feat: sort the entries before printing them

main.S

@@ -82,6 +82,12 @@ _start:
     @ r0 needs the fd, which it should already be since the call to open() set r0 to the fd.
     ldr r1, =buffer @ pointer to linux_dirent structure
     @ TODO: we can allocate the required memory on the stack instead of in the bss section I think.
+    /*
+       TODO: this only reads up to 16KB of directory entries. if a directory we pass has entries with a sum larger
+             than 16KB then we don't end up printing those.
+       Solution: a nested loop. one for reading until the syscall returns 0 to indicate end of directory, and one for
+                 looping the directories
+    */
     mov r2, #16384 @ how many bytes to read
     mov r7, #SYS_GETDENTS
     svc 0
@@ -93,19 +99,14 @@ _start:
 
     mov r4, #0 @ initialize r4 to 0
     ldr r5, =buffer
+
+    sub sp, sp, #4096 @ allocate 4KB of mem to store the directory entries.
+    mov r1, sp
+    mov r10, #0 @ dir entry counter
 1:
-    push {r0}
+    add r2, r5, #10 @ offset 10 bytes into the dirent struct to reach d_name.
-
+    str r2, [r1, r10, lsl #2] @ store the value in r2 in the memory address pointed to by r1[r10 << 2]. The shift is just (r10 * 4)
-    add r1, r5, #10 @ offset 10 bytes into the dirent struct to reach d_name.
+    add r10, r10, #1
-    bl strlen
-    mov r2, r0
-    bl print
-
-    ldr r1, =NEWLINE
-    mov r2, #1
-    bl print
-
-    pop {r0}
 
     @ load the value (as 16-bits zero-extended as opposed to loading 32-bits and then extending that ourselves with UXTH)
     @ in the memory address of r5 offset by 8 bytes to get us the d_reclen
@@ -117,6 +118,61 @@ _start:
     cmp r4, r0
     blt 1b
 
+/* bubble sort
+   TODO: improvements:
+   - a case-insensitive strcmp should be used when sorting
+   - dots at the beginning of a file name should be ignored when sorting/comparing to match ls' behavior
+*/
+    mov r5, r1 @ save the array pointer in r5 because we'll be using r1 a lot
+    mov r3, #0 @ outer loop counter
+outer:
+    mov r4, #0 @ inner loop counter
+    sub r8, r10, r3
+    sub r8, r8, #1
+    push {r3}
+
+inner:
+    ldr r1, [r5, r4, lsl #2]
+    add r6, r4, #1
+    ldr r2, [r5, r6, lsl #2]
+    push {r1-r2}
+    bl strcmp_ascii
+    pop {r1-r2}
+
+    cmp r0, #1
+    bne 2f
+    @ swap if a > b
+    mov r7, r1
+    str r2, [r5, r4, lsl #2]
+    str r7, [r5, r6, lsl #2]
+
+2:
+    add r4, r4, #1
+    cmp r4, r8
+    blt inner
+
+    pop {r3}
+    add r3, r3, #1
+    cmp r3, r10
+    blt outer
+
+print_entries:
+    ldr r1, [r5], #4
+    bl bytelen
+
+    mov r2, r0
+    bl print
+
+    ldr r1, =NEWLINE
+    mov r2, #1
+    bl print
+
+    sub r10, r10, #1
+    cmp r10, #0
+    bgt print_entries
+
+    add sp, sp, #4096
+
     mov r0, #0
     bl exit
 

string.S

@@ -1,6 +1,4 @@
 /*
-    Current implementation only counts the bytes instead of characters.
-
     @args:
         r1 => pointer to the string
     @returns:
@@ -8,7 +6,7 @@
     @clobbers:
         r2
 */
-strlen:
+bytelen:
     mov r0, #0
 1:
     ldrb r2, [r1, r0]
@@ -19,7 +17,32 @@ strlen:
     b 1b
 
 /*
-    @input r1, r2 - pointers to the strings to compare
+    @args:
-    @return r0 - 0 if equal, -1 if a < b, 1 if a > b
+        r1 => pointer to first string
+        r2 => pointer to second string
+    @returns:
+        r0 => 0 if equal, -1 if a < b, 1 if a > b
+    @clobbers:
+        r1, r2, r3
 */
-strcmp: @ TODO: implement
+strcmp_ascii:
+@ comparison loop
+1:
+    ldrb r0, [r1], #1
+    ldrb r3, [r2], #1
+    cmp r0, r3
+    bne 2f
+    cmp r0, #0
+    bne 1b
+
+    @ strings are equal
+    mov r0, #0
+    bx lr
+
+@ if we found a different byte then return the appropriate integer
+2:
+    sub r0, r0, r3
+    cmp r0, #0
+    movlt r0, #-1
+    movgt r0, #1
+    bx lr