histogram.kt

fun <T> List<T>.histogram(map: (T) -> Int, showAs: (Boolean, T) -> String): String {
    val maxHeight = map { map(it) }.max() ?: 0
    return (maxHeight downTo 0).map { i ->
      this.map {
        showAs(i <= map(it), it)
      }.joinToString("")
    }.joinToString("\n")
}

//Sample usage:

listOf(1, 2, 3, 4, 1, 2, 4).histogram({ it }, { it, e ->
  if (it) "#" else " "
})

/* sample output
   #  #
  ##  #
 ### ##
####### */