Test/coverage modulation effects 187

[raajpatre]

What does this PR do?

Adds 20 unit tests across the three LFO-modulated effects — Chorus,
Flanger, and Phaser — pushing line coverage from ~50–59% to 100% on
all three files.

The largest single gap was process_stereo(), which was completely
untested in all three effects. Six of the 20 tests target that path
directly. Beyond stereo, the new tests cover parameter edge cases
(Level/Mix = 0 and 1, negative feedback, Depth = 0), reset()
correctness for delay buffers and APF state, and behavioral
verification (e.g. 4 stages vs 12 stages produce different output,
not just finite output).

Chorus (6 tests)

• chorus_process_stereo_produces_finite_output — exercises the entire process_stereo() path (was completely untested)
• chorus_process_stereo_disabled_early_return — covers the if (!enabled_) guard in process_stereo
• chorus_level_zero_passes_input_unchanged — verifies dry passthrough when Level = 0.0
• chorus_level_one_fully_wet — verifies wet blend when Level = 1.0
• chorus_reset_clears_delay_and_lfo_phase — confirms delay_buffer_ and lfo_phase_ are both zeroed by reset()
• chorus_parameters_have_valid_ranges — verifies all 3 params have correct defaults and ranges

Flanger (6 tests)

• flanger_process_stereo_produces_finite_output — exercises process_stereo() including delay_buffer_r_ and 180° LFO offset
• flanger_process_stereo_disabled_early_return — covers the if (!enabled_) guard in process_stereo
• flanger_negative_feedback_stays_finite — covers the negative feedback range [-0.95, 0.0) which was untested
• flanger_negative_feedback_differs_from_positive — confirms negative feedback takes a genuinely different code path
• flanger_mix_zero_passes_dry_signal — verifies dry passthrough when Mix = 0.0
• flanger_reset_clears_both_buffers — confirms both delay_buffer_ and delay_buffer_r_ are zeroed by reset()

Phaser (8 tests)

• phaser_process_stereo_produces_finite_output — exercises the right-channel APF cascade (apf_xprev_r_, apf_yprev_r_, feedback_state_r_) which was completely untested
• phaser_process_stereo_disabled_early_return — covers the if (!enabled_) guard in process_stereo
• phaser_depth_zero_freezes_sweep — verifies Depth=0 freezes fc at 200 Hz regardless of LFO position
• phaser_feedback_zero_stays_finite — covers the zero-feedback path (x = dry + 0 * feedback_state_)
• phaser_feedback_max_stays_finite — verifies 0.95 maximum resonance stays finite under heavy feedback
• phaser_mix_zero_passes_dry_signal — verifies dry passthrough when Mix = 0.0
• phaser_stage_count_affects_output — confirms 4 stages and 12 stages produce meaningfully different output (stronger than the existing finite-only check)
• phaser_reset_clears_apf_state — confirms all 4 APF arrays and both feedback_state vars are zeroed by reset(), verified by comparing output against a fresh instance

Related Issue

Fixes #187

Type of Change

• 🐛 Bug fix
• ✨ New feature
• ♻️ Refactor / code cleanup
• 📝 Documentation
• ✅ Tests
• 🔧 Build / CI / Configuration

How Was This Tested?

• Platform(s) tested on: macOS (Apple Silicon)
• Test command run: ./amplitron-tests
• All 195 tests pass, 0 failures
• Coverage measured with lcov — all three target files hit 100%:

File	Before	After
chorus.cpp	59.5%	100%
flanger.cpp	52.9%	100%
phaser.cpp	51.8%	100%

Checklist

• Code compiles and builds successfully on my platform
• All existing tests pass (./amplitron-tests)
• New tests added for new functionality (if applicable)
• Documentation updated (README, code comments) if applicable
• No blocking calls on the audio thread (no std::mutex::lock() on the hot path)
• Tested on: macOS Sequoia, Apple Silicon

Screenshots / Demo

N/A — test-only change, no UI modifications.

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[raajpatre]

@sudip-mondal-2002
Submitted PR #187 to address this issue.
Adds 20 tests across Chorus, Flanger, and Phaser —
all three files now at 100% line coverage.

[raajpatre]

@sudip-mondal-2002 could you please add the appropriate
GSSoC label (level:intermediate) to this PR? Thanks!

[github-actions]

PR Preview Removed

The GitHub Pages preview for this PR has been removed because the PR was closed.

[sudip-mondal-2002]

@raajpatre Please check the conflicts

[sudip-mondal-2002]

Closing as AI slop