Day 1 part 2

01/poop.sh

@@ -9,17 +9,23 @@ main() {
   local dial=50
   local count_exact_zero=0
   local count_passed_zero=0
-  local prev dir num line
+  local prev dir num line crossings
 
   while IFS= read -r line; do
       prev=$dial
       dir="${line:0:1}"
       num="${line:1}"
 
-      # apply rotation
+      # apply rotation and count zero crossings before modulo wrap
       if [[ "$dir" == L ]]; then
+        if (( prev == 0 )); then
+          crossings=$(( num / 100 ))
+        else
+          crossings=$(( (num - prev + 100) / 100 ))
+        fi
         dial=$(( dial - num ))
       else
+        crossings=$(( (prev + num) / 100 ))
         dial=$(( dial + num ))
       fi
 
@@ -28,21 +34,11 @@ main() {
 
       # count zero hits (for part 1)
       if (( dial == 0 )); then
-        ((count_exact_zero+=1))
+        count_exact_zero=$(( count_exact_zero + 1 ))
       fi
 
-      # check if we passed 0 (for part 2)
-      if [[ "$dir" == L ]]; then
-          # moving left: crossed 0 if new dial < prev
-          if (( dial > prev )); then
-              ((count_passed_zero+=1))
-          fi
-      else
-      # moving right: crossed 0 if new dial < prev
-          if (( dial < prev )); then
-              ((count_passed_zero+=1))
-          fi
-      fi
+      # count how many times we passed 0 (for part 2)
+      count_passed_zero=$(( count_passed_zero + crossings ))
 
   #done < example.txt
   done < input.txt