Equivalent to DecimalFormat("#.######") but thread-safe and faster

fastNumberToString.kt

package com.gonnen.util

import kotlin.math.pow

private const val PRECISION = 6
private val SCALE_FACTOR = 10.0.pow(PRECISION).toLong()

/**
 * Equivalent to DecimalFormat("#.######") but thread-safe and faster.
 */
fun fastNumberToString(number: Double) = StringBuilder().apply {
    if (number.isNaN()) {
        append(number.toString())
        return@apply
    }

    require(!(number * SCALE_FACTOR + 0.5 > Long.MAX_VALUE)) { "Number too large: $number" }

    val absValue = if (number < 0) -number else number
    var scaled = (absValue * SCALE_FACTOR + 0.5).toLong()

    if (scaled == 0L) {
        append('0')
        return@apply
    }

    var digitsAdded = 0

    do {
        val digit = scaled % 10

        if (isNotEmpty() || digit != 0L) append('0' + digit.toInt())

        digitsAdded++

        if (digitsAdded == PRECISION && isNotEmpty()) append('.')

        scaled /= 10L
    } while (digitsAdded <= PRECISION || scaled != 0L)

    if (number < 0) append('-')
}.reverse().toString()