Implement Season next/prev

See #12

src/season.js

@@ -61,14 +61,36 @@ class Season extends ExtDateTime {
   }
 
   set season(season) {
-    return this.values[1] = Number(season)
+    return validate(this.values[1] = Number(season))
   }
 
   get values() {
     return V.get(this)
   }
 
-  // TODO next/prev
+  next(k = 1) {
+    let { season, year } = this
+
+    switch (true) {
+    case (season >= 21 && season <= 36):
+      [year, season] = inc(year, season, k, season - (season - 21) % 4, 4)
+      break
+    case (season >= 37 && season <= 39):
+      [year, season] = inc(year, season, k, 37, 3)
+      break
+    case (season >= 40 && season <= 41):
+      [year, season] = inc(year, season, k, 40, 2)
+      break
+    default:
+      throw new RangeError(`Cannot compute next/prev for season ${season}`)
+    }
+
+    return new Season(year, season)
+  }
+
+  prev(k = 1) {
+    return this.next(-k)
+  }
 
   get min() { // eslint-disable-line complexity
     switch (this.season) {
@@ -155,4 +177,15 @@ class Season extends ExtDateTime {
   }
 }
 
+function validate(season) {
+  if (isNaN(season) || season < 21 || season === Infinity)
+    throw new RangeError(`invalid division of year: ${season}`)
+  return season
+}
+
+function inc(year, season, by, base, size) {
+  const m = (season + by) - base
+  return [year + Math.floor(m / size), validate(base + (m % size + size) % size)]
+}
+
 module.exports = Season

test/interval.js

@@ -74,9 +74,14 @@ describe('Interval', () => {
       expect(S80.toEDTF()).to.be.eql('1980-21/1980-24')
     })
 
-    //it('includes', () => {
-    //  expect(S80.includes(new Date(1980, 8))).to.be.true
-    //  expect(S80.includes(new Season(1980, 22))).to.be.true
-    //})
+    it('.covers', () => {
+      expect(S80.covers(new Season(1980, 22))).to.be.true
+      expect(S80.covers(new Date(1980, 8))).to.be.true
+    })
+
+    it('.includes', () => {
+      expect(S80.includes(new Season(1980, 22))).to.be.true
+      expect(S80.includes(new Date(1980, 8))).to.be.false
+    })
   })
 })

test/season.js

@@ -20,6 +20,24 @@ describe('Season', () => {
     })
   })
 
+  describe('.next', () => {
+    it('positive', () => {
+      expect(new Season(2018, 21).next().toEDTF()).to.eql('2018-22')
+      expect(new Season(2018, 21).next(2).toEDTF()).to.eql('2018-23')
+      expect(new Season(2018, 21).next(3).toEDTF()).to.eql('2018-24')
+      expect(new Season(2018, 21).next(4).toEDTF()).to.eql('2019-21')
+      expect(new Season(2018, 25).next(9).toEDTF()).to.eql('2020-26')
+    })
+
+    it('negative', () => {
+      expect(new Season(2018, 24).next(-1).toEDTF()).to.eql('2018-23')
+      expect(new Season(2018, 24).next(-2).toEDTF()).to.eql('2018-22')
+      expect(new Season(2018, 24).next(-3).toEDTF()).to.eql('2018-21')
+      expect(new Season(2018, 24).next(-4).toEDTF()).to.eql('2017-24')
+      expect(new Season(2018, 24).next(-9).toEDTF()).to.eql('2016-23')
+    })
+  })
+
   describe('.edtf', () => {
     it('default', () =>
       expect(new Season().edtf).to.match(/^\d\d\d\d-21$/))