Awk + print from the third field until end of line each word with one space

IMO the best way to do this would be to use the programming/scripting language you know best and:

1. load small.txt into an in-memory hash/map/associative array keyed on the words
2. Process huge.txt line by line, adding the column looked up from the hash and writing the result into an output file
3. Buffer input and output so that it happens in chunks of at least 4K

sed '/class\|namespace\|struct/ {
   : loop
   s/{ *specific_text;$/{/
   t
   n
   b loop
   }'

It works like this:

• Until class or namespace or struct is found, sed does nothing more than its default action: it prints incoming lines as they are.
• When class or namespace or struct is found, sed enters a loop.
  • : loop is a label, beginning of the loop.
  • b loop branches to the label, it's the end of the loop.
  • There are two ways to leave the loop:
    • end of input,
    • t which branches to the end of the script.
  • t will only work if the preceding s is successful.
  • If t doesn't work, the script continues to n which prints the current line and moves to the next.

In other words both the outside and the inside of the loop print incoming lines, except only in the loop we are trying to do the substitution with s. A successful substitution leaves the loop. The tool will loop again if another class or namespace or struct is encountered.

In yet other words this is exactly what you want (I think):

to change the first line that matches my very specific pattern, AND which is the first among its kind after the class|namespace|struct keyword.

Notes:

• The script doesn't care what is between class (or namespace or struct) and the specific pattern.
• I adjusted your patterns. In particular
  • I allowed spaces between { and specific_text because your test cases have spaces;
  • I used $ instead of ^, because ^ in your pattern is literal, it makes no sense (maybe you confused ^ and $ anchors).

Example input:

foo
bar
class A {      specific_text;
baz { specific_text;
qux
this is my namespace

whatever
abc {specific_text;
foo
{ specific_text;
{specific_text;

The output:

foo
bar
class A {
baz { specific_text;
qux
this is my namespace

whatever
abc {
foo
{ specific_text;
{specific_text;

Altered lines:

• class A { specific_text;
• abc {specific_text; (because of namespace few lines before)