1 files changed, 191 insertions, 0 deletions

diff --git a/src/treewalk/hashtable.h b/src/treewalk/hashtable.h
new file mode 100644
index 0000000..8f210e3
--- /dev/null
+++ b/src/treewalk/hashtable.h
@@ -0,0 +1,191 @@
+#ifndef BDL_HASHTABLE_H
+#define BDL_HASHTABLE_H
+
+#include "darray.h"
+#include "objects.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 {
+    Object *key;
+    Object *value;
+} HashTablePair;
+
+typedef struct HashTable {
+    // All available key-value pairs as a dynamic array.
+    HashTablePair *pairs;
+
+    // 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);
+}
+
+uint64_t
+ht_hash(const HashTable *table, const Object *key) {
+    uint64_t hash = 0;
+    switch (key->type) {
+        case OBJ_TYPE_FIXNUM: {
+            hash = key->fixnum;
+        } break;
+        case OBJ_TYPE_STRING: {
+            hash = _xor_shift_hash(key->string, array_size(key->string));
+        } break;
+        case OBJ_TYPE_SYMBOL: {
+            hash = _xor_shift_hash(key->symbol, array_size(key->symbol));
+        } break;
+        case OBJ_TYPE_BOOL:
+        case OBJ_TYPE_NIL:
+        case OBJ_TYPE_PAIR:
+        case OBJ_TYPE_LAMBDA:
+        case OBJ_TYPE_PROCEDURE:
+        case OBJ_TYPE_ERR: {
+            hash = (uintptr_t)key;
+        } break;
+    }
+    hash = _fibonacci_hash(hash, table->shift_amount);
+    return hash;
+}
+
+static inline float
+ht_load_factor(const HashTable *table) {
+    return (float)array_size(table->pairs) / (float)array_cap(table->pairs);
+}
+
+HashTable *
+ht_init(void) {
+    HashTable *table = malloc(sizeof(HashTable));
+    table->pairs = NULL;
+    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;
+    return table;
+}
+
+void
+_ht_insert(HashTable *table, const Object *key, const Object *value) {
+    size_t position = ht_hash(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;
+    while (true) {
+        if (pairs[probe_position].key == NULL) {
+            array_head(pairs)->size++;
+            break;
+        }
+        if (obj_eq(pairs[probe_position].key, key)) {
+            break;
+        }
+        if (probe_position == array_cap(pairs) - 1) {
+            probe_position = 0;
+        } else {
+            probe_position++;
+        }
+    }
+    pairs[probe_position].key = (Object *)key;
+    pairs[probe_position].value = (Object *)value;
+}
+
+void
+_ht_maybe_grow(HashTable *table) {
+    HashTablePair *pairs = table->pairs;
+    if (ht_load_factor(table) < HT_LOAD_THRESHOLD) {
+        return;
+    }
+
+    // Create a new array with 2x capacity.
+    table->pairs = NULL;
+    array_init(table->pairs, array_cap(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(pairs); i++) {
+        if (pairs[i].key != NULL) {
+            _ht_insert(table, pairs[i].key, pairs[i].value);
+        }
+    }
+
+    // Free the old array.
+    array_free(pairs);
+}
+
+void
+ht_insert(HashTable *table, const Object *key, const Object *value) {
+    _ht_maybe_grow(table);
+    _ht_insert(table, key, value);
+    return;
+}
+
+Object *
+ht_lookup(const HashTable *table, const Object *key) {
+    size_t position = ht_hash(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 (obj_eq(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;
+    }
+    if (table->pairs == NULL) {
+        return;
+    }
+    array_free(table->pairs);
+    free(table);
+}
+
+#endif // BDL_HASHTABLE_H