Int.zeroPrefixed

ZeroPrefixed.kt

/**
 * Kotlin stdlib doesn't have a 'String.format()' that's not relying on JVM.
 * https://youtrack.jetbrains.com/issue/KT-25506/Stdlib-Stringformat-in-common
 *
 * To keep things in common code, this "formatter" simply prefixes an Int string with zeros
 * if needed.
 * For example, 1.zeroPrefixed(2) returns "01".
 */
fun Int.zeroPrefixed(
  maxLength: Int,
): String {
  if (this < 0 || maxLength < 1) return ""
  
  val string = this.toString()
  val currentStringLength = string.length
  return if (maxLength <= currentStringLength) {
    string
  } else {
    val diff = maxLength - currentStringLength
    var prefixedZeros = ""
    repeat(diff) {
      prefixedZeros += "0"
    }
    "$prefixedZeros$string"
  }
}

ZeroPrefixedTests.kt

import kotlin.test.Test
import kotlin.test.assertEquals

class ZeroPrefixedTests {
  
  @Test
  fun `format 1 to 01`() {
    val actual = 1.zeroPrefixed(maxLength = 2)
    val expected = "01"
    assertEquals(expected, actual)
  }
  
  @Test
  fun `format 2 to 02`() {
    val actual = 2.zeroPrefixed(maxLength = 2)
    val expected = "02"
    assertEquals(expected, actual)
  }
  
  @Test
  fun `format 10 to 10`() {
    val actual = 10.zeroPrefixed(maxLength = 2)
    val expected = "10"
    assertEquals(expected, actual)
  }
  
  @Test
  fun `format 10 to 010`() {
    val actual = 10.zeroPrefixed(maxLength = 3)
    val expected = "010"
    assertEquals(expected, actual)
  }
  
  @Test
  fun `format 2 to 002`() {
    val actual = 2.zeroPrefixed(maxLength = 3)
    val expected = "002"
    assertEquals(expected, actual)
  }
  
  @Test
  fun `format 2 to 00002`() {
    val actual = 2.zeroPrefixed(maxLength = 5)
    val expected = "00002"
    assertEquals(expected, actual)
  }
  
  @Test
  fun `format 12 to 00012`() {
    val actual = 12.zeroPrefixed(maxLength = 5)
    val expected = "00012"
    assertEquals(expected, actual)
  }
  
  @Test
  fun `format 12345 to 12345`() {
    val actual = 12345.zeroPrefixed(maxLength = 5)
    val expected = "12345"
    assertEquals(expected, actual)
  }
  
  @Test
  fun `format negative int to empty string`() {
    val actual = (-1).zeroPrefixed(maxLength = 5)
    val expected = ""
    assertEquals(expected, actual)
  }
  
  @Test
  fun `format to empty string if maxLength is below 1`() {
    val actual = 123.zeroPrefixed(maxLength = 0)
    val expected = ""
    assertEquals(expected, actual)
  }
}