# overstrk.sed - implement the overstrike printer control function.
# Created by E.Clay Buchanan <ecbyahoo@yahoo.com> Oct 27, 2002
#
# All lines start with a printer control character.  The overstrike
# printer control character is a plus sign, +.  All nonblank characters
# on the overstrike line overwrite the character on the previous line at
# the same column position except for the overwritten lines printer
# control character which is preserved.
#
# The pattern space buffer is loaded with two lines. If line 2 is an
# overstrike line the plus sign is replaced with a blank to preserve
# the original lines printer control character. Then an empty merge
# buffer is created by inserting a newline at the front of pattern
# space giving: ^\n<line 1>\n<line 2>$
#
# One by one, move either a character from line 2 (if non blank) or
# from line 1 to the merge buffer until the end of line 1 or line 2 is
# reached.  Delete the newline character at the end of the merge buffer
# and at the end of line 1 effectively appending the remaining text from
# line 1 or 2 with the merge buffer. This leaves one overstruck line in
# pattern space.  Append the following line into pattern space and
# repeat.
#
# ----------------------------------------------------------------------
#
# To: sed-users@yahoogroups.com 
# From: "ecbyahoo" <ecbyahoo@yahoo.com> 
# Data: Tue, 29 Oct 2002 21:06:36 -0000 
# Assunto: Re: Subject: how to superimpose several lines 
#        
# [...]
# I created the general overstrike problem:
# 
# The input file consists of a series of records with the first 
# character of each record being a printer control character. In
# the case of an overstrike printer control character, a "+",
# the previous line is overstruck with all nonblank characters 
# from the current line maintaining all column positions and 
# maintaining the previous line's printer control character.
# There is no limit on the number of overstrike records.
# 
# The following input file, input.txt:
# 
#  111
# + 222
#   333
# +444
#  555
# +666
#  7 7 7 
# + 8 8 8
#  999
# +888
# +777
# +666
#  1  1  1
# + 22 22 22
# +  3  3  3
# -222
# +  333
# 
# should produce the result:
# 
#  1222
#  4443
#  666
#  787878
#  666
#  123123123
# -22333
# 
# The following sed command line:
# 
# sed -f overstrk.sed 
# 
# where overstrk.sed is:

:top
N               # always have two lines in pattern space
/\n+/{          # was second line an overstrike line?
   s/\n+/\n /   # yes, delete plus sign
   s/^/\n/      # create merge buffer at start of pattern space
   :merge
   s/^\(.*\)\n.\(.*\n\)\([^ ]\)\(.*\)$/\1\3\n\2\4/;tmerge #overstrike
   s/^\(.*\)\n\(.\)\(.*\n\) \(.*\)$/\1\2\n\3\4/;tmerge    #do not
   s/\n//g      # combine merged buffer with remaining line 1 or 2
   btop
}
P                     # print line
D                     # delete first line, repeat till done


# does the job.  Many thanks to aurelio for his sed debugger!
#
# -------------------------------------------------------------------------
#
# Aurelio posted a GNU sed 3.02 working version (no s//\n/):
#
# :top
# N                  ;# always have two lines in pattern space
# /\n+/{             ;# was second line an overstrike line?
#    s/\(\n\)+/\1 /  ;# yes, delete plus sign
#    H;s/.*//;x      ;# create merge buffer at start of pattern space
#    :merge
#    s/^\(.*\)\(\n\).\(.*\n\)\([^ ]\)\(.*\)$/\1\4\2\3\5/;tmerge ;#overstrike
#    s/^\(.*\)\(\n\)\(.\)\(.*\n\) \(.*\)$/\1\3\2\4\5/;tmerge    ;#do not
#    s/\n//g         ;# combine merged buffer with remaining line 1 or 2
#    btop
# }
# P                  ;# print line
# D                  ;# delete first line, repeat till done