ClickHouse

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#include <Functions/FunctionFactory.h>
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#include <Functions/FunctionStringToString.h>
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#include <Functions/StringHelpers.h>
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#include <base/find_symbols.h>
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namespace DB
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{
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struct FirstLine
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{
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    static size_t getReserveLengthForElement() { return 16; }
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    static void execute(Pos data, size_t size, Pos & res_data, size_t & res_size)
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    {
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        res_data = data;
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        const Pos end = data + size;
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        const Pos pos = find_first_symbols<'\r', '\n'>(data, end);
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        res_size = pos - data;
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    }
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};
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struct NameFirstLine
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{
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    static constexpr auto name = "firstLine";
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};
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using FunctionFirstLine = FunctionStringToString<ExtractSubstringImpl<FirstLine>, NameFirstLine>;
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REGISTER_FUNCTION(FirstLine)
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{
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    factory.registerFunction<FunctionFirstLine>(FunctionDocumentation{
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        .description = "Returns first line of a multi-line string.",
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        .syntax = "firstLine(string)",
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        .arguments = {{.name = "string", .description = "The string to process."}},
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        .returned_value = {"The first line of the string or the whole string if there is no line separators."},
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        .examples = {
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            {.name = "Return first line", .query = "firstLine('Hello\\nWorld')", .result = "'Hello'"},
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            {.name = "Return whole string", .query = "firstLine('Hello World')", .result = "'Hello World'"},
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        }});
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}
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}
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