Add startOfWeekYear helper

STATUS.md

@@ -139,7 +139,7 @@
 
 - [x] `getWeekYear`
 - [ ] `setWeekYear`
-- [ ] `startOfWeekYear`
+- [x] `startOfWeekYear`
 
 ### Decade helpers
 

__tests__/startOfWeekYear_test.re

@@ -0,0 +1,43 @@
+open Jest;
+
+open Js.Date;
+
+describe("startOfWeekYear", () => {
+  open ExpectJs;
+
+  test(
+    "returns the date with the time set to 00:00:00 and the date set to the first day of a week year",
+    () => {
+      let date = makeWithYMD(~year=2020., ~month=4., ~date=10., ());
+      let expectedDate =
+        makeWithYMDHMS(
+          ~year=2019.,
+          ~month=11.,
+          ~date=29.,
+          ~hours=0.,
+          ~minutes=0.,
+          ~seconds=0.,
+          (),
+        );
+
+      date |> ReDate.startOfWeekYear |> expect |> toEqual(expectedDate);
+    },
+  );
+
+  test("allows to specify which day is the first day of the week", () => {
+    let date = makeWithYMD(~year=2020., ~month=4., ~date=10., ());
+    let expectedDate =
+      setHoursMSMs(
+        makeWithYMD(~year=2019., ~month=11., ~date=30., ()),
+        ~hours=0.,
+        ~minutes=0.,
+        ~seconds=0.,
+        ~milliseconds=0.,
+        (),
+      )
+      |> fromFloat;
+    let startOfWeekYear' = ReDate.startOfWeekYear(~weekStartsOn=Monday);
+
+    date |> startOfWeekYear' |> expect |> toEqual(expectedDate);
+  });
+});

docs/week_numbering.md

@@ -18,3 +18,22 @@ let getWeekYear' = ReDate.getWeekYear(~weekStartsOn=Monday);
 
 date |> getWeekYear';
 ```
+
+#### startOfWeekYear
+
+> Return the start of a local week-numbering year.
+
+`let startOfWeekYear: (~weekStartsOn: day=?, Js.Date.t) => Js.Date.t`
+
+```reason
+let date = Js.Date.makeWithYMD(~year=2018., ~month=0., ~date=12., ());
+
+date |> ReDate.startOfWeekYear;
+```
+
+```reason
+let date = Js.Date.makeWithYMD(~year=2018., ~month=10., ~date=14., ());
+let startOfWeekYear' = ReDate.startOfWeekYear(~weekStartsOn=Monday);
+
+date |> startOfWeekYear';
+```

src/ReDate.re

@@ -289,6 +289,11 @@ module Internal = {
     |> float_of_int;
   };
 
+  let startOfWeekYear = (~weekStartsOn=Sunday, date) => {
+    let year = date |> getWeekYear(~weekStartsOn);
+    year |> makeDateWithY |> startOfWeek(~weekStartsOn);
+  };
+
   let isLeap = year =>
     year mod 400 === 0 || year mod 4 === 0 && year mod 100 !== 0;
 
@@ -684,6 +689,8 @@ let differenceInYears = Internal.differenceIn(Years);
 
 let getWeekYear = Internal.getWeekYear;
 
+let startOfWeekYear = Internal.startOfWeekYear;
+
 /* ——[Interval helpers]——————————— */
 
 let isWithinInterval = (interval, date) => {

src/ReDate.rei

@@ -217,6 +217,8 @@ let differenceInYears: (Js.Date.t, Js.Date.t) => int;
 
 let getWeekYear: (~weekStartsOn: day=?, Js.Date.t) => float;
 
+let startOfWeekYear: (~weekStartsOn: day=?, Js.Date.t) => Js.Date.t;
+
 /* ——[Interval helpers]——————————— */
 
 let isWithinInterval: (interval, Js.Date.t) => bool;