From a7a12c7c546205fd86786050370f05f16ccb559e Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Le=CC=81o?= <leogaudinoff@gmail.com>
Date: Tue, 23 Jul 2024 18:41:14 +0200
Subject: [PATCH] feat: implement ft_strcpy and its explanation

---
 Makefile    |  2 +-
 README.md   | 60 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 ft_strcpy.s | 15 ++++++++++++++
 libasm.h    |  4 +++-
 main.c      | 43 ++++++++++++++++++++++++++++++++------
 5 files changed, 116 insertions(+), 8 deletions(-)
 create mode 100644 ft_strcpy.s

diff --git a/Makefile b/Makefile
index 9bcd23e..f0254f8 100644
--- a/Makefile
+++ b/Makefile
@@ -5,7 +5,7 @@ NC = \033[0m
 NAME = libasm.a
 
 SOURCES = ft_strlen.s \
-		#   ft_strcpy.s \
+		  ft_strcpy.s \
 		#   ft_strcmp.s \
 		#   ft_write.s \
 		#   ft_read.s \
diff --git a/README.md b/README.md
index 9cd59a4..c149007 100644
--- a/README.md
+++ b/README.md
@@ -287,3 +287,63 @@ end:
 ```
 
 And that's it! We have implemented the `ft_strlen` function in assembly.
+
+## ft_strcpy
+
+The `ft_strcpy` function is a function that copies a string into another string, and returns a pointer to the destination string.
+
+The logic is very similar to the `ft_strlen` function:
+
+1. Set a counter to 0.
+2. Look at the first character of the source string.
+3. Copy it to the destination string.
+4. Increment the counter.
+5. Look at the next character and copy it.
+6. If it is not the null-terminator, increment the counter and go back to step 5.
+7. If it is the null-terminator, exit the loop and return a pointer to the destination string.
+
+### Implementation
+
+A pointer to our `dst` string is passed in `rdi`, and a pointer to our `src` string is passed in `rsi`.
+
+We can start the same way we did with `ft_strlen` by setting the counter to 0.
+
+```nasm
+ft_strcpy:
+	mov rcx, 0
+```
+
+We can then start our loop.
+
+We need to copy every character in `rsi` to `rdi`. However, **in assembly, we can't copy data directly from one address to another** (`mov [rdi], [rsi]` would not work).
+
+We therefore need to copy the data from the source address to a register, and then copy it to the destination address.
+
+> We could use any register to store the character (like `r8` as seen before), but it is more appropriate to use `al` for this purpose, as it is a register that is meant to store a single byte.
+
+```nasm
+loop:
+	mov al, [rsi + rcx]
+	mov [rdi + rcx], al
+	inc rcx
+	cmp al, 0
+	jne loop
+```
+
+In this loop:
+1. We copy the character in `rsi` to `al`.
+2. We copy `al` to `rdi`.
+3. We increment the counter.
+4. We check if the character is the null-terminator.
+5. If it is not, we go back to the beginning of the loop.
+
+Finally, we need to return the pointer to the destination string.
+
+Given that we received this pointer in `rdi` and that we did not move it, we can simply copy it to `rax` and return.
+
+```nasm
+mov rax, rdi
+ret
+```
+
+## ft_strcmp
diff --git a/ft_strcpy.s b/ft_strcpy.s
new file mode 100644
index 0000000..8dc36de
--- /dev/null
+++ b/ft_strcpy.s
@@ -0,0 +1,15 @@
+global _ft_strcpy
+
+_ft_strcpy:
+	xor	rcx, rcx
+
+while:
+	mov al, [rsi + rcx]
+	mov [rdi + rcx], al
+	inc rcx
+	cmp al, 0
+	jne while
+
+break:
+	mov rax, rdi
+	ret
diff --git a/libasm.h b/libasm.h
index 017036b..e846301 100644
--- a/libasm.h
+++ b/libasm.h
@@ -4,10 +4,12 @@
 #define NC "\033[0m"
 #define RED "\033[0;31m"
 #define GREEN "\033[0;32m"
+#define BOLD "\033[1m"
 
 #include <stdio.h>
 #include <string.h>
 
-int	ft_strlen(char *str);
+int		ft_strlen(char *str);
+char	*ft_strcpy(char *dst, char *src);
 
 #endif
diff --git a/main.c b/main.c
index 0169cf7..1620af6 100644
--- a/main.c
+++ b/main.c
@@ -2,7 +2,7 @@
 
 void		test_strlen(void)
 {
-	printf("\nTesting ft_strlen\n");
+	printf(BOLD "\nTesting ft_strlen\n" NC);
 
 	char	*strings[] = {
 		"test12345",
@@ -19,17 +19,48 @@ void		test_strlen(void)
 		int	expected = strlen(strings[i]);
 		int	result = ft_strlen(strings[i]);
 
-		if (expected == result) {
-			printf(GREEN "OK" NC);
-		} else {
-			printf(RED "KO" NC);
-		}
+		printf(expected == result ? GREEN "OK" NC : RED "KO" NC);
 		printf("\t Expected: %d\tResult: %d\n", expected, result);
 	}
 }
 
+void	test_strcpy(void)
+{
+	printf(BOLD "\nTesting ft_strcpy\n" NC);
+
+	char	*strings[] = {
+		"test12345",
+		"1",
+		"test",
+		"",
+		"zzzzzzzz",
+	};
+
+	int	numStrings = sizeof(strings) / sizeof(strings[0]);
+
+	for (int i = 0; i < numStrings; i++) {
+		// Test return value
+		char	dummy_dest[100];
+
+		void	*ref_ptr = strcpy(dummy_dest, strings[i]);
+		void	*test_ptr = ft_strcpy(dummy_dest, strings[i]);
+		printf(ref_ptr == test_ptr ? GREEN "POINTER OK" NC : RED "POINTER KO" NC);
+		printf("\tExpected: %p\tResult: %p\n", ref_ptr, test_ptr);
+
+		// Test destination string
+		char	ref_dest[100];
+		char	test_dest[100];
+
+		strcpy(ref_dest, strings[i]);
+		ft_strcpy(test_dest, strings[i]);
+		printf(strcmp(ref_dest, test_dest) == 0 ? GREEN "COPY OK" NC : RED "COPY KO" NC);
+		printf("\t\tExpected: %s\tResult: %s\n", ref_dest, test_dest);
+	}
+}
+
 int	main(void)
 {
 	test_strlen();
+	test_strcpy();
 	return (0);
 }