Fix drop_start on JavaScript for multi-byte strings#925
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Mhm encoding it twice seems to be a bit of a waste, maybe it's fine for this to have a native JS implementation? |
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Yes I think so. Normally I would want to avoid that, but performance is key in the standard library, so we can do things that would otherwise be unappealing. |
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I'll take a look at a JS implementation for drop_start |
The JavaScript target reused unsafe_byte_slice with Erlang byte offsets, but JS strings are indexed by UTF-16 code units, so drop_start returned wrong results for multi-byte input. Add a target-specific implementation that slices off the grapheme prefix using code-unit lengths.
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@lpil @giacomocavalieri I've pushed a new version that changes |
giacomocavalieri
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Jun 4, 2026
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string_byte_sliceusedString.prototype.slice, which operates on UTF-16 code units, but it is called with UTF-8 byte offsets frombyte_size. Encode to UTF-8, slice the byte array, then decode back.This is a simple fix, but there is a downside: the string param to
drop_startwill be encoded to UTF-8 twice, once forbyte_sizeand again forstring_byte_slice. A better path might be to liftdrop_startto a native implementation in JS.Closes #924