Rabin-Karp uses a rolling hash to filter candidate matches, then compares tokens to guarantee correctness despite hash collisions..
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 | #include <stdio.h> #include <stdlib.h> #include <stdint.h> #include <limits.h> #include <errno.h> // Rabin Karp Pattern Matching in C // // PURPOSE // Rabin-Karp uses a rolling hash to filter candidate matches, then compares tokens to // guarantee correctness despite hash collisions. // // METHOD // Hash the pattern and first window modulo a small prime. At each position verify matching // hashes, then remove the old token and append the new one. // // COMPLEXITY // Expected O(n+m) time on suitable data; O(nm) worst case due to verification. O(1) // auxiliary space. // // CONTRACT // Tokens are integers in [0,255]. Empty pattern matches at zero. Returns first index or -1; // not a cryptographic hash. // Public APIs infer array lengths except in C, which accepts pointer/length pairs. Vertex // counts and matrix dimensions remain explicit. Arrays must contain signed 32-bit integers. // Sorted order, correct tree topology, and caller-owned data-structure invariants are // preconditions; checking them on every query would change query complexity. // Scalar and shape checks are included. Value-range checks are included where needed to // prevent invalid indexing or nontermination. Allocation sizes are capped at 4,000,000 // workspace entries. Arithmetic outside Python raises an error on 32-bit overflow; Python // uses arbitrary precision. Native allocation failures propagate. In-place outputs may be // partly updated after arithmetic failure. These are single-threaded, in-memory // implementations. // // LANGUAGE DETAILS // C99: checked calloc/free workspace, const input pointers, explicit size_t lengths. Errors // return INT_MIN with errno=EINVAL, ERANGE, or ENOMEM. Check errno before using a result. // Input/output arrays must not alias. No variable-length stack arrays. // // EXAMPLE // Expected return: 2. // Run: gcc -std=c99 -O2 example.c -o example && ./example // Tests for this exact source artifact run before publication. The accompanying source // archive includes unit cases, runtime results, and source hashes. static int alg_min(int a, int b) { return a < b ? a : b; } static int alg_max(int a, int b) { return a > b ? a : b; } static int alg_checked(int64_t v, int *error) { if (v < INT_MIN || v > INT_MAX) { *error = 1; return 0; } return (int)v; } static int alg_add(int a, int b, int *e) { return alg_checked((int64_t)a+b,e); } static int alg_sub(int a, int b, int *e) { return alg_checked((int64_t)a-b,e); } static int alg_mul(int a, int b, int *e) { return alg_checked((int64_t)a*b,e); } int rabin_karp(const int *text, size_t text_length, const int *pattern, size_t pattern_length) { int alg_error = 0; int alg_result = INT_MIN; errno = 0; if (text_length > 1000000 || (text_length && !text)) { errno = EINVAL; alg_result = INT_MIN; goto alg_cleanup; } if (pattern_length > 1000000 || (pattern_length && !pattern)) { errno = EINVAL; alg_result = INT_MIN; goto alg_cleanup; } int n = (int)text_length; if (alg_error) { errno = ERANGE; alg_result = INT_MIN; goto alg_cleanup; } int m = (int)pattern_length; if (alg_error) { errno = ERANGE; alg_result = INT_MIN; goto alg_cleanup; } if (!(((n >= 0) && (n <= 1000000)))) { errno = EINVAL; alg_result = INT_MIN; goto alg_cleanup; } if (alg_error) { errno = ERANGE; alg_result = INT_MIN; goto alg_cleanup; } if (!(((m >= 0) && (m <= 1000000)))) { errno = EINVAL; alg_result = INT_MIN; goto alg_cleanup; } if (alg_error) { errno = ERANGE; alg_result = INT_MIN; goto alg_cleanup; } if (!(((int)pattern_length == m))) { errno = EINVAL; alg_result = INT_MIN; goto alg_cleanup; } if (alg_error) { errno = ERANGE; alg_result = INT_MIN; goto alg_cleanup; } if (!(((int)text_length == n))) { errno = EINVAL; alg_result = INT_MIN; goto alg_cleanup; } if (alg_error) { errno = ERANGE; alg_result = INT_MIN; goto alg_cleanup; } for (int alg_i = 0; alg_i < (int)text_length; alg_i++) { int alg_value = text[alg_i]; if (!(((alg_value >= 0) && (alg_value <= 255)))) { errno = EINVAL; alg_result = INT_MIN; goto alg_cleanup; } if (alg_error) { errno = ERANGE; alg_result = INT_MIN; goto alg_cleanup; } } for (int alg_i = 0; alg_i < (int)pattern_length; alg_i++) { int alg_value = pattern[alg_i]; if (!(((alg_value >= 0) && (alg_value <= 255)))) { errno = EINVAL; alg_result = INT_MIN; goto alg_cleanup; } if (alg_error) { errno = ERANGE; alg_result = INT_MIN; goto alg_cleanup; } } int alg_condition_1 = (m == 0); if (alg_error) { errno = ERANGE; alg_result = INT_MIN; goto alg_cleanup; } if (alg_condition_1) { alg_result = 0; if (alg_error) { errno = ERANGE; alg_result = INT_MIN; goto alg_cleanup; } goto alg_cleanup; } int alg_condition_2 = (m > n); if (alg_error) { errno = ERANGE; alg_result = INT_MIN; goto alg_cleanup; } if (alg_condition_2) { alg_result = alg_sub(0, 1, &alg_error); if (alg_error) { errno = ERANGE; alg_result = INT_MIN; goto alg_cleanup; } goto alg_cleanup; } int power = 1; if (alg_error) { errno = ERANGE; alg_result = INT_MIN; goto alg_cleanup; } int target_hash = 0; if (alg_error) { errno = ERANGE; alg_result = INT_MIN; goto alg_cleanup; } int window_hash = 0; if (alg_error) { errno = ERANGE; alg_result = INT_MIN; goto alg_cleanup; } int i = 0; if (alg_error) { errno = ERANGE; alg_result = INT_MIN; goto alg_cleanup; } int j = 0; if (alg_error) { errno = ERANGE; alg_result = INT_MIN; goto alg_cleanup; } while (1) { int alg_condition = (i < m); if (alg_error) { errno = ERANGE; alg_result = INT_MIN; goto alg_cleanup; } if (!alg_condition) break; target_hash = (alg_add(alg_mul(target_hash, 256, &alg_error), pattern[i], &alg_error) % 10007); if (alg_error) { errno = ERANGE; alg_result = INT_MIN; goto alg_cleanup; } window_hash = (alg_add(alg_mul(window_hash, 256, &alg_error), text[i], &alg_error) % 10007); if (alg_error) { errno = ERANGE; alg_result = INT_MIN; goto alg_cleanup; } int alg_condition_3 = (i < alg_sub(m, 1, &alg_error)); if (alg_error) { errno = ERANGE; alg_result = INT_MIN; goto alg_cleanup; } if (alg_condition_3) { power = (alg_mul(power, 256, &alg_error) % 10007); if (alg_error) { errno = ERANGE; alg_result = INT_MIN; goto alg_cleanup; } } i = alg_add(i, 1, &alg_error); if (alg_error) { errno = ERANGE; alg_result = INT_MIN; goto alg_cleanup; } } i = 0; if (alg_error) { errno = ERANGE; alg_result = INT_MIN; goto alg_cleanup; } while (1) { int alg_condition = (i <= alg_sub(n, m, &alg_error)); if (alg_error) { errno = ERANGE; alg_result = INT_MIN; goto alg_cleanup; } if (!alg_condition) break; int alg_condition_4 = (target_hash == window_hash); if (alg_error) { errno = ERANGE; alg_result = INT_MIN; goto alg_cleanup; } if (alg_condition_4) { j = 0; if (alg_error) { errno = ERANGE; alg_result = INT_MIN; goto alg_cleanup; } while (1) { int alg_condition = ((j < m) && (text[alg_add(i, j, &alg_error)] == pattern[j])); if (alg_error) { errno = ERANGE; alg_result = INT_MIN; goto alg_cleanup; } if (!alg_condition) break; j = alg_add(j, 1, &alg_error); if (alg_error) { errno = ERANGE; alg_result = INT_MIN; goto alg_cleanup; } } int alg_condition_5 = (j == m); if (alg_error) { errno = ERANGE; alg_result = INT_MIN; goto alg_cleanup; } if (alg_condition_5) { alg_result = i; if (alg_error) { errno = ERANGE; alg_result = INT_MIN; goto alg_cleanup; } goto alg_cleanup; } } int alg_condition_6 = (i < alg_sub(n, m, &alg_error)); if (alg_error) { errno = ERANGE; alg_result = INT_MIN; goto alg_cleanup; } if (alg_condition_6) { window_hash = (alg_add(alg_sub(window_hash, (alg_mul(text[i], power, &alg_error) % 10007), &alg_error), 10007, &alg_error) % 10007); if (alg_error) { errno = ERANGE; alg_result = INT_MIN; goto alg_cleanup; } window_hash = (alg_add(alg_mul(window_hash, 256, &alg_error), text[alg_add(i, m, &alg_error)], &alg_error) % 10007); if (alg_error) { errno = ERANGE; alg_result = INT_MIN; goto alg_cleanup; } } i = alg_add(i, 1, &alg_error); if (alg_error) { errno = ERANGE; alg_result = INT_MIN; goto alg_cleanup; } } alg_result = alg_sub(0, 1, &alg_error); if (alg_error) { errno = ERANGE; alg_result = INT_MIN; goto alg_cleanup; } goto alg_cleanup; alg_cleanup: return alg_result; } int main(void) { int text[] = {1, 2, 1, 2, 3}; int pattern[] = {1, 2, 3}; int result = rabin_karp(text, 5, pattern, 3); printf("%d\n", result); if (errno) { perror("algorithm"); return 1; } return 0; } |
