Rewrite input lines for variable fields in AWK - awk

Question

I want to rewrite the following lines (given sample of 3 lines, but it runs for about 500 lines)
sl_no, id, value1, value2, value3, total, invalid, no_option, gross_total
1,1,570,0,0,.......,655,0,1,656
2,2,394,5,24,......,477,0,4,481
3,3,489,9,2,.......,704,0,2,706
as
sl_no, id, 0, no_option
sl_no, id,loop, value1
sl_no, id,loop, value2
sl_no, id,loop, value3
i.e.
1,1,0,1
1,1,1,570
1,1,2,0
1,1,3,0
2,2,0,4
2,2,1,394
2,2,2,5
2,2,3,24
3,3,0,2
3,3,1,489
3,3,2,9
3,3,3,2
value_1 to value_3 is variable i.e. it can be value_1 to value_15 or it can be value_1 to value_8 or so
I tried with AWK but hard coding of variable value list is not manageable
$ awk -F, '{printf("%d,%s,0,%d\n%d,%s,1,%d\n%d,%s,2,%d\n%d,%s,3,%d\n",$1,$2,$(NF-1),$1,$2,$3,$1,$2,$4,$1,$2,$5)}' input.txt > output.txt

Answer 1

With a for loop:
awk 'BEGIN{ FS=OFS="," }
{ i=0; print $1,$2,i,$(NF-1) }
{ for(i=3; i<NF-4; i++){ print $1,$2,i-2,$i } }' file
Output:
1,1,0,1
1,1,1,570
1,1,2,0
1,1,3,0
2,2,0,4
2,2,1,394
2,2,2,5
2,2,3,24
3,3,0,2
3,3,1,489
3,3,2,9
3,3,3,2
See: 8 Powerful Awk Built-in Variables – FS, OFS, RS, ORS, NR, NF, FILENAME, FNR

Answer 2

Awk is a programming language, you can just loop over the fields.
Assuming the last four are fixed, you want to loop from the third through the fourth last.
awk -F , '{ printf "%i,%i,%i,%i\n", $1, $2, 0, $(NF-1)
for(i=3; i<NF-4; ++i)
printf "%i,%i,%i,%i\n", $1, $2, i-2, $i }' input.txt >output.txt