| 1 | /* -*-c-*- |
| 2 | * |
| 3 | * Prosaic C implementation of a `same-fringe' solver. |
| 4 | */ |
| 5 | |
| 6 | #include <assert.h> |
| 7 | #include <stdio.h> |
| 8 | #include <stdlib.h> |
| 9 | |
| 10 | /*----- Utilities ---------------------------------------------------------*/ |
| 11 | |
| 12 | static const char *progname = "?"; |
| 13 | |
| 14 | /* Mournfully announce an error and quit. */ |
| 15 | static void bail(const char *m) |
| 16 | { fprintf(stderr, "%s: %s\n", progname, m); exit(EXIT_FAILURE); } |
| 17 | |
| 18 | /*----- Our node structure ------------------------------------------------*/ |
| 19 | |
| 20 | struct node { |
| 21 | struct node *left; |
| 22 | struct node *right; |
| 23 | int data; |
| 24 | }; |
| 25 | |
| 26 | /* Make a new node and return it. */ |
| 27 | static struct node *makenode(int data, struct node *left, struct node *right) |
| 28 | { |
| 29 | struct node *n = malloc(sizeof(*n)); |
| 30 | |
| 31 | if (!n) bail("no memory"); |
| 32 | n->data = data; n->left = left; n->right = right; |
| 33 | return (n); |
| 34 | } |
| 35 | |
| 36 | /* Free node N and its subtrees. */ |
| 37 | static void freetree(struct node *n) |
| 38 | { if (n) { freetree(n->left); freetree(n->right); free(n); } } |
| 39 | |
| 40 | /* Recursive parser, used by `parsetree': read from string, updating `*p' as |
| 41 | * we go. |
| 42 | */ |
| 43 | static struct node *rparsetree(const char **p) |
| 44 | { |
| 45 | struct node *left, *right; |
| 46 | int data; |
| 47 | |
| 48 | switch (**p) { |
| 49 | case '(': |
| 50 | (*p)++; |
| 51 | left = rparsetree(p); |
| 52 | data = *(*p)++; |
| 53 | if (!data) bail("no data"); |
| 54 | right = rparsetree(p); |
| 55 | if (**p != ')') bail("missing )"); |
| 56 | (*p)++; |
| 57 | return (makenode(data, left, right)); |
| 58 | default: |
| 59 | return (0); |
| 60 | } |
| 61 | } |
| 62 | |
| 63 | /* Parse a tree description from the string `p'. |
| 64 | * |
| 65 | * The syntax is as follows. |
| 66 | * |
| 67 | * tree ::= empty | `(' tree char tree `)' |
| 68 | * |
| 69 | * where the ambiguity is resolved by always treating `(' as starting a tree |
| 70 | * if a tree is expected. |
| 71 | */ |
| 72 | static struct node *parsetree(const char *p) |
| 73 | { |
| 74 | struct node *n = rparsetree(&p); |
| 75 | |
| 76 | if (*p) bail("trailing junk"); |
| 77 | return (n); |
| 78 | } |
| 79 | |
| 80 | /*----- Iteration ---------------------------------------------------------*/ |
| 81 | |
| 82 | struct nodeiter { |
| 83 | #define MAXDEPTH 64 |
| 84 | struct node *stack[MAXDEPTH]; |
| 85 | int sp; |
| 86 | }; |
| 87 | |
| 88 | /* Helper for `nextnode' and `iternodes'. If N is not null, push it onto |
| 89 | * NI's stack, and then do the same for N's left child. |
| 90 | */ |
| 91 | static void pushnodes(struct nodeiter *ni, struct node *n) |
| 92 | { |
| 93 | int sp = ni->sp; |
| 94 | |
| 95 | while (n) { |
| 96 | assert(sp < MAXDEPTH); |
| 97 | ni->stack[sp++] = n; |
| 98 | n = n->left; |
| 99 | } |
| 100 | ni->sp = sp; |
| 101 | } |
| 102 | |
| 103 | /* Return the next node in order for the tree being traversed by NI, or null |
| 104 | * if all nodes are exhausted. |
| 105 | */ |
| 106 | static struct node *nextnode(struct nodeiter *ni) |
| 107 | { |
| 108 | struct node *n; |
| 109 | |
| 110 | if (!ni->sp) |
| 111 | return (0); |
| 112 | else { |
| 113 | n = ni->stack[--ni->sp]; |
| 114 | pushnodes(ni, n->right); |
| 115 | return (n); |
| 116 | } |
| 117 | } |
| 118 | |
| 119 | /* Initialize NI as an iterator iterating over the tree headed by N. */ |
| 120 | static void iternodes(struct nodeiter *ni, struct node *n) |
| 121 | { ni->sp = 0; pushnodes(ni, n); } |
| 122 | |
| 123 | /*------ Fringe operations ------------------------------------------------*/ |
| 124 | |
| 125 | /* Print the characters stored in the tree headed by N to stdout, in |
| 126 | * order. */ |
| 127 | static void printfringe(struct node *n) |
| 128 | { |
| 129 | struct nodeiter ni; |
| 130 | |
| 131 | for (iternodes(&ni, n); (n = nextnode(&ni)) != 0; ) |
| 132 | putchar(n->data); |
| 133 | putchar('\n'); |
| 134 | } |
| 135 | |
| 136 | /* Return nonzero if traversing the trees headed by N and NN respectively |
| 137 | * yields the same items in the same order. |
| 138 | */ |
| 139 | static int samefringep(struct node *n, struct node *nn) |
| 140 | { |
| 141 | struct nodeiter ni, nni; |
| 142 | |
| 143 | iternodes(&ni, n); iternodes(&nni, nn); |
| 144 | for (;;) { |
| 145 | n = nextnode(&ni); nn = nextnode(&nni); |
| 146 | if (!n) return (!nn); |
| 147 | else if (!nn) return (0); |
| 148 | else if (n->data != nn->data) return (0); |
| 149 | } |
| 150 | } |
| 151 | |
| 152 | /*----- Main program ------------------------------------------------------*/ |
| 153 | |
| 154 | int main(int argc, char *argv[]) |
| 155 | { |
| 156 | struct node *n, *nn; |
| 157 | |
| 158 | progname = argv[0]; |
| 159 | switch (argc) { |
| 160 | case 2: |
| 161 | n = parsetree(argv[1]); |
| 162 | printfringe(n); |
| 163 | freetree(n); |
| 164 | break; |
| 165 | case 3: |
| 166 | n = parsetree(argv[1]); nn = parsetree(argv[2]); |
| 167 | printf("%s\n", samefringep(n, nn) ? "match" : "no match"); |
| 168 | freetree(n); freetree(nn); |
| 169 | break; |
| 170 | default: |
| 171 | bail("bad args"); |
| 172 | break; |
| 173 | } |
| 174 | return (0); |
| 175 | } |
| 176 | |
| 177 | /*----- That's all, folks -------------------------------------------------*/ |