"UTF-8 decode without BOM or fail" implementation in C

gistfile1.txt

#include <stddef.h>
#include <stdbool.h>
#include <stdint.h>

bool is_valid_utf8(const uint8_t *str, size_t len)
{
  size_t bytes_seen = 0;
  size_t bytes_needed = 0;
  uint8_t lower_boundary = 0x80;
  uint8_t upper_boundary = 0xbf;
  const uint8_t *end = str + len;

  while (str != end) {
    uint8_t byte = *str++;

    if (bytes_needed == 0) {
      if (byte < 0x80)
        continue;

      if (byte < 0xc2)
        return false;

      if (byte < 0xe0) {
        bytes_needed = 1;
        continue;
      }

      if (byte < 0xf0) {
        if (byte == 0xe0)
          lower_boundary = 0xa0;
        else if (byte == 0xed)
          upper_boundary = 0x9f;
        bytes_needed = 2;
        continue;
      }

      if (byte < 0xf5) {
        if (byte == 0xf0)
          lower_boundary = 0x90;
        else if (byte == 0xf4)
          upper_boundary = 0x8f;
        bytes_needed = 3;
        continue;
      }

      return false;
    }

    if (byte < lower_boundary || byte > upper_boundary)
      return false;

    lower_boundary = 0x80;
    upper_boundary = 0xbf;

    ++bytes_seen;

    if (bytes_seen == bytes_needed) {
      bytes_needed = 0;
      bytes_seen = 0;
    }
  }

  return bytes_needed == 0;
}