Understanding awk commands; split, rec and ORS

Question

I have a piece of code, I am trying to break down and decipher. I need someone that can walk me through what is happening in each step:

FNR==NR {
  if (split($1,a, "_") ==3)
  f1[$1]
next
}
NF{
if (split($1, a, "_") ==3 && $1 in f1)
  p=1
  rec=(!rec)?$0:rec ORS $0
next
}
p{
  print rec ORS
  p=0; rec=""
}

Answer 1

NR is the total number of records processed (inclusive of the one being processed currently); FNR is the number of records processed in the current file. This will only happen for the first file.

FNR==NR {
  if (split($1,a, "_") ==3)
  f1[$1]
next
}

Similarly, NF is the number of fields in the current record. If it is a blank line, this will be zero, which is "falsy" (as opposed to "truthy"), and so this block will only execute for non-blank lines. Make a note of the code in this block where the value of p is set; that will be important later. ORS is the Output Record Separator; this is what separates each record as it is written out. By default is is a newline. Here is is conditionally used as the value for the rec variable.

NF{
if (split($1, a, "_") ==3 && $1 in f1)
  p=1
  rec=(!rec)?$0:rec ORS $0
next
}

This last block will run only if the value of p is "truthy".

p{
  print rec ORS
  p=0; rec=""
}