notcurses/src/lib/egcpool.h

#ifndef NOTCURSES_EGCPOOL
#define NOTCURSES_EGCPOOL

#include <wchar.h>
#include <stddef.h>
#include <stdlib.h>
#include <string.h>
#include <stdbool.h>

#ifdef __cplusplus
extern "C" {
#endif

// cells only provide storage for a single 7-bit character. if there's anything
// more than that, it's spilled into the egcpool, and the cell is given an
// offset. when a cell is released, the memory it owned is zeroed out, and
// recognizable as use for another cell.

typedef struct egcpool {
  char* pool;            // ringbuffer of attached extension storage
  size_t poolsize;       // total number of bytes in pool
  size_t poolused;       // bytes actively used, grow when this gets too large
  size_t poolwrite;      // next place to *look for* a place to write
} egcpool;

static inline void
egcpool_init(egcpool* p){
  memset(p, 0, sizeof(*p));
}

int egcpool_grow(egcpool* pool, size_t len);

// Eat an EGC from the UTF-8 string input. This consists of extracting a
// multibyte via mbtowc, then continuing to extract any which have zero
// width until hitting another spacing character or a NUL terminator. Writes
// the number of columns occupied to '*colcount'. Returns the number of bytes
// consumed, not including any NUL terminator. Note that neither the number
// of bytes nor columns is necessarily equivalent to the number of decoded code
// points. Such are the ways of Unicode.
static inline size_t
utf8_gce_len(const char* gcluster, int* colcount){
  size_t ret = 0;
  *colcount = 0;
  wchar_t wc;
  int r;
  do{
    r = mbtowc(&wc, gcluster, MB_CUR_MAX);
    if(r < 0){
      return -1;
    }else if(r){
      int cols = wcwidth(wc);
      if(cols){
        if(*colcount){ // this must be starting a new EGC, exit and do not claim
          break;
        }
        *colcount += cols;
      }
      ret += r;
      gcluster += r;
    }
  }while(r);
  return ret;
}

// if we're inserting a EGC of |len| bytes, ought we proactively realloc?
static inline bool
egcpool_alloc_justified(const egcpool* pool, size_t len){
  const size_t poolfree = pool->poolsize - pool->poolused;
  // proactively get more space if we have less than 10% free. this doesn't
  // guarantee that we'll have enough space to insert the string -- we could
  // theoretically have every 10th byte free, and be unable to write even a
  // two-byte egc -- so we might have to allocate after an expensive search :/.
  if(poolfree >= len && poolfree * 10 > pool->poolsize){
    return false;
  }
  return true;
}

// stash away the provided UTF8, NUL-terminated grapheme cluster. the cluster
// should not be less than 2 bytes (such a cluster should be directly stored in
// the cell). returns -1 on error, and otherwise a non-negative 24-bit offset.
// The number of bytes copied is stored to '*ulen'. The number of presentation
// columns is stored to '*cols'.
static inline int
egcpool_stash(egcpool* pool, const char* egc, size_t* ulen, int* cols){
  size_t len = utf8_gce_len(egc, cols) + 1; // count the NUL terminator
  if(len <= 2){ // should never be empty, nor a single byte + NUL
    return -1;
  }
  *ulen = len - 1;
  // the first time through, we don't force a grow unless we expect ourselves
  // to have too little space. once we've done a search, we do force the grow.
  // we should thus never have more than two iterations of this loop.
  bool searched = false;
  // we might have to realloc our underlying pool. it is possible that this EGC
  // is actually *in* that pool, in which case our pointer will be invalidated.
  // to be safe, duplicate prior to a realloc, and free along all paths.
  char* duplicated = NULL;
  do{
    if(egcpool_alloc_justified(pool, len) || searched){
      if(!duplicated){
        duplicated = strdup(egc);
      }
      if(egcpool_grow(pool, len)){
        free(duplicated);
        return -1;
      }
      egc = duplicated;
    }
    // we now look for a place to lay out this egc. we need |len| zeroes in a
    // row. starting at pool->poolwrite, look for such a range of unused
    // memory. if we find it, write it out, and update used count. if we come
    // back to where we started, force a growth and try again.
    size_t curpos = pool->poolwrite; 
    do{
      if(curpos == pool->poolsize){
        curpos = 0;
      }
      if(pool->pool[curpos]){ // can't write if there's stuff here
        ++curpos;
      }else if(pool->poolsize - curpos < len){ // can't wrap around
        if(pool->poolwrite > curpos){
          break;
        }
        curpos = 0; // can this skip pool->poolwrite?
      }else{ // promising! let's see if there's enough space
        size_t need = len;
        size_t trial = curpos;
        while(--need){
          if(pool->pool[++trial]){ // alas, not enough space here
            break;
          }
        }
        if(need == 0){ // found a suitable space, copy it!
          memcpy(pool->pool + curpos, egc, len - 1);
          pool->poolwrite = curpos + len;
          pool->pool[curpos + len - 1] = '\0';
          pool->poolused += len;
          free(duplicated);
          return curpos;
        }
        if(pool->poolwrite > curpos && pool->poolwrite - (len - need) < curpos){
          break;
        }
        curpos += len - need;
      }
    }while(curpos != pool->poolwrite); 
  }while( (searched = !searched) );
  free(duplicated);
  return -1; // should never get here
}

// remove the egc from the pool. start at offset, and zero out everything until
// we find a zero (our own NUL terminator). remove that number of bytes from
// the usedcount.
static inline void
egcpool_release(egcpool* pool, size_t offset){
  size_t freed = 1; // account for free(d) NUL terminator
  while(pool->pool[offset]){
    pool->pool[offset] = '\0';
    ++freed;
    if(++offset == pool->poolsize){
      offset = 0;
    }
  }
  pool->poolused -= freed;
  // FIXME ought we update pool->poolwrite?
}

static inline void
egcpool_dump(egcpool* pool){
  free(pool->pool);
  pool->pool = NULL;
  pool->poolsize = 0;
  pool->poolwrite = 0;
  pool->poolused = 0;
}

#ifdef __cplusplus
}
#endif

#endif