Add HashMap

Author: AhmedLSayed9

data_structures/linear/hash_maps/hash_map.dart

@@ -0,0 +1,140 @@
+import 'package:test/test.dart';
+
+/// Note: Hash Tables (Hash Maps) can be implemented as linear or non-linear data structure.
+/// Often, they are implemented as a linear data structure using arrays.
+///
+/// Dart's different built-in hash maps and the default one:
+/// https://stackoverflow.com/questions/49224999/does-dart-automatically-determine-which-map-type-to-use
+
+/// The prime number in the hash is helpful in spreading out the keys more uniformly.
+/// It's also helpful if the array that you're putting values into has a prime length.
+/// You don't need to know why, but here are some sources:
+/// https://computinglife.wordpress.com/2008/11/20/why-do-hash-functions-use-prime-numbers
+/// https://www.quora.com/Does-making-array-size-a-prime-number-help-in-hash-table-implementation-Why
+const _mapLength = 53;
+
+/// Internal class to store each key-value pair
+class Entry<K, V> {
+  Entry(this.key, this.value);
+
+  final K key;
+  V value;
+}
+
+class HashMap<K, V> {
+  /// Internal list to store the keys and values via separate chaining
+  final _table = List<List<Entry<K, V>>?>.filled(_mapLength, null);
+
+  /// Loops through the hash table array and returns an array of keys in the table
+  List<K> keys() {
+    List<K> keys = [];
+    for (final entryList in _table) {
+      entryList?.forEach((entry) {
+        keys.add(entry.key);
+      });
+    }
+    return keys;
+  }
+
+  /// Loops through the hash table array and returns an array of values in the table
+  List<V> values() {
+    List<V> values = [];
+    for (final entryList in _table) {
+      entryList?.forEach((entry) {
+        values.add(entry.value);
+      });
+    }
+    return values;
+  }
+
+  /// Pseudocode:
+  /// - Accepts a key and a value.
+  /// - Hashes the key.
+  /// - Stores the key-value pair in the hash table array via separate chaining.
+  /// - Note: If the key-value pair is already there, override the value and return.
+  void set(K key, V value) {
+    final index = _hash(key);
+    final entry = Entry(key, value);
+
+    if (_table[index] case final entryList?) {
+      //check if any entry at that index has the same key.
+      for (final entry in entryList) {
+        if (entry.key == key) {
+          entry.value = value;
+          return;
+        }
+      }
+      entryList.add(entry);
+    } else {
+      _table[index] = [entry];
+    }
+  }
+
+  /// Pseudocode:
+  /// - Accepts a key.
+  /// - Hashes the key.
+  /// - Retrieves the key-value pair in the hash table and return the value.
+  /// - If the key isn't found, returns null.
+  V? get(K key) {
+    final index = _hash(key);
+    if (_table[index] case final entryList?) {
+      for (final entry in entryList) {
+        if (entry.key == key) return entry.value;
+      }
+    }
+    return null;
+  }
+
+  int _hash(K key) {
+    return key.hashCode % _table.length;
+    /*
+    //Manual hash that works on strings only
+    int total = 0;
+    const prime = 31;
+    for (int i = 0; i < math.min(key.length, 100); i++) {
+      final value = key.codeUnitAt(i) - 96;
+      total = ((total * prime) + value) % _table.length;
+    }
+    return total;
+    */
+  }
+}
+
+void main() {
+  test('should add entries to the HashMap and retrieve with the key', () {
+    final map = HashMap();
+
+    map.set(1, 'Ahmed');
+    map.set(2, 'ElSayed');
+    map.set('stringKey', 'Dart');
+
+    expect(map.get(1), 'Ahmed');
+    expect(map.get(2), 'ElSayed');
+    expect(map.get(3), null);
+    expect(map.get('stringKey'), 'Dart');
+  });
+
+  test('should override the value when adding entry with an existing key', () {
+    final map = HashMap();
+
+    map.set(1, 'Ahmed');
+    expect(map.get(1), 'Ahmed');
+
+    map.set(1, 'ElSayed');
+    expect(map.get(1), 'ElSayed');
+    expect(map.keys(), [1]);
+    expect(map.values(), ['ElSayed']);
+  });
+
+  test('should retrieve all keys/values of the map', () {
+    final map = HashMap();
+
+    map.set(1, 'Ahmed');
+    map.set(2, 'ElSayed');
+    map.set(3, 3);
+    map.set('stringKey', 'Dart');
+
+    expect(map.keys(), ['stringKey', 3, 2, 1]);
+    expect(map.values(), ['Dart', 3, 'ElSayed', 'Ahmed']);
+  });
+}