From 8b102c2c61dc611b76d0376a1666227d56cc550c Mon Sep 17 00:00:00 2001
From: Mindaugas Bernotas <mindaugas@bernotas.lt>
Date: Sun, 7 Dec 2025 14:18:52 +0200
Subject: [PATCH] Solve day 7

---
 day7/example.txt |  16 ++++++
 day7/main.go     | 145 +++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 161 insertions(+)
 create mode 100644 day7/example.txt
 create mode 100644 day7/main.go

diff --git a/day7/example.txt b/day7/example.txt
new file mode 100644
index 0000000..57a2466
--- /dev/null
+++ b/day7/example.txt
@@ -0,0 +1,16 @@
+.......S.......
+...............
+.......^.......
+...............
+......^.^......
+...............
+.....^.^.^.....
+...............
+....^.^...^....
+...............
+...^.^...^.^...
+...............
+..^...^.....^..
+...............
+.^.^.^.^.^...^.
+...............
diff --git a/day7/main.go b/day7/main.go
new file mode 100644
index 0000000..6204abf
--- /dev/null
+++ b/day7/main.go
@@ -0,0 +1,145 @@
+package main
+
+import (
+	"bufio"
+	"flag"
+	"fmt"
+	"io"
+	"log"
+	"os"
+	"slices"
+)
+
+func main() {
+	debug := flag.Bool("debug", false, "Debug.")
+	flag.Parse()
+
+	f := os.Stdin
+	if flag.NArg() > 0 && flag.Arg(0) != "-" {
+		var err error
+		f, err = os.Open(flag.Arg(0))
+		if err != nil {
+			log.Fatal(err)
+		}
+		defer f.Close()
+	}
+
+	diagram, err := readDiagram(f)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	splits := diagram.Propagate()
+	var timelines int
+	for _, row := range slices.Backward(diagram) {
+		for _, c := range row {
+			if c.Marker != BeamMarker {
+				continue
+			}
+			timelines += c.WaysToReach
+		}
+		break
+	}
+
+	if *debug {
+		for _, row := range diagram {
+			for _, c := range row {
+				if c.Marker == BeamMarker {
+					if c.WaysToReach < 10 {
+						fmt.Print(c.WaysToReach)
+					} else {
+						b := 'A' + c.WaysToReach - 10
+						if b > 'Z' {
+							b = int(c.Marker)
+						}
+						fmt.Printf("%c", b)
+					}
+				} else {
+					fmt.Printf("%c", c.Marker)
+				}
+			}
+			fmt.Println()
+		}
+	}
+
+	fmt.Println("beam split", splits, "times into", timelines, "timelines")
+}
+
+type Marker byte
+
+const (
+	BeamEmitterMarker Marker = 'S'
+	BeamMarker        Marker = '|'
+	SplitterMarker    Marker = '^'
+	EmptyMarker       Marker = '.'
+)
+
+type Cell struct {
+	Marker      Marker
+	WaysToReach int
+}
+
+type diagram [][]Cell
+
+func (d diagram) Propagate() (splits int) {
+	splitoff := func(x, y, dx int) {
+		x1 := x + dx
+		if x1 < 0 || x1 >= len(d[y]) {
+			return // out of range
+		}
+		switch d[y][x1].Marker {
+		case EmptyMarker:
+			d[y][x1] = Cell{
+				Marker:      BeamMarker,
+				WaysToReach: d[y-1][x].WaysToReach,
+			}
+		case BeamMarker:
+			d[y][x1].WaysToReach += d[y-1][x].WaysToReach
+		}
+	}
+
+	propagateCell := func(x, y int) {
+		switch d[y-1][x].Marker {
+		case BeamEmitterMarker:
+			d[y-1][x].WaysToReach = 1
+		case BeamMarker:
+		default:
+			return
+		}
+		// we have a beam above
+
+		switch d[y][x].Marker {
+		case EmptyMarker:
+			d[y][x] = Cell{
+				Marker:      BeamMarker,
+				WaysToReach: d[y-1][x].WaysToReach,
+			}
+		case BeamMarker:
+			d[y][x].WaysToReach += d[y-1][x].WaysToReach
+		case SplitterMarker:
+			splitoff(x, y, -1)
+			splitoff(x, y, +1)
+			splits++
+		}
+	}
+
+	for y := 1; y < len(d); y++ {
+		for x := range d[y] {
+			propagateCell(x, y)
+		}
+	}
+	return splits
+}
+
+func readDiagram(r io.Reader) (diagram, error) {
+	var diagram diagram
+	sc := bufio.NewScanner(r)
+	for sc.Scan() {
+		cells := make([]Cell, 0, len(sc.Bytes()))
+		for _, b := range sc.Bytes() {
+			cells = append(cells, Cell{Marker: Marker(b)})
+		}
+		diagram = append(diagram, cells)
+	}
+	return diagram, sc.Err()
+}