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@bluzky
Created December 11, 2021 16:19
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Elixir datetime parser with regex
defmodule DateTimeParser do
@mapping %{
"H" => "(?<hour>\\d{2})",
"I" => "(?<hour12>\\d{2})",
"M" => "(?<minute>\\d{2})",
"S" => "(?<second>\\d{2})",
"d" => "(?<day>\\d{2})",
"m" => "(?<month>\\d{2})",
"y" => "(?<year2>\\d{2})",
"Y" => "(?<year>-?\\d{4})",
"z" => "(?<tz>[+-]?\\d{4})",
"Z" => "(?<tz_name>[a-zA-Z_\/]+)",
"p" => "(?<p>PM|AM)",
"P" => "(?<P>pm|am)",
"%" => "%"
}
@doc """
Parse string to datetime struct
**Example**
parse("2021-20-10", "%Y-%M-%d")
Support format
| format | description| value example |
| -- | -- | -- |
| H | 24 hour | 00 - 23 |
| I | 12 hour | 00 - 12 |
| M | minute| 00 - 59 |
| S | second | 00 - 59 |
| d | day | 01 - 31 |
| m | month | 01 -12 |
| y | 2 digits year | 00 - 99 |
| Y | 4 digits year | |
| z | timezone offset | +0100, -0330 |
| Z | timezone name | UTC+7, Asia/Ho_Chi_Minh |
| p | PM or AM | |
| P | pm or am | |
"""
def parse!(dt_string, format \\ "%Y-%m-%dT%H:%M:%SZ") do
case parse(dt_string, format) do
{:ok, dt} -> dt
{:error, message} -> raise "Parse string #{dt_string} with error: #{message}"
end
end
def parse(dt_string, format \\ "%Y-%m-%dT%H:%M:%SZ") do
format
|> build_regex
|> Regex.named_captures(dt_string)
|> cast_data
|> to_datetime
end
def build_regex(format) do
keys = Map.keys(@mapping) |> Enum.join("")
Regex.compile!("([^%]*)%([#{keys}])([^%]*)")
|> Regex.scan(format)
|> Enum.map(fn [_, s1, key, s2] ->
[s1, Map.get(@mapping, key), s2]
end)
|> to_string()
|> Regex.compile!()
end
@default_value %{
day: 1,
month: 1,
year: 0,
hour: 0,
minute: 0,
second: 0,
utc_offset: 0,
tz_name: "UTC",
shift: "AM"
}
def cast_data(nil), do: {:error, "invalid datetime"}
def cast_data(captures) do
captures
|> Enum.reduce_while([], fn {part, value}, acc ->
case cast(part, value) do
{:ok, data} -> {:cont, [data | acc]}
{:error, _} = error -> {:halt, error}
end
end)
|> case do
{:error, _} = error -> error
data -> Enum.into(data, @default_value)
end
end
@value_rages %{
"hour" => [0, 23],
"hour12" => [0, 12],
"minute" => [0, 59],
"second" => [0, 59],
"day" => [0, 31],
"month" => [1, 12],
"year2" => [0, 99]
}
defp cast("P", value) do
cast("p", String.upcase(value))
end
defp cast("p", value) do
{:ok, {:shift, value}}
end
defp cast("tz", value) do
{hour, minute} = String.split_at(value, 3)
with {:ok, {_, hour}} <- cast("offset_h", hour),
{:ok, {_, minute}} <- cast("offset_m", minute) do
sign = div(hour, abs(hour))
{:ok, {:utc_offset, sign * (abs(hour) * 3600 + minute * 60)}}
else
_ -> {:error, "#{value} is invalid timezone offset"}
end
end
defp cast("tz_name", value) do
{:ok, {:tz_name, value}}
end
defp cast(part, value) do
value = String.to_integer(value)
valid =
case Map.get(@value_rages, part) do
[min, max] ->
value >= min and value <= max
_ ->
true
end
if valid do
{:ok, {String.to_atom(part), value}}
else
{:error, "#{value} is not a valid #{part}"}
end
end
defp to_datetime({:error, _} = error), do: error
defp to_datetime(%{year2: value} = data) do
current_year = DateTime.utc_now() |> Map.get(:year)
year = div(current_year, 100) * 100 + value
data
|> Map.put(:year, year)
|> Map.delete(:year2)
|> to_datetime()
end
defp to_datetime(%{hour12: hour} = data) do
# 12AM is not valid
if hour == 12 and data.shift == "AM" do
{:error, "12AM is invalid value"}
else
hour =
cond do
hour == 12 and data.shift == "PM" -> hour
data.shift == "AM" -> hour
data.shift == "PM" -> hour + 12
end
data
|> Map.put(:hour, hour)
|> Map.delete(:hour12)
|> to_datetime()
end
end
defp to_datetime(data) do
with {:ok, date} <- Date.new(data.year, data.month, data.day),
{:ok, time} <- Time.new(data.hour, data.minute, data.second),
{:ok, datetime} <- DateTime.new(date, time) do
datetime = DateTime.add(datetime, -data.utc_offset, :second)
if data.tz_name != "UTC" do
DateTime.shift_zone(datetime, data.tz_name)
else
{:ok, datetime}
end
end
end
end
@toddholmberg
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Thank you!

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