From 0a7d808290893e7bc5d009902c4c453cf64eea4a Mon Sep 17 00:00:00 2001
From: Jesse Gumm <gumm@sigma-star.com>
Date: Fri, 20 Nov 2020 14:14:25 -0600
Subject: [PATCH] Add age functions

---
 CHANGELOG.markdown |  4 ++++
 README.markdown    | 11 ++++++++-
 src/qdate.erl      | 60 +++++++++++++++++++++++++++++++++++++++++++++-
 3 files changed, 73 insertions(+), 2 deletions(-)

diff --git a/CHANGELOG.markdown b/CHANGELOG.markdown
index 37f504a..2c16875 100644
--- a/CHANGELOG.markdown
+++ b/CHANGELOG.markdown
@@ -1,3 +1,7 @@
+## 0.6.0 (in development)
+
+* Add `age` and `age_days` functions
+
 ## 0.5.0
 
 * Add `range_X` functions for getting a list of dates/times within a range
diff --git a/README.markdown b/README.markdown
index 25b9e56..d12f64d 100644
--- a/README.markdown
+++ b/README.markdown
@@ -845,6 +845,16 @@ with qdate:
 
 Also note that the range functions are *inclusive*.
 
+
+## Age Comparison
+
+There are two main comparisons right now, age in years, and age in days.
+
+  + `age(Date)` - Number of years since `Date`
+  + `age(FromDate, ToDate)` - Number of years between `FromDate` to `ToDate`
+  + `age_days(Date)` - Number of full days since `Date` (for example from `2pm` yesterday to `1:59pm` today is still 0.
+  + `age_days(FromDate, ToDate)` - Number of full days between `FromDate` and `ToDate`.
+
 ## Configuration Sample
 
 There is a sample configuration file can be found in the root of the qdate
@@ -879,7 +889,6 @@ See [CHANGELOG.markdown](https://github.com/choptastic/qdate/blob/master/CHANGEL
 + Add `-spec` and `-type` info for dialyzer
 + Research the viability of [ezic](https://github.com/drfloob/ezic) for a
   timezone backend replacement for `erlang_localtime`.
-+ Add age calculation stuff: `age_years(Date)`, `age_minutes(Date)`, etc.
 
 ## Conclusion
 
diff --git a/src/qdate.erl b/src/qdate.erl
index e81061e..a1608c5 100644
--- a/src/qdate.erl
+++ b/src/qdate.erl
@@ -101,6 +101,13 @@
     range_years/3
 ]).
 
+-export([
+    age/1,
+    age/2,
+    age_days/1,
+    age_days/2
+]).
+
 -export([
     register_parser/2,
     register_parser/1,
@@ -768,6 +775,42 @@ fmid({Y, M, D}) when D < 1 ->
 fmid(Date) ->
     Date.
 
+age(Birth) ->
+    age(Birth, os:timestamp()).
+
+age(Birth, Now) ->
+    %% B=Birth
+    {{BY, BM, BD}, _} = to_date(Birth),
+    %% C=Current
+    {{CY, CM, CD}, _} = to_date(Now),
+    if
+        (CM > BM);
+        (CM == BM andalso CD >= BD)
+            -> CY - BY;
+        true ->
+            (CY - BY) - 1
+    end.
+
+age_days(Birth) ->
+    age_days(Birth, os:timestamp()).
+
+age_days(Birth, Now) ->
+    case {to_date(Birth), to_date(Now)} of
+        {{_SameDay, _}, {_SameDay, _}} ->
+            0;
+        {{BirthDate, BirthTime}, {NowDate, NowTime}} ->
+            BirthDays = calendar:date_to_gregorian_days(BirthDate),
+            NowDays = calendar:date_to_gregorian_days(NowDate),
+            DiffDays = NowDays - BirthDays,
+            if
+                NowTime >= BirthTime ->
+                    DiffDays;
+                true ->
+                    DiffDays-1
+            end
+    end.
+
+
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%      Ranges        %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
@@ -1340,7 +1383,22 @@ arith_tests(_) ->
         ?_assertEqual({{2017,1,3},{0,0,0}}, beginning_week(2, {{2017,1,4},{0,0,0}})),
         ?_assertEqual({{2016,12,29},{0,0,0}}, beginning_week(4, {{2017,1,4},{0,0,0}})),
         ?_assertEqual({{2016,12,31},{0,0,0}}, beginning_week(6, {{2017,1,6},{0,0,0}})),
-        ?_assertEqual({{2017,1,1},{0,0,0}}, beginning_week(7, {{2017,1,6},{0,0,0}}))
+        ?_assertEqual({{2017,1,1},{0,0,0}}, beginning_week(7, {{2017,1,6},{0,0,0}})),
+
+        ?_assertEqual(0, age("1981-01-15", "1981-12-31")),
+        ?_assertEqual(39, age("1981-01-15", "2020-01-15")),
+        ?_assertEqual(39, age("1981-01-15", "2020-01-15 12am")),
+        ?_assertEqual(38, age("1981-01-15", "2020-01-14")),
+        ?_assertEqual(38, age("1981-01-15", "2020-01-14 11:59pm")),
+
+        %% checking pre-unix-epoch
+        ?_assertEqual(100, age("1901-01-01","2001-01-01")),
+        ?_assertEqual(20, age("1900-01-01", "1920-01-01")),
+        
+        ?_assertEqual(0, age_days("2020-11-20 12am","2020-11-20 11:59:59pm")),
+        ?_assertEqual(1, age_days("2020-11-19 11:59:59pm","2020-11-20 11:59:59pm")),
+        ?_assertEqual(7, age_days("2020-11-20","2020-11-27")),
+        ?_assertEqual(7, age_days("2020-11-27","2020-12-04"))
 
     ]}.