From e25bdb3fd682fcdcaa776fbdb88a603325639675 Mon Sep 17 00:00:00 2001
From: crate <crateimage@gmail.com>
Date: Fri, 21 Mar 2025 00:29:27 +0000
Subject: [PATCH] Add kaprekar.sh

---
 kaprekar.sh | 54 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 54 insertions(+)
 create mode 100644 kaprekar.sh

diff --git a/kaprekar.sh b/kaprekar.sh
new file mode 100644
index 0000000..66db9d0
--- /dev/null
+++ b/kaprekar.sh
@@ -0,0 +1,54 @@
+#!/bin/bash
+
+# Kaprekar's Routine Implementation
+# For four-digit numbers, this routine always leads to 6174 (Kaprekar's constant)
+# Usage: ./kaprekar.sh <four-digit-number>
+
+if [ $# -ne 1 ]; then
+    echo "Usage: $0 <four-digit-number>"
+    exit 1
+fi
+
+if ! [[ $1 =~ ^[0-9]+$ ]]; then
+    echo "Error: Input must be a number"
+    exit 1
+fi
+
+num=$1
+
+num=$(printf "%04d" $num)
+
+if [[ $(echo $num | grep -o . | sort -u | wc -l) -eq 1 ]]; then
+    echo "Error: Number must have at least two different digits"
+    exit 1
+fi
+
+echo "Starting with number: $num"
+iteration=0
+KAPREKAR_CONSTANT=6174
+
+while [ "$num" != "$KAPREKAR_CONSTANT" ] && [ $iteration -lt 7 ]; do
+    iteration=$((iteration + 1))
+    
+    num=$(printf "%04d" $num)
+    
+    ascending=$(echo $num | grep -o . | sort | tr -d '\n')
+    descending=$(echo $num | grep -o . | sort -r | tr -d '\n')
+    
+    difference=$((10#$descending - 10#$ascending))
+    
+    num=$(printf "%04d" $difference)
+    
+    echo "Iteration $iteration: $descending - $ascending = $num"
+    
+    if [ "$num" -eq 0 ]; then
+        echo "Error: Reached 0. Make sure the input has at least two different digits."
+        exit 1
+    fi
+done
+
+if [ "$num" = "$KAPREKAR_CONSTANT" ]; then
+    echo "Reached Kaprekar's constant ($KAPREKAR_CONSTANT) in $iteration iterations!"
+else
+    echo "Failed to reach Kaprekar's constant in 7 iterations."
+fi
\ No newline at end of file