# this subroutine bubble sorts vectors of words
#
# inputs
#   r0 - start of vector
#   r1 - number of elements to sort
#
# no outputs
#
# locals
#   r4 - current pointer
#   r5 - inner counter
#   r6 - keep_going flag
#   r7 - first element
#   r8 - second element

.equ datum_size,4
.globl _bubble
.text
_bubble:
        stmfd        sp!, {r4, r5, r6, r7, r8, lr}        @ save variables to stack
        cmp        r1, #1                                @ number of elements must be > 1
        ble        end_outer                        @ stop if nothing to do

        sub        r5, r1, #1                        @ need n-1 comparisons
        mov        r4, r0                                @ initialise current pointer
        mov        r6, #0                                @ this register set when we swap

loop_start:
        ldr        r7, [r4], #datum_size                @ load one element
        ldr        r8, [r4]                        @ and next one
        cmp        r7, r8                                @ compare them
        ble        no_swap                                @ branch if second greater

        mov        r6, #1                                @ set keep_going flag
        sub        r4, r4, #datum_size                @ reset pointer to first element
        swp        r8, r8, [r4]                        @ exchange value in r8 and address in r4
        str        r8, [r4, #datum_size]!                @ store new r8 to incremented address
no_swap:
        subs        r5, r5, #1                        @ decrement counter
        bne        loop_start                        @ and restart loop if more needed

end_inner:
        cmp        r6, #0                                @ check keep_going flag
        beq        end_outer                        @ and leave if not set

        mov        r6, #0                                @ clear keep_going flag 
        mov        r4, r0                                @ reset pointer
        sub     r5, r1, #1                        @ reset counter
        b        loop_start                        @ start another iteration

end_outer:
        ldmfd   sp!, {r4, r5, r6, r7, r8, pc}        @ restore state from stack and leave subroutime