got it right the first time... except for overflow. Gotta remember Long exists.

src/Day07.java

@@ -0,0 +1,76 @@
+package src;
+
+import java.io.FileReader;
+import java.time.Duration;
+import java.time.Instant;
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Map;
+import java.util.Scanner;
+import java.util.Set;
+
+public class Day07 {
+    public static void main(String[] args) {
+        Instant start = Instant.now();
+        part1();
+        Instant middle = Instant.now();
+        part2();
+        Instant end = Instant.now();
+        IO.println("part 1: " + Duration.between(start, middle).toMillis() + " ms");
+        IO.println("part 2: " + Duration.between(middle, end).toMillis() + " ms");
+    }
+
+    private static void part1() {
+        try (Scanner scanner = new Scanner(new FileReader("resources/inputs/07/input"))) {
+            int i = 0;
+            int splits = 0;
+            Set<Integer> beams = new HashSet<>();
+            while (scanner.hasNextLine()) {
+                String row = scanner.nextLine();
+                if (i == 0) {
+                    beams.add(row.indexOf("S"));
+                } else {
+                    for (int j = 0; j < row.length(); j++) {
+                        if (row.charAt(j) == '^' && beams.contains(j)) {
+                            beams.add(j - 1);
+                            beams.add(j + 1);
+                            beams.remove(j);
+                            splits++;
+                        }
+                    }
+                }
+                i++;
+            }
+            IO.println(splits);
+        } catch (Exception e) {
+            IO.println(e.getMessage());
+        }
+    }
+
+    private static void part2() {
+        try (Scanner scanner = new Scanner(new FileReader("resources/inputs/07/input"))) {
+            int i = 0;
+            Map<Integer, Long> beams = new HashMap<>();
+            while (scanner.hasNextLine()) {
+                String row = scanner.nextLine();
+                if (i == 0) {
+                    beams.put(row.indexOf("S"), 1L);
+                } else {
+                    for (int j = 0; j < row.length(); j++) {
+                        if (row.charAt(j) == '^' && beams.containsKey(j)) {
+                            Long count = beams.remove(j);
+                            beams.merge(j - 1, count, (a, b) -> a + b);
+                            beams.merge(j + 1, count, (a, b) -> a + b);
+                        }
+                    }
+                }
+                i++;
+            }
+            IO.println(beams.values().stream().reduce(0L, Long::sum));
+        } catch (Exception e) {
+            IO.println(e.getMessage());
+        }
+    }
+}