jenv: refactor and optimize logic

plugins/jenv/jenv.plugin.zsh

-_homebrew-installed() {
-    type brew &> /dev/null
-}
-
-_jenv-from-homebrew-installed() {
-    brew --prefix jenv &> /dev/null
-}
-
 jenvdirs=("$HOME/.jenv" "/usr/local/jenv" "/opt/jenv")
-if _homebrew-installed && _jenv-from-homebrew-installed ; then
-    jenvdirs+=($(brew --prefix jenv) "${jenvdirs[@]}")
-fi
 
 FOUND_JENV=0
-for jenvdir in "${jenvdirs[@]}" ; do
-    if [ -d $jenvdir/bin -a $FOUND_JENV -eq 0 ] ; then
+for jenvdir in $jenvdirs; do
+    if [[ -d "${jenvdir}/bin" ]]; then
         FOUND_JENV=1
-        export PATH="${jenvdir}/bin:$PATH"
-        eval "$(jenv init - zsh)"
+        break
+    fi
+done
 
-        function jenv_prompt_info() {
-          echo "$(jenv version-name)"
-        }
+if [[ $FOUND_JENV -eq 0 ]]; then
+    if (( $+commands[brew] )) && jenvdir="$(brew --prefix jenv)"; then
+        FOUND_JENV=1
     fi
-    if [ -d $jenvdir/versions -a $FOUND_JENV -eq 0 ] ; then
+fi
+
+if [[ $FOUND_JENV -eq 1 ]]; then
+    export PATH="${jenvdir}/bin:$PATH"
+    eval "$(jenv init - zsh)"
+
+    function jenv_prompt_info() { jenv version-name 2>/dev/null }
+
+    if [[ -d "${jenvdir}/versions" ]]; then
         export JENV_ROOT=$jenvdir
     fi
-done
-unset jenvdir
-
-if [ $FOUND_JENV -eq 0 ] ; then
+else
     function jenv_prompt_info() { echo "system: $(java -version 2>&1 | cut -f 2 -d ' ')" }
 fi
+
+unset jenvdir FOUND_JENV