From 1fbb07c54523c7a576bfff1cb689e155dd55f15a Mon Sep 17 00:00:00 2001
From: Prefetch
Date: Sat, 2 Mar 2024 19:36:12 +0100
Subject: Add first five days

---
 03/lib.scm | 99 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 99 insertions(+)
 create mode 100644 03/lib.scm

(limited to '03/lib.scm')

diff --git a/03/lib.scm b/03/lib.scm
new file mode 100644
index 0000000..6e1e4d1
--- /dev/null
+++ b/03/lib.scm
@@ -0,0 +1,99 @@
+(library (lib)
+  (export solve-part1 solve-part2)
+  (import (chezscheme))
+
+  ; Is this direction up or down?
+  (define (vertical? dir)
+    (= 0 (vector-ref dir 0)))
+
+  ; Rotate stepping direction counterclockwise 90 degrees
+  (define (rotate-ccw dir)
+    (cond
+      ((equal? dir '#(1 0))
+       '#(0 1))
+      ((equal? dir '#(0 1))
+       '#(-1 0))
+      ((equal? dir '#(-1 0))
+       '#(0 -1))
+      ((equal? dir '#(0 -1))
+       '#(1 0))))
+
+  ; Follow the spiral to find (x,y) `pos' of the `address'th square.
+  ; The idea is to take `steps' steps in the current direction `dir',
+  ; rotate `dir', take `steps' steps again, rotate, increment `steps',
+  ; then start over, and so on. So the sequence starts like this:
+  ;   1 step right, 1 step up,
+  ;   2 steps left, 2 steps down,
+  ;   3 steps right, 3 steps up,
+  ;   4 steps left, 4 steps down,
+  ;   etc.
+  ; Note: for each value of `steps', the last stage is always up/down.
+  (define (get-position address)
+    (let loop ((pos '#(0 0))
+               (dir '#(1 0))
+               (steps 1)
+               (count 0)
+               (total 1))
+      (if (= total address)
+          pos
+          (if (= count (- steps 1))
+              ; Yes, this is the last step before we need to turn
+              (loop
+                ; Take the step, i.e. add direction to position
+                (vector-map + pos dir)
+                ; Rotate counterclockwise after this step
+                (rotate-ccw dir)
+                ; If this step is vertical, we need to increment
+                (+ steps (if (vertical? dir) 1 0))
+                ; Reset step counter for the current direction
+                0
+                ; Keep track of total steps since square one
+                (+ total 1))
+              ; No, this is an ordinary step, no rotation needed
+              (loop
+                (vector-map + pos dir)
+                dir
+                steps
+                (+ count 1)
+                (+ total 1))))))
+
+  ; Given a position, return all eight positions adjacent to it
+  (define (get-adjacent pos)
+    (map
+      (lambda (dir) (vector-map + pos dir))
+      '(#(1 0) #(1 1) #(0 1) #(-1 1) #(-1 0) #(-1 -1) #(0 -1) #(1 -1))))
+
+  ; Given a position, sum the values of all initialized adjacent squares
+  (define (sum-adjacent pos memory)
+    (fold-left
+      (lambda (sum adj)
+        ; Try to retrieve `pair' from memory
+        (let ((pair (assoc adj memory)))
+          (+ sum
+             ; If position `(car pair)' has been written,
+             ; add its value `(cdr pair) to the sum total.
+             (if pair (cdr pair) 0))))
+      0
+      (get-adjacent pos)))
+
+  (define (solve-part1 target)
+    (let* ((xy (get-position target))
+           (x (vector-ref xy 0))
+           (y (vector-ref xy 1)))
+      (+ (abs x) (abs y))))
+
+  (define (solve-part2 target)
+    (let loop ((address 2)
+               (memory '((#(0 0) . 1))))
+      (let ((value (cdar memory)))
+        ; Has the most recently written `value' crossed the threshold?
+        (if (> value target)
+            value
+            ; If not, move on, writing a new `val' to the next `pos'
+            (let* ((pos (get-position address))
+                   (val (sum-adjacent pos memory)))
+              (loop
+                (+ address 1)
+                (cons (cons pos val) memory)))))))
+
+)
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