New, LRU mode; better than removed autocs feature.

* cdlog.sh (cdlog_lru): New Boolean configuration variable,
blank/false by default.
(cdlog.chdir): If cdlog_lru is true, perform the rotation
through to the most recent occurrence of the new directory.

* README.md: Documented.

2 files changed, 45 insertions, 6 deletions

diff --git a/README.md b/README.md
index 248ff71..8c54ac4 100644
--- a/README.md
+++ b/README.md
@@ -87,6 +87,24 @@ When one or more persisted session are present, `cdlog`'s initialization
 mentions this, suggesting that the `cdr` command may be used to recover
 to one of the sessions.
 
+## LRU Mode
+
+When the configuration variable `cdlog_lru` is set to the value `y`, LRU mode
+is enabled in the `cdlog.chdir` function, affecting the `cd` and `mcd`
+commands. LRU mode modifies the FIFO algorithm in the following way.
+
+If the directory change successfully takes place to a directory which
+already exists in the FIFO, then the most recent (topmost) occurrence
+of that directory is removed from the FIFO before the previous
+directory is pushed into the FIFO. Thus, in effect, a rotation is
+taking place: the existing entry in the FIFO becomes the current
+directory, and is removed; the entries above it shift down to take
+its place, and the previous current directory becomes the top of
+the FIFO.
+
+LRU mode keeps duplicate directories out of the FIFO and while
+promoting recently used directories toward the top.
+
 ## How is this better?
 
 * It's not hard-coded C code inside the shell; it consists of a small amount of
diff --git a/cdlog.sh b/cdlog.sh
index b712b2e..b7d814d 100644
--- a/cdlog.sh
+++ b/cdlog.sh
@@ -3,6 +3,9 @@
 # Globals
 declare -A cdlog_alias            # cd aliases
 
+# Configuration variables
+cdlog_lru=${cdlog_lru-}
+
 # Update after FIFO change.
 cdlog.update()
 {
@@ -145,7 +148,6 @@ cdlog.recover()
 cdlog.chdir()
 {
   local cur=$PWD
-  local i
   local def
 
   if [ $# -eq 2 -a "$1" = -P ] ; then
@@ -165,13 +167,32 @@ cdlog.chdir()
     # only if we successfully change to a different
     # directory do the following
 
-    c9=$c8; c8=$c7; c7=$c6
-    c6=$c5; c5=$c4; c4=$c3
-    c3=$c2; c2=$c1
+    if [ $cdlog_lru ] ; then
+      local nx=
+      local pv=$cur
+      local i
+      local d=("" "$c1" "$c2" "$c3" "$c4" "$c5" "$c6" "$c7" "$c8" "$c9")
+      unset d[0]
+
+      for ((i = 1; i <= 9; i++)); do
+        nx=${d[$i]}
+        d[$i]=$pv
+        if [ "$nx" = "$PWD" ] ; then
+          break
+        fi
+        pv=$nx
+      done
 
-    c1=$cur
+      cdlog.args "${d[@]}"
+    else
+      c9=$c8; c8=$c7; c7=$c6
+      c6=$c5; c5=$c4; c4=$c3
+      c3=$c2; c2=$c1
 
-    cdlog.update
+      c1=$cur
+
+      cdlog.update
+    fi
   fi
 }