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ClémentClément
Clément
authored and
Clément
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Working on dictionary implementation.
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‎source/code/projects/Dictionary/Dictionary/Dictionary.cs‎

Lines changed: 283 additions & 0 deletions
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using System;
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using System.Collections.Generic;
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public class CDictionary<TKey, TValue>
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where TKey : IComparable<TKey>
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{
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// Two enumerated types that we will be using
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// for our implementation of Dictionary.
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public enum StatusType
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{
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Empty,
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Active,
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Deleted,
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};
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public enum CollisionRes
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{
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Linear,
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Quad,
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Double,
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};
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private class Cell
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{
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public StatusType Status { get; set; }
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public TValue Value { get; set; }
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public TKey Key { get; set; }
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public Cell(
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TKey aKey = default(TKey),
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TValue aValue = default(TValue),
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StatusType aStatus = StatusType.Empty
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)
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{
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Key = aKey;
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Value = aValue;
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Status = aStatus;
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}
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public override string ToString()
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{
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return Key + ":" + Value;
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}
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}
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// The hash table is an array of Cells,
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// and a collision strategy.
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private Cell[] table;
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private readonly CollisionRes Strategy;
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public override string ToString()
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{
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string returned = "";
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int i = 0;
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foreach (Cell pos in table)
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{
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returned += $"Position {i}: {pos}\n";
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i++;
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}
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return returned;
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}
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public CDictionary(
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int size = 31,
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CollisionRes aCollisionStrategy = CollisionRes.Double
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)
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{
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table = new Cell[PrimeHelper.NextPrime(size)];
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Strategy = aCollisionStrategy;
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}
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public void Clear()
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{
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for (int i = 0; i < table.Length; i++)
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if (table[i] != null)
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table[i].Status = StatusType.Deleted; // Reuse cells by setting them to Empty
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}
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public void Add(TKey aKey, TValue aValue)
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{
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/*
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* First, we find an empty cell (e.g. cell is null, status empty or deleted)
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* - We computer a possible index:
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* - We first use GetHashCode() to generate a hash code,
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* - then shift it using collisionR.
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* - We check if the cell at this index is available,
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* - If it is not, we try with the next one,
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* - If all cells are occupied, we throw an error.
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*/
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int count = 0;
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int index = GetIndex(aKey, count);
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// Collision resolution
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while (
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table[index] != null
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&& !table[index].Status.Equals(StatusType.Deleted)
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)
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{
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count++;
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if (count == table.Length) // If table is full, throw an exception.
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throw new ApplicationException("Table is full.");
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index = GetIndex(aKey, count);
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}
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if (table[index] == null) // table slot is empty (e.g. never been used)
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table[index] = new Cell(
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aKey,
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aValue,
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StatusType.Active
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);
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else if (
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table[index].Key.Equals(aKey) == true
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&& table[index].Status == StatusType.Active
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) // duplicate key found
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throw new ArgumentException(
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"Dictionary Error: Don't add duplicate keys: "
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+ aKey.ToString()
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);
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else if (table[index].Status == StatusType.Deleted) // previously used item, reuse the cell
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{
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table[index].Value = aValue;
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table[index].Key = aKey;
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table[index].Status = StatusType.Active;
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}
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else
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throw new ApplicationException(
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"Something went wrong in Add method."
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);
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}
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/// <summary>
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/// Returns the data associated with the key
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/// </summary>
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/// <param name="aKey"></param>
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/// <returns>data item</returns>
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public TValue Find(TKey aKey)
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{
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// search until found or empty
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int count = 0;
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int index = GetIndex(aKey, count);
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while (
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table[index] != null
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&& table[index].Status != StatusType.Deleted
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&& !table[index].Key.Equals(aKey)
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)
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{
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count++;
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if (count == table.Length) // in case table is full, kicks out of inf loop
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throw new ApplicationException("Table is full");
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index = GetIndex(aKey, count);
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}
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if (table[index] == null)
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throw new KeyNotFoundException(
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"The key " + aKey.ToString() + " was not found"
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);
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else if (
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table[index].Status == StatusType.Active
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&& table[index].Key.Equals(aKey) == true
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)
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return table[index].Value;
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else
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throw new ApplicationException(
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"Something went wrong in Find method."
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);
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}
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// The following is used to compute the
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// integer hash code of the key, and shift it if needed
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// using countP.
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private int GetIndex(TKey aKey, int countP)
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{
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// countP captures the number of times we had to solve
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// a collision.
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return (
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Math.Abs(aKey.GetHashCode())
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+ collisionR(countP, aKey)
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) % table.Length;
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}
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// This is the how collision are handled.
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// It depends on the strategy picked.
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// This overall strategy is called open addressing.
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// https://en.wikibooks.org/wiki/Data_Structures/Hash_Tables#Open_addressing
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private int collisionR(int i, TKey aKey)
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{
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if (i == 0)
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return 0;
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else
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{
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if (Strategy == CollisionRes.Linear)
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return i++;
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else if (Strategy == CollisionRes.Quad)
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return i * i;
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else if (Strategy == CollisionRes.Double)
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// This is double hashing.
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return i * (31 - (aKey.GetHashCode() % 31)); // i * hash2(aKey) where hash2 is 31 - (key % 31) and will always be > 0
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else
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throw new ApplicationException(
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"Unknown collision startegy."
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);
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}
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}
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public void Remove(TKey aKey)
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{
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//int index = Search(aKey, IsDeletedOrFound);
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int count = 0;
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int index = GetIndex(aKey, count);
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while (
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table[index] != null
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&& (
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table[index].Status == StatusType.Deleted
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|| !table[index].Key.Equals(aKey)
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)
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)
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{
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count++;
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if (count == table.Length) // in case table is full, kicks out of inf loop
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throw new ApplicationException("Table is full");
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index = GetIndex(aKey, count);
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}
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// Search will keep looking until found or empty cell.
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if (table[index] == null)
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throw new KeyNotFoundException(
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"Cannot delete missing key: " + aKey.ToString()
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);
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else if (
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table[index].Status == StatusType.Active
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&& table[index].Key.Equals(aKey)
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)
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table[index].Status = StatusType.Deleted; // Found it! Mark the cell as deleted.
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else
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throw new ApplicationException(
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"Something went wrong in the Remove method."
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);
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}
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// The following allows the use of [].
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public TValue this[TKey aKey]
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{
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get { return Find(aKey); }
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set
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{
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// find empty cell (e.g. cell is null, status empty or deleted)
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int count = 0;
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int index = GetIndex(aKey, count);
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while (
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table[index] != null
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&& !table[index].Status.Equals(StatusType.Deleted)
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)
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{
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count++;
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if (count == table.Length) // in case table is full, kicks out of inf loop
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throw new ApplicationException("Table is full");
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index = GetIndex(aKey, count);
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}
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// table slot is empty
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if (table[index] == null)
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table[index] = new Cell(
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aKey,
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value,
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StatusType.Active
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);
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// duplicate key found
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else if (
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table[index].Key.Equals(aKey) == true
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&& table[index].Status == StatusType.Active
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)
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table[index].Value = value;
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// previously used item, reuse it
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else if (table[index].Status == StatusType.Deleted)
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{
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table[index].Value = value;
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table[index].Key = aKey;
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table[index].Status = StatusType.Active;
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}
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else
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throw new ApplicationException(
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"Something went wrong in [] set."
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);
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}
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}
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}

‎source/code/projects/Dictionary/Dictionary/PrimeHelper.cs‎

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/*
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* Why prime numbers are needed is explained for example
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* at
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* https://cs.stackexchange.com/questions/11029
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*/
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public static class PrimeHelper
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{
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public static bool IsPrime(int n)
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{
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// "A prime number is a natural number greater than 1 that is not a product of two smaller natural numbers."
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// https://en.wikipedia.org/wiki/Prime_number
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if (n < 2)
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return false;
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if (n == 2 || n == 3)
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return true;
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if (n % 2 == 0)
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return false;
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for (int i = 3; i * i <= n; i += 2)
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if (n % i == 0)
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return false;
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return true;
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}
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public static int NextPrime(int n)
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{
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if (n < 2)
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{
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n = 2;
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}
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else
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{
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// Since 2 is the only even prime,
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// we make the n even if it is divisible
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// by 2.
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if (n % 2 == 0)
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n++;
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while (!IsPrime(n))
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{
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n += 2;
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}
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}
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return n;
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}
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}

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