xtc/docs/ROADMAP.md

Roadmap

Phase 0 — Baseline

Inspect repository/toolchain, verify Makefile and minimal executable, document dependencies. No system changes without permission.

Baseline implementation: headless startup/shutdown executable, core tests, separate debug build, optional FLTK shell build, and header dependencies. See BUILD.md for toolchain, commands, licensing and limitations.

Phase 1 — C core and offline processing

Graph data structures, validation, cycle detection, topological ordering, preallocated buffers, test oscillator, gain and offline render tests.

Implemented: mono ports, transactional cycle rejection, independent compiled plans, precomputed input routing, oscillator/gain/meter/output, deterministic variable-block tests and lifecycle/error coverage. See AUDIO_ENGINE.md for API limits and compatibility behavior.

Phase 2 — Real-time backend

Choose and implement a Linux backend. Document callback semantics, start/stop, device errors, shutdown, and safe graph publication.

Implemented: optional JACK-compatible mono backend, explicit audio port routing, format/xrun/loss status, start/stop/reopen, and bounded pending/retired plan handoff using lock-free atomics. Hardware-independent failure/concurrency tests and a 30-second software-loopback run pass. See decision 0004 and AUDIO_ENGINE.md. Physical hardware playback/endurance and live server-loss recovery remain unverified; GUI engine controls are implemented in Phase 3.

Phase 3 — FLTK shell

Main window, transport controls, view switching, backend status, clean startup and shutdown through the C API.

Implemented: dark FLTK shell, Play/Stop/Reconnect, view/menu shortcuts, backend state/format/xrun/process status, persistent command errors, timer-based refresh and retirement collection, and callback-safe close/retry. Mock-backend GUI tests and live GUI/JACK start/stop/reconnect/close tests pass under Xvfb. Sequencer, mixer and project-file actions remain placeholders; initial graph is silent. See GUI.md.

Phase 4 — Modular editor

Draw nodes/ports/edges; create, move and connect nodes; reject invalid edges; safely submit edits to the C core.

Implemented: scrollable node/port/wire canvas; add/select/move/delete nodes; drag connections, select/delete wires, and show invalid-edit feedback. C owns stable IDs, read-only snapshots and transactional edits. Exact accepted graph snapshots are submitted to the backend; prepared/queued/active state is exposed through revision acknowledgements. Core/render, GUI rollback, mouse smoke and live adoption tests pass. See GUI.md and AUDIO_ENGINE.md. Preset parameters, mono-only processing, fixed canvas and no persistence/undo/zoom remain limits.

Phase 5 — Project and sequencer

Versioned persistence, stable IDs, tracks, clips, timeline, selection, zoom, scrolling, and transport synchronization.

Implemented: bounded version-1 project persistence with transactional loads and temporary-file/fsync/rename saves; stable track/clip/node IDs; silent regions; track and region creation/deletion, selection, move/trim, timeline zoom/scroll; sample-counted playhead, pause/resume and stopped seek. Project/graph arrangement roundtrips, malformed-input, GUI rollback/gesture and live transport tests pass. See PROJECT_FORMAT.md and GUI.md. Constant tempo/4/4, silent metadata regions, no undo, asset playback, MIDI or persisted GUI layout are explicit limits.

Phase 6 — Audio clips and recording

Audio import/playback, recording through bounded buffers, missing-asset handling, no file I/O in callbacks.

Implemented: bounded PCM16/24/32 and float32 WAV import (mono/stereo downmix), preloaded overlap-summed clip plans with linear rate conversion, and an audio source node using ordinary graph routing. A lock-free SPSC ring transfers JACK input to a control-thread WAV writer; Stop finalizes and adds successful takes. Version 2 metadata stores external asset references and reads v1. Missing assets remain silent with reporting and manual relinking. Core, callback, GUI and live software record/save/reopen/play acceptance tests pass. See AUDIO_FILES.md for memory/format limits, overflow recovery and deferred physical-device validation. No new dependency or callback I/O was introduced.

Phase 7 — MIDI and instrument

MIDI input/events/clips, simple internal instrument, note-off/panic handling, then a basic piano roll.

Implemented: bounded source-relative note events with stable IDs; sample-offset JACK MIDI input; a velocity-scaled sine instrument and separate graph source; panic, note-off, stop and seek handling; and a piano roll for drawing, moving, resizing and deleting notes. Version 3 saves notes and reads v1/v2 projects. Core, fault-injection, GUI and live software MIDI/save/reopen/play tests pass. Acceptance uses drawn notes; MIDI recording and SMF import are deferred. See MIDI.md for capacities, immediate note-off, routing and hardware limits.

Phase 8 — Mixer and editing

Gain/pan, meters, mute/solo, automation, undo/redo, keyboard workflow.

Implemented: selectable per-track channel strips with gain/pan and mute/solo; stereo graph/JACK output and output peak meters; editable gain/pan automation lanes with sample interpolation; bounded 32-edit C snapshot undo/redo and mixer keyboard controls. Format v4 persists mixing/automation and reads v1–v3. Core, GUI rollback/gesture/history and live stereo software playback tests pass. See MIXER.md for routing, mono compatibility, pan law, history behavior and limits. Track inserts/sends, live fader adoption, MIDI capture, per-track meters and advanced automation remain deferred. No project-file editing is needed for the implemented composition/mixing workflow.

Phase 9 — Extensibility and release

Native node API, evaluate plugin hosting, licensing review, packaging, installation tests, performance and recovery tests.

Implemented: versioned native C processor lifecycle and graph source/effect API, explicit instrument-buffer integration, example and error/retirement tests; external-hosting evaluation (deferred); dependency/license review; deterministic source/release targets; core/GUI staged install/uninstall and installed C/C++ consumer tests; headless version/help/metadata validation; offline performance and process-crash/write-failure recovery tests. Source format stays v4, reading v1–v3. See NATIVE_API.md, RELEASING.md, LINUX.md, LICENSES.md, PERFORMANCE.md and RECOVERY.md. Native graphs remain session-only, with unsupported saves preserving files. No stable binary plugin ABI, external host, system package, autosave or physical-device/endurance qualification is claimed.

Phase 10 — Engine Correctness and Reliability

Goal: Verify that the existing implementation behaves correctly under realistic workloads before adding more features.

Tasks

• Audit graph compilation, topological ordering, port compatibility, and cycle detection.
• Test graph replacement while audio is running, including safe reclamation of old processing graphs.
• Verify that audio-thread operations are bounded and non-blocking.
• Test repeated initialization, start/stop cycles, device failures, shutdown, and partial initialization.
• Run AddressSanitizer, UndefinedBehaviorSanitizer, leak checks, and static analysis where available.
• Add regression tests for every bug discovered.
• Document known limitations and thread ownership for shared resources.

Acceptance criteria

• Reproducible tests cover engine lifecycle, graph changes, error handling, and DSP output.
• No known critical real-time safety defects remain.
• Resource ownership and cleanup behavior are documented.

Implemented: graph/DSP and ownership audit; stereo source-alias correction with exact-output regression tests; maximum-size reverse-order graph/cycle rollback; allocation-failure sweeps and callback allocation/free guards; 2000 concurrent native-plan replacements with control-thread destruction and balanced lifetimes; repeatable ASan/UBSan/leak and Clang static-analysis targets. Core, no-JACK, GUI/mock/live JACK, recovery and TSan publication/recorder tests pass. See RELIABILITY.md for ownership, synchronization, bounded-work analysis, commands and evidence. Physical-device/endurance validation and trusted native callback behavior remain explicit limits; no critical realtime safety defect is known within the documented contracts.

Phase 11 — Complete Minimal Music-Making Workflow

Goal: Ensure a user can create, save, reopen, and play a small musical project without manually editing project files.

Tasks

• Verify project creation, track creation, clip arrangement, routing, and transport as one integrated workflow.
• Ensure play, pause, stop, seek, and return-to-start behavior are consistent.
• Save, close, reopen, and continue editing projects.
• Provide useful defaults and actionable error messages.
• Keep the sequencer as the default workspace, with the modular graph available when needed.
• Test that GUI state and engine state remain synchronized.

Acceptance criteria

• A user can create and reopen a small project and reproduce the same arrangement and routing.
• Common operations do not require manual project-file edits.
• Transport and GUI state remain consistent during normal playback and error conditions.

Implemented: explicit Pause/F7 and To start/Home controls, position-preserving Stop compatibility, recording guards, sequencer selection on New/Open, first-use composition/routing guidance, and GUI status refresh after failed installation. A real GUI/backend workflow test creates MIDI and audio, arranges/routes, saves, closes/destroys the window, reopens in a fresh window, compares exact stereo rendering and stable clip/routing metadata, then plays/seeks/pauses/resumes, continues editing with undo/redo and saves/reopens again. Mock failure tests and live JACK/GUI/software MIDI tests pass. See WORKFLOW.md for instructions, transport semantics, test commands and limits. External playback routing remains explicit; physical-device listening and endurance remain unqualified.

Phase 12 — Audio Quality and Transport Correctness

Goal: Make timing and audio output predictable and musically reliable.

Tasks

• Define precise behavior for tempo and meter changes.
• Verify conversion between musical positions and sample positions without accumulated timing drift.
• Test clips and events that cross processing-block boundaries.
• Prevent clicks at clip boundaries and during playback start/stop where appropriate.
• Implement gain ramps and parameter smoothing where abrupt changes could create artifacts.
• Test silence, clipping, denormal values, channel layouts, and unusual supported buffer sizes.
• Document latency reporting and identify whether latency compensation is required by the current architecture.
• Add deterministic tests for sample-accurate event scheduling where supported.

Acceptance criteria

• Automated tests verify timing conversions and expected sample output.
• Tests cover events and clips at block boundaries and tempo changes.
• Listening tests cover transitions, start/stop behavior, and representative projects.
• Known limitations involving latency and synchronization are documented.

Implemented: documented constant-tempo/fixed-4/4 snapshot policy; absolute timing and sample-offset regression matrix; configurable audio/scheduled-note boundary fades; single-owner gain ramps; GUI 5 ms start and asynchronous Pause/Stop fades with control-thread acknowledgement/deactivation; finite/subnormal output policy; exact endpoint, block-size, stereo, overload and failure/retry tests. Latency and synchronization limits are documented in AUDIO_QUALITY.md. Normal/no-JACK, sanitizer/static-analysis, TSan, GUI/workflow/live JACK and install checks pass. make quality-preview generates matched listening WAVs. Human listening and physical-device acceptance remain pending; immediate safety/recording/seek paths and live graph swaps may still click. No tempo/meter map, latency compensation, limiter or project-format change is claimed.

Phase 13 — Recording and Recovery

Goal: Protect recorded material and handle recording failures predictably.

Tasks

• Test long recordings and recording-buffer saturation.
• Handle disk-full conditions, write errors, interrupted recording, and unavailable destinations.
• Keep file I/O outside the real-time audio callback.
• Define and implement recovery or clear reporting for incomplete recordings.
• Make recording destinations and supported file formats explicit.
• Protect existing files from accidental overwrites.
• Test reopening projects when media files are missing, incomplete, or inaccessible.
• Ensure shutdown and project closure do not silently discard pending recorded data.

Acceptance criteria

• Recording failures are reported clearly and do not cause unsafe audio-thread behavior.
• Valid recorded material is preserved whenever possible.
• Incomplete recordings and missing media are detected and explained.
• Recording and recovery behavior is covered by automated or repeatable integration tests.

Implemented: no-clobber recording destinations and race-safe publication; explicit opt-in replacement API; retained failed temporaries and recovery-path reporting; streaming xtc-check --recover-recording TEMP NEW.wav salvage; quiescent destructor draining and exceptional-shutdown reporting. Virtual 30-minute recording, saturation/concurrency, destination races, disk-write failure/prefix recovery, killed writers and missing/incomplete media tests pass. See AUDIO_FILES.md and RECOVERY.md. Physical endurance, power-loss guarantees and playback of assets above the existing preload limit remain unqualified.

Phase 14 — Native Plugin, Instrument, and Effect Architecture

Goal: Provide a stable internal extension interface before hosting external plugins.

Tasks

• Finalize the native C node API, including initialization, processing, reset, and destruction.
• Define parameter metadata, defaults, valid ranges, and automation behavior.
• Specify audio channel layouts and MIDI/event handling.
• Define node state serialization and restoration requirements.
• Establish rules for node creation, failure, reconfiguration, and destruction.
• Ensure the GUI uses documented public APIs rather than internal engine structures.
• Provide a small example instrument and effect implemented against the public interface.
• Document ABI/API compatibility expectations for native extensions.

Acceptance criteria

• A developer can implement a small native instrument or effect using documented interfaces.
• Invalid parameters, unsupported layouts, and failed initialization are handled safely.
• Node state can be saved and restored where required.
• No C++ or FLTK types leak into the public C API.

Implemented: ABI 2 parameter-only C processor lifecycle with copied descriptors, metadata/default/range validation, sample-offset parameter/MIDI ordering, transactional portable configuration restoration, and public gain/instrument examples. Lifecycle, invalid input, event ordering, in-place output, reset, callback failure and state compatibility tests pass. See NATIVE_API.md. ABI 1 graph compatibility is retained; native graph persistence and opaque plugin state are explicitly outside this parameter-only interface.

Phase 15 — External Plugin Hosting

Goal: Add external plugin support only after the internal node lifecycle is reliable.

Tasks

• Evaluate candidate plugin formats for licensing, dependencies, maintenance, and Linux support.
• Record the selected format and rationale in an architecture decision document.
• Implement plugin discovery and loading outside the real-time callback.
• Map plugin audio ports, parameters, MIDI/events, and channel layouts to the engine's node model.
• Support plugin state save/load and clear reporting for missing or incompatible plugins.
• Test plugins with different channel counts, parameter sets, and processing requirements.
• Define how plugin failures are reported and how the engine recovers.
• Document crash-isolation limitations; an in-process plugin must not be assumed safe merely because the host is safe.

Acceptance criteria

• A small, documented set of compatible third-party plugins can be loaded, connected, saved, reopened, and played.
• Missing or incompatible plugins do not silently corrupt the project.
• Plugin loading and destruction do not occur in the real-time callback.
• The chosen format's dependencies and licensing are documented.

Implemented: bounded LADSPA 1.1 C host with explicit file/index discovery, control-thread loading/lifecycle, mono source/effect graph mapping, independent stereo instances, metadata/range checks and v5 identity/control persistence. Public-API GUI load/configure commands support ordinary routing/save/reopen and undo. Fixture/error/retirement/GUI tests and installed amp/inverter/sine acceptance pass. See decision 0009, PLUGIN_HOSTING.md and LICENSES.md. MIDI, true multichannel layouts, opaque state, editors, latency compensation and crash isolation are unsupported; plugin realtime behavior and physical listening are unqualified.

Phase 16 — Automation and Expressive Control

Goal: Make automation and MIDI control reliable, sample-timed, and usable.

Tasks

• Support automation for common parameters such as volume, pan, instrument parameters, and effect parameters.
• Define interpolation modes and behavior at clip boundaries.
• Evaluate automation at the correct time and sample offsets for each processing block.
• Smooth parameter changes where needed to avoid zipper noise or audio artifacts.
• Add MIDI controller mapping where appropriate.
• Handle note-off, panic, seek, stop, project teardown, and device disconnect correctly.
• Define deterministic ordering for MIDI and automation events that occur at the same sample position.
• Test overlapping notes, repeated note-ons, and interrupted playback.

Acceptance criteria

• Automation and MIDI control reproduce consistent results across repeated playback and project reloads.
• Held notes are released safely on stop, panic, seek, and relevant failure paths.
• Event ordering and interpolation behavior are documented and tested.

Implemented: persisted sample-evaluated LADSPA parameter lanes with linear/step modes, range/discrete validation and public GUI editing; absolute automation position through seek and plan handoff; fixed live CC7/10/64 mappings with volume/ pan ramps, sustain/retrigger safety, and exact block-partition regressions. Existing track Gain/Pan lanes cover audio and scheduled instrument volume. Format v6 migrates v1–v5. See CONTROL.md. MIDI capture/learn, pitch bend, new instrument timbres and physical listening remain outside the supported scope.

Phase 17 — Editing Workflow, Undo/Redo, and Usability

Goal: Make common editing tasks efficient and protect users from accidental changes.

Tasks

• Make common operations undoable, including create, delete, move, trim, duplicate, and connect.
• Define undo behavior for recording and destructive operations.
• Add consistent keyboard shortcuts and context menus for frequently used actions.
• Implement predictable snapping, grid settings, zoom, scrolling, and selection behavior.
• Improve clip labels, track organization, and visual feedback.
• Make destructive actions difficult to trigger accidentally.
• Keep the GUI responsive while audio is running.
• Ensure undo/redo updates project state and the active engine graph consistently.
• Test keyboard focus and accessibility of essential controls where practical.

Acceptance criteria

• Common editing workflows can be completed without unnecessary dialogs or manual workarounds.
• Undo/redo restores the intended project state and corresponding engine state.
• GUI operations do not block the real-time audio thread.
• Interaction behavior is documented and covered by relevant tests.

Implemented: transactional full-source clip duplication, clip/track names, track ordering, configurable timeline snap, Ctrl+D/F2/Alt+arrow commands and a context menu; kind/name clip labels and focus regressions. Snapshot/GUI tests verify duplicate/rename/order undo/redo, source-note retention, capacity rollback and existing graph/backend history behavior. Recording undo removes references, never media. See WORKFLOW.md. Background loading, persisted UI layout and broad screen-reader/accessibility qualification remain explicit limits.

Phase 18 — Project Format, Compatibility, and Portability

Goal: Keep project files valid, recoverable, and maintainable over time.

Tasks

• Version the project format and implement explicit migration logic when the format changes.
• Validate project data before constructing engine objects from it.
• Handle malformed files, unknown node types, missing assets, and unavailable plugins.
• Define project-folder behavior and reliable media path handling.
• Make saving robust against interrupted writes, for example by writing a temporary file and replacing the destination safely where supported.
• Test save/load round trips and migration from every format version that is supported.
• Keep the format documented and preferably human-inspectable.
• Avoid silently discarding unknown data during load/save unless the format policy explicitly requires it.

Acceptance criteria

• Supported older projects load correctly or produce clear, actionable compatibility errors.
• Malformed project files do not crash the application or trigger unsafe allocations.
• Missing media and plugins are reported clearly.
• Save/load round trips and format migrations have automated tests.

Implemented: v7 folder-relative media persistence with absolute runtime references, transactional resolution independent of launch cwd, Save As identity preservation, and parent-directory fsync after atomic publication. Checked-in v1–v6 migrations (including missing plugins), moved-folder exact playback, missing-media retention, truncation/mutation and unknown-record/error tests pass. See PROJECT_FORMAT.md. No media is copied/deleted automatically; absolute external/plugin dependencies, path bounds, post-rename sync errors and physical power-loss limits are explicit.

Phase 19 — Performance, Scalability, and Stress Testing

Goal: Measure performance and establish predictable behavior as projects grow.

Tasks

• Create repeatable benchmarks for increasing node counts and audio channel counts.
• Measure processing time at supported sample rates and block sizes.
• Profile graph compilation separately from real-time audio processing.
• Identify unnecessary buffer copies, allocations, and redundant processing.
• Test large arrangements, many clips, automation events, and active meters.
• Define bounded processing limits and meaningful overload diagnostics.
• Test under CPU pressure and during frequent GUI redraws.
• Optimize only after profiling identifies a meaningful bottleneck.
• Record baseline benchmark results so regressions can be detected.

Acceptance criteria

• Performance targets and test conditions are documented.
• Repeatable benchmarks cover representative small and large projects.
• Overload behavior is defined and does not cause unbounded work in the callback.
• Significant performance regressions can be detected through repeatable tests.

Implemented: 24-case node/channel/rate/block matrix with separate compilation and warmed process timings, recorded machine/compiler baseline and opt-in regression budgets; maximum track/clip/note/automation/meter capacity checks and publication/GUI tests under CPU contention. Measurements justified skipping unused live-controller processing; exact output tests remain passing. See PERFORMANCE.md and performance-baseline.json for results, thresholds and overload policy. Arbitrary plugin costs and physical realtime/endurance are unqualified.

Phase 20 — Release Engineering, Packaging, and Maintenance

Goal: Make the application straightforward to build, install, use, diagnose, and maintain.

Tasks

• Provide reproducible release builds and clean build instructions.
• Add Makefile targets for building, testing, installing, and uninstalling.
• Package the application and required resources for the initial supported Linux target.
• Review licenses and attribution for direct and bundled dependencies.
• Document supported audio backends, required system packages, and configuration.
• Add useful crash and error diagnostics without compromising real-time processing.
• Test fresh installations, upgrades, missing dependencies, and removal.
• Publish a basic user manual, developer documentation, and a known-limitations list.
• Define a release checklist and a policy for handling regressions and project-format compatibility.

Acceptance criteria

• A person other than the primary developer can build or install the application using the documented steps.
• The installed application can complete the supported core workflow.
• Installation and removal are tested on the initial target environment.
• Dependencies, licensing, limitations, and troubleshooting steps are documented.
• A release can be reproduced from a clean checkout with recorded build instructions.

Implemented: normalized Linux binary/development archives with SHA-256/file/ ELF-dependency manifests, headless compiled-feature diagnostics, repeat-install upgrade/repair and installed C/C++ save/reopen/render checks. Existing deterministic source/release, no-dependency, sanitizer, staged uninstall and GUI/live tests are retained. USER_MANUAL.md, KNOWN_LIMITATIONS.md and MAINTENANCE.md consolidate setup, operation, licensing/diagnostics, release and regression policy. No public release or distro registration is performed. Independent human installation/usability, physical-device/endurance and listening acceptance remain pending and must be completed before claiming production qualification.

Phase 21 — Advanced MIDI and Composition

Goal

Make MIDI composition expressive and efficient without turning the application into an unnecessarily complicated notation editor.

Tasks

• Improve piano-roll editing for note creation, selection, movement, resizing, and deletion.
• Support configurable snap and grid resolution, including triplets where practical.
• Add velocity editing and editing for supported MIDI note properties.
• Support MIDI channel and note-range filtering where appropriate.
• Improve quantization, transposition, duplication, and pattern repetition.
• Ensure MIDI event ordering is deterministic, especially when note-off and note-on events share a timestamp.
• Define handling for overlapping notes, repeated notes, and malformed MIDI events.
• Add regression tests for MIDI editing and playback scheduling.

Acceptance criteria

• Common MIDI editing tasks can be completed without manually editing project files.
• Editing preserves unrelated notes and event properties.
• MIDI scheduling remains deterministic at buffer boundaries and under transport changes.
• MIDI operations are covered by automated tests.

Implemented: transactional C filtered note transforms, nearest-grid quantization, velocity/channel edits, transposition, bounded fresh-ID repetition and deletion; piano-roll Notes menu, keyboard commands, configurable straight/triplet grids, channel/pitch filtering and filtered-set selection. Existing mouse move/resize and project undo/redo remain supported. Core and GUI regression tests demonstrate property/other-clip preservation, rollback, undo/redo, adjacent/overlapping notes, equal-offset caller ordering, exact block partitions and seek chase. See MIDI.md. No format change, MIDI capture, SMF import, lasso selection or piano-roll zoom; physical-device and listening validation remain pending.

Acceptance evidence:

• Menu/keyboard/mouse commands edit notes without project-file editing.
• Filtered batch tests preserve unmatched notes and unrelated properties.
• Exact scheduling tests cover buffer boundaries and transport seeks.
• Core composition and GUI undo/redo regressions exercise these operations.

Phase 22 — Audio Clip Editing and Utility Operations

Goal

Provide focused, non-destructive audio editing tools for arranging and preparing recorded or imported material.

Tasks

• Support moving, trimming, splitting, duplicating, and deleting audio clips.
• Define clip-boundary behavior, including fades and source offsets.
• Add non-destructive clip gain and basic fade-in/fade-out controls.
• Support time-stretching and pitch-shifting only after selecting a suitable implementation and quality target; otherwise document them as future work.
• Ensure edits do not unexpectedly modify source audio files.
• Add waveform display caching that never blocks the audio thread.
• Test edits at project boundaries, source-file boundaries, and very short clip lengths.
• Document which operations are destructive and non-destructive.

Acceptance criteria

• Basic clip editing is predictable and undoable.
• Source files remain unchanged unless a documented destructive operation is explicitly invoked.
• Clip boundaries do not produce unintended clicks from invalid offsets or discontinuities.
• Editing and waveform generation never perform blocking work in the real-time audio callback.

Implemented: transactional audio/silent-clip splitting with stable IDs and source offsets; persisted linear clip gain and explicit in/out fades in v8 (v1–v7 migrate to unity/no explicit fades); undoable sequencer menu/context/keyboard controls; immutable bounded source-peak summaries shared across clips and drawn from cached memory. Existing move/trim/duplicate/delete remain supported; very short clip hit testing now matches drawing. See AUDIO_FILES.md and PROJECT_FORMAT.md. Stretch/pitch processing and crossfades remain future work; waveform loading is synchronous on the control thread, and physical listening remains pending.

Acceptance evidence:

• Core/GUI regressions cover split/duplicate/delete and snapshot undo.
• Exact byte comparison verifies source files survive all metadata edits.
• Boundary tests cover invalid offsets, short tails, one-frame media, explicit fade endpoints, gain, block partitions, seeks and capacity rollback.
• Waveform construction stays on control-thread paths; redraw reads cached peaks and audio rendering retains immutable/preloaded callback storage.

Phase 23 — Modular Workflow and Routing Refinement

Goal

Preserve the modular spirit of the application while making complex projects understandable and manageable.

Tasks

• Refine graph and routing views so signal paths are easy to inspect.
• Make connections, disconnections, and invalid routing states clear.
• Provide useful names and visual distinctions for tracks, instruments, effects, buses, and I/O ports.
• Support track grouping or folders if the project model can represent them cleanly.
• Add routing presets or reusable track templates only if they do not complicate the core model.
• Define behavior for cycles and feedback paths; reject unsupported cycles explicitly.
• Make graph changes transactional so failed edits do not leave partially updated routing.
• Add tests for graph changes while playback is running.

Acceptance criteria

• Users can inspect and understand a project's signal path.
• Invalid graph edits fail safely and leave the previous valid graph intact.
• Graph updates follow the documented real-time synchronization strategy.
• GUI routing concepts map cleanly to the public C engine API.

Phase 24 — Mixing and Mastering Essentials

Goal

Provide the essential mixing controls needed to finish a small or medium-sized project.

Tasks

• Complete track and bus gain, pan, mute, solo, and metering behavior.
• Define how solo states affect buses, sends, and sidechains.
• Add peak meters with clear units and documented ballistics.
• Add master-bus monitoring and clipping indicators.
• Support routing to buses and auxiliary sends if the graph model supports them.
• Make gain staging explicit and avoid hidden or surprising level changes.
• Document meter tap points and whether values are pre-fader, post-fader, or post-effects.
• Test mute, solo, pan, gain, and routing interactions.

Acceptance criteria

• Mixer controls behave consistently across tracks, buses, and master output.
• Meter readings correspond to documented signal points and units.
• Solo and mute behavior is covered by automated tests.
• No mixer operation introduces allocations, locks, or other prohibited work in the audio callback.

Phase 25 — Plugin and Instrument Usability

Goal

Make instruments and effects practical to use, manage, and diagnose in real projects.

Tasks

• Improve plugin discovery, insertion, removal, replacement, and ordering.
• Show useful plugin identity and status information, including load failures.
• Define parameter metadata, ranges, defaults, units, and formatting.
• Support parameter reset and safe default values.
• Preserve plugin state across save/load where the plugin API supports it.
• Define behavior when a saved plugin is missing or incompatible.
• Manage plugin UI lifetime separately from engine object lifetime.
• Add tests for plugin lifecycle, state restoration, and failure paths.
• Document plugin API versioning and compatibility expectations.

Acceptance criteria

• Users can identify and manage loaded instruments and effects without editing configuration files.
• Missing or incompatible plugins produce actionable diagnostics and do not silently corrupt projects.
• Plugin state restoration is tested for supported plugin types.
• The engine remains safe if plugin initialization or processing fails, within the documented failure model.

Phase 26 — Automation and Control Mapping

Goal

Enable repeatable changes to mix and instrument parameters over time, with clear timing and ownership rules.

Tasks

• Define a common parameter identity and metadata model.
• Support automation lanes for a carefully selected set of parameters.
• Define interpolation modes and behavior at clip boundaries.
• Specify how automation interacts with live parameter changes and MIDI control.
• Add control mapping for selected external MIDI controllers if feasible.
• Schedule automation deterministically without allocating memory in the audio callback.
• Define behavior when a target parameter or plugin is missing.
• Test automation at buffer boundaries, during seeks, and across loop points.
• Document precedence rules for automation, controller input, and direct user changes.

Acceptance criteria

• Supported parameters can be automated and saved in a project.
• Playback produces deterministic automation values for a given project and transport position.
• Missing targets fail safely and are reported to the user.
• Automation processing meets the engine's real-time safety requirements.

Phase 27 — Editing Workflow, Undo/Redo, and Usability

Goal

Make common editing actions quick, discoverable, and safe to reverse.

Tasks

• Audit user-visible editing operations for undo/redo support.
• Implement command-based edits with explicit ownership and rollback behavior.
• Group related low-level changes into one meaningful undo step.
• Define undo behavior for operations affecting engine state or external files.
• Improve keyboard shortcuts, selection behavior, focus handling, and context menus.
• Use confirmation only for genuinely destructive or difficult-to-reverse actions.
• Provide clear feedback for long-running operations and failures.
• Add GUI tests or reproducible manual test procedures for critical workflows.
• Keep presentation logic out of the core C engine.

Acceptance criteria

• Documented editing operations have predictable undo/redo behavior.
• Undo and redo restore the project model and relevant engine state consistently.
• Failed operations do not leave partial changes behind.
• Common tasks can be completed using documented keyboard and mouse interactions.

Phase 28 — Project Format, Compatibility, and Portability

Goal

Make project files robust, diagnosable, and maintainable across application versions and supported systems.

Tasks

• Version the project format and document its compatibility policy.
• Validate project input before applying it to the live engine.
• Handle missing media, plugins, and external dependencies with clear diagnostics.
• Define relative-path, absolute-path, and media-relocation behavior.
• Add safe migration routines for supported older versions.
• Preserve unknown optional fields where practical, or document when they cannot be preserved.
• Test malformed files, truncated files, oversized input, and invalid values.
• Make saves atomic where the platform permits, using a temporary file and replacement strategy.
• Document portability limits, including plugin and audio-backend dependencies.

Acceptance criteria

• Invalid project files are rejected safely without partially replacing the current project.
• Save failures do not silently destroy the previous valid project file.
• Version incompatibilities and missing dependencies are clearly reported.
• Supported project versions have migration and regression tests.

Phase 29 — Performance, Scalability, and Stress Testing

Goal

Establish measurable performance expectations and verify that larger projects remain stable.

Tasks

• Define representative test projects and target buffer sizes.
• Measure audio callback duration, deadline misses, CPU use, memory use, and graph rebuild costs.
• Test projects with increasing numbers of tracks, graph nodes, connections, MIDI events, and automation points.
• Test long playback sessions and repeated start, stop, seek, and loop operations.
• Stress-test device changes, plugin failures, missing media, and project reloads.
• Use profiling to identify bottlenecks before introducing optimizations.
• Verify that the real-time callback does not allocate, block on mutexes, perform file I/O, or wait on GUI work.
• Document expected limits and known performance bottlenecks.
• Add sanitizers and static analysis to regular development builds where supported.

Acceptance criteria

• Performance is measured against documented test cases, not subjective impressions alone.
• Stress tests can run repeatedly and produce reproducible results.
• No known real-time safety violation remains in the engine's own callback path.
• Performance regressions can be detected by repeatable tests or benchmark reports.

Phase 30 — Release Engineering, Packaging, and Maintenance

Goal

Turn the application into a maintainable release that users can build, install, run, and troubleshoot.

Tasks

• Define supported operating systems, compiler versions, and audio backends.
• Document build dependencies and provide clear build and test commands.
• Add install and uninstall targets that respect PREFIX and standard directory variables.
• Provide appropriate package metadata and desktop integration where relevant.
• Verify installation paths for binaries, libraries, headers, resources, documentation, and example projects.
• Add version reporting and a concise diagnostic mode where useful.
• Prepare release notes, known limitations, and upgrade guidance.
• Test clean builds from a fresh checkout and installation into a staging directory.
• Define a release checklist covering tests, warnings, packaging, and project compatibility.
• Document how users can report reproducible bugs and how maintainers should triage them.

Acceptance criteria

• A clean checkout can be built and tested using documented commands.
• Installation works with a configurable prefix and does not overwrite unrelated user files.
• The release includes documentation, known limitations, and a reproducible version identifier.
• The project has a repeatable process for preparing and validating releases.

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Recommended Execution Order

Reorder the phases when the audit reveals a blocking dependency. Do not mark a phase complete merely because code exists: its acceptance criteria must be demonstrated by builds, tests, or documented manual verification.

Definition of Done for Each Phase

A phase is complete only when:

• The project builds with warnings enabled.
• Relevant automated tests pass, and manual tests are documented where automation is impractical.
• Error paths, resource ownership, and thread boundaries have been reviewed.
• Real-time audio constraints remain satisfied.
• Documentation is updated alongside API or behavior changes.
• No unrelated functionality was silently removed.
• Untested behavior and known limitations are reported honestly.
• The phase's acceptance criteria are explicitly checked off.

Do not skip phases without documenting why and preserving acceptance criteria.