feat($slice): array slicing operator, both arities#6
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Adds $slice to the expression ops table. Both Mongo forms compile to a jsonb_array_elements ... WITH ORDINALITY subquery re-aggregated with jsonb_agg(e order by ord) so element order is preserved: - [arr, n]: ord <= n for positive n; for negative n, last |n| via ord > jsonb_array_length(arr) + n. - [arr, skip, n]: start = greatest(case when skip < 0 then len + skip else skip end, 0), take ord in (start, start + n]. Negative skip counts from the end and clamps at 0, matching Mongo. Tests: 2 unit (SQL shape: ordinality window, ordered agg, skip/take bounds, params) + 2 integration in $project and $addFields covering first-n, last-n, n past length, zero n, skip+take, negative position, position past start. README expression list updated. 88 pass / 0 fail. tsc clean. Closes #2 Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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Summary
Implements MongoDB's
$sliceexpression operator (#2), both forms:{ $slice: ["$arr", n] }— firstnelements, or last|n|whennis negative{ $slice: ["$arr", skip, n] }—nelements starting atskip; negativeskipcounts from the end and clamps at the start, matching Mongo semanticsBoth compile to a
jsonb_array_elements ... WITH ORDINALITYsubquery re-aggregated withjsonb_agg(e ORDER BY ord)so element order is preserved. All user-supplied values stay parameterized.Tests
$slicein$projectand$addFieldsagainst real Postgres: first-n, last-n, n past length, zero n, skip+take, negative position, position past starttsc --noEmitcleanREADME expression list updated.
Closes #2
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