From fcc7ea20efc9479be8dacbedd0ca3f4be2ffd97e Mon Sep 17 00:00:00 2001
From: Tangui <29804907+tanguilp@users.noreply.github.com>
Date: Wed, 29 May 2024 00:52:17 +0300
Subject: [PATCH] Implement `Duration.to_iso8601/1` (#13604)

---
 lib/elixir/lib/calendar/duration.ex           | 95 +++++++++++++++++++
 .../test/elixir/calendar/duration_test.exs    | 41 ++++++++
 2 files changed, 136 insertions(+)

diff --git a/lib/elixir/lib/calendar/duration.ex b/lib/elixir/lib/calendar/duration.ex
index 8b523ae1be..29fa77e01c 100644
--- a/lib/elixir/lib/calendar/duration.ex
+++ b/lib/elixir/lib/calendar/duration.ex
@@ -124,6 +124,8 @@ defmodule Duration do
   """
   @type duration :: t | [unit_pair]
 
+  @microseconds_per_second 1_000_000
+
   @doc """
   Creates a new `Duration` struct from given `unit_pairs`.
 
@@ -342,4 +344,97 @@ defmodule Duration do
         raise ArgumentError, ~s/failed to parse duration "#{string}". reason: #{inspect(reason)}/
     end
   end
+
+  @doc """
+  Converts the given `duration` to an [ISO 8601-2:2019](https://en.wikipedia.org/wiki/ISO_8601) formatted string.
+
+  Note this function implements the *extension* of ISO 8601:2019. This extensions allows weeks to
+  appear between months and days: `P3M3W3D`, making it fully compatible with any `Duration` struct.
+
+  ## Examples
+
+      iex> Duration.to_iso8601(%Duration{year: 3})
+      "P3Y"
+      iex> Duration.to_iso8601(%Duration{day: 40, hour: 12, minute: 42, second: 12})
+      "P40DT12H42M12S"
+      iex> Duration.to_iso8601(%Duration{second: 30})
+      "PT30S"
+
+      iex> Duration.to_iso8601(%Duration{})
+      "PT0S"
+
+      iex> Duration.to_iso8601(%Duration{second: 1, microsecond: {2_200, 3}})
+      "PT1.002S"
+      iex> Duration.to_iso8601(%Duration{second: 1, microsecond: {-1_200_000, 4}})
+      "PT-0.2000S"
+  """
+
+  @spec to_iso8601(t) :: String.t()
+  def to_iso8601(duration)
+
+  def to_iso8601(%Duration{
+        year: 0,
+        month: 0,
+        week: 0,
+        day: 0,
+        hour: 0,
+        minute: 0,
+        second: 0,
+        microsecond: {0, _}
+      }) do
+    "PT0S"
+  end
+
+  def to_iso8601(%Duration{} = d) do
+    IO.iodata_to_binary([?P, to_iso8601_duration_date(d), to_iso8601_duration_time(d)])
+  end
+
+  defp to_iso8601_duration_date(d) do
+    [
+      if(d.year == 0, do: [], else: [Integer.to_string(d.year), ?Y]),
+      if(d.month == 0, do: [], else: [Integer.to_string(d.month), ?M]),
+      if(d.week == 0, do: [], else: [Integer.to_string(d.week), ?W]),
+      if(d.day == 0, do: [], else: [Integer.to_string(d.day), ?D])
+    ]
+  end
+
+  defp to_iso8601_duration_time(%Duration{hour: 0, minute: 0, second: 0, microsecond: {0, _}}) do
+    []
+  end
+
+  defp to_iso8601_duration_time(d) do
+    [
+      ?T,
+      if(d.hour == 0, do: [], else: [Integer.to_string(d.hour), ?H]),
+      if(d.minute == 0, do: [], else: [Integer.to_string(d.minute), ?M]),
+      second_component(d)
+    ]
+  end
+
+  defp second_component(%Duration{second: 0, microsecond: {0, _}}) do
+    []
+  end
+
+  defp second_component(%Duration{second: 0, microsecond: {_, 0}}) do
+    ~c"0S"
+  end
+
+  defp second_component(%Duration{microsecond: {_, 0}} = d) do
+    [Integer.to_string(d.second), ?S]
+  end
+
+  defp second_component(%Duration{microsecond: {ms, p}} = d) do
+    total_ms = d.second * @microseconds_per_second + ms
+    second = total_ms |> div(@microseconds_per_second) |> abs()
+    ms = total_ms |> rem(@microseconds_per_second) |> abs()
+    sign = if total_ms < 0, do: ?-, else: []
+
+    [
+      sign,
+      Integer.to_string(second),
+      ?.,
+      ms |> Integer.to_string() |> String.pad_leading(6, "0") |> binary_part(0, p),
+      ?S
+    ]
+  end
 end
diff --git a/lib/elixir/test/elixir/calendar/duration_test.exs b/lib/elixir/test/elixir/calendar/duration_test.exs
index ed6a23f27f..f3dccf3447 100644
--- a/lib/elixir/test/elixir/calendar/duration_test.exs
+++ b/lib/elixir/test/elixir/calendar/duration_test.exs
@@ -312,4 +312,45 @@ defmodule DurationTest do
                    Duration.from_iso8601!("P4.5YT6S")
                  end
   end
+
+  test "to_iso8601/1" do
+    assert %Duration{year: 1, month: 2, day: 3, hour: 4, minute: 5, second: 6}
+           |> Duration.to_iso8601() == "P1Y2M3DT4H5M6S"
+
+    assert %Duration{week: 3, hour: 5, minute: 3} |> Duration.to_iso8601() == "P3WT5H3M"
+    assert %Duration{hour: 5, minute: 3} |> Duration.to_iso8601() == "PT5H3M"
+    assert %Duration{year: 1, month: 2, day: 3} |> Duration.to_iso8601() == "P1Y2M3D"
+    assert %Duration{hour: 4, minute: 5, second: 6} |> Duration.to_iso8601() == "PT4H5M6S"
+    assert %Duration{year: 1, month: 2} |> Duration.to_iso8601() == "P1Y2M"
+    assert %Duration{day: 3} |> Duration.to_iso8601() == "P3D"
+    assert %Duration{hour: 4, minute: 5} |> Duration.to_iso8601() == "PT4H5M"
+    assert %Duration{second: 6} |> Duration.to_iso8601() == "PT6S"
+    assert %Duration{second: 1, microsecond: {600_000, 1}} |> Duration.to_iso8601() == "PT1.6S"
+    assert %Duration{second: -1, microsecond: {-600_000, 1}} |> Duration.to_iso8601() == "PT-1.6S"
+
+    assert %Duration{second: -1, microsecond: {-234_567, 6}} |> Duration.to_iso8601() ==
+             "PT-1.234567S"
+
+    assert %Duration{second: 1, microsecond: {123_456, 6}} |> Duration.to_iso8601() ==
+             "PT1.123456S"
+
+    assert %Duration{year: 3, week: 4, day: -3, second: -6} |> Duration.to_iso8601() ==
+             "P3Y4W-3DT-6S"
+
+    assert %Duration{second: -4, microsecond: {-230_000, 2}} |> Duration.to_iso8601() ==
+             "PT-4.23S"
+
+    assert %Duration{second: -4, microsecond: {230_000, 2}} |> Duration.to_iso8601() ==
+             "PT-3.77S"
+
+    assert %Duration{second: 2, microsecond: {-1_200_000, 4}} |> Duration.to_iso8601() ==
+             "PT0.8000S"
+
+    assert %Duration{second: 1, microsecond: {-1_200_000, 3}} |> Duration.to_iso8601() ==
+             "PT-0.200S"
+
+    assert %Duration{microsecond: {-800_000, 2}} |> Duration.to_iso8601() == "PT-0.80S"
+    assert %Duration{microsecond: {-800_000, 0}} |> Duration.to_iso8601() == "PT0S"
+    assert %Duration{microsecond: {-1_200_000, 2}} |> Duration.to_iso8601() == "PT-1.20S"
+  end
 end