gistfile1.c

#include <ruby.h>

#define LINECOUNT_ACCURATE

static VALUE rb_String_line_count(int argc, VALUE * argv, VALUE self) {
  unsigned char *str = (unsigned char *)RSTRING_PTR(self);
  size_t len = RSTRING_LEN(self), n=0, i=0;
  VALUE count_trailing;

  if (len > 256) {
    for (; i < len; i += 4) {
      /* Count lines, but do it fast.
     * Access the string by word chunks and XOR with "\n\n\n\n"
     * which should set any bytes equal to '\n' (0xA) to zero.
     * Then we perform some int underflowing to check if any
     * bytes are zero. The bytes that are zero get marked with
     * 0x80, so we can shift by 7 to move the MSB of each byte
     * into the LSB, and then add all the LSBs.
     */
      unsigned int word = *(unsigned int *)(str + i) ^ 0x0A0A0A0AUL;
      word = (word - 0x01010101UL) & ~word & 0x80808080UL;

      if (word) {
        unsigned char *bytes = (unsigned char *)&word;
        word >>= 7;
        n += bytes[0] + bytes[1] + bytes[2] + bytes[3];
      }
    }
  }

  for(; i<len; i++) {
    if (str[i] == '\n') n++;
  }

  if (rb_scan_args(argc, argv, "01", &count_trailing) == 1 && count_trailing) {
    n++;
  }

  return ULONG2NUM(n);
}

void Init_string_ext() {
  rb_define_method(rb_cString, "line_count", rb_String_line_count, -1);
}