edit the only lines which matches some word

I have to search a file for lines with some specific words and edit these lines as follows. The other lines should remain the same. For example:

    eltu r21, r24, r23
    brz r21, outer_end
    add r27, r0, r25
    jp outer_end
    add r27, r0, r22
   outer_end:

to be modified to:

    eltu r21, r24, r23
line2:  brz r21, $(outer_end-line2)
    add r27, r0, r25
line4:  jp $(outer_end-line4-4)
    add r27, r0, r22
outer_end:

Can any one please help how to use AWK to achieve this.

Regards

Answer

awk solution:

$ cat tst.awk
/brz|jp/{ 
   last=$NF;
   NF--;
   printf("line%s:t%s $(%s-line%s)n", NR, $0, last, NR); next }
NF>1{ print $0; next }
{ print $1 }

Explanation:

When brz or jp matches, save last field in last, remove last field with NF-- and print the resulting line.

If it is a line containing multiple fields NF>1 then just print it. Last possibility is a label line, only the first field needs to be printed.

In action:

awk -f tst.awk input.txt
    eltu r21, r24, r23
line2:  brz r21, $(outer_end-line2)
    add r27, r0, r25
line4:  jp $(outer_end-line4)
    add r27, r0, r22
outer_end: