# Efficient Data Storage and Retrieval

#### Beyond Data Integrity

In addition to data integrity, hash functions can also be used as a tagging or identification mechanism in data storage and data structures. This provides **efficiency gains**.

* **Message digests** are of **fixed length** but still **unique**.
* Hash functions can take **almost anything** as an input value.
* Many **data structures** in information systems use hash functions to **identify or index data**.

#### The Role of Hash Tables

At the foundation of computer systems, hash tables (also known as **associative arrays, maps, or dictionaries**) are a core data structure that make **memory access as efficient as possible**.

* **All other data structures** can be built from hash tables.
* Some programming languages (e.g., **AWK**) rely on them almost exclusively.
* Like arrays, hash tables store data at an **index**, but instead of using sequential numbers, they use a **hash of the key** to determine the index.

#### Arrays vs. Hash Tables

In an array of size 10, values are stored at indexes `0..9`:

```
package main 

import ( 
	"fmt" 
) 

func main() { 
	var array [10]string = [10]string{"one", "two", "three", "four", "five", "six", "seven", "eight", "nine", "ten"} 

	for i := 0; i < 9; i++ { 
		fmt.Printf("%s, ", array[i]) 
	}  

	fmt.Printf("%s\n", array[9]) 

} 

//Output: one, two, three, four, five, six, seven, eight, nine, ten 
```

In a hash table, however, the **hash of the key modulus the array length** determines the index where the value is stored.

```
package main 

import ( 
	"bytes" 
	"encoding/binary" 
	"fmt" 
	bk "github.com/libsv/go-bk/crypto" 
	"math/big" 
) 
 
type Cell struct { 
	key   string 
	value string 
	next  *Cell 
} 

type HashTable struct { 
	Cells []*Cell 
} 
 
func main() { 
	hashTable := NewHashTable() 
	hashTable.Insert("0", "zero") 
	hashTable.Insert("1", "one") 
	hashTable.Insert("2", "two") 
	hashTable.Insert("3", "three") 
	hashTable.Insert("4", "four") 
	hashTable.Insert("5", "five") 
	hashTable.Insert("6", "six") 
	hashTable.Insert("7", "seven") 
	hashTable.Insert("8", "eight") 
	hashTable.Insert("9", "nine") 
  
	fmt.Printf("%s\n", hashTable) 
 
} 

func (hashTable *HashTable) String() string { 
 
	var out bytes.Buffer  

	for _, cell := range hashTable.Cells {  

		if cell != nil {  

			fmt.Fprintf(&out, "%s ", cell.value) 

			if cell.next != nil { 
				for nextCell := cell.next; nextCell != nil; nextCell = nextCell.next { 
					fmt.Fprintf(&out, "%s ", nextCell) 
				} 
			} 
		} 
	} 

  	return out.String() 
} 
 
func (cell *Cell) String() string { 

	var out bytes.Buffer 

	for ; cell != nil; cell = cell.next { 
  
		fmt.Fprintf(&out, "%s", cell.value) 
  
	}  

	return out.String() 

}   

func NewHashTable() *HashTable { 
	return &HashTable{Cells: make([]*Cell, 50)} 

}   

func GetIndex(key string) (index int) {   

	bInt := new(big.Int) 
	bInt.SetBytes(bk.Sha256d([]byte(key))) 

	return int(binary.BigEndian.Uint64(bInt.Bytes()) % 50) 
} 

func (hashTable *HashTable) Insert(key string, value string) { 

	index := GetIndex(key) 

	if hashTable.Cells[index] == nil { 

		hashTable.Cells[index] = &Cell{key: key, value: value} 

	} else {  

		head := hashTable.Cells[index] 

		for ; head != nil; head = head.next { 

			if head.key == key { 
				head.value = value 
			} 
			return 
		} 
 
		head.next = &Cell{key: key, value: value} 
	} 
}   

func (hashTable *HashTable) Get(key string) (string, bool) { 

	index := GetIndex(key)   

	if hashTable.Cells[index] != nil {  

		head := hashTable.Cells[index] 

		for ; ; head = head.next { 
 
			if head.key == key { 
				return head.value, true 
			}  

			if head.next == nil { 
				break 
			} 
		} 

	} 

	return "", false 
}  

//Output: three four five nine eight zero six seven one two 
```

#### Why Hash Tables Are Efficient

Notice the **output is not in order** like the array. That’s because the index is determined by the hash of the key, not its position.

✅ **This makes hash tables extremely efficient:**

* The **index is pre-computed**, so **retrieval is fast**.
* **Almost any input** can be converted into an index.


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