backend/internal/util/util.go

package util

import "io"
import "image/color"

// IndexOf returns the index of needle within haystack. If needle does not exist
// within haystack, it returns -1.
func IndexOf[T comparable] (haystack []T, needle T) int {
	for index, test := range haystack {
		if test == needle {
			return index
		}
	}
	return -1
}

// Remove removes an element from slice at index.
func Remove[T any] (slice []T, index int) []T {
	return append(slice[:index], slice[index + 1:]...)
}

// Insert inserts an element into slice at index.
func Insert[T any] (slice []T, index int, element T) []T {
	slice = append(slice[:index + 1], slice[index:]...)
	slice[index] = element
	return slice
}

// Transparent returns whether or not a color has transparency.
func Transparent (c color.Color) bool {
	_, _, _, a := c.RGBA()
	return a != 0xFFFF
}

// Set is a set of unique items, built on top of map.
type Set[T comparable] map[T] struct { }

// Empty returns true if there are no items in the set.
func (set Set[T]) Empty () bool {
	return set == nil || len(set) == 0
}

// Has returns true if the set contains item.
func (set Set[T]) Has (item T) bool {
	if set == nil {
		return false
	}
	_, ok := set[item]
	return ok
}

// Add adds an item to the set.
func (set Set[T]) Add (item T) {
	set[item] = struct { } { }
}

// Pop removes the first accessible item from the set and returns it.
func (set Set[T]) Pop () (item T) {
	for item := range set {
		delete(set, item)
		return item
	}
	return
}

// Memo holds a cached value.
type Memo[T any] struct {
	cache  T
	valid  bool
	update func () T
}

// NewMemo creates a new Memo which will take its value from the specified
// update callback.
func NewMemo[T any] (update func () T) Memo[T] {
	return Memo[T] {
		update: update,
	}
}

// Value returns the Memo's value, updating it if the current cached value is
// invalid.
func (this *Memo[T]) Value () T {
	if !this.valid {
		this.cache = this.update()
	}
	return this.cache
}

// Invalidate marks the Memo's value as invalid, which will cause it to be
// updated the next time Value is called.
func (this *Memo[T]) Invalidate () {
	this.valid = false
}

// InvalidateTo invalidates the Memo and sets its value. The new value will be
// entirely inaccessible. This is only intended to be used for setting a
// reference to nil
func (this *Memo[T]) InvalidateTo (value T) {
	this.Invalidate()
	this.cache = value
}

// Cycler stores a value and an accompanying io.Closer. When the value is set,
// the closer associated with the previous value is closed.
type Cycler[T any] struct {
	value  T
	closer io.Closer
}

// Value returns the cycler's value.
func (this *Cycler[T]) Value () T {
	return this.value
}

// Set sets the value and associated closer, closing the previous one.
func (this *Cycler[T]) Set (value T, closer io.Closer) (err error) {
	if this.closer != nil {
		err = this.closer.Close()
	}
	this.value  = value
	this.closer = closer
	return err
}

// Close closes the associated closer early.
func (this *Cycler[T]) Close () error {
	err := this.closer.Close()
	this.closer = nil
	return err
}