@ Jakub Obuchowski 385877

.arm
.global grayscale
.func grayscale

.text
grayscale:
    push {r3, r4, r5, r6, r7, r8, r9, lr}

    @ r3 - width * height
    mul r3, r1, r2

    @ set factors for average
    mov r7, #77
    mov r8, #151
    mov r9, #28

main_loop_begin:
    cmp r3, #0
    beq main_loop_end

    mov r4, #0
    mov r5, #0
    mov r6, #0

    @ get r, g, b values of current cell
    ldrb r4, [r0]
    add r0, #1
    ldrb r5, [r0]
    add r0, #1
    ldrb r6, [r0]
    sub r0, #2

    @ transform values
    mul r4, r7
    mul r5, r8
    mul r6, r9

    @ sum
    add r4, r5
    add r4, r6

    @ divide
    lsr r4, #8

    @ r value of current cell
    strb r4, [r0]
    add r0, #3

    sub r3, #1
    b main_loop_begin

main_loop_end:
    pop {r3, r4, r5, r6, r7, r8, r9, lr}
    bx lr
.end