[FFmpeg-cvslog] configure: speed up flatten_extralibs_wrapper()

[Avi Halachmi (:avih)]

ffmpeg | branch: release/4.0 | Avi Halachmi (:avih) <avihpit at yahoo.com> | Mon Jul 30 22:39:15 2018 +0300| [42355d12dbfd92b11fa6c28858ea041bcf94bd7b] | committer: James Almer

configure: speed up flatten_extralibs_wrapper()

x50 - x200 faster.

Currently configure spends 50-70% of its runtime inside a single
function: flatten_extralibs[_wrapper] - which does string processing.

During its run, nearly 20K command substitutions (subshells) are used,
including its callees unique() and resolve(), which is the reason
for its lengthy run.

This commit avoids all subshells during its execution, speeding it up
by about two orders of magnitude, and reducing the overall configure
runtime by 50-70% .

resolve() is rewritten to avoid subshells, and in unique() and
flatten_extralibs() we "inline" the filter[_out] functionality.

Note that logically, "unique" functionality has more than one possible
output (depending on which of the recurring items is kept). As it
turns out, other parts expect the last recurring item to be kept
(which was the original behavior of uniqie()). This patch preservs
its output order.

Tested-by: Michael Niedermayer <michael at niedermayer.cc>
Tested-by: Helmut K. C. Tessarek <tessarek at evermeet.cx>
Tested-by: Dave Yeo <daveryeo at telus.net>
Tested-by: Reino Wijnsma <rwijnsma at xs4all.nl>
Signed-off-by: James Almer <jamrial at gmail.com>
(cherry picked from commit 58b81ac621aea1428aa9e7ca20ad45fc18dba9f7)

> http://git.videolan.org/gitweb.cgi/ffmpeg.git/?a=commit;h=42355d12dbfd92b11fa6c28858ea041bcf94bd7b
---

 configure | 46 +++++++++++++++++++++++++++++++++-------------
 1 file changed, 33 insertions(+), 13 deletions(-)

diff --git a/configure b/configure
index 827abfe694..26d602e40e 100755
--- a/configure
+++ b/configure
@@ -836,22 +836,37 @@ prepend(){
     eval "$var=\"$* \$$var\""
 }
 
+reverse () {
+    eval '
+        reverse_out=
+        for v in $'$1'; do
+            reverse_out="$v $reverse_out"
+        done
+        '$1'=$reverse_out
+    '
+}
+
+# keeps the last occurence of each non-unique item
 unique(){
-    var=$1
-    uniq_list=""
-    for tok in $(eval echo \$$var); do
-        uniq_list="$(filter_out $tok $uniq_list) $tok"
+    unique_out=
+    eval unique_in=\$$1
+    reverse unique_in
+    for v in $unique_in; do
+        # " $unique_out" +space such that every item is surrounded with spaces
+        case " $unique_out" in *" $v "*) continue; esac  # already in list
+        unique_out="$unique_out$v "
     done
-    eval "$var=\"${uniq_list}\""
+    reverse unique_out
+    eval $1=\$unique_out
 }
 
 resolve(){
-    var=$1
-    tmpvar=
-    for entry in $(eval echo \$$var); do
-        tmpvar="$tmpvar $(eval echo \$${entry})"
+    resolve_out=
+    eval resolve_in=\$$1
+    for v in $resolve_in; do
+        eval 'resolve_out="$resolve_out$'$v' "'
     done
-    eval "$var=\"${tmpvar}\""
+    eval $1=\$resolve_out
 }
 
 add_cppflags(){
@@ -6682,14 +6697,19 @@ if test $target_os = "haiku"; then
 fi
 
 flatten_extralibs(){
-    unset nested_entries
+    nested_entries=
     list_name=$1
     eval list=\$${1}
     for entry in $list; do
         entry_copy=$entry
         resolve entry_copy
-        append nested_entries $(filter '*_extralibs' $entry_copy)
-        flat_entries=$(filter_out '*_extralibs' $entry_copy)
+        flat_entries=
+        for e in $entry_copy; do
+            case $e in
+                *_extralibs) nested_entries="$nested_entries$e ";;
+                          *) flat_entries="$flat_entries$e ";;
+            esac
+        done
         eval $entry="\$flat_entries"
     done
     append $list_name "$nested_entries"