add optional kubecontext shortening for gke and eks contexts

This is done via an optional array parameter that lists
shortening stragies to be applied. Currently there are
only two supported strategies: 'gke' and 'eks'.

  # Shorten gke and eks cluster names:
  #
  #   - gke_projectname_availability-zone_cluster-01         => cluster-01
  #   - arn:aws:eks:us-east-1:XXXXXXXXXXXX:cluster/eks-infra => eks-infra
  #
  # This transformation is applied before class matching and content
  # expansion.
  POWERLEVEL9K_KUBECONTEXT_SHORTEN=(gke eks)

For https://github.com/romkatv/powerlevel10k/issues/139.

internal/p10k.zsh

@@ -2883,6 +2883,7 @@ prompt_dir_writable() {
 ################################################################
 # Kubernetes Current Context/Namespace
 prompt_kubecontext() {
+  unset P9K_KUBECONTEXT_{NAME,CLUSTER,NAMESPACE}
   (( $+commands[kubectl] )) || return
   local cfg
   local -a key
@@ -2893,29 +2894,68 @@ prompt_kubecontext() {
   done
 
   if ! _p9k_cache_get $0 "${key[@]}"; then
-    local ctx="$(kubectl config view -o=jsonpath='{.current-context}')"
-    if [[ -n $ctx ]]; then
-      local p="{.contexts[?(@.name==\"$ctx\")].context.namespace}"
-      local ns="${$(kubectl config view -o=jsonpath=$p):-default}"
-      if [[ $ctx != $ns && ($ns != default || $_POWERLEVEL9K_KUBECONTEXT_SHOW_DEFAULT_NAMESPACE == 1) ]]; then
-        ctx+="/$ns"
+    local context cluster namespace
+    () {
+      local cfg && cfg=(${(f)"$(kubectl config view -o=yaml)"}) || return
+      local ctx=(${(@M)cfg:#current-context:[[:space:]]*})
+      (( $#ctx == 1 )) || return
+      context=${${ctx[1]}[(w)2]}  # TODO: figure out how values with spaces look in yaml
+      local -i pos=${cfg[(i)contexts:]}
+      (( pos <= $#cfg )) || return
+      shift $pos cfg
+      pos=${cfg[(i)  name: $context]}
+      (( pos <= $#cfg )) || return
+      (( --pos ))
+      for ((; pos > 0; --pos)); do
+        local line=$cfg[pos]
+        case $line in
+          '- context:')      return;;
+          '    cluster:'*)   cluster=${line[(w)2]};;
+          '    namespace:'*) namespace=${line[(w)2]};;
+        esac
+      done
+    }
+    local text=$context
+    local ns=${namespace:-default}
+    if [[ $context != $ns && ($ns != default || $_POWERLEVEL9K_KUBECONTEXT_SHOW_DEFAULT_NAMESPACE == 1) ]]; then
+      text+="/$ns"
+    fi
+    local shorten
+    for shorten in $_POWERLEVEL9K_KUBECONTEXT_SHORTEN; do
+      case $shorten in
+        gke)
+          # gke_projectname_availability-zone_cluster-01 => cluster-01
+          if [[ $cluster == gke_* ]]; then
+            local parts=(${(s:_:)cluster})
+            (( $#parts > 3 )) && text=$parts[4,-1]
+            break
           fi
+          ;;
+        eks)
+          # arn:aws:eks:us-east-1:XXXXXXXXXXXX:cluster/eks-infra
+          if [[ $cluster == (#b)arn:aws:eks:[[:alnum:]-]##:[[:digit:]]##:cluster/(*) ]]; then
+            [[ -n $match[1] ]] && text=$match[1]
+            break
           fi
+      esac
+    done
     local suf
-    if [[ -n $ctx ]]; then
+    if [[ -n $text ]]; then
       local pat class
       for pat class in "${_POWERLEVEL9K_KUBECONTEXT_CLASSES[@]}"; do
-        if [[ $ctx == ${~pat} ]]; then
+        if [[ $text == ${~pat} ]]; then
           [[ -n $class ]] && suf=_${(U)class}
           break
         fi
       done
     fi
-    _p9k_cache_set "$ctx" "$suf"
+    _p9k_cache_set "$context" "$cluster" "$namespace" "$text" "$suf"
   fi
-
   [[ -n $_p9k_cache_val[1] ]] || return
-  _p9k_prompt_segment $0$_p9k_cache_val[2] magenta white KUBERNETES_ICON 0 '' "${_p9k_cache_val[1]//\%/%%}"
+  typeset -g P9K_KUBECONTEXT_NAME=$_p9k_cache_val[1]
+  typeset -g P9K_KUBECONTEXT_CLUSTER=$_p9k_cache_val[2]
+  typeset -g P9K_KUBECONTEXT_NAMESPACE=$_p9k_cache_val[3]
+  _p9k_prompt_segment $0$_p9k_cache_val[5] magenta white KUBERNETES_ICON 0 '' "${_p9k_cache_val[4]//\%/%%}"
 }
 
 ################################################################
@@ -3676,6 +3716,7 @@ _p9k_init_params() {
   _p9k_declare -e POWERLEVEL9K_NODEENV_LEFT_DELIMITER "["
   _p9k_declare -e POWERLEVEL9K_NODEENV_RIGHT_DELIMITER "]"
   _p9k_declare -b POWERLEVEL9K_KUBECONTEXT_SHOW_DEFAULT_NAMESPACE 1
+  _p9k_declare -a POWERLEVEL9K_KUBECONTEXT_SHORTEN -- gke eks
   # Defines context classes for the purpose of applying different styling to different contexts.
   #
   # POWERLEVEL9K_KUBECONTEXT_CLASSES must be an array with even number of elements. The first