[mazkev]

Summary

This PR addresses boundary inconsistencies across scan/search methods (PreviousSet, NextSet, NextSetMany), eliminates an undefined shift-by-64 in PreviousSet/PreviousClear, and fixes a signed integer cast in wordsNeededUnbound.

Problem Details & Fixes

1. Logical length bounds check in PreviousSet, NextSet, and NextSetMany:

   • In PR Keep PreviousClear within the bitset length #235 (fix/previous-clear-logical-length), PreviousClear was updated to check if i >= b.length { return 0, false } so queries beyond the logical length are cleanly rejected.
   • However, PreviousSet, NextSet, and NextSetMany did not perform this check.
   • For example, with b := New(10).Set(5):
     • b.PreviousSet(20) returned (5, true) because index 20 falls within the first 64-bit backing word.
     • b.PreviousSet(64) returned (0, false) because word 1 was not allocated in b.set.
     • In both cases the queried index was out of bounds (>= b.length). Returning different results based solely on backing slice allocation boundaries is unexpected.
   • Added if i >= b.length { return 0, false } to PreviousSet, NextSet, and NextSetMany to match PreviousClear, NextClear, and Test.

2. Shift-by-64 in PreviousSet and PreviousClear:

   • Both methods previously used word & ((1 << (wordsIndex(i) + 1)) - 1). When wordsIndex(i) == 63, this evaluated 1 << 64 on a uint64.
   • Updated to word & (allBits >> (wordSize - 1 - wordsIndex(i))), avoiding shifting by 64 bits and working portably across compilers and architectures without arithmetic subtraction.

3. Signed integer conversion in wordsNeededUnbound:

   • wordsNeededUnbound had (int(i) + wordMask) >> log2WordSize. For large values of i where the high bit was set (e.g. 1 << 63), converting int(i) produced a negative integer, causing negative return values.
   • Restored to int((i + wordMask) >> log2WordSize).

Tests Added

• TestPreviousSetBeyondLength: Verifies that PreviousSet returns (0, false) for indices beyond the bitset's logical length.
• TestNextSetBeyondLength: Verifies that NextSet returns (0, false) for indices beyond length.
• TestNextSetManyBeyondLength: Verifies that NextSetMany returns empty results for indices beyond length.
• TestWordsNeededUnboundSignedCast: Verifies non-negative return values for large uint inputs.
• TestPreviousSetWordBoundary: Verifies correct behavior when wordsIndex(i) == 63.
• All existing tests and fuzz tests pass (go test ./... and go vet ./... across amd64 and 386).