From 7bc5df124766662f356fc9d988fc7635d6329100 Mon Sep 17 00:00:00 2001
From: MsK` <msk@lywenn.eu.org>
Date: Sat, 4 Dec 2021 23:41:59 +0100
Subject: [PATCH] day 4

---
 day-4.input | 601 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 day-4.scm   | 150 +++++++++++++
 2 files changed, 751 insertions(+)
 create mode 100644 day-4.input
 create mode 100644 day-4.scm

diff --git a/day-4.input b/day-4.input
new file mode 100644
index 0000000..f372bfd
--- /dev/null
+++ b/day-4.input
@@ -0,0 +1,601 @@
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diff --git a/day-4.scm b/day-4.scm
new file mode 100644
index 0000000..ef8040c
--- /dev/null
+++ b/day-4.scm
@@ -0,0 +1,150 @@
+(import (srfi :43))
+
+; returns a list of numbers parsed from the first line of the file,
+; separated by commas
+(define (parse-draws file)
+  (let loop [(draws '()) (n 0)]
+    (let [(c (get-char file))]
+      (cond
+	[(char=? c #\,) (loop (cons n draws) 0)]
+	[(char-whitespace? c) (reverse (cons n draws))]
+	[else (loop draws (+ (* n 10) (string->number (string c))))]))))
+
+; returns numbers 0 to 9 from ascii character
+(define (char->number c)
+  (- (char->integer c) 48)) ; 48 is ASCII's number zero
+
+; returns a number from two characters
+; ex: '0' and '9' => 9
+;     ' ' and '6' => 6
+;     '1' and '2' => 12
+(define (char-pair->number c1 c2)
+  (if (char-numeric? c1)
+    [+ (* 10 (char->number c1)) (char->number c2)]
+    [char->number c2]))
+
+; returns a vector of 25 numbers
+; parsed from input file where numbers are separated by whitespace
+(define (read-board file)
+  (let parse-loop [(i 0) (numbers '())]
+    (if (= i 25)
+      [list->vector (reverse numbers)]
+      [let [(c2 (get-char file))
+	    (c1 (get-char file))]
+	(get-char file) ; consume space or empty line
+	(parse-loop
+	  (+ i 1)
+	  (cons (char-pair->number c1 c2) numbers))])))
+
+; returns a list of vectors of 25 numbers parsed from input file
+(define (read-boards file)
+  (reverse
+    (let loop [(boards '())]
+      (get-char file) ; consume empty line
+      (if (eof-object? (peek-char file))
+	boards
+	[loop (cons (read-board file) boards)]))))
+
+; returns a number where bit N is set if N'th number
+; in board matches given number
+; bits that are already set in mark are kept
+(define (mark-bingo number board mark)
+  (let loop [(i 0)]
+    (if (< i 25)
+      [if (= (vector-ref board i) number)
+	[bitwise-ior mark (bitwise-arithmetic-shift-left 1 i)]
+	[loop (+ i 1)]]
+      mark)))
+
+; marks the corresponding n'th bit in each board if 
+; an n'th element of the corresponding board matches input number
+(define (mark-bingos! number boards marks)
+  (let loop [(board (car boards))
+	     (boards (cdr boards))
+	     (index 0)]
+    (vector-set! marks index
+		 (mark-bingo number board (vector-ref marks index)))
+    (when (not (null? boards))
+      (loop (car boards) (cdr boards) (+ index 1)))))
+
+; all possible bitfields corresponding to a line or column of bingo
+(define bingos 
+  '#(; lines
+     #b1111100000000000000000000
+     #b0000011111000000000000000
+     #b0000000000111110000000000
+     #b0000000000000001111100000
+     #b0000000000000000000011111
+     ; columns
+     #b1000010000100001000010000
+     #b0100001000010000100001000
+     #b0010000100001000010000100
+     #b0001000010000100001000010
+     #b0000100001000010000100001))
+
+; check if any bitfield represented grid of marked bingo cell is bingo!
+(define (check-bingos marks)
+  (vector-index
+    (lambda (mark)
+      (vector-any
+	(lambda (mask)
+	  (= mask (bitwise-and mask mark)))
+	bingos))
+    marks))
+
+; computes the score of the winning board
+(define (score boards marks index last-draw)
+  (let [(board (list-ref boards index))
+	(mark (vector-ref marks index))]
+    (let loop [(score 0) (index 0)]
+      (if (< index 25)
+	[if (bitwise-bit-set? mark index)
+	  [loop score (+ index 1)]
+	  [loop (+ score (vector-ref board index)) (+ index 1)]]
+	[* last-draw score]))))
+
+; returns list with the index of the board that won and the score
+; #f if none won after all draws
+(define (bingo draws boards marks)
+  (let loop [(draw (car draws))
+	     (draws (cdr draws))]
+    (mark-bingos! draw boards marks)
+    (let [(check (check-bingos marks))]
+      (if check
+	[list check (score boards marks check draw)]
+	[if (null? draws)
+	  #f
+	  [loop (car draws) (cdr draws)]]))))
+
+; returns a list without its n-th element
+(define (except list n-th)
+  (let loop [(list list) (i 0)]
+    (if (= i n-th)
+      (cdr list)
+      (cons (car list) (loop (cdr list) (+ i 1))))))
+
+(define (last-winning draws boards)
+  (let loop [(boards boards)
+	     (prev-score 0)
+	     (marks (make-vector (length boards) 0))]
+    (if (null? boards)
+      prev-score
+      [let [(index+score (bingo draws boards marks))]
+	(if index+score
+	  [loop 
+	    (except boards (car index+score))
+	    (cadr index+score)
+	    (make-vector (length boards) 0)]
+	  [cadr index+score])])))
+
+(call-with-input-file
+  "day-4.input"
+  (lambda (file)
+    (let* [(draws (parse-draws file))
+	   (boards (read-boards file))]
+      (printf "part 1: ~% ")
+      (let* [(marks (make-vector (length boards) 0))
+	    (index+score (bingo draws boards marks))]
+	(printf "bingo! ~a~%" (cadr index+score)))
+      (printf "part 2: ~% bingo! ~a~%" 
+	      (last-winning draws boards)))))