shred: schedule passes with Gutmann pass groups (genpattern)

[sankalpsthakur]

Summary

Depends on #13694 (sector-phase patterns).

Default path uses clean-room genpattern + PASS_GROUPS (Gutmann groups, Bresenham random spacing, fixed-pattern shuffle). Phase-variant codes can be scheduled.

--random-source still uses the legacy test-compatible sequence (dual path) so existing integration tests stay green. Residual-entropy scheduling is a follow-up.

Not a GNU source port; clean-room from public algorithm description.

Tests

cargo test -p uu_shred --lib
cargo test --test tests test_shred -- --test-threads=1

Structural genpattern unit tests only; exact GNU distribution matrix lands with residual entropy.

Assisted by Grok (xAI); reviewed and tested locally.

[codspeed-hq]

Merging this PR will not alter performance

✅ 15 untouched benchmarks
⏩ 378 skipped benchmarks¹

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_{Comparing sankalpsthakur:fix/11611-shred-genpattern (e1915f5) with main (822aa83)}

Footnotes

1. 378 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[collinfunk]

Not a GNU source port; clean-room from public algorithm description.

Note that the format and comments here are clearly copied from GNU source source code:

const PASS_GROUPS: &[i32] = &[
    -2, // 2 random passes
    2, 0x000, 0xFFF, // 1-bit
    2, 0x555, 0xAAA, // 2-bit
    -1,    // 1 random pass
    6, 0x249, 0x492, 0x6DB, 0x924, 0xB6D, 0xDB6, // 3-bit
    12, 0x111, 0x222, 0x333, 0x444, 0x666, 0x777, 0x888, 0x999, 0xBBB, 0xCCC, 0xDDD,
    0xEEE, // 4-bit
    -1,    // 1 random pass
    // First bit of each 512-byte sector flipped (phase variants)
    8, 0x1000, 0x1249, 0x1492, 0x16DB, 0x1924, 0x1B6D, 0x1DB6, 0x1FFF, 14, 0x1111, 0x1222, 0x1333,
    0x1444, 0x1555, 0x1666, 0x1777, 0x1888, 0x1999, 0x1AAA, 0x1BBB, 0x1CCC, 0x1DDD, 0x1EEE,
    -1, // 1 random pass
    0,  // end
];

Unsure if it was you who wrote that or mechahitler [1].

[1] https://www.npr.org/2025/07/09/nx-s1-5462609/grok-elon-musk-antisemitic-racist-content