utf16LeToUtf8ArrayListImpl

fn utf16LeToUtf8ArrayListImpl(
    result: *std.array_list.Managed(u8),
    utf16le: []const u16,
    comptime surrogates: Surrogates,
) (switch (surrogates) {
    .cannot_encode_surrogate_half => Utf16LeToUtf8AllocError,
    .can_encode_surrogate_half => Allocator.Error,
})!void {
    assert(result.unusedCapacitySlice().len >= utf16le.len);

    var remaining = utf16le;
    vectorized: {
        const chunk_len = std.simd.suggestVectorLength(u16) orelse break :vectorized;
        const Chunk = @Vector(chunk_len, u16);

        // Fast path. Check for and encode ASCII characters at the start of the input.
        while (remaining.len >= chunk_len) {
            const chunk: Chunk = remaining[0..chunk_len].*;
            const mask: Chunk = @splat(mem.nativeToLittle(u16, 0x7F));
            if (@reduce(.Or, chunk | mask != mask)) {
                // found a non ASCII code unit
                break;
            }
            const ascii_chunk: @Vector(chunk_len, u8) = @truncate(mem.nativeToLittle(Chunk, chunk));
            // We allocated enough space to encode every UTF-16 code unit
            // as ASCII, so if the entire string is ASCII then we are
            // guaranteed to have enough space allocated
            result.addManyAsArrayAssumeCapacity(chunk_len).* = ascii_chunk;
            remaining = remaining[chunk_len..];
        }
    }

    switch (surrogates) {
        .cannot_encode_surrogate_half => {
            var it = Utf16LeIterator.init(remaining);
            while (try it.nextCodepoint()) |codepoint| {
                const utf8_len = utf8CodepointSequenceLength(codepoint) catch unreachable;
                assert((utf8Encode(codepoint, try result.addManyAsSlice(utf8_len)) catch unreachable) == utf8_len);
            }
        },
        .can_encode_surrogate_half => {
            var it = Wtf16LeIterator.init(remaining);
            while (it.nextCodepoint()) |codepoint| {
                const utf8_len = utf8CodepointSequenceLength(codepoint) catch unreachable;
                assert((wtf8Encode(codepoint, try result.addManyAsSlice(utf8_len)) catch unreachable) == utf8_len);
            }
        },
    }
}