van_eck.go

package main

import "fmt"

// VanEck generates the first n elements in the Van-Eck sequence
// https://oeis.org/A181391
// Thanks to: https://www.youtube.com/watch?v=etMJxB-igrc
func VanEck(limit int64) []int64 {

	// prepare the basics. The output, and the lookup
	sequence := make([]int64, limit, limit)
	lastSeen := map[int64]int64{}

	// first in the sequence is a zero
	// NOTE: van eck can start at any other number too.
	// But we are traditionalists :)
	sequence[0] = 0

	var i int64
	for i = 1; i < limit; i++ {

		// check the last item, and see when it was see before.
		prev := sequence[i-1]
		index, seen := lastSeen[prev]

		// update the when was seen (note: we update this only after we've
		// checked the previous occurrence )
		lastSeen[prev] = i - 1

		// if it was seen earlier, we know who many steps did it take
		var steps int64
		if seen {
			steps = i - 1 - index
		}

		// now, we know the i-th item
		sequence[i] = steps
	}

	return sequence
}

func main() {
	fmt.Printf("%v\n", VanEck(2000))
}