feat: Rewrite the trie example (#927)

The trie example had some issues;

* It did not follow the code convention in CONTRIBUTING.md
* The createTrieNode used an inefficient zeroing method (looping over
  the entries) which also does not zero out holes in the structure (e.g.
  an alternative would be to use "*node = &(TrieNode){0}", but calloc
  does all that anyway
* It used an inefficient and clumsy printArray method
* It used strlen inside the algorithm; this new method could get rid of
  any strlen/strnlen usage (inserts/searches could be sanitized by
  snprintf)
* This version can allow for a custom mapping function, e.g. if NULL is
  a valid separator (say that you want a trie for certain binary
  packages)
* The previous version actually contained out-of-bounds array indexing;
  there were no checks for out-of-bound indexing and words in the word
  list did contain out of bounds words. It's a surprise it was working
  so well.
* This version just returns 'int' to allow for error checks (instead of
  a printf inside the algorithm), and uses double pointers for return
  values (good practice)
* The usage example contained unnecessary mallocs, switched that out for
  scanf. The example is just an example after all, in real applications
  you'd have better input sanitazion.

data_structures/trie/dictionary.txt

@@ -11398,7 +11398,6 @@ ancylostome
 ancylostomiasis
 ancyroid
 and
-and/or
 anda
 andabata
 andabatarian
@@ -23796,8 +23795,6 @@ azymous
 b
 bhoy
 bs
-b/l
-b/s
 ba
 baa
 baaed
@@ -41542,10 +41539,6 @@ byzants
 bz
 c
 cs
-c/d
-c/f
-c/m
-c/o
 ca
 ca
 cacanny
@@ -126581,7 +126574,6 @@ hailweed
 haily
 haimsucken
 hain
-hain"t
 haint
 hainberry
 hainch
@@ -128730,7 +128722,6 @@ hdlc
 hdqrs
 hdwe
 he
-he"ll
 hed
 hell
 hes
@@ -139010,7 +139001,6 @@ ill
 im
 is
 ive
-i/c
 ia
 iago
 iamatology
@@ -145956,7 +145946,6 @@ inpours
 inpush
 input
 inputs
-input/output
 inputfile
 inputs
 inputted
@@ -151770,7 +151759,6 @@ isuretine
 isuroid
 isz
 it
-it"ll
 itd
 itll
 its
@@ -155260,7 +155248,6 @@ kb
 kbar
 kbps
 kc
-kc/s
 kcal
 kea
 keach
@@ -156978,7 +156965,6 @@ klva
 klystron
 klystrons
 km
-km/sec
 kmel
 kmet
 kmole
@@ -158105,7 +158091,6 @@ lenvoy
 loeil
 ls
 ltre
-l/w
 la
 laager
 laagered
@@ -164745,8 +164730,6 @@ ller
 lloyds
 llyn
 lm
-lm/ft
-lm/m
 ln
 lndg
 lnr
@@ -167403,7 +167386,6 @@ lyttas
 lyxose
 m
 ms
-m/s
 ma
 maam
 maad
@@ -185958,8 +185940,6 @@ n
 ngana
 nimporte
 ns
-n/a
-n/f
 na
 naa
 naam
@@ -198979,8 +198959,6 @@ oclock
 oer
 oertop
 os
-o/c
-o/s
 oad
 oadal
 oaf
@@ -345815,8 +345793,6 @@ vyingly
 vyrnwy
 w
 ws
-w/
-w/o
 wa
 wa
 waac

data_structures/trie/trie.c

@@ -3,6 +3,9 @@
 
 /*-----character - 97 used for get the character from the ASCII value-----*/
 
+// needed for strnlen
+#define _POSIX_C_SOURCE 200809L
+
 #include <stdbool.h>
 #include <stdio.h>
 #include <stdlib.h>
@@ -11,177 +14,193 @@
 #define ALPHABET_SIZE 26
 
 /*--Node in the Trie--*/
-typedef struct TrieNode
-{
-    struct TrieNode *children[ALPHABET_SIZE];
-    char character;
-    bool isEndOfWord;
+struct trie {
+    struct trie *children[ALPHABET_SIZE];
+    bool end_of_word;
+};
 
-} TrieNode;
 
-/*--Create new node--*/
-TrieNode *createTrieNode()
+/*--Create new trie node--*/
+int trie_new (
+    struct trie ** trie
+)
 {
-    TrieNode *node;
-    node = malloc(sizeof(TrieNode));
-    node->isEndOfWord = false;
-    int i = 0;
-    while (i < ALPHABET_SIZE)
-    {
-        node->children[i] = NULL;
-        i++;
+    *trie = calloc(1, sizeof(struct trie));
+    if (NULL == *trie) {
+        // memory allocation failed
+        return -1;
     }
-    return node;
+    return 0;
 }
 
+
 /*--Insert new word to Trie--*/
-void insert(TrieNode *root, char *word)
+int trie_insert (
+    struct trie * trie,
+    char *word,
+    unsigned word_len
+)
 {
-    /*----Addition of the word done by recurcively----*/
+    int ret = 0;
 
-    // Check wheather word character pointer is NULL
-    if ((strlen(word) - 1) != 0)
-    {
-        char character = *word;
-        if (root->children[character - 97] == NULL)
-        {
-            TrieNode *node = NULL;
-            node = createTrieNode();
-            node->character = character;
-            root->children[character - 97] = node;
+    // this is the end of this word; add an end-of-word marker here and we're
+    // done.
+    if (0 == word_len) {
+        trie->end_of_word = true;
+        return 0;
     }
-        word++;
-        insert(root->children[character - 97], word);
+
+    // if you have some more complex mapping, you could introduce one here. In
+    // this easy example, we just subtract 'a' (97) from it, meaning that 'a' is 0,
+    // 'b' is 1, and so on.
+    const unsigned int index = word[0] - 'a';
+
+    // this index is outside the alphabet size; indexing this would mean an
+    // out-of-bound memory access (bad!). If you introduce a separate map
+    // function for indexing, then you could move the out-of-bounds index in
+    // there.
+    if (ALPHABET_SIZE <= index) {
+        return -1;
     }
-    else
-    {
-        root->isEndOfWord = true;
+
+    // The index does not exist yet, allocate it.
+    if (NULL == trie->children[index]) {
+        ret = trie_new(&trie->children[index]);
+        if (-1 == ret) {
+            // creating new trie node failed
+            return -1;
         }
-    return;
     }
     
+    // recurse into the child node
+    return trie_insert(
+        /* trie = */ trie->children[index],
+        /* word = */ word + 1,
+        /* word_len = */ word_len - 1
+    );
+}
+
+
 /*--Search a word in the Trie--*/
-TrieNode *search(TrieNode *root, char *word)
+int trie_search(
+    struct trie * trie,
+    char *word,
+    unsigned word_len,
+    struct trie ** result
+)
 {
-    TrieNode *temp;
-    while (*word != '\0')
-    {
-        char character = *word;
-        if (root->children[character - 97] != NULL)
-        {
-            temp = root->children[character - 97];
-            word++;
-            root = temp;
-        }
-        else
-        {
-            printf("No possible words!!\n");
-            return NULL;
-        }
-    }
-    return root;
+    // we found a match
+    if (0 == word_len) {
+        *result = trie;
+        return 0;
     }
 
-/*---Print a word in the array--*/
-void printArray(char chars[], int len)
-{
-    int i;
-    for (i = 0; i < len; i++)
-    {
-        printf("%c", chars[i]);
+    // same here as in trie_insert, if you have a separate index mapping, add
+    // it here. In this example, we just subtract 'a'.
+    const unsigned int index = word[0] - 'a';
+
+    // This word contains letters outside the alphabet length; it's invalid.
+    // Remember to do this to prevent buffer overflows.
+    if (ALPHABET_SIZE <= index) {
+        return -1;
     }
-    printf("\n");
+
+    // No match
+    if (NULL == trie->children[index]) {
+        return -1;
+    }
+
+    // traverse the trie
+    return trie_search(
+        /* trie = */ trie->children[index],
+        /* word = */ word + 1,
+        /* word_len = */ word_len - 1,
+        /* result = */ result
+    );
 }
 
 /*---Return all the related words------*/
-void printPathsRecur(TrieNode *node, char prefix[], int filledLen)
+void trie_print (
+    struct trie * trie,
+    char prefix[],
+    unsigned prefix_len
+)
 {
-    if (node == NULL)
-        return;
 
-    prefix[filledLen] = node->character;
-    filledLen++;
-
-    if (node->isEndOfWord)
-    {
-        printArray(prefix, filledLen);
+    // An end-of-word marker means that this is a complete word, print it.
+    if (true == trie->end_of_word) {
+        printf("%.*s\n", prefix_len, prefix);
     }
 
-    int i;
-    for (i = 0; i < ALPHABET_SIZE; i++)
-    {
-        printPathsRecur(node->children[i], prefix, filledLen);
+    // However, there can be longer words with the same prefix; traverse into
+    // those as well.
+    for (int i = 0; i < ALPHABET_SIZE; i++) {
+
+        // No words on this character
+        if (NULL == trie->children[i]) {
+            continue;
+        }
+
+        // If you have a separate index mapping, then you'd need the inverse of
+        // the map here. Since we subtracted 'a' for the index, we can just add
+        // 'a' to get the inverse map function.
+        prefix[prefix_len] = i + 'a';
+
+        // traverse the print into the child
+        trie_print(trie->children[i], prefix, prefix_len + 1);
     }
 }
 
-/*--Travel through the Trie and return words from it--*/
-void traverse(char prefix[], TrieNode *root)
-{
-    TrieNode *temp = NULL;
-    temp = search(root, prefix);
-    int j = 0;
-    while (prefix[j] != '\0')
-    {
-        j++;
-    }
-    printPathsRecur(temp, prefix, j - 1);
-}
 
 /*------Demonstrate purposes uses text file called dictionary -------*/
 
-#define NUMBER_OF_WORDS (354935)
-#define INPUT_WORD_SIZE (100)
+int main() {
+    int ret = 0;
+    struct trie * root = NULL;
+    struct trie * trie = NULL;
+    char word[100] = {0};
 
-/*----Get input from the user------*/
-char *receiveInput(char *s)
-{
-    scanf("%99s", s);
-    return s;
+    // Create a root trie
+    ret = trie_new(&root);
+    if (-1 == ret) {
+        fprintf(stderr, "Could not create trie\n");
+        exit(1);
     }
 
-int main()
-{
-    // Read the file dictionary
-    int word_count = 0;
-    char *words[NUMBER_OF_WORDS];
+    // open the dictionary file
     FILE *fp = fopen("dictionary.txt", "r");
-
-    if (fp == 0)
-    {
+    if (NULL == fp) {
         fprintf(stderr, "Error while opening dictionary file");
         exit(1);
     }
 
-    words[word_count] = malloc(INPUT_WORD_SIZE);
-
-    while (fgets(words[word_count], INPUT_WORD_SIZE, fp))
-    {
-        word_count++;
-        words[word_count] = malloc(INPUT_WORD_SIZE);
+    // insert all the words from the dictionary
+    while (1 == fscanf(fp, "%100s\n", word)) {
+        ret = trie_insert(root, word, strnlen(word, 100));
+        if (-1 == ret) {
+            fprintf(stderr, "Could not insert word into trie\n");
+            exit(1);
+        }
     }
 
-    // Push the words in to Trie
-    TrieNode *root = NULL;
-    root = createTrieNode();
-    int i;
-    for (i = 0; i < NUMBER_OF_WORDS; i++)
-    {
-        insert(root, words[i]);
-    }
-
-    while (1)
-    {
+    while (1) {
         printf("Enter keyword: ");
-        char str[100];
-        receiveInput(str);
+        if (1 != scanf("%100s", word)) {
+            break;
+        }
+
         printf(
             "\n==========================================================\n");
         printf("\n********************* Possible Words ********************\n");
 
-        // Find the word through the Trie
-        traverse(str, root);
+        ret = trie_search(root, word, strnlen(word, 100), &trie);
+        if (-1 == ret) {
+            printf("No results\n");
+            continue;
+        }
 
-        printf(
-            "\n==========================================================\n");
+        trie_print(trie, word, strnlen(word, 100));
+
+        printf("\n==========================================================\n");
     }
 }