Fix Android waveform IndexOutOfBoundsException on long files

[sjoenk]

Summary

• Fix an IndexOutOfBoundsException (negative index) in getWaveform / getWaveformBytes on Android for medium-to-large audio files (e.g. a 5-minute MP3).
• Root cause was a 32-bit integer overflow: the per-window offset i * totalSamples exceeds Int.MAX_VALUE once a file decodes to ~millions of samples, wrapping to a negative index.
• The window-bound math now uses 64-bit arithmetic, and the RMS reduction is extracted into a testable helper with regression coverage.

Closes #45

Changes

• android/.../AudioDecoderPlugin.kt: extract the RMS/normalization reduction into an internal fun computeWaveform(...) and compute the window start with (i.toLong() * totalSamples / numberOfSamples).toInt() so large files no longer overflow.
• android/.../AudioDecoderPluginTest.kt: add three tests — a regression test with 15,567,358 samples (the length from the bug report), an empty-input test, and an absolute-normalization test.
• example/integration_test/: add an on-device regression test that synthesizes a long (3M-frame) WAV at runtime via a new buildPcmWav helper and runs getWaveform over it, catching the overflow end-to-end without bundling a large asset.
• CHANGELOG.md / pubspec.yaml: release 0.8.1 documenting the fix.
• example/pubspec.lock: refresh the resolved local package version to 0.8.1.

Test plan

• Run the native unit tests from example/android/: ./gradlew :audio_decoder:testDebugUnitTest — all 14 tests pass, including computeWaveform_largeSampleCount_doesNotOverflow.
• On a real Android device (Samsung SM A266B, API 36), run getWaveform over a long file without an IndexOutOfBoundsException — verified via the new integration test using a synthetic 3M-frame WAV that crosses the same 32-bit overflow threshold (not a literal 4–6 min MP3, but exercises the identical code path).
• Confirm waveforms for short files are unchanged — the short-file getWaveform / getWaveformBytes integration tests pass on device, and the example app renders waveforms correctly.

Time spent

⏱️ Estimated time spent: 3 hours

[Copilot]

Pull request overview

Fixes a 32-bit integer overflow in the Android waveform reduction that crashed getWaveform/getWaveformBytes on long files (issue #45). The window-offset arithmetic now uses 64-bit math, and the reduction is extracted into a testable helper with new unit and integration coverage. Version bumped to 0.8.1.

Changes:

• Extract RMS reduction into internal fun computeWaveform(...) and compute window starts via (i.toLong() * totalSamples / numberOfSamples).toInt().
• Add Android unit tests (large-sample regression, empty input, absolute normalization) and a Dart integration regression test using a new buildPcmWav helper.
• Release 0.8.1 (CHANGELOG, pubspec, example lockfile).

Reviewed changes

Copilot reviewed 6 out of 7 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
android/src/main/kotlin/nl/silversoft/audio_decoder/AudioDecoderPlugin.kt	Extracts computeWaveform helper; uses 64-bit math for window offsets.
android/src/test/kotlin/nl/silversoft/audio_decoder/AudioDecoderPluginTest.kt	Adds 3 unit tests covering the helper, including the overflow regression.
example/integration_test/test_helpers.dart	Adds buildPcmWav helper to synthesize long PCM WAV files.
example/integration_test/plugin_integration_test.dart	Adds on-device regression test using a 3M-frame synthetic WAV.
CHANGELOG.md	Documents the 0.8.1 fix.
pubspec.yaml	Bumps version to 0.8.1.
example/pubspec.lock	Refreshes resolved local package version.

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