Added java solution, updated readme.

README.md

@@ -2,7 +2,8 @@
 
 This repository has my solutions for this year's Advent of Code challenges. Each day's challenge will be placed into a directory named `day_N`, where `N` is the day of the challenge. Each day will have a solution program named `solution.*` (with the filetype depending on which programming language I used).
 
-- For D programs, you can simply run them with `rdmd solution.d`.
+- For D programs, you can run them with `rdmd solution.d`.
 - For Lua programs, you can run them with `lua solution.lua`.
+- For Java programs, you can run them with `java solution.java`.
 
 Note that I try and solve the problems as fast as possible, but then I go back and clean up my solution, so what you see isn't really my ugly first attempt.

day_1/solution.java

@@ -0,0 +1,64 @@
+import java.nio.file.Files;
+import java.nio.file.Path;
+
+class solution {
+    private interface DigitFinder {
+        int getDigit(String s, int idx);
+    }
+
+    private static int parseDigits(DigitFinder df, String s) {
+        int a = -1, b = -1;
+        for (int i = 0; i < s.length(); i++) {
+            int an = df.getDigit(s, i);
+            int bn = df.getDigit(s, s.length() - i - 1);
+            if (a == -1 && an != -1) a = an;
+            if (b == -1 && bn != -1) b = bn;
+            if (a != -1 && b != -1) break;
+        }
+        return a * 10 + b;
+    }
+
+    private static class Part1DigitFinder implements DigitFinder {
+        public int getDigit(String s, int idx) {
+            if (Character.isDigit(s.charAt(idx))) {
+                return s.charAt(idx) - '0';
+            }
+            return -1;
+        }
+    }
+
+    private static void part1() throws Exception {
+        var df = new Part1DigitFinder();
+        int sum = Files.readAllLines(Path.of("part_1_input.txt")).stream()
+            .mapToInt(line -> parseDigits(df, line))
+            .sum();
+        System.out.println(sum);
+    }
+
+    private static class Part2DigitFinder implements DigitFinder {
+        public int getDigit(String s, int idx) {
+            if (Character.isDigit(s.charAt(idx))) return s.charAt(idx) - '0';
+            String[] words = {"one", "two", "three", "four", "five", "six", "seven", "eight", "nine"};
+            for (int i = 0; i < words.length; i++) {
+                String word = words[i];
+                if (s.length() - idx >= word.length() && s.substring(idx, idx + word.length()).equals(word)) {
+                    return i + 1;
+                }
+            }
+            return -1;
+        }
+    }
+
+    private static void part2() throws Exception {
+        var df = new Part2DigitFinder();
+        int sum = Files.readAllLines(Path.of("part_2_input.txt")).stream()
+            .mapToInt(line -> parseDigits(df, line))
+            .sum();
+        System.out.println(sum);
+    }
+
+    public static void main(String[] args) throws Exception {
+        part1();
+        part2();
+    }
+}