format/mutator.go

package main

import (
	"reflect"
	"strconv"
)

type NumMutator struct {
	Op    NumOperator
	Var   bool
	Value int
}

type NumOperator string

const (
	NumOperatorAdd NumOperator = "+"
	NumOperatorSub NumOperator = "-"
	NumOperatorMul NumOperator = "*"
	NumOperatorDiv NumOperator = "/"
)

func NewNumOperatorFromString(str string) NumOperator {
	switch str {
	case "+":
		return NumOperatorAdd
	case "-":
		return NumOperatorSub
	case "*":
		return NumOperatorMul
	case "/":
		return NumOperatorDiv
	default:
		panic("invalid number operator: " + str)
	}
}

var numMutatorCache = map[string]interface{}{}

func numMut2func[T int64 | float64](mutation string) (f func(value T, vars []string) T) {
	if mutation == "" {
		return func(value T, vars []string) T { return value }
	}

	// caching
	if v, ok := numMutatorCache[mutation]; ok {
		return v.(func(value T, vars []string) T)
	}
	defer func() { numMutatorCache[mutation] = f }()

	matches := numMutationPattern.FindAllStringSubmatch(mutation, -1)
	mutators := make([]NumMutator, 0, len(matches))
	var err error
	for _, match := range matches {
		mut := NumMutator{Op: NewNumOperatorFromString(match[1])}
		if match[3] == "" {
			mut.Value, err = strconv.Atoi(match[2])
			mut.Var = false
			if err != nil {
				panic("invalid number in number mutator: " + match[2])
			}
		} else {
			mut.Var = true
			mut.Value, err = strconv.Atoi(match[3])
			if err != nil {
				panic("invalid back reference group in number mutator: " + match[2])
			}
		}
		mutators = append(mutators, mut)
	}

	numberParser := number_parser[T]()

	return func(value T, vars []string) T {
		for _, mutator := range mutators {
			var otherValue T
			if mutator.Var {
				other := numberParser(vars[mutator.Value])
				otherValue = T(other)
			} else {
				otherValue = T(mutator.Value)
			}

			switch mutator.Op {
			case NumOperatorAdd:
				value += otherValue
			case NumOperatorSub:
				value -= otherValue
			case NumOperatorMul:
				value *= otherValue
			case NumOperatorDiv:
				value /= otherValue
			default:
			}
		}

		return value
	}
}

func number_parser[T int64 | float64]() func(str string) T {
	typeOfT := reflect.TypeOf(new(T)).Elem()
	typeOfInt64 := reflect.TypeOf(new(int64)).Elem()
	typeOfFloat64 := reflect.TypeOf(new(float64)).Elem()

	if typeOfT == typeOfInt64 {
		return func(str string) T {
			num, err := strconv.Atoi(str)
			if err != nil {
				panic("expected integer but found " + str)
			}
			return T(num)
		}
	} else if typeOfT == typeOfFloat64 {
		return func(str string) T {
			num, err := strconv.ParseFloat(str, 64)
			if err != nil {
				panic("expected float but found " + str)
			}
			return T(num)
		}
	}

	panic("invalid number type")
}