From 0758ee4e559a83fea7e1fc6611a22a1568ca63d6 Mon Sep 17 00:00:00 2001
From: Alex Karle <alex@alexkarle.com>
Date: Tue, 13 Dec 2022 20:52:48 -0500
Subject: [PATCH] day12: Add part 2 solution

---
 2022/12/sol.scm | 28 ++++++++++++++++++++++++----
 1 file changed, 24 insertions(+), 4 deletions(-)

diff --git a/2022/12/sol.scm b/2022/12/sol.scm
index 99d258b..ae39d4a 100755
--- a/2022/12/sol.scm
+++ b/2022/12/sol.scm
@@ -37,7 +37,7 @@
 (define (ident x) x)
 
 
-(define (neigh grid pos)
+(define (neigh grid pos dir)
   (define (in-bounds? pt)
     (let ((n (vector-length grid))
           (m (vector-length (vector-ref grid 0))))
@@ -53,7 +53,9 @@
            (j2 (caddr pt))
            (l1 (get grid i j))
            (l2 (get grid i2 j2)))
-      (and (<= l2 (add1 l1)))))
+      (if (eq? dir 'up)
+          (and (<= l2 (add1 l1)))
+          (and (>= (add1 l2) l1)))))
   (let* ((d (car pos))
          (i (cadr pos))
          (j (caddr pos)))
@@ -75,7 +77,24 @@
             (if (equal? (cdr pos) goal)
                 (car pos)
                 (let ((neighs (filter (lambda (p) (not (hash-table-exists? seen (cdr p))))
-                                      (neigh grid pos))))
+                                      (neigh grid pos 'up))))
+                  (for-each (lambda (p) (hash-table-set! seen (cdr p) 1)) neighs)
+                  (loop (append (cdr Q) neighs)))))))))
+
+;; Part 2 is a simple flip of the search: don't search for one goal loc
+;; search starting at E and until depth 0 (with inverted 'in-reach?)
+(define (dist-down grid start)
+  ;; BFS to find the goal, each item is (D i j) where D is depth
+  (let ((seen (make-hash-table)))
+    (let loop ((Q (list (cons 0 start))))
+      (if (null? Q)
+          #f
+          (let ((pos (car Q)))
+            (hash-table-set! seen (cdr pos) 'seen)
+            (if (equal? (get grid (cadr pos) (caddr pos)) 0)
+                (car pos)
+                (let ((neighs (filter (lambda (p) (not (hash-table-exists? seen (cdr p))))
+                                      (neigh grid pos 'down))))
                   (for-each (lambda (p) (hash-table-set! seen (cdr p) 1)) neighs)
                   (loop (append (cdr Q) neighs)))))))))
 
@@ -86,7 +105,8 @@
          (lgrid (make-grid lines lvl))
          (start (find cgrid #\S))
          (goal (find cgrid #\E)))
-    (print (dist lgrid start goal))))
+    (print (dist lgrid start goal))
+    (print (dist-down lgrid goal))))
 
 ; for repl
 (define ln '("Sabqponm"
--
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