Kotlin List.padStart and List.padEnd extensions

ListPadExt.kt

fun <TYPE> List<TYPE>.padEndOrCompact(size: Int, fallback: TYPE): List<TYPE> {
  if (size < 0) throw IllegalArgumentException("Desired length $size is less than zero.")
  return (0 until size).map { this.getOrNull(it) ?: fallback }
}

fun <TYPE> List<TYPE>.padEnd(size: Int, fallback: TYPE): List<TYPE> {
  if (size < 0) throw IllegalArgumentException("Desired length $size is less than zero.")
  return when {
    size > this.size -> this.padEndOrCompact(size, fallback)
    else -> this
  }
}

fun <TYPE> List<TYPE>.padStartOrCompact(size: Int, fallback: TYPE): List<TYPE> {
  if (size < 0) throw IllegalArgumentException("Desired length $size is less than zero.")
  return ((0 until size - this.size).map { fallback } + this).takeLast(size)
}

fun <TYPE> List<TYPE>.padStart(size: Int, fallback: TYPE): List<TYPE> {
  if (size < 0) throw IllegalArgumentException("Desired length $size is less than zero.")
  return when {
    size > this.size -> this.padStartOrCompact(size, fallback)
    else -> this
  }
}

ListPadExtTest.kt

class ListPadExtTest {
  @Test
  fun `test padEndOrCompact() function`() {
    "0123456789".map(Char::toString).let { size10list ->
      size10list.padEndOrCompact(11, "x").run {
        assertThat(this).hasSize(11)
        assertThat(this.first()).isEqualTo("0")
        assertThat(this.last()).isEqualTo("x")
      }
      size10list.padEndOrCompact(9, "x").run {
        assertThat(this).hasSize(9)
        assertThat(this.first()).isEqualTo("0")
        assertThat(this.last()).isEqualTo("8")
      }
    }
  }

  @Test
  fun `test padEnd() function`() {
    "0123456789".map(Char::toString).let { size10list ->
      size10list.padEnd(11, "x").run {
        assertThat(this).hasSize(11)
        assertThat(this.first()).isEqualTo("0")
        assertThat(this.last()).isEqualTo("x")

      }
      size10list.padEnd(9, "x").run {
        assertThat(this).hasSize(size10list.size)
        assertThat(this.first()).isEqualTo(size10list.first())
        assertThat(this.last()).isEqualTo(size10list.last())
      }
    }
  }

  @Test
  fun `test padStartOrCompact() function`() {
    "0123456789".map(Char::toString).let { size10list ->
      size10list.padStartOrCompact(11, "x").run {
        assertThat(this).hasSize(11)
        assertThat(this.first()).isEqualTo("x")
        assertThat(this.last()).isEqualTo("9")
      }
      size10list.padStartOrCompact(9, "x").run {
        assertThat(this).hasSize(9)
        assertThat(this.first()).isEqualTo("1")
        assertThat(this.last()).isEqualTo("9")
      }
    }
  }

  @Test
  fun `test padStart() function`() {
    "0123456789".map(Char::toString).let { size10list ->
      size10list.padStart(11, "x").run {
        assertThat(this).hasSize(11)
        assertThat(this.first()).isEqualTo("x")
        assertThat(this.last()).isEqualTo("9")
      }
      size10list.padStart(9, "x").run {
        assertThat(this).hasSize(size10list.size)
        assertThat(this.first()).isEqualTo(size10list.first())
        assertThat(this.last()).isEqualTo(size10list.last())
      }
    }
  }
}