Add hashtable for Environment tracking

1 files changed, 180 insertions, 0 deletions

diff --git a/src/hashtable.h b/src/hashtable.h
new file mode 100644
index 0000000..faa8591
--- /dev/null
+++ b/src/hashtable.h
@@ -0,0 +1,180 @@
+#ifndef BDL_HASHTABLE_H
+#define BDL_HASHTABLE_H
+
+#include "darray.h"
+
+// Minimum table capacity.
+#define HT_MIN_CAP   4
+#define HT_MIN_SHIFT 2
+
+// Adjust the load factor threshold at which the table will grow on insertion.
+#define HT_LOAD_THRESHOLD 0.8
+
+typedef struct HashTablePair {
+    void *key;
+    void *value;
+} HashTablePair;
+
+struct HashTable;
+typedef uint64_t (HashFunction)(const struct HashTable *table, void *bytes);
+typedef bool (EqFunc)(void *a, void *b);
+
+typedef struct HashTable {
+    // All available key-value pairs as a dynamic array.
+    HashTablePair *pairs;
+
+    // Hash function.
+    HashFunction *hash_func;
+
+    // Equality function.
+    EqFunc *eq_func;
+
+    // This table expects the number of buckets to grow in powers of two. To
+    // speedup the default hashing, we memoize the number of bits equivalent to
+    // that power of 2:
+    //
+    //     cap := 1024 = 2 ^ 10, shift_amount := 10
+    //
+    uint8_t shift_amount;
+} HashTable;
+
+// Hash a byte stream using a circular shift + XOR hash function.
+static inline uint64_t
+_xor_shift_hash(const char *key, size_t n) {
+    uint64_t hash = 0x65d9d65f6a19574f;
+    char *last = (char *)key + n;
+    while (key != last) {
+        hash ^= (uint64_t)*key++;
+        hash = (hash << 8) | (hash >> (64 - 8));
+    }
+    return hash;
+}
+
+// Use Fibonacci hashing to map a hash to a value in range of the table.
+static inline uint64_t
+_fibonacci_hash(uint64_t hash, size_t shift_amount) {
+    return (hash * UINT64_C(11400714819323198485)) >> (64 - shift_amount);
+}
+
+static inline float
+ht_load_factor(const HashTable *table) {
+    return (float)array_size(table->pairs) / (float)array_cap(table->pairs);
+}
+
+HashTable *
+ht_init(HashFunction *hash_func, EqFunc *eq_func) {
+    HashTable *table = malloc(sizeof(HashTable));
+    *table = (HashTable){0};
+    array_init(table->pairs, HT_MIN_CAP);
+    for (size_t i = 0; i < array_cap(table->pairs); i++) {
+        table->pairs[i] = (HashTablePair){NULL, NULL};
+    }
+    table->shift_amount = HT_MIN_SHIFT;
+    table->hash_func = hash_func;
+    table->eq_func = eq_func;
+    return table;
+}
+
+void
+_ht_insert(HashTable *table, void *key, void *value) {
+    size_t position = table->hash_func(table, key);
+    size_t probe_position = position;
+
+    // Verify the key in that position is free. If not, use linear probing to
+    // find the next free slot.
+    HashTablePair *pairs = table->pairs;
+    bool update = false;
+    while (true) {
+        if (pairs[probe_position].key == NULL) {
+            array_head(pairs)->size++;
+            break;
+        }
+        if (table->eq_func(pairs[probe_position].key, key)) {
+            update = true;
+            break;
+        }
+        if (probe_position == array_cap(pairs) - 1) {
+            probe_position = 0;
+        } else {
+            probe_position++;
+        }
+    }
+
+    if (!update) {
+        pairs[probe_position].key = key;
+        pairs[probe_position].value = value;
+    } else {
+        pairs[probe_position].value = value;
+    }
+}
+
+void
+_ht_maybe_grow(HashTable *table) {
+    if (ht_load_factor(table) < HT_LOAD_THRESHOLD) {
+        return;
+    }
+
+    // Create a new array with 2x capacity.
+    HashTablePair *old_pairs = table->pairs;
+    table->pairs = NULL;
+    array_init(table->pairs, array_cap(old_pairs) * 2);
+    for (size_t i = 0; i < array_cap(table->pairs); i++) {
+        table->pairs[i] = (HashTablePair){NULL, NULL};
+    }
+    table->shift_amount++;
+
+    // Hash everything in the table for the new array capacity.
+    for (size_t i = 0; i < array_cap(old_pairs); i++) {
+        if (old_pairs[i].key != NULL) {
+            _ht_insert(table, old_pairs[i].key, old_pairs[i].value);
+        }
+    }
+
+    // Free old arrays.
+    array_free(old_pairs);
+}
+
+void
+ht_insert(HashTable *table, void *key, void *value) {
+    _ht_maybe_grow(table);
+    _ht_insert(table, key, value);
+    return;
+}
+
+void *
+ht_lookup(const HashTable *table, void *key) {
+    size_t position = table->hash_func(table, key);
+    size_t probe_position = position;
+
+    // Verify the key in that position is the same. If not perform linear
+    // probing to find it.
+    HashTablePair *pairs = table->pairs;
+    while (true) {
+        if (pairs[probe_position].key == NULL) {
+            return NULL;
+        }
+        if (table->eq_func(pairs[probe_position].key, key)) {
+            break;
+        }
+        if (probe_position == array_cap(pairs) - 1) {
+            probe_position = 0;
+        } else {
+            probe_position++;
+        }
+        if (probe_position == position) {
+            return NULL;
+        }
+    }
+    return pairs[probe_position].value;
+}
+
+void
+ht_free(HashTable *table) {
+    if (table == NULL) {
+        return;
+    }
+    array_free(table->pairs);
+    free(table);
+}
+
+#endif // BDL_HASHTABLE_H