Bit String Previous Set Position
Find the last set bit strictly before an explicit position; null preserves absence instead of inventing an index.
Description
Find the last set bit strictly before an explicit position; null preserves absence instead of inventing an index.
A previous-set query locates the nearest one strictly before a boundary position.
When to use this tool
Find the last set bit strictly before an explicit position; null preserves absence instead of inventing an index.
- Prepare the supplied bits, position fields for this operation.
- Compare the complete example with the representation used by your own input before carrying the result into another step.
How to use the tool and how it works
- Enter bits, position. Use the exact text syntax, units and index conventions explained below.
- Run the tool. If input validation fails, correct the supplied fields before interpreting any result.
- Read result.position. Interpret false, null and empty lists using the distinctions below; they can be valid outputs.
Require position from zero through the bitstring length. Collect set indexes smaller than position and return the last, or null when absent.1
| Input field | Meaning and representation | Accepted input |
|---|---|---|
| bits | bits. | Binary text containing only 0 and 1; at most 128 characters |
| position | position. | Integer from 0 to bits.length inclusive |
| Output field | Interpretation |
|---|---|
| result | Object containing the named outputs below. |
| result.position | Zero-based matching position, or null when the requested match is absent. |
{
"input": {
"bits": "10100",
"position": 2
},
"output": {
"result": {
"position": 0
}
}
}Limitations and assumptions
- The boundary itself is excluded. Index zero and a missing result must be distinguished from null.
- Every bitstring is at most 128 characters and contains only literal zero and one.
- All declared input fields are required and unexpected top-level fields are rejected. Numbers must be finite; lists are limited to 128 entries and ordinary text fields to 8,192 UTF-16 code units, with tighter limits where stated.
- JSON text inputs must parse within bounded nesting and size limits. Numeric outputs outside the safe finite range are rejected; large exact integer results use decimal strings. Indexes and half-open spans are zero-based unless a rank is explicitly described as one-based.
Alternative or Complementary methods
Choose a packed-integer or textual-bit representation deliberately before combining operations. Preserve the documented index direction, width and exclusive boundaries; do not substitute a rank query for select.
References
Similar or alternative tools
- Bit String Next Set Position
Find the first set bit at or after an explicit position; null preserves absence instead of inventing an index.
- Bit String Hamming Position List
Return every differing position between equal-width bit strings, preserving Hamming witnesses rather than just a count.