Peer extraction stage 2: ONE send-spec builder for app + service (CaptureSpecBuilder)

The WASAPI outputs-or-apps spec assembly - the exact logic the standing 'service must
mirror the app's send behaviour' rule exists for - lived as two hand-mirrored copies in
MainForm.ApplySendSources and ServiceSendHost.BuildSendSpecs. Now ONE builder:
- Devices mode: loopback specs, 'Use Windows default' sentinel resolved to the LIVE
  default render endpoint, duplicates collapsed, resolved-default reported via out
  param (drives the app's re-route-on-default-change detection).
- Applications mode: one process-loopback spec per running PID per chosen app name.
The app layers WASAPI inputs + ASIO pairs on top; the service adds neither (by design).
Both callers rewired; behaviour identical by construction and pinned three ways: the
service parity test, the follower test (now also pinning the out param, follower+
explicit-device dedup to one spec, and not-running apps contributing nothing), and the
main-window profile round-trip.

Gate 61/61.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/RemSound.App/CaptureSpecBuilder.cs

@@ -0,0 +1,57 @@
+using RemSound.Core;
+using RemSound.Sender;
+
+namespace RemSound.App;
+
+/// <summary>
+/// The WASAPI send-spec assembly shared by the main window and the send-only service — the very rule
+/// the standing "service must mirror the app's send behaviour" applies to, which previously existed as
+/// two hand-mirrored copies (MainForm.ApplySendSources and ServiceSendHost.BuildSendSpecs). Devices
+/// mode: loopback specs with the "Use Windows default" sentinel resolved to the LIVE default render
+/// endpoint and duplicates collapsed. Applications mode: one process-loopback spec per running PID of
+/// each chosen app name (child processes ride along via the include-tree capture). The app layers its
+/// WASAPI inputs and ASIO pairs on top; the service deliberately adds neither.
+/// </summary>
+internal static class CaptureSpecBuilder
+{
+    /// <summary>Devices mode. <paramref name="followedDefaultId"/> reports the endpoint the follower
+    /// resolved to (null when not following, or no default exists) — the app compares it on device
+    /// changes to know when the Windows default moved and a re-route is due.</summary>
+    internal static List<CaptureSourceSpec> BuildOutputSpecs(
+        IEnumerable<(string Id, string Name)> outputs, out string? followedDefaultId)
+    {
+        var specs = new List<CaptureSourceSpec>();
+        var added = new HashSet<string>(StringComparer.OrdinalIgnoreCase);
+        followedDefaultId = null;
+        foreach (var (id, name) in outputs)
+        {
+            if (AudioDefaultFollower.IsLoopbackSend(id))
+            {
+                var def = AudioDefaultFollower.ResolveDefaultRenderId();
+                followedDefaultId = def;
+                if (def is not null && added.Add(def))
+                    specs.Add(new CaptureSourceSpec(def, CaptureKind.Loopback, "Windows default audio device"));
+            }
+            else if (added.Add(id))
+            {
+                specs.Add(new CaptureSourceSpec(id, CaptureKind.Loopback, name));
+            }
+        }
+        return specs;
+    }
+
+    /// <summary>Applications mode. Apps not currently running contribute nothing until they reappear —
+    /// the reconcile timer and the session-start watcher keep re-applying, so they're caught on open.</summary>
+    internal static List<CaptureSourceSpec> BuildApplicationSpecs(IEnumerable<string> appNames)
+    {
+        var specs = new List<CaptureSourceSpec>();
+        foreach (var name in appNames.Distinct(StringComparer.OrdinalIgnoreCase))
+        {
+            foreach (var pid in AudioAppEnumerator.PidsForProcessName(name))
+            {
+                specs.Add(new CaptureSourceSpec(ProcessLoopbackId.Format(pid), CaptureKind.ProcessLoopback, name));
+            }
+        }
+        return specs;
+    }
+}

src/RemSound.App/MainForm.cs

@@ -5942,44 +5942,17 @@ public sealed class MainForm : Form
         // Build the unified spec list from all three send-side lists. The CompositeCaptureBackend
         // splits this set internally into WASAPI specs (sent to MixingEngine) and ASIO specs
         // (sent to AsioCaptureBackend). Both run in parallel and their outputs are summed.
-        var specs = new List<CaptureSourceSpec>();
-        var addedLoopbackIds = new HashSet<string>(StringComparer.OrdinalIgnoreCase);
-        string? followedDefaultLoopback = null;
+        // The WASAPI outputs-or-apps portion is the SHARED builder (CaptureSpecBuilder) — the same code
+        // the service assembles its specs with, so the two can no longer drift apart.
         var appsMode = ProcessLoopbackCapture.IsSupported && sendModeList.SelectedIndex == SendModeApplicationsIndex;
-        if (!appsMode)
-        {
-            // Devices mode (classic): loopback-capture whole output devices from the ticked list.
-            foreach (var item in sendOutputDevicesList.CheckedItems.OfType<AudioDeviceChoice>())
-            {
-                if (item.IsDefaultFollower)
-                {
-                    // Loopback-capture whatever Windows currently uses as the default OUTPUT, and follow it.
-                    followedDefaultLoopback = ResolveDefaultDeviceId(NAudio.CoreAudioApi.DataFlow.Render);
-                    if (!string.IsNullOrEmpty(followedDefaultLoopback) && addedLoopbackIds.Add(followedDefaultLoopback))
-                    {
-                        specs.Add(new CaptureSourceSpec(followedDefaultLoopback, CaptureKind.Loopback, "Windows default audio device"));
-                    }
-                }
-                else if (item.DeviceId is { } id && addedLoopbackIds.Add(id))
-                {
-                    specs.Add(new CaptureSourceSpec(id, CaptureKind.Loopback, item.Name));
-                }
-            }
-        }
-        else
-        {
-            // Applications mode: one process-loopback spec per running process of each ticked app name.
-            // Apps not currently running contribute nothing until they reappear (the reconcile timer and
-            // the session-start watcher keep the list fresh and re-apply). Child processes are captured
-            // too (the process-loopback include-tree mode), so a browser's audio renderers are covered.
-            foreach (var name in CheckedSendApplicationNames())
-            {
-                foreach (var pid in AudioAppEnumerator.PidsForProcessName(name))
-                {
-                    specs.Add(new CaptureSourceSpec(ProcessLoopbackId.Format(pid), CaptureKind.ProcessLoopback, name));
-                }
-            }
-        }
+        string? followedDefaultLoopback = null;
+        var specs = appsMode
+            ? CaptureSpecBuilder.BuildApplicationSpecs(CheckedSendApplicationNames())
+            : CaptureSpecBuilder.BuildOutputSpecs(
+                sendOutputDevicesList.CheckedItems.OfType<AudioDeviceChoice>()
+                    .Where(c => c.DeviceId is not null)
+                    .Select(c => (c.DeviceId!, c.Name)),
+                out followedDefaultLoopback);
         lastFollowedDefaultLoopbackId = followedDefaultLoopback;
         var addedInputIds = new HashSet<string>(StringComparer.OrdinalIgnoreCase);
         string? followedDefaultInput = null;

src/RemSound.App/SelfTest.cs

@@ -962,7 +962,16 @@ internal static class SelfTest
         if (expected is null) return Skip("no Windows default output device on this box to resolve the follower against");
         Check(specs.Any(s => s.Kind == CaptureKind.Loopback && s.DeviceId == expected),
             "the follower must resolve to a loopback spec on the current Windows default output");
-        return "follower flagged + sentinel shared with the app; service resolves it to the live default render endpoint";
+
+        // Shared-builder details: the resolved default reports via the out param (drives the app's
+        // re-route detection), and a follower + the SAME device ticked explicitly collapse to one spec.
+        var built = CaptureSpecBuilder.BuildOutputSpecs(
+            new[] { (AudioDefaultFollower.LoopbackSendId, "follow"), (expected, "explicit") }, out var followed);
+        Check(followed == expected, "the resolved default must be reported via the out param");
+        Check(built.Count == 1, "the follower and the same explicit device must collapse to ONE spec");
+        Check(CaptureSpecBuilder.BuildApplicationSpecs(new[] { "zzremsound_no_such_process" }).Count == 0,
+            "an app that isn't running must contribute no specs (it's caught on open by the watcher)");
+        return "sentinel shared + resolved to the live default; builder dedups and reports the followed id";
     }
 
     /// <summary>The encoder-boundary clamp is now ONE shared rule (SampleClamp) used by all three

src/RemSound.App/ServiceSendHost.cs

@@ -557,35 +557,14 @@ public sealed class ServiceSendHost : IDisposable
         var specs = new List<CaptureSourceSpec>();
         var appsMode = ProcessLoopbackCapture.IsSupported
             && string.Equals(p.WasapiSendMode, "applications", StringComparison.OrdinalIgnoreCase);
-        if (appsMode)
-        {
-            // Specific applications only — matches the main app. The old "send all applications"
-            // path (SendAllApplications) was removed from both the app and the service; the profile
-            // flag is ignored here so a stale profile can't resurrect whole-system capture.
-            foreach (var name in p.SelectedSendApplications.Distinct(StringComparer.OrdinalIgnoreCase))
-                foreach (var pid in AudioAppEnumerator.PidsForProcessName(name))
-                    specs.Add(new CaptureSourceSpec(ProcessLoopbackId.Format(pid), CaptureKind.ProcessLoopback, name));
-        }
-        else
-        {
-            var addedOutputs = new HashSet<string>(StringComparer.OrdinalIgnoreCase);
-            foreach (var id in p.SelectedWasapiSendOutputs.Distinct())
-            {
-                if (AudioDefaultFollower.IsLoopbackSend(id))
-                {
-                    // "Use Windows default output" — resolve to the live default render endpoint. Re-run
-                    // on every ApplyProfile (which a default-device change triggers via the notifier), so
-                    // the service FOLLOWS the default. Same sentinel + resolver the main app uses.
-                    var def = AudioDefaultFollower.ResolveDefaultRenderId();
-                    if (def is not null && addedOutputs.Add(def))
-                        specs.Add(new CaptureSourceSpec(def, CaptureKind.Loopback, "Windows default audio device"));
-                }
-                else if (addedOutputs.Add(id))
-                {
-                    specs.Add(new CaptureSourceSpec(id, CaptureKind.Loopback, id));
-                }
-            }
-        }
+        // Both branches are the SHARED builder (CaptureSpecBuilder) — the same code the main window
+        // assembles its specs with, so the two can no longer drift apart. Apps mode = specific apps
+        // only (the "send all" path was removed from both sides; the stale profile flag is ignored);
+        // devices mode resolves the "Use Windows default" sentinel to the live default on every
+        // ApplyProfile, which a default-device change re-triggers via the notifier — so it FOLLOWS.
+        specs.AddRange(appsMode
+            ? CaptureSpecBuilder.BuildApplicationSpecs(p.SelectedSendApplications)
+            : CaptureSpecBuilder.BuildOutputSpecs(p.SelectedWasapiSendOutputs.Distinct().Select(id => (id, id)), out _));
         // The service never sends WASAPI inputs (mics/line-ins) — outputs and specific apps only. Any
         // SelectedWasapiSendInputs left in an old profile is deliberately ignored here.
         return specs;

src/RemSound.Core/SampleClamp.cs

@@ -0,0 +1,23 @@
+namespace RemSound.Core;
+
+/// <summary>
+/// The encoder-boundary hard clamp, shared by all three capture paths (mix engine, ASIO backend,
+/// push-mode WASAPI backend) — which used to carry three private copies of it. Clamps every sample to
+/// [−1, +1] and returns how many were clipped, so callers batch ONE Interlocked.Add per buffer instead
+/// of per-sample interlocked increments on the real-time path (the ASIO copy did up to four per frame).
+/// Exactly ±1 is NOT clipping — only samples beyond the range count.
+/// </summary>
+public static class SampleClamp
+{
+    public static long ClampBuffer(Span<float> samples)
+    {
+        long clipped = 0;
+        for (var i = 0; i < samples.Length; i++)
+        {
+            var v = samples[i];
+            if (v > 1f) { samples[i] = 1f; clipped++; }
+            else if (v < -1f) { samples[i] = -1f; clipped++; }
+        }
+        return clipped;
+    }
+}