12 files changed, 743 insertions, 305 deletions

diff --git a/src/core/CMakeLists.txt b/src/core/CMakeLists.txt
index 2722355..a8ec17e 100644
--- a/src/core/CMakeLists.txt
+++ b/src/core/CMakeLists.txt
@@ -4,6 +4,7 @@ set(
    ${CMAKE_CURRENT_SOURCE_DIR}/knowledge.c
    ${CMAKE_CURRENT_SOURCE_DIR}/assimilate.c
    ${CMAKE_CURRENT_SOURCE_DIR}/create_sentences.c
+   ${CMAKE_CURRENT_SOURCE_DIR}/sequence.c
 )
 
 set(SRC_FILES ${SRC_FILES} PARENT_SCOPE)
diff --git a/src/core/assimilate.c b/src/core/assimilate.c
index 28f4dd4..49efe23 100644
--- a/src/core/assimilate.c
+++ b/src/core/assimilate.c
@@ -7,121 +7,109 @@
 
 /** Functions to assimilate sentences using a ZoO_knowledge structure *********/
 
-static int link_to
+
+static int add_sequence
 (
-   ZoO_index links_count [const restrict static 1],
-   ZoO_index * links_occurrences [const restrict static 1],
-   ZoO_index * links [const restrict static 1],
-   ZoO_index const target
+   ZoO_index links_count [const],
+   struct ZoO_knowledge_link * links [const],
+   ZoO_index const sequence [const restrict static ZoO_MARKOV_ORDER],
+   ZoO_index const target_i,
+   ZoO_index const offset
 )
 {
-   ZoO_index i, * new_p;
+   ZoO_index link_index, i;
+   struct ZoO_knowledge_link * link;
+   ZoO_index * new_p;
 
-   for (i = 0; i < *links_count; ++i)
+   if (ZoO_knowledge_get_link(links_count, links, (sequence + offset), &link_index) < 0)
    {
-      if ((*links)[i] == target)
-      {
-         if ((*links_occurrences)[i] == ZoO_INDEX_MAX)
-         {
-            ZoO_S_WARNING
-            (
-               "Maximum link occurrences count has been reached."
-            );
+      return -1;
+   }
 
-            return -1;
-         }
+   link = (*links + link_index);
+   link->occurrences += 1;
 
-         (*links_occurrences)[i] += 1;
+   for (i = 0; i < link->targets_count; ++i)
+   {
+      if (link->targets[i] == sequence[target_i])
+      {
+         link->targets_occurrences[i] += 1;
 
          return 0;
       }
    }
 
-   if (*links_count == ZoO_INDEX_MAX)
-   {
-      ZoO_S_WARNING("Maximum links count has been reached.");
-
-      return -1;
-   }
+   link->targets_count += 1;
 
    new_p =
       (ZoO_index *) realloc
       (
-         *links_occurrences,
-         (
-            (
-               /* Safe: *links_count < ZoO_INDEX_MAX */
-               (size_t) (*links_count + 1)
-            )
-            * sizeof(ZoO_index)
-         )
+         (void *) link->targets,
+         (sizeof(ZoO_index) * link->targets_count)
       );
 
    if (new_p == (ZoO_index *) NULL)
    {
-      ZoO_S_ERROR("Could not reallocate a link occurrences list.");
+      link->targets_count -= 1;
 
+      /* TODO: err. */
       return -1;
    }
 
-   new_p[*links_count] = 1;
-
-   *links_occurrences = new_p;
+   link->targets = new_p;
+   link->targets[link->targets_count - 1] = sequence[target_i];
 
    new_p =
       (ZoO_index *) realloc
       (
-         *links,
-         (
-            (
-               /* Safe: *links_count < ZoO_INDEX_MAX */
-               (size_t) (*links_count + 1)
-            ) * sizeof(ZoO_index)
-         )
+         (void *) link->targets_occurrences,
+         (sizeof(ZoO_index) * link->targets_count)
       );
 
    if (new_p == (ZoO_index *) NULL)
    {
-      ZoO_S_ERROR("Could not reallocate a link list.");
+      link->targets_count -= 1;
 
+      /* TODO: err. */
       return -1;
    }
 
-   new_p[*links_count] = target;
-
-   *links = new_p;
-
-   *links_count += 1;
+   link->targets_occurrences = new_p;
+   link->targets_occurrences[link->targets_count - 1] = 1;
 
    return 0;
 }
 
-static int link_words
+static int add_word_occurrence
 (
    struct ZoO_knowledge k [const restrict static 1],
-   ZoO_index const a,
-   ZoO_index const b
+   ZoO_index const sequence [const static ((ZoO_MARKOV_ORDER * 2) + 1)]
 )
 {
+   ZoO_index w;
    int error;
 
+   w = sequence[ZoO_MARKOV_ORDER];
+
    error =
-      link_to
+      add_sequence
       (
-         &(k->words[a].forward_links_count),
-         &(k->words[a].forward_links_occurrences),
-         &(k->words[a].forward_links),
-         b
+         &(k->words[w].forward_links_count),
+         &(k->words[w].forward_links),
+         sequence + (ZoO_MARKOV_ORDER + 1),
+         (ZoO_MARKOV_ORDER - 1),
+         0
       );
 
    error =
       (
-         link_to
+         add_sequence
          (
-            &(k->words[b].backward_links_count),
-            &(k->words[b].backward_links_occurrences),
-            &(k->words[b].backward_links),
-            a
+            &(k->words[w].backward_links_count),
+            &(k->words[w].backward_links),
+            sequence,
+            0,
+            1
          )
          | error
       );
@@ -129,83 +117,144 @@ static int link_words
    return error;
 }
 
-int ZoO_knowledge_assimilate
+static int should_assimilate
 (
-   struct ZoO_knowledge k [const static 1],
    struct ZoO_strings string [const restrict static 1],
    ZoO_index const aliases_count,
    const char * restrict aliases [const restrict static aliases_count]
 )
 {
-   int error;
-   ZoO_index curr_word, next_word;
-   ZoO_index curr_word_id, next_word_id;
-
-   curr_word = 0;
+   ZoO_index i;
 
+   /* Don't assimilate empty strings. */
    if (string->words_count == 0)
    {
       return 0;
    }
 
-   for (curr_word = 0; curr_word < aliases_count; ++curr_word)
+   /* Don't assimilate things that start with our name. */
+   for (i = 0; i < aliases_count; ++i)
    {
-      if (ZoO_IS_PREFIX(aliases[curr_word], string->words[0]))
+      if (ZoO_IS_PREFIX(aliases[i], string->words[0]))
       {
          return 0;
       }
    }
 
-   curr_word = 0;
+   return 1;
+}
+
+static int init_sequence
+(
+   struct ZoO_knowledge k [const static 1],
+   struct ZoO_strings string [const restrict static 1],
+   ZoO_index sequence [const restrict static ((ZoO_MARKOV_ORDER * 2) + 1)]
+)
+{
+   ZoO_index i;
+
+   sequence[0] = ZoO_WORD_START_OF_LINE;
 
-   if (ZoO_knowledge_learn(k, string->words[curr_word], &curr_word_id) < 0)
+   for (i = 0; i < ZoO_MARKOV_ORDER; ++i)
+   {
+      sequence[ZoO_MARKOV_ORDER - i] = ZoO_WORD_START_OF_LINE;
+
+      if (i < string->words_count)
+      {
+         if
+         (
+            ZoO_knowledge_learn
+            (
+               k,
+               string->words[i],
+               (sequence + (ZoO_MARKOV_ORDER + i + 1))
+            ) < 0
+         )
+         {
+            return -1;
+         }
+      }
+      else
+      {
+         sequence[ZoO_MARKOV_ORDER + i + 1] = ZoO_WORD_END_OF_LINE;
+      }
+   }
+
+   return 0;
+}
+
+int ZoO_knowledge_assimilate
+(
+   struct ZoO_knowledge k [const static 1],
+   struct ZoO_strings string [const restrict static 1],
+   ZoO_index const aliases_count,
+   const char * restrict aliases [const restrict static aliases_count]
+)
+{
+   int error;
+   ZoO_index sequence[(ZoO_MARKOV_ORDER * 2) + 1];
+   ZoO_index next_word, new_word, new_word_id;
+
+   if (!should_assimilate(string, aliases_count, aliases))
+   {
+      return 0;
+   }
+
+   if (init_sequence(k, string, sequence) < 0)
    {
       return -1;
    }
 
-   if (link_words(k, ZoO_WORD_START_OF_LINE, curr_word_id) < 0)
+   if (add_word_occurrence(k, sequence) < 0)
    {
       error = -1;
 
-      ZoO_WARNING
-      (
-         "Could not indicate that '"
-         ZoO_CHAR_STRING_SYMBOL
-         "' was the first word of the sentence.",
-         string->words[0]
-      );
-   }
+      /* There's a pun... */
+      ZoO_S_WARNING("Could not add a link between words.");
 
-   next_word = 1;
+      return -1;
+   }
 
    error = 0;
 
-   while (next_word < string->words_count)
+   next_word = 0;
+   new_word = ZoO_MARKOV_ORDER;
+
+   while (next_word <= string->words_count)
    {
-      /* prevents words [restrict], k [restrict] */
-      if (ZoO_knowledge_learn(k, string->words[next_word], &next_word_id) < 0)
+      if (new_word < string->words_count)
       {
-         return -1;
+         /* prevents words [restrict], k [restrict] */
+         if (ZoO_knowledge_learn(k, string->words[new_word], &new_word_id) < 0)
+         {
+            return -1;
+         }
+      }
+      else
+      {
+         new_word_id = ZoO_WORD_END_OF_LINE;
       }
 
-      if (link_words(k, curr_word_id, next_word_id) < 0)
+      memmove
+      (
+         (void *) sequence,
+         (const void *) (sequence + 1),
+         /* Accepts 0. */
+         (sizeof(ZoO_index) * (ZoO_MARKOV_ORDER * 2))
+      );
+
+      sequence[ZoO_MARKOV_ORDER * 2] = new_word_id;
+
+      if (add_word_occurrence(k, sequence) < 0)
       {
          error = -1;
 
-         ZoO_WARNING
-         (
-            "Could not add a link between words '"
-            ZoO_CHAR_STRING_SYMBOL
-            "' and '"
-            ZoO_CHAR_STRING_SYMBOL
-            "'.",
-            string->words[curr_word],
-            string->words[next_word]
-         );
+         /* There's a pun... */
+         ZoO_S_WARNING("Could not add a link between words.");
+
+         return -1;
       }
 
-      curr_word = next_word;
-      curr_word_id = next_word_id;
       /*
        * Safe:
        *  - next_word < words_count
@@ -214,19 +263,7 @@ int ZoO_knowledge_assimilate
        *  next_word < ZoO_INDEX_MAX
        */
       next_word += 1;
-   }
-
-   if (link_words(k, curr_word_id, ZoO_WORD_END_OF_LINE) < 0)
-   {
-      error = -1;
-
-      ZoO_WARNING
-      (
-         "Could not indicate that '"
-         ZoO_CHAR_STRING_SYMBOL
-         "' was the last word of the sentence.",
-         string->words[curr_word_id]
-      );
+      new_word += 1;
    }
 
    return error;
diff --git a/src/core/create_sentences.c b/src/core/create_sentences.c
index 6b69111..a3640dc 100644
--- a/src/core/create_sentences.c
+++ b/src/core/create_sentences.c
@@ -18,11 +18,10 @@
  *    (== (sum links_occurrences) occurrences).
  *    (can_store ZoO_index (length links)).
  */
-static ZoO_index pick_an_index
+static ZoO_index pick_index
 (
    ZoO_index const occurrences,
-   const ZoO_index links_occurrences [const restrict static 1],
-   const ZoO_index links [const restrict static 1]
+   const ZoO_index links_occurrences [const restrict static 1]
 )
 {
    ZoO_index result, accumulator, random_number;
@@ -62,13 +61,13 @@ static ZoO_index pick_an_index
    }
 
    /* Safe: (< result (length links)) */
-   return links[result];
+   return result;
 }
 
 static unsigned char * extend_left
 (
    struct ZoO_knowledge k [const restrict static 1],
-   ZoO_index word_id,
+   ZoO_index sequence [const restrict static ZoO_MARKOV_ORDER],
    ZoO_char current_sentence [static 1],
    size_t sentence_size [const restrict static 1],
    ZoO_index credits [const static 1]
@@ -77,17 +76,7 @@ static unsigned char * extend_left
    size_t addition_size;
    struct ZoO_knowledge_word * w;
    ZoO_char * next_sentence;
-
-   w = (k->words + word_id);
-
-   if
-   (
-      (w->special == ZoO_WORD_STARTS_SENTENCE)
-      || (w->occurrences == 0)
-   )
-   {
-      return current_sentence;
-   }
+   ZoO_index j;
 
    /* prevents current_sentence [restrict] */
    next_sentence = current_sentence;
@@ -100,15 +89,8 @@ static unsigned char * extend_left
       }
 
       *credits -= 1;
-      word_id =
-         pick_an_index
-         (
-            w->occurrences,
-            w->backward_links_occurrences,
-            w->backward_links
-         );
 
-      w = (k->words + word_id);
+      w = (k->words + sequence[ZoO_MARKOV_ORDER - 1]);
 
       switch (w->special)
       {
@@ -205,13 +187,51 @@ static unsigned char * extend_left
 
       /* prevents current_sentence [const] */
       current_sentence = next_sentence;
+
+      memmove
+      (
+         (void *) (sequence + 1),
+         (const void *) sequence,
+         /* Accepts 0. */
+         (sizeof(ZoO_index) * (ZoO_MARKOV_ORDER - 1))
+      );
+
+      if
+      (
+         ZoO_knowledge_find_link
+         (
+            w->backward_links_count,
+            w->backward_links,
+            (sequence + 1),
+            &j
+         )
+         < 0
+      )
+      {
+         ZoO_S_ERROR("Unexpectedly, no backtracking link was found.");
+
+         break;
+      }
+
+      sequence[0] =
+         w->backward_links[j].targets
+         [
+            pick_index
+            (
+               w->backward_links[j].occurrences,
+               w->backward_links[j].targets_occurrences
+            )
+         ];
+
+      /* prevents current_sentence [const] */
+      current_sentence = next_sentence;
    }
 }
 
 static unsigned char * extend_right
 (
    struct ZoO_knowledge k [const restrict static 1],
-   ZoO_index word_id,
+   ZoO_index sequence [const restrict static ZoO_MARKOV_ORDER],
    ZoO_char current_sentence [static 1],
    size_t sentence_size [const restrict static 1],
    ZoO_index credits [const static 1]
@@ -220,17 +240,7 @@ static unsigned char * extend_right
    size_t addition_size;
    struct ZoO_knowledge_word * w;
    ZoO_char * next_sentence;
-
-   w = (k->words + word_id);
-
-   if
-   (
-      (w->special == ZoO_WORD_ENDS_SENTENCE)
-      || (w->occurrences == 0)
-   )
-   {
-      return current_sentence;
-   }
+   ZoO_index j;
 
    /* prevents current_sentence [restrict] */
    next_sentence = current_sentence;
@@ -244,15 +254,7 @@ static unsigned char * extend_right
 
       *credits -= 1;
 
-      word_id =
-         pick_an_index
-         (
-            w->occurrences,
-            w->forward_links_occurrences,
-            w->forward_links
-         );
-
-      w = (k->words + word_id);
+      w = (k->words + sequence[0]);
 
       switch (w->special)
       {
@@ -340,95 +342,233 @@ static unsigned char * extend_right
 
       /* prevents current_sentence [const] */
       current_sentence = next_sentence;
+
+      memmove
+      (
+         (void *) sequence,
+         (const void *) (sequence + 1),
+         /* Accepts 0. */
+         (sizeof(ZoO_index) * (ZoO_MARKOV_ORDER - 1))
+      );
+
+      if
+      (
+         ZoO_knowledge_find_link
+         (
+            w->forward_links_count,
+            w->forward_links,
+            sequence,
+            &j
+         )
+         < 0
+      )
+      {
+         ZoO_S_ERROR("Unexpectedly, no forward link was found.");
+
+         break;
+      }
+
+      sequence[ZoO_MARKOV_ORDER - 1] =
+         w->forward_links[j].targets
+         [
+            pick_index
+            (
+               w->forward_links[j].occurrences,
+               w->forward_links[j].targets_occurrences
+            )
+         ];
    }
 }
 
-int ZoO_knowledge_extend
+static ZoO_index select_first_word
 (
    struct ZoO_knowledge k [const static 1],
    const struct ZoO_strings string [const],
-   int const ignore_first_word,
-   ZoO_char * result [const static 1]
+   int const ignore_first_word
 )
 {
-   int word_found;
-   size_t sentence_size;
-   ZoO_index i, word_id, word_min_score, word_min_id, credits;
+   ZoO_index i, word_id, word_min_score, word_min_id;
+   ZoO_index word_found;
 
-   credits = ZoO_MAX_REPLY_WORDS;
+   if (string == (struct ZoO_strings *) NULL)
+   {
+      return word_min_id = (rand() % k->words_count);
+   }
 
-   if (string != (struct ZoO_strings *) NULL)
+   if (ignore_first_word)
+   {
+      i = 1;
+   }
+   else
    {
-      word_found = 0;
+      i = 0;
+   }
 
-      if (ignore_first_word)
-      {
-         i = 1;
-      }
-      else
-      {
-         i = 0;
-      }
+   word_found = 0;
 
-      for (; i < string->words_count; ++i)
+   for (; i < string->words_count; ++i)
+   {
+      /* prevents k [restrict] */
+      if (ZoO_knowledge_find(k, string->words[i], &word_min_id) == 0)
       {
-         /* prevents k [restrict] */
-         if (ZoO_knowledge_find(k, string->words[i], &word_min_id) == 0)
-         {
-            word_found = 1;
-            word_min_score = k->words[word_min_id].occurrences;
+         word_found = 1;
+         word_min_score = k->words[word_min_id].occurrences;
 
-            break;
-         }
+         break;
       }
+   }
+
+   if (word_found == 0)
+   {
+      return word_min_id = (rand() % k->words_count);
+   }
 
-      if (word_found == 0)
+   for (; i < string->words_count; ++i)
+   {
+      if
+      (
+         (ZoO_knowledge_find(k, string->words[i], &word_id) == 0)
+         && (k->words[word_id].occurrences < word_min_score)
+      )
       {
-         word_min_id = (rand() % k->words_count);
-         word_min_score = k->words[word_min_id].occurrences;
+         word_min_score = k->words[word_id].occurrences;
+         word_min_id = word_id;
       }
+   }
 
-      for (; i < string->words_count; ++i)
-      {
-         if
+   return word_min_id;
+}
+
+static void init_sequence
+(
+   struct ZoO_knowledge k [const static 1],
+   const struct ZoO_strings string [const],
+   int const ignore_first_word,
+   ZoO_index sequence [const static (ZoO_MARKOV_ORDER * 2) + 1]
+)
+{
+   ZoO_index i, j, accumulator, random_number;
+   struct ZoO_knowledge_word * fiw;
+
+   sequence[ZoO_MARKOV_ORDER] = select_first_word(k, string, ignore_first_word);
+
+   fiw = (k->words + sequence[ZoO_MARKOV_ORDER]);
+
+   for (i = 0; i < ZoO_MARKOV_ORDER; ++i)
+   {
+      sequence[ZoO_MARKOV_ORDER - i - 1] = ZoO_WORD_START_OF_LINE;
+      sequence[ZoO_MARKOV_ORDER + i + 1] = ZoO_WORD_END_OF_LINE;
+   }
+/*
+   i = 0;
+   accumulator = 0;
+
+   random_number = (((ZoO_index) rand()) % fiw->occurrences);
+
+   while (accumulator < random_number)
+   {
+      i += 1;
+      accumulator += fiw->forward_links[i].occurrences;
+   }
+*/
+   if (fiw->forward_links_count == 0)
+   {
+      ZoO_S_FATAL("First word has no forward links.");
+   }
+
+   i = (((ZoO_index) rand()) % fiw->forward_links_count);
+
+   memcpy
+   (
+      (void *) (sequence + ZoO_MARKOV_ORDER + 1),
+      fiw->forward_links[i].sequence,
+      sizeof(ZoO_index) * (ZoO_MARKOV_ORDER - 1)
+   );
+
+   sequence[ZoO_MARKOV_ORDER * 2] =
+      fiw->forward_links[i].targets
+      [
+         pick_index
          (
-            (ZoO_knowledge_find(k, string->words[i], &word_id) == 0)
-            && (k->words[word_id].occurrences < word_min_score)
+            fiw->forward_links[i].occurrences,
+            fiw->forward_links[i].targets_occurrences
          )
-         {
-            word_min_score = k->words[word_id].occurrences;
-            word_min_id = word_id;
-         }
-      }
-   }
-   else
+      ];
+
+   for (i = 1; i <= ZoO_MARKOV_ORDER; ++i)
    {
-      word_min_id = (rand() % k->words_count);
+      fiw = (k->words + sequence[(ZoO_MARKOV_ORDER * 2) - i]);
+
+      if
+      (
+         ZoO_knowledge_find_link
+         (
+            fiw->backward_links_count,
+            fiw->backward_links,
+            sequence + (ZoO_MARKOV_ORDER - i + 1),
+            &j
+         )
+         < 0
+      )
+      {
+         ZoO_S_ERROR("Unexpectedly, no back link was found.");
+
+         break;
+      }
+
+      sequence[ZoO_MARKOV_ORDER - i] =
+         fiw->backward_links[j].targets
+         [
+            pick_index
+            (
+               fiw->backward_links[j].occurrences,
+               fiw->backward_links[j].targets_occurrences
+            )
+         ];
    }
+}
+
+int ZoO_knowledge_extend
+(
+   struct ZoO_knowledge k [const static 1],
+   const struct ZoO_strings string [const],
+   int const ignore_first_word,
+   ZoO_char * result [const static 1]
+)
+{
+   int word_found;
+   size_t sentence_size;
+   ZoO_index sequence[(ZoO_MARKOV_ORDER * 2) + 1];
+   ZoO_index first_word, credits;
+
+   credits = ZoO_MAX_REPLY_WORDS;
+
+   init_sequence(k, string, ignore_first_word, sequence);
+   first_word = sequence[ZoO_MARKOV_ORDER];
 
    /* 3: 2 spaces + '\0' */
    /* FIXME: not overflow-safe */
-   switch (k->words[word_min_id].special)
+   switch (k->words[first_word].special)
    {
       case ZoO_WORD_REMOVES_LEFT_SPACE:
       case ZoO_WORD_REMOVES_RIGHT_SPACE:
          /* word + ' ' + '\0' */
-         sentence_size = (strlen(k->words[word_min_id].word) + 2);
+         sentence_size = (strlen(k->words[first_word].word) + 2);
          break;
 
       case ZoO_WORD_HAS_NO_EFFECT:
          /* word + ' ' * 2 + '\0' */
-         sentence_size = (strlen(k->words[word_min_id].word) + 3);
+         sentence_size = (strlen(k->words[first_word].word) + 3);
          break;
 
       default:
          ZoO_WARNING
          (
             "'%s' was unexpectedly selected as pillar.",
-            k->words[word_min_id].word
+            k->words[first_word].word
          );
          /* word + '[' + ']' + ' ' * 2 + '\0' */
-         sentence_size = (strlen(k->words[word_min_id].word) + 5);
+         sentence_size = (strlen(k->words[first_word].word) + 5);
          break;
    }
 
@@ -441,7 +581,7 @@ int ZoO_knowledge_extend
       return -2;
    }
 
-   switch (k->words[word_min_id].special)
+   switch (k->words[first_word].special)
    {
       case ZoO_WORD_REMOVES_LEFT_SPACE:
          snprintf
@@ -449,7 +589,7 @@ int ZoO_knowledge_extend
             *result,
             sentence_size,
             ZoO_CHAR_STRING_SYMBOL " ",
-            k->words[word_min_id].word
+            k->words[first_word].word
          );
          break;
 
@@ -459,7 +599,7 @@ int ZoO_knowledge_extend
             *result,
             sentence_size,
             " " ZoO_CHAR_STRING_SYMBOL,
-            k->words[word_min_id].word
+            k->words[first_word].word
          );
          break;
 
@@ -469,7 +609,7 @@ int ZoO_knowledge_extend
             *result,
             sentence_size,
             " " ZoO_CHAR_STRING_SYMBOL " ",
-            k->words[word_min_id].word
+            k->words[first_word].word
          );
          break;
 
@@ -479,27 +619,36 @@ int ZoO_knowledge_extend
             *result,
             sentence_size,
             " [" ZoO_CHAR_STRING_SYMBOL "] ",
-            k->words[word_min_id].word
+            k->words[first_word].word
          );
          break;
    }
 
-   if ((word_min_score == 0) || (credits == 0))
-   {
-      return 0;
-   }
-
-   --credits;
-
    /* prevents result [restrict] */
-   *result = extend_left(k, word_min_id, *result, &sentence_size, &credits);
+   *result =
+      extend_right
+      (
+         k,
+         (sequence + ZoO_MARKOV_ORDER + 1),
+         *result,
+         &sentence_size,
+         &credits
+      );
 
    if (*result == (ZoO_char *) NULL)
    {
       return -2;
    }
 
-   *result = extend_right(k, word_min_id, *result, &sentence_size, &credits);
+   *result =
+      extend_left
+      (
+         k,
+         sequence,
+         *result,
+         &sentence_size,
+         &credits
+      );
 
    if (*result == (ZoO_char *) NULL)
    {
diff --git a/src/core/knowledge.c b/src/core/knowledge.c
index 9b4e250..a2bf708 100644
--- a/src/core/knowledge.c
+++ b/src/core/knowledge.c
@@ -3,6 +3,7 @@
 #include <stdint.h> /* defines SIZE_MAX */
 
 #include "../io/error.h"
+#include "../tool/sorted_list.h"
 
 #include "knowledge.h"
 
@@ -35,6 +36,28 @@ const ZoO_char const ZoO_knowledge_forbidden_chars[8]=
       '>'
    };
 
+static int cmp_word
+(
+   const void * const a,
+   const void * const b,
+   const void * const other
+)
+{
+   ZoO_char const * word;
+   ZoO_index const * sorted_index;
+   struct ZoO_knowledge const * k;
+
+   word = (ZoO_char const *) a;
+   sorted_index = (ZoO_index const *) b;
+   k = (struct ZoO_knowledge *) other;
+
+   return strcmp
+   (
+      (const char *) word,
+      (const char *) k->words[*sorted_index].word
+   );
+}
+
 /* See "knowledge.h". */
 int ZoO_knowledge_find
 (
@@ -43,74 +66,31 @@ int ZoO_knowledge_find
    ZoO_index result [const restrict static 1]
 )
 {
-   int cmp;
-   ZoO_index i, current_min, current_max;
+   ZoO_index r;
 
-   /* This is a binary search. */
-
-   if (k->words_count < 1)
+   if
+   (
+      ZoO_sorted_list_index_of
+      (
+         k->words_count,
+         (void const *) k->sorted_indices,
+         (void const *) word,
+         sizeof(ZoO_index),
+         cmp_word,
+         (void const *) k,
+         &r
+      )
+      == 0
+   )
    {
-      *result = 0;
+      *result = k->sorted_indices[r];
 
-      return -1;
+      return 0;
    }
 
-   current_min = 0;
-
-   /* Safe: (>= k->words_count 1) */
-   current_max = (k->words_count - 1);
-
-   for (;;)
-   {
-      /* FIXME: overflow-safe? */
-      i = ((current_min + current_max) / 2);
-
-      if (i == k->words_count)
-      {
-         *result = k->words_count;
-
-         return -1;
-      }
-
-      cmp =
-         /* XXX: Assumed to be compatible with ZoO_char */
-         strcmp
-         (
-            (char *) word,
-            (const char *) k->words[k->sorted_indices[i]].word
-         );
-
-      if (cmp > 0)
-      {
-         if ((current_min > current_max))
-         {
-            *result = (i + 1);
-
-            return -1;
-         }
+   *result = r;
 
-         /* FIXME: overflow-safe? */
-         current_min = (i + 1);
-      }
-      else if (cmp < 0)
-      {
-         if ((current_min > current_max) || (i == 0))
-         {
-            *result = i;
-
-            return -1;
-         }
-
-         /* overflow-safe */
-         current_max = (i - 1);
-      }
-      else
-      {
-         *result = k->sorted_indices[i];
-
-         return 0;
-      }
-   }
+   return -1;
 }
 
 static void word_init (struct ZoO_knowledge_word w [const restrict static 1])
@@ -121,10 +101,8 @@ static void word_init (struct ZoO_knowledge_word w [const restrict static 1])
    w->occurrences = 1;
    w->forward_links_count  = 0;
    w->backward_links_count = 0;
-   w->forward_links_occurrences  = (ZoO_index *) NULL;
-   w->backward_links_occurrences = (ZoO_index *) NULL;
-   w->forward_links  = (ZoO_index *) NULL;
-   w->backward_links = (ZoO_index *) NULL;
+   w->forward_links  = (struct ZoO_knowledge_link *) NULL;
+   w->backward_links = (struct ZoO_knowledge_link *) NULL;
 }
 
 /*
@@ -209,6 +187,21 @@ int ZoO_knowledge_initialize (struct ZoO_knowledge k [const static 1])
    return 0;
 }
 
+static void finalize_links
+(
+   ZoO_index const count,
+   struct ZoO_knowledge_link links [const restrict static count]
+)
+{
+   ZoO_index i;
+
+   for (i = 0; i < count; ++i)
+   {
+      free((void *) links[i].targets_occurrences);
+      free((void *) links[i].targets);
+   }
+}
+
 /*
  * Frees all the memory used by {w}, but not {w} itself.
  * The values of {w}'s members are set to reflect the changes.
@@ -225,32 +218,22 @@ static void finalize_word
       w->word = (ZoO_char *) NULL;
    }
 
-   if (w->forward_links_occurrences != (ZoO_index *) NULL)
+   if (w->forward_links != (struct ZoO_knowledge_link *) NULL)
    {
-      free((void *) w->forward_links_occurrences);
-
-      w->forward_links_occurrences = (ZoO_index *) NULL;
-   }
+      finalize_links(w->forward_links_count, w->forward_links);
 
-   if (w->backward_links_occurrences != (ZoO_index *) NULL)
-   {
-      free((void *) w->backward_links_occurrences);
-
-      w->backward_links_occurrences = (ZoO_index *) NULL;
-   }
-
-   if (w->forward_links != (ZoO_index *) NULL)
-   {
       free((void *) w->forward_links);
 
-      w->forward_links = (ZoO_index *) NULL;
+      w->forward_links = (struct ZoO_knowledge_link *) NULL;
    }
 
-   if (w->backward_links != (ZoO_index *) NULL)
+   if (w->backward_links != (struct ZoO_knowledge_link *) NULL)
    {
+      finalize_links(w->backward_links_count, w->backward_links);
+
       free((void *) w->backward_links);
 
-      w->backward_links = (ZoO_index *) NULL;
+      w->backward_links = (struct ZoO_knowledge_link *) NULL;
    }
 
    w->forward_links_count  = 0;
diff --git a/src/core/knowledge.h b/src/core/knowledge.h
index c9ea342..93f5f49 100644
--- a/src/core/knowledge.h
+++ b/src/core/knowledge.h
@@ -75,4 +75,22 @@ int ZoO_knowledge_extend
    ZoO_char * result [const static 1]
 );
 
+int ZoO_knowledge_find_link
+(
+   ZoO_index const links_count,
+   struct ZoO_knowledge_link links [const],
+   ZoO_index const sequence [const restrict static ZoO_SEQUENCE_SIZE],
+   ZoO_index result [const restrict static 1]
+);
+
+/* Create it if it's not found. */
+int ZoO_knowledge_get_link
+(
+   ZoO_index links_count [const],
+   struct ZoO_knowledge_link * links [const],
+   ZoO_index const sequence [const restrict static ZoO_S_LINK_SIZE],
+   ZoO_index result [const restrict static 1]
+);
+
+
 #endif
diff --git a/src/core/knowledge_types.h b/src/core/knowledge_types.h
index f2e8161..8f541e7 100644
--- a/src/core/knowledge_types.h
+++ b/src/core/knowledge_types.h
@@ -6,6 +6,14 @@
 #define ZoO_WORD_START_OF_LINE 0
 #define ZoO_WORD_END_OF_LINE   1
 
+#if ZoO_MARKOV_ORDER == 1
+   #define ZoO_SEQUENCE_SIZE 1
+#else
+   #define ZoO_SEQUENCE_SIZE ZoO_MARKOV_ORDER - 1
+#endif
+
+#define ZoO_S_LINK_SIZE (ZoO_SEQUENCE_SIZE + 1)
+
 /* XXX: are we as close to immutable as we want to be? */
 extern unsigned int const ZoO_knowledge_punctuation_chars_count;
 extern const ZoO_char const ZoO_knowledge_punctuation_chars[7];
@@ -22,6 +30,15 @@ enum ZoO_knowledge_special_effect
    ZoO_WORD_REMOVES_RIGHT_SPACE
 };
 
+struct ZoO_knowledge_link
+{
+   ZoO_index sequence[ZoO_SEQUENCE_SIZE];
+   ZoO_index occurrences;
+   ZoO_index targets_count;
+   ZoO_index * targets_occurrences;
+   ZoO_index * targets;
+};
+
 struct ZoO_knowledge_word
 {
    size_t word_size;
@@ -30,12 +47,11 @@ struct ZoO_knowledge_word
    ZoO_index occurrences;
    ZoO_index forward_links_count;
    ZoO_index backward_links_count;
-   ZoO_index * forward_links_occurrences;
-   ZoO_index * backward_links_occurrences;
-   ZoO_index * forward_links;
-   ZoO_index * backward_links;
+   struct ZoO_knowledge_link * forward_links;
+   struct ZoO_knowledge_link * backward_links;
 };
 
+
 struct ZoO_knowledge
 {
    ZoO_index words_count;
diff --git a/src/core/sequence.c b/src/core/sequence.c
new file mode 100644
index 0000000..0f4043e
--- /dev/null
+++ b/src/core/sequence.c
@@ -0,0 +1,129 @@
+#include <stdlib.h>
+#include <string.h>
+
+#include "../io/error.h"
+#include "../tool/sorted_list.h"
+
+#include "knowledge.h"
+
+static int cmp_seq_link
+(
+   const void * const a,
+   const void * const b,
+   const void * const other
+)
+{
+   ZoO_index j;
+   ZoO_index * sequence;
+   struct ZoO_knowledge_link * link;
+
+   sequence = (ZoO_index *) a;
+   link = (struct ZoO_knowledge_link *) b;
+
+   for (j = 0; j < ZoO_SEQUENCE_SIZE; ++j)
+   {
+      if (sequence[j] < link->sequence[j])
+      {
+         return -1;
+      }
+      else if (sequence[j] > link->sequence[j])
+      {
+         return 1;
+      }
+   }
+
+   return 0;
+}
+
+int ZoO_knowledge_find_link
+(
+   ZoO_index const links_count,
+   struct ZoO_knowledge_link links [const],
+   ZoO_index const sequence [const restrict static ZoO_SEQUENCE_SIZE],
+   ZoO_index result [const restrict static 1]
+)
+{
+   return
+      ZoO_sorted_list_index_of
+      (
+         links_count,
+         (void const *) links,
+         (void const *) sequence,
+         sizeof(struct ZoO_knowledge_link),
+         cmp_seq_link,
+         (void const *) NULL,
+         result
+      );
+}
+
+int ZoO_knowledge_get_link
+(
+   ZoO_index links_count [const],
+   struct ZoO_knowledge_link * links [const],
+   ZoO_index const sequence [const restrict static ZoO_SEQUENCE_SIZE],
+   ZoO_index result [const restrict static 1]
+)
+{
+   struct ZoO_knowledge_link * new_p;
+
+   if
+   (
+      ZoO_sorted_list_index_of
+      (
+         *links_count,
+         (void const *) *links,
+         (void const *) sequence,
+         sizeof(struct ZoO_knowledge_link),
+         cmp_seq_link,
+         (void const *) NULL,
+         result
+      ) == 0
+   )
+   {
+      return 0;
+   }
+
+   *links_count += 1;
+
+   new_p =
+      (struct ZoO_knowledge_link *) realloc
+      (
+         (void *) *links,
+         (sizeof(struct ZoO_knowledge_link) * (*links_count))
+      );
+
+   if (new_p == (struct ZoO_knowledge_link *) NULL)
+   {
+      *links_count -= 1;
+
+      return -1;
+   }
+
+   if (*result < (*links_count - 1))
+   {
+      memmove(
+         (void *) (new_p + *result + 1),
+         (const void *) (new_p + *result),
+         (sizeof(struct ZoO_knowledge_link) * (*links_count - 1 - *result))
+      );
+   }
+
+   *links = new_p;
+
+   new_p += *result;
+
+   memcpy
+   (
+      (void *) new_p->sequence,
+      (void const *) sequence,
+      /* can be zero */
+      (sizeof(ZoO_index) * ZoO_SEQUENCE_SIZE)
+   );
+
+   new_p->occurrences = 0;
+   new_p->targets_count = 0;
+   new_p->targets_occurrences = (ZoO_index *) NULL;
+   new_p->targets = (ZoO_index *) NULL;
+
+   return 0;
+}
diff --git a/src/io/error.h b/src/io/error.h
index e4267a0..be7359f 100644
--- a/src/io/error.h
+++ b/src/io/error.h
@@ -23,6 +23,8 @@
    #define ZoO_DEBUG_LEARNING       (0 || ZoO_DEBUG_ALL)
 #endif
 
+#define ZoO_DEBUG_NETWORK  1
+
 #ifndef ZoO_DEBUG_NETWORK
    #define ZoO_DEBUG_NETWORK        (0 || ZoO_DEBUG_ALL)
 #endif
diff --git a/src/pervasive.h b/src/pervasive.h
index d2b0344..4cc43fe 100644
--- a/src/pervasive.h
+++ b/src/pervasive.h
@@ -39,6 +39,10 @@
    #define ZoO_DEFAULT_REPLY_RATE         8
 #endif
 
+#ifndef ZoO_MARKOV_ORDER
+   #define ZoO_MARKOV_ORDER               2
+#endif
+
 typedef unsigned int ZoO_index;
 #define ZoO_INDEX_MAX UINT_MAX
 
diff --git a/src/tool/CMakeLists.txt b/src/tool/CMakeLists.txt
index 3a1d947..3ad122b 100644
--- a/src/tool/CMakeLists.txt
+++ b/src/tool/CMakeLists.txt
@@ -1,6 +1,7 @@
 set(
    SRC_FILES ${SRC_FILES}
    ${CMAKE_CURRENT_SOURCE_DIR}/strings.c
+   ${CMAKE_CURRENT_SOURCE_DIR}/sorted_list.c
 )
 
 set(SRC_FILES ${SRC_FILES} PARENT_SCOPE)
diff --git a/src/tool/sorted_list.c b/src/tool/sorted_list.c
new file mode 100644
index 0000000..880a4b5
--- /dev/null
+++ b/src/tool/sorted_list.c
@@ -0,0 +1,79 @@
+#include "./sorted_list.h"
+
+int ZoO_sorted_list_index_of
+(
+   ZoO_index const list_length,
+   const void * const sorted_list,
+   const void * const elem,
+   size_t const type_size,
+   int (*compare) (const void *, const void *, const void *),
+   const void * const other,
+   ZoO_index result [const restrict static 1]
+)
+{
+   int cmp;
+   ZoO_index i, current_min, current_max;
+   const char * sorted_list_access;
+
+   sorted_list_access = (char *) sorted_list;
+
+   /* This is a binary search. */
+
+   if (list_length == 0)
+   {
+      *result = 0;
+
+      return -1;
+   }
+
+   current_min = 0;
+
+   current_max = (list_length - 1);
+
+   for (;;)
+   {
+      /* FIXME: overflow-safe? */
+      i = ((current_min + current_max) / 2);
+
+      if (i == list_length)
+      {
+         /* FIXME: I don't see how this one can be true */
+         *result = list_length;
+
+         return -1;
+      }
+
+      cmp = compare(elem, (sorted_list_access + (i * type_size)), other);
+
+      if (cmp > 0)
+      {
+         if ((current_min > current_max))
+         {
+            *result = (i + 1);
+
+            return -1;
+         }
+
+         /* FIXME: overflow-safe? */
+         current_min = (i + 1);
+      }
+      else if (cmp < 0)
+      {
+         if ((current_min > current_max) || (i == 0))
+         {
+            *result = i;
+
+            return -1;
+         }
+
+         /* overflow-safe */
+         current_max = (i - 1);
+      }
+      else
+      {
+         *result = i;
+
+         return 0;
+      }
+   }
+}
diff --git a/src/tool/sorted_list.h b/src/tool/sorted_list.h
new file mode 100644
index 0000000..72bd893
--- /dev/null
+++ b/src/tool/sorted_list.h
@@ -0,0 +1,19 @@
+#ifndef _ZoO_TOOL_SORTED_LIST_H_
+#define _ZoO_TOOL_SORTED_LIST_H_
+
+#include <stdlib.h>
+
+#include "../pervasive.h"
+
+int ZoO_sorted_list_index_of
+(
+   ZoO_index const list_length,
+   const void * const sorted_list,
+   const void * const elem,
+   size_t const type_size,
+   int (*compare) (const void *, const void *, const void *),
+   const void * const other,
+   ZoO_index result [const restrict static 1]
+);
+
+#endif