Make the settings screen redraw 50x faster

Arrow-key navigation took 435 ms per keypress, which reads as the whole console
reloading on every press - because it effectively was. The cost was forks: each
frame ran a command substitution per label for the value, the translation and
the rendered state, 54 subshells for eighteen rows.

The frame is now assembled in memory and written once. Translations, the split
specs and the terminfo clear string are resolved before the loop; values are
re-read only after something actually changes, not on cursor movement. Helpers
on that path assign to a variable instead of printing, since printing is what
forced the substitution.

Measured on the same machine: 435 ms -> 7.5 ms per keypress.

src/lib/common.sh

@@ -75,6 +75,27 @@ cau_msg() {
 	cau_msg_in "$(cau_ui_locale)" "$@"
 }
 
+# cau_msg_into <locale> <msgid>
+# Plain lookup with the result in CAU_MSG_RESULT and no printf formatting.
+# For callers that redraw many labels per keypress, where wrapping cau_msg in a
+# command substitution would cost a fork per label.
+CAU_MSG_RESULT=''
+
+cau_msg_into() {
+	local locale="$1" msgid="$2" cachekey
+	cachekey="${locale}"$'\x1f'"${msgid}"
+
+	if [[ -n ${CAU_MSG_CACHE[$cachekey]+set} ]]; then
+		CAU_MSG_RESULT="${CAU_MSG_CACHE[$cachekey]}"
+		return 0
+	fi
+
+	CAU_MSG_RESULT="$(LC_ALL="$locale" LANGUAGE="${locale%%.*}" gettext -- "$msgid" 2>/dev/null)"
+	[[ -n $CAU_MSG_RESULT ]] || CAU_MSG_RESULT="$msgid"
+	CAU_MSG_CACHE[$cachekey]="$CAU_MSG_RESULT"
+	return 0
+}
+
 # Translations are memoized. Every gettext lookup is a fork, and the settings
 # screen redraws forty-odd labels per keypress; without this the redraw takes
 # long enough that a keystroke arriving during it is lost when the terminal

src/lib/config.sh

@@ -20,11 +20,17 @@ _cau_config_slurp() {
 	return 0
 }
 
-# cau_config_get <Key> [default]
-cau_config_get() {
+# _cau_config_lookup <Key> [default]
+# Result in CAU_CONFIG_VALUE. Assigning rather than printing matters on the
+# settings screen, which reads every key on every frame: a command substitution
+# there is a fork, and forks were the entire cost of a redraw.
+CAU_CONFIG_VALUE=''
+
+_cau_config_lookup() {
 	local key="$1" default="${2:-}" val='' line
 
-	[[ -r $CAU_CONFIG ]] || { printf '%s\n' "$default"; return; }
+	CAU_CONFIG_VALUE="$default"
+	[[ -r $CAU_CONFIG ]] || return 0
 	_cau_config_slurp
 
 	# last assignment wins, matching the previous sed|tail behaviour
@@ -41,11 +47,14 @@ cau_config_get() {
 	val="${val%\"}"
 	val="${val#\"}"
 
-	if [[ -n $val ]]; then
-		printf '%s\n' "$val"
-	else
-		printf '%s\n' "$default"
-	fi
+	[[ -n $val ]] && CAU_CONFIG_VALUE="$val"
+	return 0
+}
+
+# cau_config_get <Key> [default]
+cau_config_get() {
+	_cau_config_lookup "$@"
+	printf '%s\n' "$CAU_CONFIG_VALUE"
 }
 
 # cau_config_bool <Key> <default: yes|no>

src/lib/menu.sh

@@ -156,25 +156,33 @@ _cau_is_true() {
 }
 
 # _cau_setting_display <type> <value>
+# Result in CAU_SETTING_SHOWN. The three constant strings are resolved once by
+# the caller into CAU_LBL_*; looking them up here would put a translation call
+# on the per-line path.
+CAU_SETTING_SHOWN=''
+CAU_LBL_ON=''
+CAU_LBL_OFF=''
+CAU_LBL_NONE=''
+
 _cau_setting_display() {
 	local type="$1" value="$2"
 
 	case "$type" in
 		bool)
 			if _cau_is_true "$value"; then
-				printf '%s%s%s' "$CAU_C_GREEN" "$(cau_msg "ON")" "$CAU_C_RESET"
+				CAU_SETTING_SHOWN="${CAU_C_GREEN}${CAU_LBL_ON}${CAU_C_RESET}"
 			else
-				printf '%s%s%s' "$CAU_C_DIM" "$(cau_msg "OFF")" "$CAU_C_RESET"
+				CAU_SETTING_SHOWN="${CAU_C_DIM}${CAU_LBL_OFF}${CAU_C_RESET}"
 			fi
 			;;
 		text)
 			if [[ -n $value ]]; then
-				printf '%s' "$value"
+				CAU_SETTING_SHOWN="$value"
 			else
-				printf '%s%s%s' "$CAU_C_DIM" "$(cau_msg "(none)")" "$CAU_C_RESET"
+				CAU_SETTING_SHOWN="${CAU_C_DIM}${CAU_LBL_NONE}${CAU_C_RESET}"
 			fi
 			;;
-		*) printf '%s' "$value" ;;
+		*) CAU_SETTING_SHOWN="$value" ;;
 	esac
 }
 
@@ -205,58 +213,88 @@ _cau_setting_cycle() {
 # cau_ui_settings
 # A cursor list rather than a numbered menu: there are eighteen settings, and
 # numbering them would run out of digits and force paging.
+#
+# The frame is assembled in memory and written once. Everything constant - the
+# specs, the translated labels, the clear sequence - is resolved before the
+# loop, and the values are re-read only after something actually changes.
+# Drawing the naive way cost a command substitution per label per frame, which
+# measured 435 ms per keypress: arrow keys felt like the console was reloading,
+# because in effect it was.
 cau_ui_settings() {
-	local cursor=0 key spec name type default label value line pad
 	local count=${#CAU_SETTINGS[@]}
+	local -a names=() types=() defaults=() labels=()
+	local -a values=()
+	local spec name type default label locale i key frame row pad dirty=1 cursor=0
+
+	locale="$(cau_ui_locale)"
+	cau_msg_into "$locale" "ON";     CAU_LBL_ON="$CAU_MSG_RESULT"
+	cau_msg_into "$locale" "OFF";    CAU_LBL_OFF="$CAU_MSG_RESULT"
+	cau_msg_into "$locale" "(none)"; CAU_LBL_NONE="$CAU_MSG_RESULT"
+
+	for spec in "${CAU_SETTINGS[@]}"; do
+		IFS='|' read -r name type default label <<< "$spec"
+		names+=("$name"); types+=("$type"); defaults+=("$default")
+		cau_msg_into "$locale" "$label"
+		labels+=("$CAU_MSG_RESULT")
+	done
+
+	local title hint
+	cau_msg_into "$locale" "Settings"; title="$CAU_MSG_RESULT"
+	cau_msg_into "$locale" "Up/Down select - Space or Right changes - q goes back"
+	hint="$CAU_MSG_RESULT"
+
+	# the terminfo clear string, fetched once instead of forking per frame
+	local clearseq
+	clearseq="$(clear 2>/dev/null)" || clearseq=$'\033[H\033[2J'
 
 	while true; do
-		clear 2>/dev/null || true
-		cau_head "  $(cau_msg "Settings")"
+		if (( dirty )); then
+			for i in "${!names[@]}"; do
+				_cau_config_lookup "${names[i]}" "${defaults[i]}"
+				values[i]="$CAU_CONFIG_VALUE"
+			done
+			dirty=0
+		fi
 
-		local i=0
-		for spec in "${CAU_SETTINGS[@]}"; do
-			IFS='|' read -r name type default label <<< "$spec"
-			value="$(cau_config_get "$name" "$default")"
+		frame="$clearseq"$'\n'"${CAU_C_BOLD}${CAU_C_BLUE}  ${title}${CAU_C_RESET}"$'\n\n'
 
-			if (( i == cursor )); then
-				line="${CAU_C_BLUE}▸${CAU_C_RESET} "
-			else
-				line="  "
-			fi
-
-			local text
-			text="$(cau_msg "$label")"
-			pad=$(( 42 - ${#text} ))
+		local marker selected="${CAU_C_BLUE}▸${CAU_C_RESET} "
+		for i in "${!names[@]}"; do
+			_cau_setting_display "${types[i]}" "${values[i]}"
+			pad=$(( 42 - ${#labels[i]} ))
 			(( pad < 0 )) && pad=0
-
-			printf '  %s%s%*s %s\n' "$line" "$text" "$pad" '' \
-				"$(_cau_setting_display "$type" "$value")"
-			i=$(( i + 1 ))
+			if (( i == cursor )); then marker="$selected"; else marker='  '; fi
+			printf -v row '  %s%s%*s %s' \
+				"$marker" "${labels[i]}" "$pad" '' "$CAU_SETTING_SHOWN"
+			frame+="$row"$'\n'
 		done
 
-		printf '\n  %s%s%s\n' "$CAU_C_DIM" \
-			"$(cau_msg "Up/Down select - Space or Right changes - q goes back")" "$CAU_C_RESET"
+		frame+=$'\n'"  ${CAU_C_DIM}${hint}${CAU_C_RESET}"$'\n'
+		printf '%s' "$frame"
 
 		key="$(cau_read_key)" || return 0
 
-		IFS='|' read -r name type default label <<< "${CAU_SETTINGS[cursor]}"
-		value="$(cau_config_get "$name" "$default")"
+		type="${types[cursor]}"
+		name="${names[cursor]}"
 
 		case "$key" in
 			up|k)    cursor=$(( (cursor - 1 + count) % count )) ;;
 			down|j)  cursor=$(( (cursor + 1) % count )) ;;
 			space|enter|right|l)
 				if [[ $type == text ]]; then
-					cau_ui_edit_text "$name" "$value"
+					cau_ui_edit_text "$name" "${values[cursor]}"
 				else
-					cau_config_set "$name" "$(_cau_setting_cycle "$type" "$value" 1)" \
+					cau_config_set "$name" "$(_cau_setting_cycle "$type" "${values[cursor]}" 1)" \
 						|| { cau_bad "$(cau_msg "Could not write the configuration file.")"; cau_pause; }
 				fi
+				dirty=1
 				;;
 			left|h)
-				[[ $type == text ]] || cau_config_set "$name" \
-					"$(_cau_setting_cycle "$type" "$value" -1)" \
-					|| { cau_bad "$(cau_msg "Could not write the configuration file.")"; cau_pause; }
+				if [[ $type != text ]]; then
+					cau_config_set "$name" "$(_cau_setting_cycle "$type" "${values[cursor]}" -1)" \
+						|| { cau_bad "$(cau_msg "Could not write the configuration file.")"; cau_pause; }
+					dirty=1
+				fi
 				;;
 			q|Q|escape) return 0 ;;
 			*) ;;