feat: add makeChange selector

lib/bill-math.js

@@ -16,6 +16,38 @@ exports.makeChange = function makeChange (cassettes, amount) {
   // it still requires a fallback for cases where it might not provide any solution.
   // Example: Denominations: [3, 5, 10] | User inputs 4 times the [3] button, resulting in a 12 fiat tx
   // This algorithm resolves for 1 x [10], and can't resolve the remainder of 2
+
+  return _.length(cassettes) > 2 ? makeChangeDynamic(cassettes, amount) : makeChangeDuo(cassettes, amount)
+}
+
+function makeChangeDuo (cassettes, amount) {
+  const small = cassettes[0]
+  const large = cassettes[1]
+
+  const largeDenom = large.denomination
+  const smallDenom = small.denomination
+  const largeBills = Math.min(large.count, Math.floor(amount / largeDenom))
+
+  const amountNum = amount.toNumber()
+  
+  for (let i = largeBills; i >= 0; i--) {
+    const remainder = amountNum - largeDenom * i
+
+    if (remainder % smallDenom !== 0) continue
+
+    const smallCount = remainder / smallDenom
+    if (smallCount > small.count) continue
+
+    return [
+      {provisioned: smallCount, denomination: small.denomination, id: uuid.v4()},
+      {provisioned: i, denomination: largeDenom, id: uuid.v4()}
+    ]
+  }
+
+  return null
+}
+
+function makeChangeDynamic (cassettes, amount) {
   const cassetteMap = _.map(cassettes, it => ({
     denomination: it.denomination
   }))