sed conditional insertion between text & pattern - sed

Question

I would like to parse a file's content with text blocks & add a complementary delimiter.
Example of a good existing block:
%
sometext
-+- some signature
Example of a bad existing block:
%
sometext
%
someothertext
What I can already do is identify the pattern and insert the pattern unconditionally, like:
sed '/%$/ i\-+-' toto
-+-
%
1
-+-
%
in my test file.
How can I identify that the line above the % char is a text block, and if so, insert a -+- signature -+- line between the text and the new signature line?
Full example:
%
good signature is present
-+- signature -+-
%
bad signature is no present
%
this is also bad
%
this one is good
-+- signature -+-
must become
%
good signature is present
-+- signature -+-
%
bad signature is no present
-+- signature -+-
%
this is also bad
-+- signature -+-
%
this one is good
-+- signature -+-
The texts themselves won't change.

Answer 1

The following script:
#!/bin/sh
cat <<EOF |
%
good signature is present
-+- signature -+-
%
bad signature is no present
%
this is also bad
%
this one is good
EOF
sed -E '
# Last line is a big special - we add to hold buffer first.
${
# Give me functions in sed....
# Keep last 2 lines in hold space.
x; G; s/^.*((\n[^\n]*){2})$/\1/; x;
# Add the line.
b ADD;
}
# Check if current line does not contain -+-
/^-\+-/!{ b ADD; }
b NOADD; { : ADD;
# Check if two last lines match the pattern.
x; /^\n% *\n[a-zA-Z ]+$/{
# Last line needs to print pattern space first.
${ x; p; x; };
# Insert the line with signature.
# Flush hold space.
s/.*/-+- signature -+-/; p; s/.*//;
# Last line exits
${ d; };
}; x;
}; : NOADD
# Keep last 2 lines in hold space.
x; G; s/^.*((\n[^\n]*){2})$/\1/; x;
'
outputs:
%
good signature is present
-+- signature -+-
%
bad signature is no present
-+- signature -+-
%
this is also bad
-+- signature -+-
%
this one is good
-+- signature -+-
The general idea is that you accumulate enough state inside hold buffer so that you can make the decision on what you want to do. Then only evaluate if there is in hold buffer + pattern buffer that what you want and make an action then.
The last line handling is semi-broken and probably has to be also fixed and handled better - which is left to others.
Alternatively to storing state inside hold buffer, you can "store" state in like current control flow position inside the script. I think which method to choose is subjective and depends on the work to be done. I believe it is actually simpler here:
sed -E '
: RESTART
# Check for %
/^%/{
n;
# Check next line for words.
/^[a-zA-Z ]+$/{
# If end of line, first print, then we add.
${ p; b ADD; }
n;
# If something else, we also add.
/^-\+-/!{ b ADD; }
b NOADD; { : ADD;
# Add the signature.
x; s/.*/-+- signature -+-/p; x;
# Last line already printed - just quit.
${ d; }
# We already read next line above - restart.
b RESTART
}; : NOADD
}
}
'

Answer 2

With awk:
awk -v s='-+- signature -+-' '
$0=="%"{if(f) print s; f=1}
$0==s{f=0} 1; END{if(f) print s}' ip.txt
f is a flag that will be set if input line is % and unset if signature is found
If f is still set when the next % occurs, print the signature
END{if(f) print s} is needed if the final block didn't have a signature
Note that exact string comparison is used here to check the input lines, if there are excess whitespace you'll have to use regex instead or take care of the excess whitespace first
Using regexp instead of string matching, adjust regexp as needed:
awk -v s='-+- signature -+-' '
/^%/{if(f) print s; f=1}
/^-\+- signature/{f=0} 1;
END{if(f) print s}' ip.txt

Answer 3

This might work for you (GNU sed):
sed '1{x;s/^/-+- dummy sig -+-/;x};/^%/{:a;${G;b};n;/^%/{x;p;x;ba};/^-+-/!ba}' file
On the first line set up a dummy signature in the hold space for later use.
If a line begins % keep printing lines until either another % in which case insert a dummy signature and repeat above or a line beginning -+- in which case end processing of the leading %.
The solution may be altered to use the previous signature, like so:
sed -e '1{x;s/^/-+- dummy sig -+-/;x};/^%/{:a;${G;b};n;/^%/{x;p;x;ba};/^-+-/!ba;h}' file
N.B. That in the case of processing text between a pattern and the last line is encountered, the dummy signature is appended.