FormatAsciiBinaryTree.kt

fun Node.format(): String {
    return formatInternal().lines.joinToString(separator = "\n")
}

fun max(a: Int, b: Int, c: Int): Int {
    return Math.max(Math.max(a, b), c)
}

fun String.center(width: Int, fillChar: Char = ' ') : String {
    var s = this
    var left = true
    while (s.length < width) {
        if (left) {
            s = fillChar + s
        } else {
            s += fillChar
        }
        left = !left
    }
    return s
}

fun Node.formatInternal() : FormatParts {
    var label = value.toString()
    val (leftLines, leftPos, leftWidth) = left?.formatInternal() ?: FormatParts(mutableListOf(), 0, 0)
    val (rightLines, rightPos, rightWidth) = right?.formatInternal() ?: FormatParts(mutableListOf(), 0, 0)

    // Ensure left and right lines are the same size by appending whitespace.
    while (leftLines.size < rightLines.size) {
        leftLines.add(" ".repeat(leftWidth))
    }
    while (rightLines.size < leftLines.size) {
        rightLines.add(" ".repeat(rightWidth))
    }

    val middle = max(rightPos + leftWidth - leftPos + 1, label.length, 2)
    val pos = leftPos + (middle / 2)
    val width = leftPos + middle + rightWidth - rightPos

    val lines = mutableListOf<String>()

    // Pad the label with '.'s to match middle length
    if ((middle - label.length) % 2 == 1
            && parent != null
            && this != parent.left
            && label.length < middle) {
        label += '.'
    }
    label = label.center(middle, '.')
    if (label[0] == '.') {
        label = ' ' + label.substring(1)
    }
    if (label[label.length - 1] == '.') {
        label = label.substring(0, label.length - 1) + ' '
    }

    // Add the value
    var valueLine = ""
    valueLine += " ".repeat(leftPos)
    valueLine += label
    valueLine += " ".repeat(rightWidth - rightPos)
    lines.add(valueLine)

    // Add arrows when children exist.
    var slashLine = ""
    slashLine += " ".repeat(leftPos)
    slashLine += if (left != null) "/" else " "
    slashLine += " ".repeat(middle - 2)
    slashLine += if (right != null) "\\" else " "
    slashLine += " ".repeat(rightWidth - rightPos)
    lines.add(slashLine)

    // Add rest of the lines from the left and right children
    (0 until leftLines.size).mapTo(lines) {
        var result = leftLines[it]
        result += " ".repeat(width - leftWidth - rightWidth)
        result += rightLines[it]
        result
    }

    return FormatParts(lines, pos, width)
}

data class FormatParts(val lines: MutableList<String>, val pos: Int, val width: Int)

Draws something that looks like this:

         ...38..         
        /       \        
      ..30.     46       
     /     \   /  \      
    .12    35 42  99     
   /   \          / \    
  .1   15        57 116  
 /  \   \             \  
-10 5   20            122