From 1ae29437d03ea010da1b87dc12fd155d414e9c1f Mon Sep 17 00:00:00 2001
From: Marcin Chrzanowski <marcin.j.chrzanowski@gmail.com>
Date: Tue, 22 Aug 2017 16:58:42 -0400
Subject: Implment keepLow

---
 __tests__/dice.test.js | 26 ++++++++++++++++++++++++++
 src/dice.js            | 20 ++++++++++++++++++++
 2 files changed, 46 insertions(+)

diff --git a/__tests__/dice.test.js b/__tests__/dice.test.js
index 6fb48a2..3073fee 100644
--- a/__tests__/dice.test.js
+++ b/__tests__/dice.test.js
@@ -7,6 +7,7 @@ const {
   negative,
   explode,
   keepHigh,
+  keepLow,
   bonusAdd
 } = require('../src/dice.js')
 
@@ -406,6 +407,31 @@ describe('keep', () => {
     const die = keepHigh(constant(2), d(constant(1), constant(20)))
     testDie(die, basicDieTestSpecs(1, 20))
   })
+
+  describe('1k2d20', () => {
+    const die = keepLow(constant(1), d(constant(2), constant(20)))
+
+    testDie(die, {
+      diceCount: 1,
+      average: {
+        average: 7.175
+      },
+      variance: {
+        variance: 22.194
+      },
+      bounds: {
+        low: 1,
+        high: 20,
+        expectLow: true,
+        expectHigh: true
+      }
+    }, 800)
+  })
+
+  describe('2k1d20', () => {
+    const die = keepLow(constant(2), d(constant(1), constant(20)))
+    testDie(die, basicDieTestSpecs(1, 20))
+  })
 })
 
 describe('bonusAdd', () => {
diff --git a/src/dice.js b/src/dice.js
index 14addd4..becf7dc 100644
--- a/src/dice.js
+++ b/src/dice.js
@@ -75,6 +75,25 @@ const keepHigh = (die1, die2) => {
   }
 }
 
+const keepLow = (die1, die2) => {
+  return () => {
+    const numberToKeep = roll(die1)
+    let rolls = die2().map(die => [die, die()])
+    rolls.sort((a, b) => (a[1] - b[1]))
+    return rolls.slice(0, numberToKeep).map(pair => {
+      let returnedOriginal = false
+      return () => {
+        if (!returnedOriginal) {
+          returnedOriginal = true
+          return pair[1]
+        } else {
+          return pair[0]()
+        }
+      }
+    })
+  }
+}
+
 const bonusAdd = (die1, die2) => {
   return () => {
     return die1().map(die => {
@@ -92,4 +111,5 @@ exports.subtract = subtract
 exports.negative = negative
 exports.explode = explode
 exports.keepHigh = keepHigh
+exports.keepLow = keepLow
 exports.bonusAdd = bonusAdd
-- 
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