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Speed up LineReader::next_line #89

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Speed up LineReader::next_line #89

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58 changes: 54 additions & 4 deletions csv.h
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Original file line number Diff line number Diff line change
Expand Up @@ -441,6 +441,59 @@ namespace io{
return file_line;
}


template<int depth>
struct Bit_patterns;

// specializations for certain types
template<> struct Bit_patterns<sizeof(uint32_t)> {
static constexpr uintptr_t newline_value = 0x0a0a0a0a;
static constexpr uintptr_t add_mask = 0x7efefeff;
static constexpr uintptr_t and_mask = 0x81010100;
};

template<> struct Bit_patterns<sizeof(uint64_t)> {
static constexpr uintptr_t newline_value = 0x0a0a0a0a0a0a0a0a;
static constexpr uintptr_t add_mask = 0x7efefefefefefeff;
static constexpr uintptr_t and_mask = 0x8101010101010100;
};

constexpr int get_chars_in_line(const char* buffer, const char* buffer_end) const
{
const auto buffer_start = buffer;

if((buffer + sizeof(uintptr_t)) < buffer_end)
{
uintptr_t data = 0;
while ((data == 0) && (buffer + sizeof(uintptr_t)) < buffer_end)
{
memcpy(&data, buffer, sizeof(data));

// this is an adaption of the following bithack
// -> http://graphics.stanford.edu/~seander/bithacks.html#ZeroInWord
// normalize the bits read, so that the newline character is
// transformed to zero and all other values to non-zero
data ^= Bit_patterns<sizeof(data)>::newline_value;

auto helper = data;
data += Bit_patterns<sizeof(data)>::add_mask;
data ^= ~helper;
data &= Bit_patterns<sizeof(data)>::and_mask;

buffer += sizeof(uintptr_t);
}

buffer -= sizeof(uintptr_t);
}

while (buffer != buffer_end && *buffer != '\n')
{
++buffer;
}

return buffer - buffer_start;
}

char*next_line(){
if(data_begin == data_end)
return 0;
Expand All @@ -462,10 +515,7 @@ namespace io{
}
}

int line_end = data_begin;
while(buffer[line_end] != '\n' && line_end != data_end){
++line_end;
}
int line_end = data_begin + get_chars_in_line(&buffer[data_begin], &buffer[data_end]);

if(line_end - data_begin + 1 > block_len){
error::line_length_limit_exceeded err;
Expand Down