Implement dice subtraction

1 files changed, 13 insertions, 0 deletions

diff --git a/src/dice.js b/src/dice.js
index bc75cb7..c45bafc 100644
--- a/src/dice.js
+++ b/src/dice.js
@@ -16,10 +16,23 @@ const add = (die1, die2) => {
   }
 }
 
+const negative = (die) => {
+  return () => {
+    return die().map(value => -value)
+  }
+}
+
+const subtract = (die1, die2) => {
+  return () => {
+    return die1().concat(negative(die2)())
+  }
+}
+
 const roll = (die) => {
   return die().reduce((a, b) => (a + b))
 }
 
 exports.d = d
 exports.add = add
+exports.subtract = subtract
 exports.roll = roll