Implement age

src/personnummer.gleam

@@ -7,6 +7,17 @@ import gleam/regex
 import gleam/result
 import gleam/string
 
+/// DateTime represents the type return from Erlang `local_time`.
+pub type DateTime =
+  #(#(Int, Int, Int), #(Int, Int, Int))
+
+// An external function call to calendar.local_time/0. This is marked as public
+// to be able to use from unit tests but is not intended to be used by external
+// packages.
+// https://www.erlang.org/doc/apps/stdlib/calendar#local_time/0
+@external(erlang, "calendar", "local_time")
+pub fn now() -> DateTime
+
 /// A representation of a date in time, holding a year, month and day.
 pub type Date {
   Date(year: Int, month: Int, day: Int)
@@ -154,6 +165,20 @@ fn valid_date(date: Date) -> Bool {
   }
 }
 
+/// Get the age of the person holding the passed personnummer.
+pub fn age(pnr: Personnummer) -> Int {
+  let #(#(current_year, current_month, current_day), _) = now()
+
+  case pnr.date.month > current_month {
+    True -> current_year - pnr.date.year - 1
+    False ->
+      case pnr.date.month == current_month && pnr.date.day > current_day {
+        True -> current_year - pnr.date.year - 1
+        False -> current_year - pnr.date.year
+      }
+  }
+}
+
 /// Check if the personnummer is a coordination number (Samordningsnummer):
 /// https://skatteverket.se/privat/folkbokforing/samordningsnummer.4.5c281c7015abecc2e201130b.html
 pub fn is_coordination(pnr: Personnummer) -> Bool {

test/personnummer_test.gleam

@@ -1,4 +1,6 @@
+import gleam/int
 import gleam/list
+import gleam/string
 import gleeunit
 import gleeunit/should
 import personnummer
@@ -65,3 +67,69 @@ pub fn gender_test() {
     )
   })
 }
+
+pub fn age_test() {
+  let #(#(current_year, current_month, current_day), _) = personnummer.now()
+  let forty_years_ago = { current_year - 40 } % 1900
+
+  let #(y1, m1, d1) = case current_month > 1 {
+    True -> #(forty_years_ago, current_month - 1, current_day)
+    False -> #(forty_years_ago - 1, 12, 31)
+  }
+
+  let #(y2, m2, d2) = case current_month < 12 {
+    True -> #(forty_years_ago, current_month + 1, current_day)
+    False -> #(forty_years_ago + 1, 1, 1)
+  }
+
+  let assert Ok(forty) =
+    string.concat([
+      int.to_string(y1),
+      int.to_string(m1)
+        |> string.pad_left(to: 2, with: "0"),
+      int.to_string(d1)
+        |> string.pad_left(to: 2, with: "0"),
+      "0000",
+    ])
+    |> personnummer.new()
+
+  let assert Ok(forty_one) =
+    string.concat([
+      int.to_string(forty_years_ago - 1),
+      int.to_string(current_month)
+        |> string.pad_left(to: 2, with: "0"),
+      int.to_string(current_day)
+        |> string.pad_left(to: 2, with: "0"),
+      "0000",
+    ])
+    |> personnummer.new()
+
+  let assert Ok(thirty_nine) =
+    string.concat([
+      int.to_string(y2),
+      int.to_string(m2)
+        |> string.pad_left(to: 2, with: "0"),
+      int.to_string(d2)
+        |> string.pad_left(to: 2, with: "0"),
+      "0000",
+    ])
+    |> personnummer.new()
+
+  should.equal(
+    forty
+      |> personnummer.age(),
+    40,
+  )
+
+  should.equal(
+    forty_one
+      |> personnummer.age(),
+    41,
+  )
+
+  should.equal(
+    thirty_nine
+      |> personnummer.age(),
+    39,
+  )
+}