kit/cron/parser.go

/*
Copyright 2022 The Dapr Authors
Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at
    http://www.apache.org/licenses/LICENSE-2.0
Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
This package has been forked from https://github.com/robfig/cron available under the MIT license.
You can check the original license at:
		https://github.com/robfig/cron/blob/master/LICENSE
*/

//nolint
package cron

import (
	"fmt"
	"math"
	"strconv"
	"strings"
	"time"
)

// Configuration options for creating a parser. Most options specify which
// fields should be included, while others enable features. If a field is not
// included the parser will assume a default value. These options do not change
// the order fields are parse in.
type ParseOption int

const (
	Second         ParseOption = 1 << iota // Seconds field, default 0
	SecondOptional                         // Optional seconds field, default 0
	Minute                                 // Minutes field, default 0
	Hour                                   // Hours field, default 0
	Dom                                    // Day of month field, default *
	Month                                  // Month field, default *
	Dow                                    // Day of week field, default *
	DowOptional                            // Optional day of week field, default *
	Descriptor                             // Allow descriptors such as @monthly, @weekly, etc.
)

var places = []ParseOption{
	Second,
	Minute,
	Hour,
	Dom,
	Month,
	Dow,
}

var defaults = []string{
	"0",
	"0",
	"0",
	"*",
	"*",
	"*",
}

// A custom Parser that can be configured.
type Parser struct {
	options ParseOption
}

// NewParser creates a Parser with custom options.
//
// It panics if more than one Optional is given, since it would be impossible to
// correctly infer which optional is provided or missing in general.
//
// Examples
//
//	// Standard parser without descriptors
//	specParser := NewParser(Minute | Hour | Dom | Month | Dow)
//	sched, err := specParser.Parse("0 0 15 */3 *")
//
//	// Same as above, just excludes time fields
//	specParser := NewParser(Dom | Month | Dow)
//	sched, err := specParser.Parse("15 */3 *")
//
//	// Same as above, just makes Dow optional
//	specParser := NewParser(Dom | Month | DowOptional)
//	sched, err := specParser.Parse("15 */3")
func NewParser(options ParseOption) Parser {
	optionals := 0
	if options&DowOptional > 0 {
		optionals++
	}
	if options&SecondOptional > 0 {
		optionals++
	}
	if optionals > 1 {
		panic("multiple optionals may not be configured")
	}
	return Parser{options}
}

// Parse returns a new crontab schedule representing the given spec.
// It returns a descriptive error if the spec is not valid.
// It accepts crontab specs and features configured by NewParser.
func (p Parser) Parse(spec string) (Schedule, error) {
	if len(spec) == 0 {
		return nil, fmt.Errorf("empty spec string")
	}

	// Extract timezone if present
	loc := time.Local
	if strings.HasPrefix(spec, "TZ=") || strings.HasPrefix(spec, "CRON_TZ=") {
		var err error
		i := strings.Index(spec, " ")
		eq := strings.Index(spec, "=")
		if loc, err = time.LoadLocation(spec[eq+1 : i]); err != nil {
			return nil, fmt.Errorf("provided bad location %s: %v", spec[eq+1:i], err)
		}
		spec = strings.TrimSpace(spec[i:])
	}

	// Handle named schedules (descriptors), if configured
	if strings.HasPrefix(spec, "@") {
		if p.options&Descriptor == 0 {
			return nil, fmt.Errorf("parser does not accept descriptors: %v", spec)
		}
		return parseDescriptor(spec, loc)
	}

	// Split on whitespace.
	fields := strings.Fields(spec)

	// Validate & fill in any omitted or optional fields
	var err error
	fields, err = normalizeFields(fields, p.options)
	if err != nil {
		return nil, err
	}

	field := func(field string, r bounds) uint64 {
		if err != nil {
			return 0
		}
		var bits uint64
		bits, err = getField(field, r)
		return bits
	}

	var (
		second     = field(fields[0], seconds)
		minute     = field(fields[1], minutes)
		hour       = field(fields[2], hours)
		dayofmonth = field(fields[3], dom)
		month      = field(fields[4], months)
		dayofweek  = field(fields[5], dow)
	)
	if err != nil {
		return nil, err
	}

	return &SpecSchedule{
		Second:   second,
		Minute:   minute,
		Hour:     hour,
		Dom:      dayofmonth,
		Month:    month,
		Dow:      dayofweek,
		Location: loc,
	}, nil
}

// normalizeFields takes a subset set of the time fields and returns the full set
// with defaults (zeroes) populated for unset fields.
//
// As part of performing this function, it also validates that the provided
// fields are compatible with the configured options.
func normalizeFields(fields []string, options ParseOption) ([]string, error) {
	// Validate optionals & add their field to options
	optionals := 0
	if options&SecondOptional > 0 {
		options |= Second
		optionals++
	}
	if options&DowOptional > 0 {
		options |= Dow
		optionals++
	}
	if optionals > 1 {
		return nil, fmt.Errorf("multiple optionals may not be configured")
	}

	// Figure out how many fields we need
	max := 0
	for _, place := range places {
		if options&place > 0 {
			max++
		}
	}
	min := max - optionals

	// Validate number of fields
	if count := len(fields); count < min || count > max {
		if min == max {
			return nil, fmt.Errorf("expected exactly %d fields, found %d: %s", min, count, fields)
		}
		return nil, fmt.Errorf("expected %d to %d fields, found %d: %s", min, max, count, fields)
	}

	// Populate the optional field if not provided
	if min < max && len(fields) == min {
		switch {
		case options&DowOptional > 0:
			fields = append(fields, defaults[5]) // TODO: improve access to default
		case options&SecondOptional > 0:
			fields = append([]string{defaults[0]}, fields...)
		default:
			return nil, fmt.Errorf("unknown optional field")
		}
	}

	// Populate all fields not part of options with their defaults
	n := 0
	expandedFields := make([]string, len(places))
	copy(expandedFields, defaults)
	for i, place := range places {
		if options&place > 0 {
			expandedFields[i] = fields[n]
			n++
		}
	}
	return expandedFields, nil
}

var standardParser = NewParser(
	Minute | Hour | Dom | Month | Dow | Descriptor,
)

// ParseStandard returns a new crontab schedule representing the given
// standardSpec (https://en.wikipedia.org/wiki/Cron). It requires 5 entries
// representing: minute, hour, day of month, month and day of week, in that
// order. It returns a descriptive error if the spec is not valid.
//
// It accepts
//   - Standard crontab specs, e.g. "* * * * ?"
//   - Descriptors, e.g. "@midnight", "@every 1h30m"
func ParseStandard(standardSpec string) (Schedule, error) {
	return standardParser.Parse(standardSpec)
}

// getField returns an Int with the bits set representing all of the times that
// the field represents or error parsing field value.  A "field" is a comma-separated
// list of "ranges".
func getField(field string, r bounds) (uint64, error) {
	var bits uint64
	ranges := strings.FieldsFunc(field, func(r rune) bool { return r == ',' })
	for _, expr := range ranges {
		bit, err := getRange(expr, r)
		if err != nil {
			return bits, err
		}
		bits |= bit
	}
	return bits, nil
}

// getRange returns the bits indicated by the given expression:
//
//	number | number "-" number [ "/" number ]
//
// or error parsing range.
func getRange(expr string, r bounds) (uint64, error) {
	var (
		start, end, step uint
		rangeAndStep     = strings.Split(expr, "/")
		lowAndHigh       = strings.Split(rangeAndStep[0], "-")
		singleDigit      = len(lowAndHigh) == 1
		err              error
	)

	var extra uint64
	if lowAndHigh[0] == "*" || lowAndHigh[0] == "?" {
		start = r.min
		end = r.max
		extra = starBit
	} else {
		start, err = parseIntOrName(lowAndHigh[0], r.names)
		if err != nil {
			return 0, err
		}
		switch len(lowAndHigh) {
		case 1:
			end = start
		case 2:
			end, err = parseIntOrName(lowAndHigh[1], r.names)
			if err != nil {
				return 0, err
			}
		default:
			return 0, fmt.Errorf("too many hyphens: %s", expr)
		}
	}

	switch len(rangeAndStep) {
	case 1:
		step = 1
	case 2:
		step, err = mustParseInt(rangeAndStep[1])
		if err != nil {
			return 0, err
		}

		// Special handling: "N/step" means "N-max/step".
		if singleDigit {
			end = r.max
		}
		if step > 1 {
			extra = 0
		}
	default:
		return 0, fmt.Errorf("too many slashes: %s", expr)
	}

	if start < r.min {
		return 0, fmt.Errorf("beginning of range (%d) below minimum (%d): %s", start, r.min, expr)
	}
	if end > r.max {
		return 0, fmt.Errorf("end of range (%d) above maximum (%d): %s", end, r.max, expr)
	}
	if start > end {
		return 0, fmt.Errorf("beginning of range (%d) beyond end of range (%d): %s", start, end, expr)
	}
	if step == 0 {
		return 0, fmt.Errorf("step of range should be a positive number: %s", expr)
	}

	return getBits(start, end, step) | extra, nil
}

// parseIntOrName returns the (possibly-named) integer contained in expr.
func parseIntOrName(expr string, names map[string]uint) (uint, error) {
	if names != nil {
		if namedInt, ok := names[strings.ToLower(expr)]; ok {
			return namedInt, nil
		}
	}
	return mustParseInt(expr)
}

// mustParseInt parses the given expression as an int or returns an error.
func mustParseInt(expr string) (uint, error) {
	num, err := strconv.Atoi(expr)
	if err != nil {
		return 0, fmt.Errorf("failed to parse int from %s: %s", expr, err)
	}
	if num < 0 {
		return 0, fmt.Errorf("negative number (%d) not allowed: %s", num, expr)
	}

	return uint(num), nil
}

// getBits sets all bits in the range [min, max], modulo the given step size.
func getBits(min, max, step uint) uint64 {
	var bits uint64

	// If step is 1, use shifts.
	if step == 1 {
		return ^(math.MaxUint64 << (max + 1)) & (math.MaxUint64 << min)
	}

	// Else, use a simple loop.
	for i := min; i <= max; i += step {
		bits |= 1 << i
	}
	return bits
}

// all returns all bits within the given bounds.  (plus the star bit)
func all(r bounds) uint64 {
	return getBits(r.min, r.max, 1) | starBit
}

// parseDescriptor returns a predefined schedule for the expression, or error if none matches.
func parseDescriptor(descriptor string, loc *time.Location) (Schedule, error) {
	switch descriptor {
	case "@yearly", "@annually":
		return &SpecSchedule{
			Second:   1 << seconds.min,
			Minute:   1 << minutes.min,
			Hour:     1 << hours.min,
			Dom:      1 << dom.min,
			Month:    1 << months.min,
			Dow:      all(dow),
			Location: loc,
		}, nil

	case "@monthly":
		return &SpecSchedule{
			Second:   1 << seconds.min,
			Minute:   1 << minutes.min,
			Hour:     1 << hours.min,
			Dom:      1 << dom.min,
			Month:    all(months),
			Dow:      all(dow),
			Location: loc,
		}, nil

	case "@weekly":
		return &SpecSchedule{
			Second:   1 << seconds.min,
			Minute:   1 << minutes.min,
			Hour:     1 << hours.min,
			Dom:      all(dom),
			Month:    all(months),
			Dow:      1 << dow.min,
			Location: loc,
		}, nil

	case "@daily", "@midnight":
		return &SpecSchedule{
			Second:   1 << seconds.min,
			Minute:   1 << minutes.min,
			Hour:     1 << hours.min,
			Dom:      all(dom),
			Month:    all(months),
			Dow:      all(dow),
			Location: loc,
		}, nil

	case "@hourly":
		return &SpecSchedule{
			Second:   1 << seconds.min,
			Minute:   1 << minutes.min,
			Hour:     all(hours),
			Dom:      all(dom),
			Month:    all(months),
			Dow:      all(dow),
			Location: loc,
		}, nil
	}

	const every = "@every "
	if strings.HasPrefix(descriptor, every) {
		duration, err := time.ParseDuration(descriptor[len(every):])
		if err != nil {
			return nil, fmt.Errorf("failed to parse duration %s: %s", descriptor, err)
		}
		return Every(duration), nil
	}

	return nil, fmt.Errorf("unrecognized descriptor: %s", descriptor)
}