Algorithm/ShannonFanoEncode.cpp at master · vksahu399/Algorithm · GitHub

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//Shannon-Fano encoding algorithm is a form of source coding mainly used for lossless data compression of multimedia.

#include <iostream>
using namespace std;

struct node {
	string sym; // for storing symbol
	float pro; // for storing probability or frequency
	int arr[20];
	int top;
} p[20];

typedef struct node node;

// function to find shannon code
void shannon(int l, int h, node p[])
{
	float pack1 = 0, pack2 = 0, diff1 = 0, diff2 = 0;
	int i, d, k, j;
	if ((l + 1) == h || l == h || l > h) {
		if (l == h || l > h)
			return;
		p[h].arr[++(p[h].top)] = 0;
		p[l].arr[++(p[l].top)] = 1;
		return;
	}
	else {
		for (i = l; i <= h - 1; i++)
			pack1 = pack1 + p[i].pro;
		pack2 = pack2 + p[h].pro;
		diff1 = pack1 - pack2;
		if (diff1 < 0)
			diff1 = diff1 * -1;
		j = 2;
		while (j != h - l + 1) {
			k = h - j;
			pack1 = pack2 = 0;
			for (i = l; i <= k; i++)
				pack1 = pack1 + p[i].pro;
			for (i = h; i > k; i--)
				pack2 = pack2 + p[i].pro;
			diff2 = pack1 - pack2;
			if (diff2 < 0)
				diff2 = diff2 * -1;
			if (diff2 >= diff1)
				break;
			diff1 = diff2;
			j++;
		}
		k++;
		for (i = l; i <= k; i++)
			p[i].arr[++(p[i].top)] = 1;
		for (i = k + 1; i <= h; i++)
			p[i].arr[++(p[i].top)] = 0;

		// Invoke shannon function
		shannon(l, k, p);
		shannon(k + 1, h, p);
	}
}

// Function to sort the symbols based on their probability or frequency
void sortByProbability(int n, node p[])
{
	int i, j;
	node temp;
	for (j = 1; j <= n - 1; j++) {
		for (i = 0; i < n - 1; i++) {
			if ((p[i].pro) > (p[i + 1].pro)) {
				temp.pro = p[i].pro;
				temp.sym = p[i].sym;

				p[i].pro = p[i + 1].pro;
				p[i].sym = p[i + 1].sym;

				p[i + 1].pro = temp.pro;
				p[i + 1].sym = temp.sym;
			}
		}
	}
}

// function to display shannon codes
void display(int n, node p[])
{
	int i, j;
	cout << "\n\n\n\tSymbol\tProbability\tCode";
	for (i = n - 1; i >= 0; i--) {
		cout << "\n\t" << p[i].sym << "\t\t" << p[i].pro << "\t";
		for (j = 0; j <= p[i].top; j++)
			cout << p[i].arr[j];
	}
}

// Driver code
int main()
{
	int n, i, j;
	float total = 0;
	string ch;
	node temp;

	// Input number of symbols
	cout << "Enter number of symbols\t: ";
	n = 10;
	cout << n << endl;

	// Input symbols
	for (i = 0; i < n; i++) {
		cout << "Enter symbol " << i + 1 << " : ";
		ch = (char)(65 + i);
		cout << ch << endl;

		// Insert the symbol to node
		p[i].sym += ch;
	}

	// Input probability of symbols
	float x[] = { 0.22, 0.28, 0.15, 0.30, 0.05, 0.22, 0.45, 0.81, 0.55, 0.07 };
	for (i = 0; i < n; i++) {
		cout << "\nEnter probability of " << p[i].sym << " : ";
		cout << x[i] << endl;

		// Insert the value to node
		p[i].pro = x[i];
		total = total + p[i].pro;

		// checking max probability
		if (total > 1) {
			cout << "Invalid. Enter new values";
			total = total - p[i].pro;
			i--;
		}
	}

	p[i].pro = 1 - total;

	// Sorting the symbols based on
	// their probability or frequency
	sortByProbability(n, p);

	for (i = 0; i < n; i++)
		p[i].top = -1;

	// Find the shannon code
	shannon(0, n - 1, p);

	// Display the codes
	display(n, p);
	return 0;
}