AoC 2025 Day 4 - bash

Author: pareronia

src/main/bash/AoC2025_04.sh

@@ -0,0 +1,95 @@
+#!/bin/bash
+#
+# Advent of Code 2025 Day 4
+#
+
+year=2025
+day=04
+
+declare -a grid
+H=0
+W=0
+
+find_removable() {
+    local -a ans_removable=()
+    for ((r = 0; r < H; r++)); do
+        for ((c = 0; c < W; c++)); do
+            if [ "${grid[r]:c:1}" = '@' ]; then
+                local -i cnt=0
+                for dr in {-1,0,1}; do
+                    for dc in {-1,0,1}; do
+                        local -i rr=$((r + dr))
+                        local -i cc=$((c + dc))
+                        ((rr == r && cc == c)) && continue
+                        ((rr < 0 || rr >= H)) && continue
+                        ((cc < 0 || cc >= W)) && continue
+                        if [ "${grid[rr]:cc:1}" = '@' ]; then
+                            ((cnt++))
+                        fi
+                    done
+                done
+                if [ "$cnt" -lt 4 ]; then
+                    ans_removable+=("$r:$c")
+                fi
+            fi
+        done
+    done
+    for row in "${ans_removable[@]}"; do
+        echo "$row"
+    done
+}
+
+part1() {
+    mapfile -t grid < "$@"
+    H=${#grid[@]}
+    W=${#grid[0]}
+    mapfile -t removable < <(find_removable)
+    echo "${#removable[@]}"
+    return 0
+}
+
+part2() {
+    mapfile -t grid < "$@"
+    H=${#grid[@]}
+    W=${#grid[0]}
+    local -i ans=0
+    while true; do
+        mapfile -t removable < <(find_removable)
+        ((${#removable[@]} == 0)) && break
+        ((ans += ${#removable[@]}))
+        _debug "${#removable[@]}"
+        for rem in "${removable[@]}"; do
+            IFS=':' read -ra rc <<< "$rem"
+            r="${rc[0]}"
+            c="${rc[1]}"
+            row="${grid[r]:0:c}.${grid[r]:c+1}"
+            grid[r]="$row"
+        done
+        _debug "updated grid"
+    done
+    echo "$ans"
+    return 0
+}
+
+tests() {
+    # shellcheck disable=SC2034
+    {
+        sample=(
+            "..@@.@@@@."
+            "@@@.@.@.@@"
+            "@@@@@.@.@@"
+            "@.@@@@..@."
+            "@@.@@@@.@@"
+            ".@@@@@@@.@"
+            ".@.@.@.@@@"
+            "@.@@@.@@@@"
+            ".@@@@@@@@."
+            "@.@.@@@.@."
+        )
+    }
+
+    TEST part1 sample 13
+    TEST part2 sample 43
+}
+
+source "$(dirname "$0")/aoc_main.sh"