libowfat/fmt/fmt_utf8.c

#include "fmt.h"

size_t fmt_utf8(char *dest,uint32_t n) {
  size_t i,j;
  if (n<=0x7f) {
    if (dest) *dest=(char)n;
    return 1;
  }
  for (i=0x3f,j=1; i<=0x7fffffff; i=(i<<5)|0x1f, ++j) {
    if (i>=n) {
      --j;
      if (dest) {
	size_t k=j*6;
	// gcc -fanalyze warns here that j-1 might underflow, leading to
	// undefined behavior because right shift by more than integer
	// width is undefined. That can't happen because both n>0x7f and
	// i>=n would have to be true and we initialize i as 0x3f and
	// with each iteration j is incremented. It's a false positive.
	*dest++=(char)(((char)0xc0 >> (j-1)) | (char)(n >> k));
	while (k) {
	  *dest++=(char)(0x80 | ((n >> (k-6)) & 0x3f));
	  k-=6;
	}
      }
      return j+1;
    }
    if (i==0x7fffffff) return 0;
  }
  /* we were asked to encode a value that cannot be encoded */
  return 0;
}

/* unit tested via scan/scan_utf8.c */