text: #include "cache.h"
  #include "sha1-lookup.h"
  #include "refs.h"
  #include "commit.h"
  
  static struct replace_object {
  	unsigned char sha1[2][20];
  } **replace_object;
  
  static int replace_object_alloc, replace_object_nr;
  
  static const unsigned char *replace_sha1_access(size_t index, void *table)
  {
  	struct replace_object **replace = table;
  	return replace[index]->sha1[0];
  }
  
  static int replace_object_pos(const unsigned char *sha1)
  {
  	return sha1_pos(sha1, replace_object, replace_object_nr,
  			replace_sha1_access);
  }
  
  static int register_replace_object(struct replace_object *replace,
  				   int ignore_dups)
  {
  	int pos = replace_object_pos(replace->sha1[0]);
  
  	if (0 <= pos) {
  		if (ignore_dups)
  			free(replace);
  		else {
  			free(replace_object[pos]);
  			replace_object[pos] = replace;
  		}
  		return 1;
  	}
  	pos = -pos - 1;
  	if (replace_object_alloc <= ++replace_object_nr) {
  		replace_object_alloc = alloc_nr(replace_object_alloc);
  		replace_object = xrealloc(replace_object,
  					  sizeof(*replace_object) *
  					  replace_object_alloc);
  	}
  	if (pos < replace_object_nr)
  		memmove(replace_object + pos + 1,
  			replace_object + pos,
  			(replace_object_nr - pos - 1) *
  			sizeof(*replace_object));
  	replace_object[pos] = replace;
  	return 0;
  }
  
  static int register_replace_ref(const char *refname,
  				const unsigned char *sha1,
  				int flag, void *cb_data)
  {
  	/* Get sha1 from refname */
  	const char *slash = strrchr(refname, '/');
  	const char *hash = slash ? slash + 1 : refname;
  	struct replace_object *repl_obj = xmalloc(sizeof(*repl_obj));
  
  	if (strlen(hash) != 40 || get_sha1_hex(hash, repl_obj->sha1[0])) {
  		free(repl_obj);
  		warning("bad replace ref name: %s", refname);
  		return 0;
  	}
  
  	/* Copy sha1 from the read ref */
  	hashcpy(repl_obj->sha1[1], sha1);
  
  	/* Register new object */
  	if (register_replace_object(repl_obj, 1))
  		die("duplicate replace ref: %s", refname);
  
  	return 0;
  }
  
  static void prepare_replace_object(void)
  {
  	static int replace_object_prepared;
  
  	if (replace_object_prepared)
  		return;
  
  	for_each_replace_ref(register_replace_ref, NULL);
  	replace_object_prepared = 1;
  	if (!replace_object_nr)
  		read_replace_refs = 0;
  }
  
  /* We allow "recursive" replacement. Only within reason, though */
  #define MAXREPLACEDEPTH 5
  
  const unsigned char *do_lookup_replace_object(const unsigned char *sha1)
  {
  	int pos, depth = MAXREPLACEDEPTH;
  	const unsigned char *cur = sha1;
  
  	prepare_replace_object();
  
  	/* Try to recursively replace the object */
  	do {
  		if (--depth < 0)
  			die("replace depth too high for object %s",
  			    sha1_to_hex(sha1));
  
  		pos = replace_object_pos(cur);
  		if (0 <= pos)
  			cur = replace_object[pos]->sha1[1];
  	} while (0 <= pos);
  
  	return cur;
  }
  