autoroute: only issue pw-link for links that are missing

The loop re-issued all 24 orbit links every 2 s and every pw-link connects to
the PipeWire daemon: 12 min of CPU in 85 min, about 15% of a core at idle.
Index the existing links from the one snapshot in pure bash and link only the
absent pairs.

tools/tests/test_autoroute_master_and_control.py

@@ -128,3 +128,27 @@ def test_the_midi_through_playback_side_is_never_taken_for_the_source(tmp_path):
     lo = "Midi-Bridge:Midi Through:(playback_0) Midi Through Port-0\n" + MASTER_ON_SPEAKER
     calls, _ = _reconcile(tmp_path, lo)
     assert not [c for c in calls if "MIDI Control In" in c]
+
+
+ORBIT_1_LINKED = ("SuperCollider:out_1\n  |-> ardour:Tidal 01/audio_in 1\n"
+                  "SuperCollider:out_2\n  |-> ardour:Tidal 01/audio_in 2\n")
+
+
+def test_orbit_links_that_exist_cost_no_pw_link_call(tmp_path):
+    """The loop re-issued every orbit link each 2 s; each pw-link call connects to
+    the PipeWire daemon, which alone was ~15% of a core at idle."""
+    calls, _ = _reconcile(tmp_path, CTRL_LINKED + MASTER_ON_SPEAKER + ORBIT_1_LINKED)
+    assert calls == []
+
+
+def test_a_missing_orbit_link_is_still_made(tmp_path):
+    only_first = "SuperCollider:out_1\n  |-> ardour:Tidal 01/audio_in 1\n"
+    calls, _ = _reconcile(tmp_path, CTRL_LINKED + MASTER_ON_SPEAKER + only_first)
+    assert calls == ["SuperCollider:out_2 ardour:Tidal 01/audio_in 2"]
+
+
+def test_a_link_to_the_wrong_track_does_not_count(tmp_path):
+    """Presence is per (source, destination) pair, never per source."""
+    wrong = "SuperCollider:out_1\n  |-> ardour:Tidal 02/audio_in 1\n"
+    calls, _ = _reconcile(tmp_path, CTRL_LINKED + MASTER_ON_SPEAKER + wrong)
+    assert "SuperCollider:out_1 ardour:Tidal 01/audio_in 1" in calls

tools/tidal-ardour-autoroute.sh

@@ -34,11 +34,36 @@ link() {  # $1 = source port, $2 = dest port ; silent (already-linked is fine)
     pw-link "$1" "$2" 2>/dev/null
 }
 
-# Print the set of Ardour "Tidal NN" track numbers currently present.
+# Print the set of Ardour "Tidal NN" track numbers present in a `pw-link -i`
+# snapshot (stdin). Pure bash: this runs every tick.
 tidal_track_numbers() {
-    pw-link -i 2>/dev/null \
-        | grep -oE "ardour:Tidal [0-9]+/audio_in 1" \
-        | grep -oE "[0-9]+" | sort -un
+    local line
+    local re='^ardour:Tidal ([0-9]+)/audio_in 1$'
+    while IFS= read -r line; do
+        [[ "$line" =~ $re ]] && echo "${BASH_REMATCH[1]}"
+    done | sort -un
+}
+
+# HAVE["src>dst"]=1 for every link in a `pw-link -lo` snapshot (stdin), so a
+# link that already exists costs no pw-link call. The loop used to re-issue all
+# 24 orbit links every 2 s; every pw-link connects to the PipeWire daemon, and
+# that alone was ~15% of a core at idle (measured 2026-10-11: 12 min of CPU in
+# 85 min). Pure bash, no forks.
+declare -A HAVE=()
+index_links() {
+    local line src=""
+    HAVE=()
+    while IFS= read -r line; do
+        if [[ "$line" =~ ^[^[:space:]] ]]; then
+            src="$line"
+        elif [[ "$line" == *"|-> "* ]]; then
+            HAVE["$src>${line#*|-> }"]=1
+        fi
+    done
+}
+
+link_missing() {  # $1 = source port, $2 = dest port; a no-op when already linked
+    [[ -n "${HAVE["$1>$2"]:-}" ]] || link "$1" "$2"
 }
 
 # Print "src<TAB>dest" for every SuperCollider:out_* -> hardware playback link.
@@ -96,8 +121,10 @@ relink_master() {
 }
 
 reconcile() {
-    local lo src
+    local lo src inp
     lo="$(pw-link -lo 2>/dev/null)"           # ONE snapshot per tick, shared below
+    inp="$(pw-link -i 2>/dev/null)"
+    index_links <<<"$lo"
 
     # ADD: the control surface feed (no-op until Ardour registers the port)
     if [[ "$lo" != *"|-> $CTRL_IN"* ]]; then
@@ -112,7 +139,7 @@ reconcile() {
     # SOF DSP's IPC until the firmware wedged and every sink vanished
     # (2026-09-07). "SC must never reach hardware" is Ardour-session policy,
     # not idle-laptop policy.
-    tracks="$(tidal_track_numbers)"
+    tracks="$(tidal_track_numbers <<<"$inp")"
     [ -n "$tracks" ] || return 0
 
     # ADD: a hardware output for Master when it has none (additive, see header)
@@ -132,8 +159,8 @@ reconcile() {
         [ -n "${nn:-}" ] || continue
         k=$((10#$nn))                 # strip leading zero (NN -> k)
         l=$((2 * k - 1)); r=$((2 * k))
-        link "$SC:out_$l" "ardour:Tidal $nn/audio_in 1"
-        link "$SC:out_$r" "ardour:Tidal $nn/audio_in 2"
+        link_missing "$SC:out_$l" "ardour:Tidal $nn/audio_in 1"
+        link_missing "$SC:out_$r" "ardour:Tidal $nn/audio_in 2"
     done <<< "$tracks"
 
     # PRUNE: while Ardour is up, SuperCollider must never reach hardware