Zsh: Shrink PWD in RPROMPT dynamically

.config/zsh/themes/prompt.zsh

@@ -89,9 +89,28 @@ else
     add-zsh-hook precmd vcs_info
 fi
 
+# Reduce directory elements to first letter until directory path is reasonably
+# short. Never shrink first element.
+function _shrunkpwd()
+{
+    local dir=${(D)PWD}
+    local -i maxwidth=$(( ${COLUMNS} / 3 ))
+
+    local -a shortdir=(${(s:/:)dir})
+    for index in {2..${#shortdir}}; do
+        if [[ ${#${(j:/:)shortdir}} -gt ${maxwidth} ]]; then
+           shortdir[index]=${shortdir[index][1]}
+        fi
+    done
+
+    local out=${(j:/:)shortdir}
+    [[ ${out[1]} != '~' ]] && out[1]="/${out[1]}"
+    print -n ${out}
+}
+
 setopt PROMPT_SUBST
 
 PROMPT="%B%F{${zsh_theme_colours[prompt]}}${showuser}%b"'${vcs_info_msg_0_}'"%B%F{${zsh_theme_colours[prompt]}}%#%f%b "
-RPROMPT="%F{${zsh_theme_colours[minor]}}%(?..[%B%F{${zsh_theme_colours[highlight]}}%?%b%F{${zsh_theme_colours[minor]}}] )%4~%f"
+RPROMPT="%F{${zsh_theme_colours[minor]}}%(?..[%B%F{${zsh_theme_colours[highlight]}}%?%b%F{${zsh_theme_colours[minor]}}] )"'$(_shrunkpwd)'"%f"
 
 }