From 291d4512927bdcdf5ed37b7ca42b4f9552d11625 Mon Sep 17 00:00:00 2001
From: Matthew Butterick <mb@mbtype.com>
Date: Fri, 15 Dec 2017 02:23:02 -0500
Subject: [PATCH] d15

---
 2017/d15/main.rkt  | 28 ++++++++++++++++++++++++++++
 2017/d15/star1.rkt |  3 +++
 2017/d15/star2.rkt |  3 +++
 2017/d15/test1.rkt |  3 +++
 2017/d15/test2.rkt |  3 +++
 5 files changed, 40 insertions(+)
 create mode 100644 2017/d15/main.rkt
 create mode 100644 2017/d15/star1.rkt
 create mode 100644 2017/d15/star2.rkt
 create mode 100644 2017/d15/test1.rkt
 create mode 100644 2017/d15/test2.rkt

diff --git a/2017/d15/main.rkt b/2017/d15/main.rkt
new file mode 100644
index 0000000..ca1639b
--- /dev/null
+++ b/2017/d15/main.rkt
@@ -0,0 +1,28 @@
+#lang reader "../aoc-lang.rkt"
+(provide (rename-out [#%mb #%module-begin]) ★ ★★)
+
+(define-macro (#%mb (STARS) (Generator X starts with NUM) ...)
+  #`(#%module-begin
+     (apply STARS '(NUM ...))))
+
+(define (lower-16-bits x) (bitwise-bit-field x 0 16))
+
+(define (generator-base a b rounds [modulo-a 1] [modulo-b 1])
+  (for/fold ([a-last a]
+             [b-last b]
+             [sum 0]
+             #:result sum)
+            ([i (in-range rounds)])
+    (define a (for/fold ([a a-last])
+                        ([i (in-naturals)]
+                         #:break (and (positive? i) (zero? (modulo a modulo-a))))
+                (remainder (* 16807 a) 2147483647)))
+    (define b (for/fold ([b b-last])
+                        ([i (in-naturals)]
+                         #:break (and (positive? i) (zero? (modulo b modulo-b))))
+                (remainder (* 48271 b) 2147483647)))
+    (values a b (+ sum (if (= (lower-16-bits a) (lower-16-bits b)) 1 0)))))
+
+(define (★ a-start b-start) (generator-base a-start b-start 40000000))
+
+(define (★★ a-start b-start) (generator-base a-start b-start 5000000 4 8))
\ No newline at end of file
diff --git a/2017/d15/star1.rkt b/2017/d15/star1.rkt
new file mode 100644
index 0000000..bab0048
--- /dev/null
+++ b/2017/d15/star1.rkt
@@ -0,0 +1,3 @@
+#lang reader "main.rkt" ★ ; 650
+Generator A starts with 783
+Generator B starts with 325
\ No newline at end of file
diff --git a/2017/d15/star2.rkt b/2017/d15/star2.rkt
new file mode 100644
index 0000000..459e018
--- /dev/null
+++ b/2017/d15/star2.rkt
@@ -0,0 +1,3 @@
+#lang reader "main.rkt" ★★
+Generator A starts with 783
+Generator B starts with 325
\ No newline at end of file
diff --git a/2017/d15/test1.rkt b/2017/d15/test1.rkt
new file mode 100644
index 0000000..3ae6f92
--- /dev/null
+++ b/2017/d15/test1.rkt
@@ -0,0 +1,3 @@
+#lang reader "main.rkt" ★ ; 588
+Generator A starts with 65
+Generator B starts with 8921
\ No newline at end of file
diff --git a/2017/d15/test2.rkt b/2017/d15/test2.rkt
new file mode 100644
index 0000000..8e1b78a
--- /dev/null
+++ b/2017/d15/test2.rkt
@@ -0,0 +1,3 @@
+#lang reader "main.rkt" ★★ ; 309
+Generator A starts with 65
+Generator B starts with 8921
\ No newline at end of file