Implement keepHigh

2 files changed, 49 insertions, 1 deletions

diff --git a/__tests__/dice.test.js b/__tests__/dice.test.js
index 1274b3d..1be0744 100644
--- a/__tests__/dice.test.js
+++ b/__tests__/dice.test.js
@@ -5,7 +5,8 @@ const {
   add,
   subtract,
   negative,
-  explode
+  explode,
+  keepHigh
 } = require('../src/dice.js')
 
 const defaultNumberRolls = 500
@@ -378,3 +379,30 @@ describe('exploding dice', () => {
     })
   })
 })
+
+describe('keep', () => {
+  describe('1K2d20', () => {
+    const die = keepHigh(constant(1), d(constant(2), constant(20)))
+
+    testDie(die, {
+      diceCount: 1,
+      average: {
+        average: 13.825
+      },
+      variance: {
+        variance: 22.104
+      },
+      bounds: {
+        low: 1,
+        high: 20,
+        expectLow: true,
+        expectHigh: true
+      }
+    }, 800)
+  })
+
+  describe('2K1d20', () => {
+    const die = keepHigh(constant(2), d(constant(1), constant(20)))
+    testDie(die, basicDieTestSpecs(1, 20))
+  })
+})
diff --git a/src/dice.js b/src/dice.js
index b60af68..5e3b975 100644
--- a/src/dice.js
+++ b/src/dice.js
@@ -56,6 +56,25 @@ const explode = (die1, die2) => {
   }
 }
 
+const keepHigh = (die1, die2) => {
+  return () => {
+    const numberToKeep = roll(die1)
+    let rolls = die2().map(die => [die, die()])
+    rolls.sort((a, b) => (a[1] - b[1])).reverse()
+    return rolls.slice(0, numberToKeep).map(pair => {
+      let returnedOriginal = false
+      return () => {
+        if (!returnedOriginal) {
+          returnedOriginal = true
+          return pair[1]
+        } else {
+          return pair[0]()
+        }
+      }
+    })
+  }
+}
+
 exports.pool = pool
 exports.roll = roll
 exports.constant = constant
@@ -64,3 +83,4 @@ exports.add = add
 exports.subtract = subtract
 exports.negative = negative
 exports.explode = explode
+exports.keepHigh = keepHigh