package skill

import (
	"fmt"
	"strings"
	"time"

	"github.com/robfig/cron/v3"
)

// scheduleParser is the cron parser shared across the skills package. It
// accepts the standard 5-field syntax (minute hour dom month dow) plus
// descriptors such as @daily, @hourly, etc. We do not enable the seconds
// field — schedule cadence is governed in minutes, and a seconds field
// would invite specs that fire below the min-interval floor without
// surfacing as such in the spec text.
//
// Why standalone vs. cron.ParseStandard: ParseStandard rejects descriptors
// (@daily, @hourly). Skills callers may want to write @daily as a
// shorthand alongside the explicit "daily" / "weekly" forms we translate
// below.
var scheduleParser = cron.NewParser(
	cron.Minute | cron.Hour | cron.Dom | cron.Month | cron.Dow | cron.Descriptor,
)

// ParseSchedule turns a user-supplied schedule expression into a
// cron.Schedule. The empty string returns (nil, nil) — callers should
// treat that as "on-demand only".
//
// Why: Skill.Schedule is a string field stored verbatim; the validator,
// the scheduler runner, and any future tooling all need to round-trip
// through the same parser. Centralising it here avoids drift.
//
// Accepted shorthands:
//   - "daily"  → "0 0 * * *"   (midnight UTC every day)
//   - "weekly" → "0 0 * * 0"   (midnight UTC every Sunday)
//
// Anything else is fed through robfig/cron/v3's standard parser
// (descriptors enabled).
//
// Test: schedule_test.go covers shorthand expansion and invalid-spec
// rejection.
func ParseSchedule(expr string) (cron.Schedule, error) {
	expr = strings.TrimSpace(expr)
	if expr == "" {
		return nil, nil
	}
	switch strings.ToLower(expr) {
	case "daily":
		expr = "0 0 * * *"
	case "weekly":
		expr = "0 0 * * 0"
	}
	sched, err := scheduleParser.Parse(expr)
	if err != nil {
		return nil, fmt.Errorf("invalid schedule %q: %w", expr, err)
	}
	return sched, nil
}

// ScheduleMinInterval returns an estimate of the smallest gap between
// consecutive fire times for a parsed schedule. It samples the next two
// fire times from a couple of starting points and returns the smallest
// observed gap.
//
// Why: cron.Schedule does not expose a "smallest interval" API. The
// validator needs this to enforce a per-skill min-interval floor (so an
// admin can't accidentally register "* * * * *" and burn GPU minutes).
// Two probe points are enough to catch irregular schedules whose tightest
// gap appears at a particular point in the week (e.g. "0 9 * * 1,5",
// where Mon→Fri is 4d but Fri→Mon is 3d — both sampled).
//
// Returns 0 if sched is nil.
//
// Test: schedule_test.go covers a "* * * * *" minute-interval probe and
// the irregular Mon/Fri case.
func ScheduleMinInterval(sched cron.Schedule) time.Duration {
	if sched == nil {
		return 0
	}
	// Probe from a fixed reference and from a midweek offset. Six fire
	// times across two starts catches weekly irregularities (the worst
	// case is a schedule that fires once a week — we still get one gap
	// per probe). Using a wall-clock-independent reference keeps the
	// test deterministic.
	starts := []time.Time{
		time.Date(2024, 1, 1, 0, 0, 0, 0, time.UTC),     // Monday 00:00
		time.Date(2024, 1, 4, 12, 30, 0, 0, time.UTC),   // Thursday 12:30
		time.Date(2024, 6, 15, 23, 59, 59, 0, time.UTC), // mid-year, late
	}
	var min time.Duration
	for _, t := range starts {
		// Sample three consecutive fires per start to capture two gaps.
		f1 := sched.Next(t)
		f2 := sched.Next(f1)
		f3 := sched.Next(f2)
		for _, gap := range []time.Duration{f2.Sub(f1), f3.Sub(f2)} {
			if gap <= 0 {
				continue
			}
			if min == 0 || gap < min {
				min = gap
			}
		}
	}
	return min
}