Zsh prompt: refine shrunken path algorithm

- Allow short elements
- Try to keep last element
  - Shorten mid-section if not possible

.config/zsh/themes/prompt.zsh

@@ -90,24 +90,37 @@ else
 fi
 
 # Reduce directory elements to first letter until directory path is reasonably
-# short. Never shrink first element.
+# short. Never shrink first element. Don't shrink relatively short elements. Try
+# to not shrink last element.
 function _shrunkpwd()
 {
     local dir=${(D)PWD}
-    local -i maxwidth=$(( ${COLUMNS} / 3 ))
-    if [[ ${#dir} -le ${maxwidth} ]]; then
+    local -i max_width=$(( ${COLUMNS} / 3 ))
+    if [[ ${#dir} -le ${max_width} ]]; then
         print -n ${dir}
         return
     fi
 
-    local -a shortdir=(${(s:/:)dir})
-    for index in {2..${#shortdir}}; do
-        if [[ ${#${(j:/:)shortdir}} -gt ${maxwidth} ]]; then
-           shortdir[index]=${shortdir[index][1]}
+    # Maybe shorten all elements but the first and last
+    local -a short_dir=(${(s:/:)dir})
+    local -i allowed_element_width=$(( ${max_width} / ${#short_dir} -1 ))
+    for index in {2..$(( ${#short_dir} - 1 ))}; do
+        if [[ ${#short_dir[index]} -gt ${allowed_element_width} ]] && \
+               [[ ${#${(j:/:)short_dir}} -gt ${max_width} ]]; then
+            short_dir[index]=${short_dir[index][1]}
         fi
     done
 
-    local out=${(j:/:)shortdir}
+    # If the last element is too long, replace the mid-section with …
+    if [[ ${#short_dir[-1]} -gt ${allowed_element_width} ]]; then
+        local -i current_width=${#${(j:/:)short_dir}}
+        if [[ ${current_width} -gt ${max_width} ]]; then
+            local -i half_length=$(( ( ${max_width} - ( ${current_width} - ${#short_dir[-1]} ) ) / 2 - 1 ))
+            short_dir[-1]=${short_dir[-1][1,${half_length}]}"…"${short_dir[-1][$(( ${half_length} * -1 )),-1]}
+        fi
+    fi
+
+    local out=${(j:/:)short_dir}
     [[ ${out[1]} != '~' ]] && out[1]="/${out[1]}"
     print -n ${out}
 }