.text
    .global main

main:
    push {r4, r5, r6, r7, lr}

    mov r0, #0
    mov r1, #32

    mov r6, #32 // size
    mov r7, #32 // available size

    bl realloc

    mov r4, r0 // *values
    mov r5, #0 // counter

    mov r3, r0 // current pointer

loop:
    ldr r0, stdin_label
    ldr r0, [r0]
    bl fgetc

    cmp r0, #-1
    beq print_reversed

    b add_char

end_loop:
    mov r0, r3
    bl free

    pop {r4, r5, r6, r7, lr}
    bx lr

add_char:
    str r0, [r3]

    add r3, #4 // update pointer
    add r5, #1 // ++counter

    sub r7, #4
    cmp r7, #0 // realloc
    beq reallocate

    b loop

print_reversed:
    sub r3, #4
    sub r5, #1

    push {r3} // to save value in r3
    ldr r0, [r3]
    ldr r1, stdout_label
    ldr r1, [r1]
    bl fputc
    pop {r3}

    cmp r5, #0
    beq end_loop

    b print_reversed

reallocate:

    add r7, r6 // available_size = size
    mov r1, #2
    mul r6, r6, r1
    mov r1, r6 // size *= 2

    mov r0, r4
    bl realloc

    mov r4, r0 // update *values

    // update pointer
    mov r1, r5
    mov r2, #4
    mul r1, r1, r2
    add r1, r4
    mov r3, r1

    b loop

stdin_label:
    .word stdin

stdout_label:
    .word stdout