// Package calendar is the one calendar data model the rest of maven shares:
// an Event, the iCal text it is parsed from and rendered to, and the fact
// encoding that puts it in the store.
//
// It exists because three separate features read or write the same events and
// must agree on their shape: the CalDAV read side (cmd/mavcaldav, Vikunja
// #126/#127), the write-only render target that publishes maven's own
// reminders as a calendar (#127), and the day plan that recites them (#128).
// Before this package the parse lived inline in cmd/mavcaldav and the fact key
// format was a Sprintf in two places.
//
// The package is pure: no HTTP, no store, no clock of its own. Callers own the
// impurity, the way internal/morning and internal/loop do.
package calendar

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

// Fact sources. A calendar event reaches the store as a
// `facts (kind=env, key=calendar_event_..., source=<one of these>)` row, and
// the source is the whole provenance story:
//
//   - SourcePersonal — maven's own Radicale, read AND rendered to. Canonical
//     state stays in sqlite; the calendar is a render target (#127).
//   - SourceWork — a work calendar, read-only by definition (#126). Nothing in
//     maven ever writes to it: no code path pairs this source with a PUT.
//   - SourceAmbient — inferred from an Android notification-listener relay
//     rather than read from a server (#126). Confidence is below 1.0 because a
//     notification is a signal about an event, not the event.
const (
	SourcePersonal = "poll:caldav"
	SourceWork     = "poll:caldav:work"
	SourceAmbient  = "ambient:notif"
)

// AmbientConfidence — the confidence a notification-derived event is stored
// with. A parsed notification line is evidence, not a reading of the calendar,
// so it must never be indistinguishable from one (#126).
const AmbientConfidence = 0.6

// Sources lists every source a calendar event may legitimately carry, for the
// store query that reads the calendar back out. Ordered from most to least
// trusted.
func Sources() []string {
	return []string{SourcePersonal, SourceWork, SourceAmbient}
}

// ReadOnlySource reports whether events from this source may never be written
// back. The work calendar is read-only by definition — see #126: maven holding
// a credential that can write to an employer's calendar is the thing the task
// exists to avoid.
func ReadOnlySource(source string) bool {
	return source == SourceWork || source == SourceAmbient
}

// Event — one calendar entry. UID is the iCal UID when the event was parsed
// from a server and the identity maven renders under when it publishes one;
// Start/End are instants. All-day events are not modelled: the busy gate and
// the day plan both need a time of day, and an all-day marker answers neither.
type Event struct {
	UID     string
	Summary string
	Start   time.Time
	End     time.Time
}

// FactKey is the store key for an event: one key per day per summary, stable
// across polls so re-reading an unchanged calendar rewrites nothing.
//
// The date prefix is load-bearing — store.CalendarEvents selects a day range
// by key prefix, not by a timestamp column.
func FactKey(e Event) string {
	return fmt.Sprintf("calendar_event_%s_%s", e.Start.Format("20060102"), safeKey(e.Summary))
}

// FactValue is the human-readable rendering stored as the fact value, and the
// string the day plan and the query path read back.
func FactValue(e Event) string {
	return fmt.Sprintf("%s @ %s-%s", e.Summary, e.Start.Format("15:04"), e.End.Format("15:04"))
}

// KeyPrefixForDay is the fact-key prefix covering one calendar day. The store
// range-scans between two of these.
func KeyPrefixForDay(day time.Time) string {
	return fmt.Sprintf("calendar_event_%s", day.Format("20060102"))
}

// Busy reports whether any event covers the instant now — the read the loop
// gate uses to suppress nudges during a meeting.
func Busy(events []Event, now time.Time) bool {
	for _, e := range events {
		if !now.Before(e.Start) && now.Before(e.End) {
			return true
		}
	}
	return false
}

// Overlapping returns the events intersecting [from, to), sorted by start.
func Overlapping(events []Event, from, to time.Time) []Event {
	var out []Event
	for _, e := range events {
		if e.End.After(from) && e.Start.Before(to) {
			out = append(out, e)
		}
	}
	sort.Slice(out, func(i, j int) bool { return out[i].Start.Before(out[j].Start) })
	return out
}

// safeKey makes a summary safe to use inside a fact key (ASCII alphanumerics
// and dashes). Non-Latin summaries collapse to their punctuation, which is why
// the day prefix carries the identity and this only disambiguates within a day.
func safeKey(s string) string {
	var b strings.Builder
	for _, r := range s {
		switch {
		case (r >= 'a' && r <= 'z') || (r >= 'A' && r <= 'Z') || (r >= '0' && r <= '9') || r == '-':
			b.WriteRune(r)
		case r == ' ' || r == '_':
			b.WriteRune('-')
		}
	}
	return b.String()
}