From 6f4b02af33de3652aa353fedb9d9c3fe56f23d0d Mon Sep 17 00:00:00 2001
From: Bert Peters <bert@bertptrs.nl>
Date: Wed, 3 Dec 2025 09:54:40 +0100
Subject: [PATCH] Implement 2025 day 3 in bash

---
 2025/day02/README.md  |  2 +-
 2025/day03/README.md  |  4 +++
 2025/day03/sample.txt |  4 +++
 2025/day03/solve.sh   | 64 +++++++++++++++++++++++++++++++++++++++++++
 4 files changed, 73 insertions(+), 1 deletion(-)
 create mode 100644 2025/day03/README.md
 create mode 100644 2025/day03/sample.txt
 create mode 100755 2025/day03/solve.sh

diff --git a/2025/day02/README.md b/2025/day02/README.md
index 5c438d6..831fb84 100644
--- a/2025/day02/README.md
+++ b/2025/day02/README.md
@@ -1,4 +1,4 @@
-## Day 02: Terraform
+# Day 02: Terraform
 
 The code assumes an input file at `../inputs/02.txt`. Other than that, simply try to run as follows:
 
diff --git a/2025/day03/README.md b/2025/day03/README.md
new file mode 100644
index 0000000..8ec7d41
--- /dev/null
+++ b/2025/day03/README.md
@@ -0,0 +1,4 @@
+# Day 03: Bash
+
+Simple as can be, simply run `./solve.sh <input file>` and get both answers. While I allowed myself
+to use `coreutils` with bash, they turned out to be entirely unnecessary. Pure bash just works.
diff --git a/2025/day03/sample.txt b/2025/day03/sample.txt
new file mode 100644
index 0000000..7255fca
--- /dev/null
+++ b/2025/day03/sample.txt
@@ -0,0 +1,4 @@
+987654321111111
+811111111111119
+234234234234278
+818181911112111
diff --git a/2025/day03/solve.sh b/2025/day03/solve.sh
new file mode 100755
index 0000000..36e42ba
--- /dev/null
+++ b/2025/day03/solve.sh
@@ -0,0 +1,64 @@
+#!/bin/bash
+
+if [[ $# -lt 1 ]]; then
+    echo "Usage: $0 <input file>" >&2
+    exit 1
+fi
+
+joltage() {
+    local first second c i
+
+    first=0
+    second=0
+
+    for ((i = 0; i < ${#1}; i++)); do
+        c="${1:$i:1}"
+
+        if [[ $second -gt $first ]]; then
+            first="$second"
+            second="$c"
+        elif [[ $c -gt $second ]]; then
+            second="$c"
+        fi
+    done
+
+    echo "$first$second"
+}
+
+joltage2() {
+    local c i d e n digits
+
+    digits=(0 0 0 0 0 0 0 0 0 0 0 0)
+
+    for ((i = 0; i < ${#1}; i++)); do
+        c="${1:$i:1}"
+
+        for ((d = 0; d < 11; d++)); do
+            if [[ ${digits[((d + 1))]} -gt ${digits[$d]} ]]; then
+                for ((e = d; e < 11; e++)); do
+                    n=$((e + 1))
+                    digits[e]=${digits[n]}
+                done
+                digits[11]="0"
+                break
+            fi
+        done
+
+        if [[ $c -gt ${digits[11]} ]]; then
+            digits[11]="$c"
+        fi
+    done
+
+    printf "%s" "${digits[@]}"
+}
+
+total=0
+total2=0
+
+while IFS="" read -r line || [[ -n "$p" ]]; do
+    ((total += $(joltage "$line")))
+    ((total2 += $(joltage2 "$line")))
+done <"$1"
+
+echo "$total"
+echo "$total2"