BNT renderer

bnt.go

package main

import (
	"fmt"
)

func main() {
	fmt.Println(renderBNT(19))
}

func renderBNT(n int) string {
	indices := ""
	var prev []int
	for i := 0; i < n; i++ {
		indices += fmt.Sprintf("%*d ", (i&1)*4-2, i)
		prev = append(prev, i*3+(i&1))
	}
	rows := []string{indices}
	for len(prev) > 1 {
		var s []rune
		row := prev[:0]
		for i := 0; i < len(prev); i += 2 {
			l := prev[i]
			for len(s) <= l {
				s = append(s, ' ')
			}
			if i+1 == len(prev) {
				s[l] = '│'
				row = append(row, l)
				break
			}
			r := prev[i+1]
			for len(s) <= r {
				s = append(s, '─')
			}
			s[l], s[(l+r)/2], s[r] = '┌', '┴', '┐'
			row = append(row, (prev[i]+prev[i+1])/2)
		}
		rows = append(rows, string(s))
		prev = row
	}
	s := ""
	for i := len(rows) - 1; i >= 0; i-- {
		s += rows[i] + "\n"
	}
	return s
}