
	scoreDueLater    = 5
	scorePerWeight   = 15 // "срочно" / "важно" / the web form's select
	scorePerWeekOld  = 1  // so nothing rots at the bottom forever
	scoreAgeCap      = 10
	// MaxWeight — the highest importance hint capture accepts. Three rungs is
	// as many as anyone can rank by hand honestly.
	MaxWeight = 3
)

// Rank scores every item and returns them ordered: confirmed work first, then
// candidates, each by score descending, oldest first on a tie.
//
// Candidates never outrank open work, whatever their due date. A task Maven
// derived from something she read is a suggestion until he confirms it, and
// putting her guess above his own stated work would be reading his priorities
// back to him wrong.
func Rank(items []Item, now time.Time) []Ranked {
	out := make([]Ranked, 0, len(items))
	for _, it := range items {
		score, reason := score(it, now)
		out = append(out, Ranked{Item: it, Score: score, Reason: reason})
	}
	sort.SliceStable(out, func(i, j int) bool {
		ci, cj := out[i].Status == StatusCandidate, out[j].Status == StatusCandidate
		if ci != cj {
			return !ci // open before candidate
		}
		if out[i].Score != out[j].Score {
			return out[i].Score > out[j].Score
		}
		return out[i].Created.Before(out[j].Created) // oldest first, FIFO
	})
	return out
}

// score — the per-item scoring function. Returns the score and the dominant
// reason. Deadline beats weight when both are present: a date is a fact about
// the world, a weight is how he felt when he filed it.
func score(it Item, now time.Time) (float64, string) {
	var total float64
	reason := ""

	if it.Due != nil {
		days := dayDelta(*it.Due, now)
		switch {
		case days < 0:
			late := -days
			bonus := float64(late * scoreOverduePer)
			if bonus > scoreOverdueCap {
				bonus = scoreOverdueCap
			}
			total += scoreOverdue + bonus
			reason = "просрочено"
			if late == 1 {
				reason = "просрочено на день"
			} else if late > 1 {
				reason = fmt.Sprintf("просрочено на %d дн.", late)
			}
		case days == 0:
			total += scoreDueToday
			reason = "сегодня"
		case days == 1:
			total += scoreDueTomorrow
			reason = "завтра"
		case days <= 7:
			total += scoreDueWeek
			reason = fmt.Sprintf("через %d дн.", days)
		default:
			total += scoreDueLater
		}
	}

	w := it.Weight
	if w > MaxWeight {
		w = MaxWeight
	}
	if w > 0 {
		total += float64(w * scorePerWeight)
		if reason == "" {
			reason = "важно"
		}
	}

	if !it.Created.IsZero() {
		weeks := int(now.Sub(it.Created).Hours() / (24 * 7))
		if weeks > 0 {
			age := float64(weeks * scorePerWeekOld)
			if age > scoreAgeCap {
				age = scoreAgeCap
			}
			total += age
			if reason == "" && weeks >= 2 {
				reason = "давно в списке"
			}
		}
	}
	return total, reason
}

// dayDelta — calendar days from now to due, in due's own location. Whole days,
// not hours: a task due today is due today whether it is 09:00 or 23:00, and an
// hours-based comparison would call this evening's task "overdue" all afternoon.
func dayDelta(due, now time.Time) int {
	loc := due.Location()
	d := time.Date(due.Year(), due.Month(), due.Day(), 0, 0, 0, 0, loc)
	n := now.In(loc)
	n = time.Date(n.Year(), n.Month(), n.Day(), 0, 0, 0, 0, loc)
	return int(d.Sub(n).Hours() / 24)
}

// SpokenLimit — how many tasks the spoken list names before it summarises the
// rest. A recital of twenty items is noise; five is a list he can hold.
const SpokenLimit = 5

// FormatRU renders a ranked list the way Maven says it. Confirmed work first,
// with the reason attached where there is one; candidates named as
// unconfirmed, never recited as his work.
//
// One renderer for the voice reply and the web page, for the same reason
// DayPlan.Spoken is built core-side: two formatters drift, and then she says
// one order and shows another.
func FormatRU(ranked []Ranked) string {
	var open, cands []Ranked
	for _, r := range ranked {
		if r.Status == StatusCandidate {
			cands = append(cands, r)
		} else {
			open = append(open, r)
		}
	}
	if len(open) == 0 && len(cands) == 0 {
		return "задач нет."
	}

	var b strings.Builder
	if len(open) > 0 {
		b.WriteString("сначала: ")
		b.WriteString(joinRU(open, SpokenLimit, true))
		b.WriteString(".")
	}
	if len(cands) > 0 {
		if b.Len() > 0 {
			b.WriteString(" ")
		}
		b.WriteString("ещё я нашла, но ты не подтвердил: ")
		b.WriteString(joinRU(cands, SpokenLimit, false))
		b.WriteString(".")
	}
	return b.String()
}

// joinRU lists up to limit tasks, then says how many are left. withReasons
// attaches the parenthesised reason — candidates are listed bare, since their
// due dates are Maven's reading of a mail and not something he stated.
func joinRU(rs []Ranked, limit int, withReasons bool) string {
	shown := rs
	rest := 0
	if len(rs) > limit {
		shown, rest = rs[:limit], len(rs)-limit
	}
	parts := make([]string, 0, len(shown))
	for _, r := range shown {
		if withReasons && r.Reason != "" {
			parts = append(parts, r.Text+" ("+r.Reason+")")
		} else {
			parts = append(parts, r.Text)
		}
	}
	s := strings.Join(parts, "; ")
	if rest > 0 {
		s += fmt.Sprintf("; и ещё %d", rest)
	}
	return s
}