Add implementation for part 2.

2017/README.md

@@ -27,6 +27,7 @@ The current plan, in no particular order:
 - [x] PHP - [Day 19](./day-19/solution.php)
 - [ ] Prolog
 - [ ] Python
+- [x] R - [Day 20](./day-20/solution.r)
 - [x] Ruby - [Day 08](./day-08/solution.rb)
 - [x] Rust - [Day 14](./day-14/solution.rs)
 - [x] Scala - [Day 11](./day-11/solution.scala)

2017/day-20/solution.r

@@ -14,4 +14,16 @@ speeds <- rowSums(abs(input[,4:6]))
 perm <- order(accs, speeds)
 print(perm[1] - 1)
 
+for (i in 1:100) {
+	# Update positions, speeds
+	for (coord in 1:3) {
+		input[,coord+3] <- input[,coord+3] + input[,coord+6]
+		input[,coord] <- input[,coord] + input[,coord+3]
+	}
 
+	dups = !(duplicated(input[,1:3]) | duplicated(input[,1:3], fromLast=TRUE))
+
+	input = input[dups,]
+}
+
+print(nrow(input))