Advent of Code 2021 - Day 3

Author: CodingNagger

README.md

@@ -20,5 +20,4 @@ Last year I ran a [cruise log for my 25 days competing in the Advent of Code](ht
 
 - [The Advent of Code 2021 Day 1 log, starting slow](https://www.codingnagger.com/2021/12/01/the-advent-of-code-2021-day-1-log-starting-slow/)
 - [The Advent of Code 2021 Day 2 log, sneaky release](https://www.codingnagger.com/2021/12/02/the-advent-of-code-2021-day-2-log-sneaky-release/)
-
-
+- [The Advent of Code 2021 Day 3 log, sleepy brain](https://www.codingnagger.com/2021/12/03/the-advent-of-code-2021-day-3-log-sleepy-brain/)

src/main/java/com/codingnagger/App.java

@@ -3,6 +3,7 @@
 import com.codingnagger.days.Day;
 import com.codingnagger.days.Day1;
 import com.codingnagger.days.Day2;
+import com.codingnagger.days.Day3;
 import com.codingnagger.utils.InputLoader;
 
 import java.io.IOException;
@@ -15,9 +16,9 @@ public class App {
     public static void main(String[] args) throws IOException {
         System.out.println("Advent of Code 2021");
 
-        List<String> input = InputLoader.Load("day2.txt");
+        List<String> input = InputLoader.Load("day3.txt");
 
-        Day day = new Day2();
+        Day day = new Day3();
 
         System.out.println("Part 1:");
         System.out.println(day.partOne(input));

src/main/java/com/codingnagger/days/Day3.java

@@ -0,0 +1,103 @@
+package com.codingnagger.days;
+
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.stream.Collectors;
+
+public class Day3 implements Day {
+
+    @Override
+    public String partOne(List<String> input) {
+        BitInfo bitInfo = new BitInfo();
+        bitInfo.compute(input);
+        return "" + bitInfo.gamma * bitInfo.epsilon;
+    }
+
+    @Override
+    public String partTwo(List<String> input) {
+        List<String> filteredGammaInput = new ArrayList<>(input);
+        List<String> filteredEpsilonInput = new ArrayList<>(input);
+        BitInfo bitInfo = new BitInfo();
+
+        int cursor = 0;
+
+        while (cursor <= input.get(0).length()) {
+            final int lambdaCursor = cursor;
+
+            if (filteredGammaInput.size() > 1) {
+                bitInfo.compute(filteredGammaInput);
+                filteredGammaInput = filteredGammaInput.stream().filter(line -> line.charAt(lambdaCursor) == bitInfo.gammaBits.charAt(lambdaCursor)).collect(Collectors.toList());
+            }
+
+            if (filteredEpsilonInput.size() > 1) {
+                bitInfo.compute(filteredEpsilonInput);
+                filteredEpsilonInput = filteredEpsilonInput.stream().filter(line -> line.charAt(lambdaCursor) == bitInfo.epsilonBits.charAt(lambdaCursor)).collect(Collectors.toList());
+            }
+
+            cursor++;
+        }
+
+        return "" + toDecimal(filteredGammaInput.get(0)) * toDecimal(filteredEpsilonInput.get(0));
+
+
+    }
+
+    int toDecimal(String bits) {
+        int result = 0;
+        for (int i = 0; i < bits.length(); i++) {
+            double a = toDigit(bits.charAt(i));
+            int n = bits.length()-i-1;
+            result += n == 0 ? a : Math.pow(2 * a, n);
+        }
+        return result;
+    }
+
+    class BitInfo {
+        int gamma;
+        String gammaBits;
+        int epsilon;
+        String epsilonBits;
+
+        void compute(List<String> input) {
+
+            if (input.isEmpty()) {
+                gammaBits = "";
+                epsilonBits = "";
+                gamma = 0;
+                epsilon = 0;
+                return;
+            }
+
+            int positionSize = input.get(0).length();
+            int[][] data = new int[positionSize][2];
+
+            for (int i = 0; i < positionSize; i++) {
+                data[i] = new int[]{0, 0};
+            }
+
+            input.forEach(line -> {
+                for (int i = 0; i < positionSize; i++) {
+                    data[i][toDigit(line.charAt(i))]++;
+                }
+            });
+
+            StringBuilder gammaBitsSb = new StringBuilder();
+            StringBuilder epsilonBitsSb = new StringBuilder();
+
+            for (int i = 0; i < positionSize; i++) {
+                gammaBitsSb.append(data[i][1] >= data[i][0] ? 1 : 0);
+                epsilonBitsSb.append(data[i][1] >= data[i][0] ? 0 : 1);
+            }
+
+            gammaBits = gammaBitsSb.toString();
+            gamma = toDecimal(gammaBits);
+            epsilonBits = epsilonBitsSb.toString();
+            epsilon = toDecimal(epsilonBits);
+        }
+    }
+
+    private int toDigit(char c) {
+        return c == '1' ? 1 : 0;
+    }
+}

src/main/java/com/codingnagger/days/Day4.java

@@ -0,0 +1,15 @@
+package com.codingnagger.days;
+
+import java.util.List;
+
+public class Day4 implements Day {
+    @Override
+    public String partOne(List<String> input) {
+        return null;
+    }
+
+    @Override
+    public String partTwo(List<String> input) {
+        return null;
+    }
+}