feat(api): derive Mix waveform datum density from duration (~333 samples/sec, capped/floored) instead of fixed 2048 buckets

DeepDrftContent/Processors/MixWaveformResolution.cs

@@ -0,0 +1,52 @@
+namespace DeepDrftContent.Processors;
+
+/// <summary>
+/// Derives the bucket count for a Mix loudness datum from the audio's duration, so the stored
+/// profile captures at a constant <em>time</em> resolution instead of a fixed bucket count.
+///
+/// Rationale (phase-9 Mix Visualizer redesign spec §F): the max-zoom window shows one quarter note
+/// at 180 BPM = 333 ms of audio, and a smooth glassy curve wants ~100+ sample points across that
+/// window. A fixed 2048-bucket datum gives fractions of a sample per 333 ms window on any real-length
+/// mix (a 30-minute mix gets ~0.38 buckets), so long content is badly under-sampled. Capturing at a
+/// constant ≈333 samples/sec (≈3 ms/sample) makes a 333 ms window hold ~111 samples regardless of mix
+/// length — the direct expression of "high enough resolution regardless of content length."
+///
+/// This is the orchestration-side derivation (duration → bucket count); the actual compute/store stays
+/// in <see cref="WaveformProfileService"/>, which is content-agnostic and parameterized by bucket count.
+/// </summary>
+public static class MixWaveformResolution
+{
+    /// <summary>≈333 samples/sec (≈3 ms/sample): one quarter note at 180 BPM (333 ms) holds ~111 samples.</summary>
+    public const int SamplesPerSecond = 333;
+
+    /// <summary>
+    /// Upper cap on bucket count (~2,000,000 samples ≈ a 100-minute mix at 333/s). Past this length we
+    /// accept slightly-below-target density rather than an unbounded datum (spec §F mitigation #1).
+    /// </summary>
+    public const int MaxBucketCount = 2_000_000;
+
+    /// <summary>
+    /// Floor on bucket count. Keeps the historical 2048-bucket density as the minimum so a degenerate
+    /// near-zero or very-short mix still yields a usable profile rather than zero/handful of buckets.
+    /// </summary>
+    public const int MinBucketCount = 2048;
+
+    /// <summary>
+    /// Maps a track's duration (seconds) to a bucket count of <c>ceil(durationSeconds × 333)</c>,
+    /// clamped to [<see cref="MinBucketCount"/>, <see cref="MaxBucketCount"/>]. Non-finite or negative
+    /// durations fall to the floor. A 60-minute mix → ~1.2M buckets; a 3-minute mix → ~60k.
+    /// </summary>
+    public static int BucketCountForDuration(double durationSeconds)
+    {
+        if (double.IsNaN(durationSeconds) || durationSeconds <= 0)
+            return MinBucketCount;
+
+        // Guard against overflow before the cast: anything at/above the cap clamps anyway.
+        var raw = Math.Ceiling(durationSeconds * SamplesPerSecond);
+        if (raw >= MaxBucketCount)
+            return MaxBucketCount;
+
+        var buckets = (int)raw;
+        return buckets < MinBucketCount ? MinBucketCount : buckets;
+    }
+}

DeepDrftContent/Processors/WaveformProfileService.cs

@@ -42,7 +42,10 @@ public class WaveformProfileService
     /// <paramref name="entryKey"/> in <paramref name="vaultName"/> (defaults to
     /// <see cref="VaultConstants.WaveformProfiles"/> when null). Bucket resolution defaults to
     /// <see cref="WaveformProfileOptions.BucketCount"/> (512) when <paramref name="bucketCount"/> is null;
-    /// pass a higher value (e.g., 2048) for the Mix high-res datum. Returns false (and logs) on any
+    /// callers pass an explicit count for higher-resolution data — e.g. the Mix datum derives its count
+    /// from the audio duration (≈333 samples/sec, see <c>MixWaveformResolution</c>) so long mixes are not
+    /// under-sampled. This service is content-agnostic: it captures however many buckets it is told to and
+    /// does not itself decide the count. Returns false (and logs) on any
     /// failure — a missing profile is handled gracefully downstream, so callers on the upload path
     /// log-and-continue rather than failing the upload. Does not throw for expected failure modes.
     /// </summary>