From 88030e217f1ff59ed10fd65cd385fd603e17251c Mon Sep 17 00:00:00 2001
From: Rob Rix <rob.rix@github.com>
Date: Tue, 6 Oct 2015 11:30:41 -0400
Subject: [PATCH] Short-circuit degenerate cases.

---
 prototype/Doubt/Algorithm.swift | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/prototype/Doubt/Algorithm.swift b/prototype/Doubt/Algorithm.swift
index 274a5fa9f..d8d3f6bf6 100644
--- a/prototype/Doubt/Algorithm.swift
+++ b/prototype/Doubt/Algorithm.swift
@@ -101,6 +101,9 @@ public enum FreeAlgorithm<A, B> {
 			return f(Dictionary(elements: deleted + inserted + patched)).evaluate(equals)
 
 		case let .Roll(.ByIndex(a, b, f)):
+			if a.isEmpty { return f(b.map { Diff.Pure(Patch.Insert($0)) }).evaluate(equals) }
+			if b.isEmpty { return f(a.map { Diff.Pure(Patch.Delete($0)) }).evaluate(equals) }
+
 			let cost: Diff -> Int = { diff in
 				diff.map(const(1)).iterate { syntax in
 					switch syntax {