table.c

#include <stdlib.h>
#include <string.h>

#include "memory.h"
#include "object.h"
#include "table.h"
#include "value.h"

#define TABLE_MAX_LOAD 0.75

void initTable(Table* table) {
  table->count = 0;
  table->capacity = 0;
  table->entries = NULL;
};

void freeTable(Table* table) {
  FREE_ARRAY(Entry, table->entries, table->capacity);
  initTable(table);
}

static Entry* findEntry(Entry* entries, int capacity, ObjString* key) {
  uint32_t index = key->hash % capacity;
  Entry* tombstone = NULL;
  for (;;) {
    Entry* entry = &entries[index];

    if (entry->key == NULL) {
      // we could either have an empty entry or a tombstone
      if (IS_NIL(entry->value)) {
        // here we have an empty entry so return early
        // but reuse the first tombstone we saw if possible
        return tombstone != NULL ? tombstone : entry;
      } else {
        // in this case, entry->value is true, so it's a tombstone
        // and we keep probing
        if (tombstone == NULL) tombstone = entry;
      }
    } else if (entry->key == key) {
      // found actual bucket
      return entry;
    }

    // linear probing: keep looking forward until
    // key matches or we hit an empty bucket
    index = (index + 1) % capacity;
  }
}

bool tableGet(Table* table, ObjString* key, Value* value) {
  if (table->count == 0) return false;

  Entry* entry = findEntry(table->entries, table->capacity, key);
  if (entry->key == NULL) return false;

  *value = entry->value;
  return true;
}

static void adjustCapacity(Table* table, int capacity) {
  // prepare a new clean allocated table with future capacity
  Entry* entries = ALLOCATE(Entry, capacity);
  for (int i = 0; i < capacity; i++) {
    entries[i].key = NULL;
    entries[i].value = NIL_VAL;
  }

  // for present capacity, figure out the new table's buckets for current keys
  table->count = 0;
  for (int i = 0; i < table->capacity; i++) {
    Entry* entry = &table->entries[i];
    if (entry->key == NULL) continue;

    Entry* dest = findEntry(entries, capacity, entry->key);
    dest->key = entry->key;
    dest->value = entry->value;
    table->count++;
  }

  // release the old table
  FREE_ARRAY(Entry, table->entries, table->capacity);
  table->entries = entries;
  table->capacity = capacity;
}

// sets key/value in the table; returns true if new value is written
bool tableSet(Table* table, ObjString* key, Value value) {
  if (table->count + 1 > table->capacity * TABLE_MAX_LOAD) {
    int capacity = GROW_CAPACITY(table->capacity);
    adjustCapacity(table, capacity);
  }

  // findEntry either finds the actual key's bucketj or the first empty bucket
  Entry* entry = findEntry(table->entries, table->capacity, key);
  bool isNewKey = entry->key == NULL;
  if (isNewKey && IS_NIL(entry->value)) table->count++;

  entry->key = key;
  entry->value = value;
  return isNewKey;
}

bool tableDelete(Table *table, ObjString *key) {
  if (table->count == 0) return false;

  // find the entry
  Entry* entry = findEntry(table->entries, table->capacity, key);
  if (entry->key == NULL) return false;

  // if we found an entry, add a tombstone. here it's a null key with true value
  // since that combination won't be confused with an actual key/value
  entry->key = NULL;
  entry->value = BOOL_VAL(true);
  return true;
};

// it's kind of like a ruby Hash.merge
void tableAddAll(Table* from, Table* to) {
  for (int i = 0; i < from->capacity; i++) {
    Entry* entry = &from->entries[i];
    if (entry->key != NULL) {
      tableSet(to, entry->key, entry->value);
    }
  }
}

// find a string in the table
ObjString* tableFindString(Table* table, const char* chars, int length, uint32_t hash) {
  if (table->count == 0) return NULL;

  uint32_t index = hash % table->capacity;

  // this looks like findEntry but we memcmp chars instead of matching entry->key to key
  for (;;) {
    Entry* entry = &table->entries[index];
    if (entry->key == NULL) {
      // Stop if we find an empty non-tombstone entry.
      if (IS_NIL(entry->value)) return NULL;
    } else if (entry->key->length == length &&
        entry->key->hash == hash &&
        memcmp(entry->key->chars, chars, length) == 0) {
      // We found it.
      return entry->key;
    }

    index = (index + 1) % table->capacity;
  }
}