initial commit

Makefile

@@ -0,0 +1,10 @@
+ASMFLAGS = 
+CFLAGS = -I lib -O3
+
+build: src/strutils.asm src/main.c
+	nasm $(ASMFLAGS) -f elf64 -o out/strutils.o src/strutils.asm
+	#gcc $(CFLAGS) -o out/main.o -c src/main.c
+	#ld -o bin/main out/main.o out/strutils.o
+	gcc $(CFLAGS) -o bin/main \
+		src/main.c \
+		out/strutils.o

lib/strutils.h

@@ -0,0 +1 @@
+void count_vowels(char *str, int *result);

src/main.c

@@ -0,0 +1,16 @@
+#include <stdio.h>
+#include "strutils.h"
+
+int main(void)
+{
+    char source_string[1024];
+    int amount_of_vowels;
+
+    fgets(source_string, 1023, stdin);
+
+    count_vowels(source_string, &amount_of_vowels);
+
+    printf("%d\n", amount_of_vowels);
+    
+    return 0;
+}

src/strutils.asm

@@ -0,0 +1,81 @@
+global count_vowels
+
+section .note.GNU-stack
+
+section .text
+
+vowel_found:
+    inc r12
+    jmp next_char
+
+; void count_vowels(char *str, int *result);
+;                          |          |
+;                          v          v
+;                        %rdi       %rsi
+count_vowels:
+    ; back up the pointers
+    push rsi
+    push rdi
+
+    ; find string length
+    mov rcx, 1024
+    mov  al, 0
+    ; rdi is set by caller
+
+    repne scasb
+
+    ; not wasting time on JNE
+    ; as ZF should always equal to 1
+
+    mov rdi, rcx
+    mov rcx, 1024
+    sub rcx, rdi ; strlen -> rcx
+    
+    ; recover *str
+    pop rdi
+
+    ; loop through the letters
+    xor r12, r12 ; reset counter
+
+    loop1:
+    mov  bl, [rdi+rcx-1]
+    cmp bl, 0x3F
+    jle next_char ; if the symbol code <= 0x3F,
+                  ; it can never be a letter
+
+    and  bl, 0x5F ; 0x5F -> 01011111
+                  ;         ^ ^
+                  ;         | cut off 6th bit to cast
+                  ;         | all letters to uppercase
+                  ;         |
+                  ;         cut off any signed nonsense
+    
+    cmp bl, 'A'
+    je vowel_found
+
+    cmp bl, 'Y'
+    jg next_char
+    je vowel_found
+
+    cmp bl, 'E'
+    je vowel_found
+
+    cmp bl, 'I'
+    je vowel_found
+
+    cmp bl, 'O'
+    je vowel_found
+
+    cmp bl, 'U'
+    je vowel_found
+
+    next_char:
+    dec rcx
+    jnz loop1
+
+    pop rsi
+
+    ; copy result to target memory location
+    mov [rsi], r12d
+
+    ret