// SPDX-License-Identifier: GPL-2.0-or-later

  /*
   * lib/ts_kmp.c		Knuth-Morris-Pratt text search implementation
   *

   * Authors:	Thomas Graf <tgraf@suug.ch>
   *
   * ==========================================================================
   * 
   *   Implements a linear-time string-matching algorithm due to Knuth,
   *   Morris, and Pratt [1]. Their algorithm avoids the explicit
   *   computation of the transition function DELTA altogether. Its
   *   matching time is O(n), for n being length(text), using just an
   *   auxiliary function PI[1..m], for m being length(pattern),
   *   precomputed from the pattern in time O(m). The array PI allows
   *   the transition function DELTA to be computed efficiently
   *   "on the fly" as needed. Roughly speaking, for any state
   *   "q" = 0,1,...,m and any character "a" in SIGMA, the value
   *   PI["q"] contains the information that is independent of "a" and
   *   is needed to compute DELTA("q", "a") [2]. Since the array PI
   *   has only m entries, whereas DELTA has O(m|SIGMA|) entries, we
   *   save a factor of |SIGMA| in the preprocessing time by computing
   *   PI rather than DELTA.
   *
   *   [1] Cormen, Leiserson, Rivest, Stein
   *       Introdcution to Algorithms, 2nd Edition, MIT Press

   *   [2] See finite automaton theory

   */

  #include <linux/module.h>
  #include <linux/types.h>
  #include <linux/string.h>

  #include <linux/ctype.h>

  #include <linux/textsearch.h>
  
  struct ts_kmp
  {
  	u8 *		pattern;
  	unsigned int	pattern_len;

  	unsigned int	prefix_tbl[];

  };
  
  static unsigned int kmp_find(struct ts_config *conf, struct ts_state *state)
  {
  	struct ts_kmp *kmp = ts_config_priv(conf);
  	unsigned int i, q = 0, text_len, consumed = state->offset;
  	const u8 *text;

  	const int icase = conf->flags & TS_IGNORECASE;

  
  	for (;;) {
  		text_len = conf->get_next_block(consumed, &text, conf, state);
  
  		if (unlikely(text_len == 0))
  			break;
  
  		for (i = 0; i < text_len; i++) {

  			while (q > 0 && kmp->pattern[q]
  			    != (icase ? toupper(text[i]) : text[i]))

  				q = kmp->prefix_tbl[q - 1];

  			if (kmp->pattern[q]
  			    == (icase ? toupper(text[i]) : text[i]))

  				q++;
  			if (unlikely(q == kmp->pattern_len)) {
  				state->offset = consumed + i + 1;
  				return state->offset - kmp->pattern_len;
  			}
  		}
  
  		consumed += text_len;
  	}
  
  	return UINT_MAX;
  }
  
  static inline void compute_prefix_tbl(const u8 *pattern, unsigned int len,

  				      unsigned int *prefix_tbl, int flags)

  {
  	unsigned int k, q;

  	const u8 icase = flags & TS_IGNORECASE;

  
  	for (k = 0, q = 1; q < len; q++) {

  		while (k > 0 && (icase ? toupper(pattern[k]) : pattern[k])
  		    != (icase ? toupper(pattern[q]) : pattern[q]))

  			k = prefix_tbl[k-1];

  		if ((icase ? toupper(pattern[k]) : pattern[k])
  		    == (icase ? toupper(pattern[q]) : pattern[q]))

  			k++;
  		prefix_tbl[q] = k;
  	}
  }
  
  static struct ts_config *kmp_init(const void *pattern, unsigned int len,

  				  gfp_t gfp_mask, int flags)

  {
  	struct ts_config *conf;
  	struct ts_kmp *kmp;

  	int i;

  	unsigned int prefix_tbl_len = len * sizeof(unsigned int);
  	size_t priv_size = sizeof(*kmp) + len + prefix_tbl_len;
  
  	conf = alloc_ts_config(priv_size, gfp_mask);
  	if (IS_ERR(conf))
  		return conf;

  	conf->flags = flags;

  	kmp = ts_config_priv(conf);
  	kmp->pattern_len = len;

  	compute_prefix_tbl(pattern, len, kmp->prefix_tbl, flags);

  	kmp->pattern = (u8 *) kmp->prefix_tbl + prefix_tbl_len;

  	if (flags & TS_IGNORECASE)
  		for (i = 0; i < len; i++)
  			kmp->pattern[i] = toupper(((u8 *)pattern)[i]);
  	else
  		memcpy(kmp->pattern, pattern, len);

  
  	return conf;
  }
  
  static void *kmp_get_pattern(struct ts_config *conf)
  {
  	struct ts_kmp *kmp = ts_config_priv(conf);
  	return kmp->pattern;
  }
  
  static unsigned int kmp_get_pattern_len(struct ts_config *conf)
  {
  	struct ts_kmp *kmp = ts_config_priv(conf);
  	return kmp->pattern_len;
  }
  
  static struct ts_ops kmp_ops = {
  	.name		  = "kmp",
  	.find		  = kmp_find,
  	.init		  = kmp_init,
  	.get_pattern	  = kmp_get_pattern,
  	.get_pattern_len  = kmp_get_pattern_len,
  	.owner		  = THIS_MODULE,
  	.list		  = LIST_HEAD_INIT(kmp_ops.list)
  };
  
  static int __init init_kmp(void)
  {
  	return textsearch_register(&kmp_ops);
  }
  
  static void __exit exit_kmp(void)
  {
  	textsearch_unregister(&kmp_ops);
  }
  
  MODULE_LICENSE("GPL");
  
  module_init(init_kmp);
  module_exit(exit_kmp);