From 1bc6e9dd7fa439b7126770d58ac8662aaa0e4441 Mon Sep 17 00:00:00 2001
From: Nat Lasseter <user@4574.co.uk>
Date: Tue, 10 Dec 2019 11:05:55 +0000
Subject: Day 10

---
 day10/part2 | 101 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 101 insertions(+)
 create mode 100755 day10/part2

(limited to 'day10/part2')

diff --git a/day10/part2 b/day10/part2
new file mode 100755
index 0000000..f931a36
--- /dev/null
+++ b/day10/part2
@@ -0,0 +1,101 @@
+#!/usr/bin/env ruby
+
+input = $stdin.readlines.map(&:strip).map(&:chars)
+              .map{ |l| l.map{ |c| c == "#" } }
+
+H = input.length
+W = input[0].length
+
+def count(grid, x, y, angles)
+  c = 0
+  angles.each do |dx, dy|
+    tx = x
+    ty = y
+    loop do
+      tx += dx
+      ty += dy
+      break if tx < 0 || tx >= W
+      break if ty < 0 || ty >= H
+      if grid[ty][tx] then
+        c += 1
+        break
+      end
+    end
+  end
+  c
+end
+
+def rotate(n, arr)
+  arr.map do |x, y|
+    case n
+    when 1
+      [-y, x]
+    when 2
+      [-x, -y]
+    when 3
+      [y, -x]
+    end
+  end
+end
+
+def coords_of(n, grid, x, y, angles)
+  c = 0
+  loop do
+    tx = x
+    ty = y
+    angles.each do |dx, dy|
+      s = 1
+      loop do
+        tx = x + s*dx
+        ty = y + s*dy
+        break if tx < 0 || tx >= W
+        break if ty < 0 || ty >= H
+        if grid[ty][tx] then
+          c += 1
+          return [tx, ty] if c == 200
+          break
+        else
+          s += 1
+        end
+      end
+    end
+  end
+end
+
+angles = []
+(0...W).each do |x|
+  (0...H).each do |y|
+    angles << [x, -y] if x.gcd(y) == 1
+  end
+end
+
+angles.sort! do |a,b|
+  b[0].to_f/b[1] <=> a[0].to_f/a[1]
+end
+
+angles.shift
+
+angles = angles +
+         rotate(1, angles) +
+         rotate(2, angles) +
+         rotate(3, angles)
+
+maxcount = 0
+stationx = nil
+stationy = nil
+(0...H).each do |y|
+  (0...W).each do |x|
+    if input[y][x] then
+      c = count(input, x, y, angles)
+      if c > maxcount
+        maxcount = c
+        stationx = x
+        stationy = y
+      end
+    end
+  end
+end
+
+asteroid200 = coords_of(200, input, stationx, stationy, angles)
+
+puts (100 * asteroid200[0]) + asteroid200[1]
-- 
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