From 9878e51904527ab9b0b9e88b75c26b41eca3923d Mon Sep 17 00:00:00 2001
From: Stefan Harmuth <stefan.harmuth@docomodigital.com>
Date: Mon, 25 Oct 2021 04:00:33 +0200
Subject: [PATCH] day19; p2 being horribly slow

---
 day19/day.go   | 56 ++++++++++++++++++++++++++++++++++++++++++++++++++
 inputs/19      |  1 +
 inputs/19_test |  1 +
 main.go        |  3 ++-
 4 files changed, 60 insertions(+), 1 deletion(-)
 create mode 100644 day19/day.go
 create mode 100644 inputs/19
 create mode 100644 inputs/19_test

diff --git a/day19/day.go b/day19/day.go
new file mode 100644
index 0000000..12da140
--- /dev/null
+++ b/day19/day.go
@@ -0,0 +1,56 @@
+package day19
+
+import (
+	"math"
+	"tools"
+)
+
+func getNextIndex(m map[int]int, start, max int) int {
+	index := start
+	for {
+		index++
+		if index > max {
+			index = 0
+		}
+		if _, ok := m[index]; ok {
+			return index
+		}
+	}
+}
+
+func Part1(puzzle tools.AoCPuzzle) interface{} {
+	elfCount := puzzle.GetInputInt()
+	elfs := make(map[int]int)
+	for i := 0; i < elfCount; i++ {
+		elfs[i] = 1
+	}
+
+	index := 0
+	for len(elfs) > 1 {
+		nextIndex := getNextIndex(elfs, index, elfCount)
+		delete(elfs, nextIndex)
+		index = getNextIndex(elfs, nextIndex, elfCount)
+	}
+
+	return index + 1
+}
+
+func Part2(puzzle tools.AoCPuzzle) interface{} {
+	elfCount := puzzle.GetInputInt()
+	var elfs []int
+	for i := 0; i < elfCount; i++ {
+		elfs = append(elfs, i+1)
+	}
+
+	// Does one half not always eliminate the other half +/- 1? Not Quite!!!
+	// Does it eliminate half of the "upper" half? As in "every second one"? Also not Quite!
+	// 0..4; 0 eliminates 2 (index + floor(half), 1 eliminates 4 (1 + half(len)), 3 eliminates 1 (index + floor(half))
+	// (1:?)2:1:2:1:...until len(2) (start with 1 for equal lens and 2 for unequal lens)
+	for len(elfs) > 1 {
+		half := int(math.Floor(float64(len(elfs) / 2)))
+		elfs = append(elfs, elfs[0])
+		elfs = append(elfs[1:half], elfs[half+1:]...)
+	}
+
+	return elfs[0]
+}
diff --git a/inputs/19 b/inputs/19
new file mode 100644
index 0000000..bdf0aa3
--- /dev/null
+++ b/inputs/19
@@ -0,0 +1 @@
+3004953
\ No newline at end of file
diff --git a/inputs/19_test b/inputs/19_test
new file mode 100644
index 0000000..7813681
--- /dev/null
+++ b/inputs/19_test
@@ -0,0 +1 @@
+5
\ No newline at end of file
diff --git a/main.go b/main.go
index f804b3f..e5e2cca 100644
--- a/main.go
+++ b/main.go
@@ -18,6 +18,7 @@ import (
 	"aoc2016/day16"
 	"aoc2016/day17"
 	"aoc2016/day18"
+	"aoc2016/day19"
 	"flag"
 	"fmt"
 	"os"
@@ -53,7 +54,7 @@ func initDayFunctions() {
 		16: {1: day16.Part1, 2: day16.Part2},
 		17: {1: day17.Part1, 2: day17.Part2},
 		18: {1: day18.Part1, 2: day18.Part2},
-		//		19: {1: day19.Part1, 2: day19.Part2},
+		19: {1: day19.Part1, 2: day19.Part2},
 		//		20: {1: day20.Part1, 2: day20.Part2},
 		//		21: {1: day21.Part1, 2: day21.Part2},
 		//		22: {1: day22.Part1, 2: day22.Part2},