From a47ab3cdc6e215d19e8209658dabf0645d85754b Mon Sep 17 00:00:00 2001
From: Kwarde <kevin@kwarde.com>
Date: Sun, 6 Jul 2025 19:13:05 +0200
Subject: [PATCH] strcat: Better logic for determining max amount of characters
 to copy

---
 src/string.asm | 27 +++++++++++----------------
 1 file changed, 11 insertions(+), 16 deletions(-)

diff --git a/src/string.asm b/src/string.asm
index a91e1fc..d257102 100644
--- a/src/string.asm
+++ b/src/string.asm
@@ -60,30 +60,25 @@ strcpy:
 ;	rsi*	(arg) Pointer to src[]
 ;	rdx	(arg) Size of dest[]
 ;	rcx*	Counter for movsb
-;	r8*	Backup for rdi
+;	r8*	Backup for rdi (point to start of dest)
 ;	r9*	Backup for rsi
-;	r10*	Current length of dest[]
+;	r10*	Backup for rdi (point to end of dest)
 strcat:
 	mov	r8, rdi
 	mov	r9, rsi
+	call	strlen
+	sub	rdx, rax
+	dec	rdi
+	mov	r10, rdi
 	mov	rdi, rsi
 	call	strlen
-	mov	rcx, rax
-	mov	rdi, r8
-	call	strlen
-	mov	r10, rax
-	mov	rdi, r8
-	add	rcx, rax
-	cmp	rcx, rdx
-	cmova	rcx, rdx
-	cmp	rcx, rdx
+	cmp	rax, rdx
 	jb	.copy
-	dec	rcx
-	.copy:	
-	mov	rdi, r8
+	dec	rdx
+	.copy:
+	mov	rcx, rdx
 	mov	rsi, r9
-	add	rdi, r10
-	sub	rcx, r10
+	mov	rdi, r10
 	cld
 	rep	movsb
 	mov	byte [rdi], EOS