java/2016/21: add solution for part 2

Author: saser

java/src/main/java/com/github/saser/adventofcode/year2016/day21/Day21.java

@@ -3,6 +3,7 @@
 import java.io.BufferedReader;
 import java.io.Reader;
 import java.util.Arrays;
+import java.util.Collections;
 import java.util.List;
 import java.util.regex.Matcher;
 import java.util.regex.Pattern;
@@ -20,52 +21,77 @@ public static Result part2(Reader r) {
     }
 
     private static Result solve(Reader r, int part) {
-        var chars = "abcdefgh".toCharArray();
+        var chars = (part == 1 ? "abcdefgh" : "fbgdceah").toCharArray();
         var instructions = new BufferedReader(r)
                 .lines()
-                .collect(Collectors.toUnmodifiableList());
-        apply(chars, instructions);
+                .collect(Collectors.toList());
+        apply(chars, instructions, part == 2);
         return Result.ok(new String(chars));
     }
 
-    private static void apply(char[] chars, List<String> instructions) {
+    private static void apply(char[] chars, List<String> instructions, boolean reverse) {
         var swapPositionRE = Pattern.compile("swap position (\\d+) with position (\\d+)");
         var swapLetterRE = Pattern.compile("swap letter (\\w) with letter (\\w)");
         var rotateStepsRE = Pattern.compile("rotate (left|right) (\\d+) steps?");
         var rotateLetterRE = Pattern.compile("rotate based on position of letter (\\w)");
         var reverseRE = Pattern.compile("reverse positions (\\d+) through (\\d+)");
         var moveRE = Pattern.compile("move position (\\d+) to position (\\d+)");
+        if (reverse) {
+            Collections.reverse(instructions);
+        }
         for (var instruction : instructions) {
             Matcher matcher;
             matcher = swapPositionRE.matcher(instruction);
             if (matcher.matches()) {
                 var x = Integer.parseInt(matcher.group(1));
                 var y = Integer.parseInt(matcher.group(2));
-                swap(chars, x, y);
+                if (reverse) {
+                    swap(chars, y, x);
+                } else {
+                    swap(chars, x, y);
+                }
                 continue;
             }
             matcher = swapLetterRE.matcher(instruction);
             if (matcher.matches()) {
                 var x = find(chars, matcher.group(1).charAt(0));
                 var y = find(chars, matcher.group(2).charAt(0));
-                swap(chars, x, y);
+                if (reverse) {
+                    swap(chars, y, x);
+                } else {
+                    swap(chars, x, y);
+                }
                 continue;
             }
             matcher = rotateStepsRE.matcher(instruction);
             if (matcher.matches()) {
                 var right = matcher.group(1).equals("right");
                 var steps = Integer.parseInt(matcher.group(2));
+                if (reverse) {
+                    right = !right;
+                }
                 rotate(chars, right, steps);
                 continue;
             }
             matcher = rotateLetterRE.matcher(instruction);
             if (matcher.matches()) {
                 var index = find(chars, matcher.group(1).charAt(0));
-                var steps = 1 + index;
-                if (index >= 4) {
-                    steps++;
+                int steps;
+                if (reverse) {
+                    if (index == 0) {
+                        steps = 1;
+                    } else if (index % 2 == 1) {
+                        steps = index / 2 + 1;
+                    } else {
+                        steps = 5 + index / 2;
+                    }
+                } else {
+                    steps = 1 + index;
+                    if (index >= 4) {
+                        steps++;
+                    }
                 }
-                rotate(chars, true, steps);
+                rotate(chars, !reverse, steps);
                 continue;
             }
             matcher = reverseRE.matcher(instruction);
@@ -79,7 +105,11 @@ private static void apply(char[] chars, List<String> instructions) {
             if (matcher.matches()) {
                 var x = Integer.parseInt(matcher.group(1));
                 var y = Integer.parseInt(matcher.group(2));
-                move(chars, x, y);
+                if (reverse) {
+                    move(chars, y, x);
+                } else {
+                    move(chars, x, y);
+                }
                 continue;
             }
             throw new IllegalArgumentException(String.format("invalid instruction: %s", instruction));

java/src/test/java/com/github/saser/adventofcode/year2016/day21/Day21Test.java

@@ -18,22 +18,13 @@ public void part1Actual() throws IOException {
         }
     }
 
-    // @Test
-    // public void part2Example() {
-    //     var input = new StringReader("");
-    //     var output = "";
-    //     var result = Day21.part2(input);
-    //     Assert.assertEquals("no error", "", result.error);
-    //     Assert.assertEquals("correct output", output, result.answer);
-    // }
-
-    // @Test
-    // public void part2Actual() throws IOException {
-    //     try (var input = new FileReader("inputs/2016/21")) {
-    //         var output = "";
-    //         var result = Day21.part2(input);
-    //         Assert.assertEquals("no error", "", result.error);
-    //         Assert.assertEquals("correct output", output, result.answer);
-    //     }
-    // }
+     @Test
+     public void part2Actual() throws IOException {
+         try (var input = new FileReader("inputs/2016/21")) {
+             var output = "gcehdbfa";
+             var result = Day21.part2(input);
+             Assert.assertEquals("no error", "", result.error);
+             Assert.assertEquals("correct output", output, result.answer);
+         }
+     }
 }