text: #include <stdio.h>
  #include <assert.h>
  #include "jv_unicode.h"
  #include "jv_utf8_tables.h"
  
  // jvp_utf8_backtrack returns the beginning of the last codepoint in the
  // string, assuming that start is the last byte in the string.
  // If the last codepoint is incomplete, returns the number of missing bytes via
  // *missing_bytes.  If there are no leading bytes or an invalid byte is
  // encountered, NULL is returned and *missing_bytes is not altered.
  const char* jvp_utf8_backtrack(const char* start, const char* min, int *missing_bytes) {
    assert(min <= start);
    if (min == start) {
      return min;
    }
    int length = 0;
    int seen = 1;
    while (start >= min && (length = utf8_coding_length[(unsigned char)*start]) == UTF8_CONTINUATION_BYTE) {
      start--;
      seen++;
    }
    if (length == 0 || length == UTF8_CONTINUATION_BYTE || length - seen < 0) {
      return NULL;
    }
    if (missing_bytes) *missing_bytes = length - seen;
    return start;
  }
  
  const char* jvp_utf8_next(const char* in, const char* end, int* codepoint_ret) {
    assert(in <= end);
    if (in == end) {
      return 0;
    }
    int codepoint = -1;
    unsigned char first = (unsigned char)in[0];
    int length = utf8_coding_length[first];
    if ((first & 0x80) == 0) {
      /* Fast-path for ASCII */
      codepoint = first;
      length = 1;
    } else if (length == 0 || length == UTF8_CONTINUATION_BYTE) {
      /* Bad single byte - either an invalid byte or an out-of-place continuation byte */
      length = 1;
    } else if (in + length > end) {
      /* String ends before UTF8 sequence ends */
      length = end - in;
    } else {
      codepoint = ((unsigned)in[0]) & utf8_coding_bits[first];
      for (int i=1; i<length; i++) {
        unsigned ch = (unsigned char)in[i];
        if (utf8_coding_length[ch] != UTF8_CONTINUATION_BYTE){
          /* Invalid UTF8 sequence - not followed by the right number of continuation bytes */
          codepoint = -1;
          length = i;
          break;
        }
        codepoint = (codepoint << 6) | (ch & 0x3f);
      }
      if (codepoint < utf8_first_codepoint[length]) {
        /* Overlong UTF8 sequence */
        codepoint = -1;
      }
      if (0xD800 <= codepoint && codepoint <= 0xDFFF) {
        /* Surrogate codepoints can't be encoded in UTF8 */
        codepoint = -1;
      }
      if (codepoint > 0x10FFFF) {
        /* Outside Unicode range */
        codepoint = -1;
      }
    }
    assert(length > 0);
    *codepoint_ret = codepoint;
    return in + length;
  }
  
  int jvp_utf8_is_valid(const char* in, const char* end) {
    int codepoint;
    while ((in = jvp_utf8_next(in, end, &codepoint))) {
      if (codepoint == -1) return 0;
    }
    return 1;
  }
  
  /* Assumes startchar is the first byte of a valid character sequence */
  int jvp_utf8_decode_length(char startchar) {
  	if ((startchar & 0x80) == 0) return 1;         // 0___ ____
  	else if ((startchar & 0xE0) == 0xC0) return 2; // 110_ ____
  	else if ((startchar & 0xF0) == 0xE0) return 3; // 1110 ____
  	else return 4;                                 // 1111 ____
  }
  
  int jvp_utf8_encode_length(int codepoint) {
    if (codepoint <= 0x7F) return 1;
    else if (codepoint <= 0x7FF) return 2;
    else if (codepoint <= 0xFFFF) return 3;
    else return 4;
  }
  
  int jvp_utf8_encode(int codepoint, char* out) {
    assert(codepoint >= 0 && codepoint <= 0x10FFFF);
    char* start = out;
    if (codepoint <= 0x7F) {
      *out++ = codepoint;
    } else if (codepoint <= 0x7FF) {
      *out++ = 0xC0 + ((codepoint & 0x7C0) >> 6);
      *out++ = 0x80 + ((codepoint & 0x03F));
    } else if(codepoint <= 0xFFFF) {
      *out++ = 0xE0 + ((codepoint & 0xF000) >> 12);
      *out++ = 0x80 + ((codepoint & 0x0FC0) >> 6);
      *out++ = 0x80 + ((codepoint & 0x003F));
    } else {
      *out++ = 0xF0 + ((codepoint & 0x1C0000) >> 18);
      *out++ = 0x80 + ((codepoint & 0x03F000) >> 12);
      *out++ = 0x80 + ((codepoint & 0x000FC0) >> 6);
      *out++ = 0x80 + ((codepoint & 0x00003F));
    }
    assert(out - start == jvp_utf8_encode_length(codepoint));
    return out - start;
  }
  