2 files changed, 54 insertions, 1 deletions
diff --git a/__tests__/dice.test.js b/__tests__/dice.test.js
index 45c859b..275e0a9 100644
--- a/__tests__/dice.test.js
+++ b/__tests__/dice.test.js
@@ -1,4 +1,4 @@
-const { d } = require('../src/dice.js')
+const { d, add } = require('../src/dice.js')
 
 const defaultNumberRolls = 500
 
@@ -128,6 +128,29 @@ const basicDieTestSpecs = (number, sides) => {
     }
   }
 }
+
+const combinedDiceTestSpecs = (dieSpecs) => {
+  const individualTestSpecs =
+    dieSpecs.map(spec => (basicDieTestSpecs(spec.number, spec.sides)))
+
+  const combineSpecField = (fieldGetter) => {
+    return individualTestSpecs.reduce((total, spec) => {
+      return total + fieldGetter(spec)
+    }, 0)
+  }
+
+  return {
+    diceCount: combineSpecField(spec => (spec.diceCount)),
+    average: {
+      average: combineSpecField(spec => (spec.average.average))
+    },
+    variance: {
+      variance: combineSpecField(spec => (spec.variance.variance))
+    },
+    bounds: {
+      low: combineSpecField(spec => (spec.bounds.low)),
+      high: combineSpecField(spec => (spec.bounds.high)),
+    }
   }
 }
 
@@ -138,8 +161,31 @@ const describeBasicDie = (number, sides, numberRolls = defaultNumberRolls) => {
   })
 }
 
+const describeCompoundDice = (diceSpecs, numberRolls = defaultNumberRolls) => {
+  const dieString = diceSpecs.reduce((string, spec, index) => (
+    string + `${spec.number}d${spec.sides}` +
+      (index < diceSpecs.length - 1 ? ' + ' : '')
+  ), '')
+
+  const die = diceSpecs.slice(1).reduce((die, spec) => {
+    return add(die, d(spec.number, spec.sides))
+  }, d(diceSpecs[0].number, diceSpecs[0].sides))
+
+  describe(dieString, () => testDie(die, combinedDiceTestSpecs(diceSpecs),
+    numberRolls))
+}
+
 describe('basic dice', () => {
   describeBasicDie(1, 6)
   describeBasicDie(2, 8, 500)
   describeBasicDie(20, 1)
 })
+
+describe('add', () => {
+  describeCompoundDice([ { number: 1, sides: 6 }, { number: 1, sides: 4 } ])
+  describeCompoundDice([
+    { number: 3, sides: 6 },
+    { number: 2, sides: 8 },
+    { number: 1, sides: 1 }
+  ])
+})
diff --git a/src/dice.js b/src/dice.js
index 2914317..bc75cb7 100644
--- a/src/dice.js
+++ b/src/dice.js
@@ -10,9 +10,16 @@ const d = (number, sides) => {
   }
 }
 
+const add = (die1, die2) => {
+  return () => {
+    return die1().concat(die2())
+  }
+}
+
 const roll = (die) => {
   return die().reduce((a, b) => (a + b))
 }
 
 exports.d = d
+exports.add = add
 exports.roll = roll