package core

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

func GetNextUpdate(lastUpdated time.Time, spec string) (nextUpdate time.Time, err error) {
	var nextUpdates []time.Time
	switch {
	case strings.HasPrefix(spec, "every "):
		nextUpdates, err = parseEverySpec(lastUpdated, spec[len("every "):])
	case strings.HasPrefix(spec, "at "):
		nextUpdates, err = parseAtSpec(lastUpdated, spec[len("at "):])
	default:
		return time.Time{}, fmt.Errorf("unknown spec format: %v", spec)
	}
	if err != nil {
		return time.Time{}, err
	}
	for _, next := range nextUpdates {
		if next.After(lastUpdated) && (nextUpdate.IsZero() || next.Before(nextUpdate)) {
			nextUpdate = next
		}
	}
	return
}

// Get the next instance of the every-spec after the base time.
// An every-spec is a Go duration string.
func parseEverySpec(base time.Time, everySpec string) (nextUpdates []time.Time, err error) {
	var duration time.Duration
	duration, err = time.ParseDuration(everySpec)
	if err == nil {
		next := base.Round(duration)
		if !next.After(base) {
			next = next.Add(duration)
		}
		nextUpdates = []time.Time{next}
	}
	return
}

// Get the next instances of the at-spec times after the base time.
// An at-spec is in the patterm HH:MM[,HH:MM,[...]].
func parseAtSpec(base time.Time, atSpec string) (nextUpdates []time.Time, err error) {
	timeSpecs := strings.Split(atSpec, ",")
	for _, timeSpec := range timeSpecs {
		var hour, minute int
		_, err = fmt.Sscanf(timeSpec, "%d:%d", &hour, &minute)
		if err != nil {
			return nil, fmt.Errorf("could not parse %s: %v", timeSpec, err)
		}
		// The time instance on the same day as the base time
		specOfDay := time.Date(base.Year(), base.Month(), base.Day(), hour, minute, 0, 0, base.Location())
		// Bump it forward one day if it's before the base time
		if !specOfDay.After(base) {
			specOfDay = specOfDay.Add(24 * time.Hour)
		}
		nextUpdates = append(nextUpdates, specOfDay)
	}
	return
}