Rendering a Markdown Table in Kotlin

RenderMarkdown.kt

fun renderMarkdownTable(
  header: List<String>,
  rows: List<List<Any?>>
): String {

  val columnsAreNumeric: List<Boolean> = rows
    .map { row -> row.map { cell -> cell == null || cell is Number } }
    .reduce { areNumbersSoFar, row ->
      areNumbersSoFar.mapIndexed { index, isNumberSoFar ->
        isNumberSoFar && row[index]
      }
    }

  val formattedRows: List<List<String>> = rows
    .map { row ->
      row.mapIndexed { index, cell ->
        val isNumeric = columnsAreNumeric[index]
        if (isNumeric) {
          cell?.formatAsNumber() 
        } else {
          cell?.toString() 
        } ?: ""
      }
    }

  val maxSizes: List<Int> = (listOf(header) + formattedRows)
    .map { row -> row.map { cell -> cell.length } }
    .reduce { maxSizesSoFar, row ->
      maxSizesSoFar.mapIndexed { index, maxSizeSoFar ->
        maxOf(maxSizeSoFar, row[index])
      }
    }

  val headerLine: List<String> = header
    .mapIndexed { index, cell ->
      val padding = maxSizes[index]
      cell.padEnd(padding)
    }

  val horizontalLine: List<String> = List(header.size) { index ->
    val padding = maxSizes[index]
    val isNumeric = columnsAreNumeric[index]
    val content = if (isNumeric) "---:" else "---"
    content.padStart(padding, '-')
  }

  val rowLines: List<List<String>> = formattedRows
    .map { row ->
      row.mapIndexed { index, cell ->
        val padding = maxSizes[index]
        val isNumeric = columnsAreNumeric[index]
        if (isNumeric) {
          cell.padStart(padding)
        } else {
          cell.padEnd(padding)
        }
      }
    }

  return (listOf(headerLine, horizontalLine) + rowLines).joinToString("\n") {
    it.joinToString(prefix = "| ", separator = " | ", postfix = " |")
  }
}

fun Any.formatAsNumber(): String = NumberFormat.getInstance().format(this)