From f755b9a48178563ec24dcf76f6101ddcc67e91b0 Mon Sep 17 00:00:00 2001
From: Rui Ribeiro <42305006+ruiribeiro04@users.noreply.github.com>
Date: Mon, 22 Sep 2025 16:55:06 +0100
Subject: [PATCH] added permutations

---
 permutations/Makefile       |  25 +++++
 permutations/README.md      | 200 ++++++++++++++++++++++++++++++++++++
 permutations/permutations.c |  98 ++++++++++++++++++
 permutations/permutations.h |  14 +++
 permutations/subject.txt    |  27 +++++
 5 files changed, 364 insertions(+)
 create mode 100644 permutations/Makefile
 create mode 100644 permutations/README.md
 create mode 100644 permutations/permutations.c
 create mode 100644 permutations/permutations.h
 create mode 100644 permutations/subject.txt

diff --git a/permutations/Makefile b/permutations/Makefile
new file mode 100644
index 0000000..cc23f7c
--- /dev/null
+++ b/permutations/Makefile
@@ -0,0 +1,25 @@
+NAME = permutations
+
+CC = gcc
+CFLAGS = -Wall -Wextra -Werror
+
+SRCS = permutations.c
+OBJS = $(SRCS:.c=.o)
+
+all: $(NAME)
+
+$(NAME): $(OBJS)
+	$(CC) $(CFLAGS) -o $(NAME) $(OBJS)
+
+%.o: %.c permutations.h
+	$(CC) $(CFLAGS) -c $< -o $@
+
+clean:
+	rm -f $(OBJS)
+
+fclean: clean
+	rm -f $(NAME)
+
+re: fclean all
+
+.PHONY: all clean fclean re
\ No newline at end of file
diff --git a/permutations/README.md b/permutations/README.md
new file mode 100644
index 0000000..4621e13
--- /dev/null
+++ b/permutations/README.md
@@ -0,0 +1,200 @@
+# Permutations Exercise - Simple Explanation 🔄
+
+## What is a Permutation? 🤔
+
+Imagine you have some letters, like the letters in your name. A **permutation** is just a fancy word for "all the different ways you can arrange those letters."
+
+Think of it like this:
+- If you have the letters **A** and **B**, you can arrange them as **AB** or **BA**
+- If you have **A**, **B**, and **C**, you can make **ABC**, **ACB**, **BAC**, **BCA**, **CAB**, **CBA**
+
+It's like having alphabet blocks and seeing how many different words you can spell by changing the order!
+
+## The Challenge 🎯
+
+Your mission is to write a program that:
+1. Takes a word (like "abc") 
+2. Shows ALL possible ways to rearrange the letters
+3. Shows them in **alphabetical order** (like in a dictionary)
+
+## Real Examples 📝
+
+Let's see what our program does:
+
+### Example 1: Single Letter
+```bash
+./permutations a
+```
+**Output:** `a`
+
+*Why?* There's only one letter, so there's only one way to arrange it!
+
+### Example 2: Two Letters
+```bash
+./permutations ab
+```
+**Output:**
+```
+ab
+ba
+```
+
+*Why?* We can put 'a' first or 'b' first. That's it!
+
+### Example 3: Three Letters
+```bash
+./permutations abc
+```
+**Output:**
+```
+abc
+acb
+bac
+bca
+cab
+cba
+```
+
+*Why?* Think of it step by step:
+- Start with 'a': we can make **abc** and **acb**
+- Start with 'b': we can make **bac** and **bca**  
+- Start with 'c': we can make **cab** and **cba**
+
+## How Our Code Works 🛠️
+
+### Step 1: Getting Ready
+```c
+// We take the word you give us
+char *str = argv[1];  // This is your word like "abc"
+
+// We make a copy so we don't mess up the original
+copy = malloc(sizeof(char) * (len + 1));
+```
+
+### Step 2: Sorting First (The Secret Sauce! ✨)
+```c
+ft_sort_string(copy);  // This puts letters in order: "cba" becomes "abc"
+```
+
+**Why do we sort first?** 
+- If someone gives us "cba", we sort it to "abc" first
+- This way, we always start with letters in alphabetical order
+- Then when we make all arrangements, they come out in the right order!
+
+### Step 3: The Magic Swapping Function
+```c
+void ft_swap(char *a, char *b)
+{
+    char temp = *a;  // Remember the first letter
+    *a = *b;         // Put the second letter in first position
+    *b = temp;       // Put the first letter in second position
+}
+```
+
+**What's swapping?** It's like switching two cards in your hand:
+- You have cards **A** and **B**
+- After swapping, you have **B** and **A**
+
+### Step 4: The Next Permutation Magic 🪄
+```c
+int next_permutation(char *str, int len)
+{
+    int i = len - 2;
+    while (i >= 0 && str[i] >= str[i + 1])  // Find rightmost character smaller than next
+        i--;
+    if (i < 0)
+        return (0);  // No more permutations
+    
+    int j = len - 1;
+    while (str[j] <= str[i])  // Find rightmost character greater than str[i]
+        j--;
+    ft_swap(&str[i], &str[j]);  // Swap them
+    ft_reverse(str, i + 1, len - 1);  // Reverse the suffix
+    return (1);  // Successfully generated next permutation
+}
+```
+
+**How does this work?** Think of it like counting in a special way:
+1. **Find** the rightmost place where we can "increment" (make it bigger)
+2. **Swap** with the next bigger character available
+3. **Reverse** everything after that position to get the smallest arrangement
+4. **Repeat** until no more permutations exist
+
+It's like a smart odometer that counts through all possible letter arrangements in perfect alphabetical order!
+
+### Step 5: The Main Loop 🔄
+```c
+ft_sort_string(copy);           // Start with alphabetically first arrangement
+puts(copy);                     // Print the first permutation
+while (next_permutation(copy, len))  // While there are more permutations
+    puts(copy);                 // Print each one
+```
+
+**How does this work?**
+1. **Start** with the smallest (first) arrangement: "abc"
+2. **Print** it
+3. **Generate** the next alphabetical arrangement using `next_permutation`
+4. **Print** it
+5. **Repeat** until `next_permutation` says "no more!"
+
+It's like flipping through a dictionary - each page (permutation) comes in perfect alphabetical order!
+
+## Why This Approach is Cool 😎
+
+### The Step-by-Step Process 📋
+When we have "abc", our program works like this:
+
+```
+1. Start: abc (sorted, print it!)
+2. Next:  acb (swap 'b' and 'c', print it!)
+3. Next:  bac (find next in alphabetical order, print it!)
+4. Next:  bca (continue the pattern, print it!)
+5. Next:  cab (keep going, print it!)
+6. Next:  cba (last one, print it!)
+7. Done:  No more permutations possible
+```
+
+**The Magic:** Each step finds the next arrangement that would come after the current one in a dictionary! It's like having a super-smart assistant who knows exactly what comes next alphabetically.
+
+### Memory Management 🧠
+```c
+copy = malloc(sizeof(char) * (len + 1));  // Ask for memory
+// ... do our work ...
+free(copy);  // Give memory back when done
+```
+
+We're polite programmers - we clean up after ourselves!
+
+### Error Handling 🛡️
+```c
+if (argc != 2)      // Did you give us exactly one word?
+    return (1);     // If not, we quit
+
+if (!copy)          // Did we get the memory we asked for?
+    return (1);     // If not, we quit safely
+```
+
+We check if things went wrong and handle it gracefully.
+
+## Fun Facts! 🎉
+
+- With 1 letter: **1** permutation
+- With 2 letters: **2** permutations  
+- With 3 letters: **6** permutations
+- With 4 letters: **24** permutations
+- With 5 letters: **120** permutations
+
+**Pattern?** For n letters, you get n × (n-1) × (n-2) × ... × 1 permutations!
+
+## Try It Yourself! 🚀
+
+1. Compile the program: `gcc -Wall -Wextra -Werror permutations.c -o permutations`
+2. Try it with your name: `./permutations john`
+3. Try it with short words: `./permutations cat`
+4. See how the results are always in alphabetical order!
+
+Remember: The computer is doing exactly what we told it to do, step by step, just like following a recipe! 👨‍🍳
+
+---
+
+*Made with ❤️ for curious minds who want to understand how permutations work!*
\ No newline at end of file
diff --git a/permutations/permutations.c b/permutations/permutations.c
new file mode 100644
index 0000000..7903081
--- /dev/null
+++ b/permutations/permutations.c
@@ -0,0 +1,98 @@
+#include "permutations.h"
+
+int	ft_strlen(char *str)
+{
+	int	len;
+
+	len = 0;
+	while (str[len])
+		len++;
+	return (len);
+}
+
+void	ft_swap(char *a, char *b)
+{
+	char	temp;
+
+	temp = *a;
+	*a = *b;
+	*b = temp;
+}
+
+void	ft_sort_string(char *str)
+{
+	int	i;
+	int	j;
+	int	len;
+
+	len = ft_strlen(str);
+	i = 0;
+	while (i < len - 1)
+	{
+		j = i + 1;
+		while (j < len)
+		{
+			if (str[i] > str[j])
+				ft_swap(&str[i], &str[j]);
+			j++;
+		}
+		i++;
+	}
+}
+
+void	ft_reverse(char *str, int start, int end)
+{
+	while (start < end)
+	{
+		ft_swap(&str[start], &str[end]);
+		start++;
+		end--;
+	}
+}
+
+int	next_permutation(char *str, int len)
+{
+	int	i;
+	int	j;
+
+	i = len - 2;
+	while (i >= 0 && str[i] >= str[i + 1])
+		i--;
+	if (i < 0)
+		return (0);
+	j = len - 1;
+	while (str[j] <= str[i])
+		j--;
+	ft_swap(&str[i], &str[j]);
+	ft_reverse(str, i + 1, len - 1);
+	return (1);
+}
+
+int	main(int argc, char **argv)
+{
+	char	*str;
+	char	*copy;
+	int		len;
+	int		i;
+
+	if (argc != 2)
+		return (1);
+	str = argv[1];
+	len = ft_strlen(str);
+	copy = malloc(sizeof(char) * (len + 1));
+	if (!copy)
+		return (1);
+	i = 0;
+	while (i < len)
+	{
+		copy[i] = str[i];
+		i++;
+	}
+	copy[len] = '\0';
+	ft_sort_string(copy);
+	puts(copy);
+	while (next_permutation(copy, len))
+		puts(copy);
+	free(copy);
+	return (0);
+}
\ No newline at end of file
diff --git a/permutations/permutations.h b/permutations/permutations.h
new file mode 100644
index 0000000..1ee8c38
--- /dev/null
+++ b/permutations/permutations.h
@@ -0,0 +1,14 @@
+#ifndef PERMUTATIONS_H
+# define PERMUTATIONS_H
+
+# include <stdlib.h>
+# include <unistd.h>
+# include <stdio.h>
+
+void	ft_swap(char *a, char *b);
+void	ft_sort_string(char *str);
+void	ft_reverse(char *str, int start, int end);
+int		next_permutation(char *str, int len);
+int		ft_strlen(char *str);
+
+#endif
\ No newline at end of file
diff --git a/permutations/subject.txt b/permutations/subject.txt
new file mode 100644
index 0000000..52fece3
--- /dev/null
+++ b/permutations/subject.txt
@@ -0,0 +1,27 @@
+Assignment name  : permutations
+Expected files   : *.c *.h
+Allowed functions: puts, malloc, calloc, realloc, free, write
+---------------------------------------------------------------
+
+Write a program that will print all the permutations of a string given as argument.
+
+The solutions must be given in alphabetical order.
+
+We will not try your program with strings containing duplicates (eg: 'abccd').
+
+For example this should work:
+
+$> ./permutations a | cat -e
+a$
+
+$> ./permutations ab | cat -e
+ab$
+ba$
+
+$> ./permutations abc | cat -e
+abc$
+acb$
+bac$
+bca$
+cab$
+cba$
\ No newline at end of file