ffsshhttiikk What I do
- Calculate entropy and information content
- Design efficient encodings
- Analyze channel capacity
- Implement compression algorithms
When to use me
When working with data compression, encoding, or communication systems.
Entropy
Shannon Entropy
import math
from collections import Counter
class InformationTheory:
"""Information theory calculations"""
@staticmethod
def entropy(probabilities: List[float]) -> float:
"""Calculate Shannon entropy H(X) = -Σ p(x) log₂ p(x)"""
return -sum(p * math.log2(p)
for p in probabilities if p > 0)
@staticmethod
def entropy_from_frequencies(data: List) -> float:
"""Calculate entropy from data"""
counter = Counter(data)
total = len(data)
probabilities = [count / total for count in counter.values()]
return InformationTheory.entropy(probabilities)
@staticmethod
def joint_entropy(data: List[tuple]) -> float:
"""Calculate joint entropy H(X,Y)"""
counter = Counter(data)
total = len(data)
probabilities = [count / total for count in counter.values()]
return InformationTheory.entropy(probabilities)
@staticmethod
def conditional_entropy(data: List[tuple]) -> float:
"""Calculate conditional entropy H(Y|X)"""
# H(Y|X) = H(X,Y) - H(X)
return InformationTheory.joint_entropy(data) - \
InformationTheory.entropy([x[0] for x in data])
@staticmethod
def mutual_information(x: List, y: List) -> float:
"""Calculate mutual information I(X;Y) = H(X) + H(Y) - H(X,Y)"""
data = list(zip(x, y))
h_x = InformationTheory.entropy_from_frequencies(x)
h_y = InformationTheory.entropy_from_frequencies(y)
h_xy = InformationTheory.joint_entropy(data)
return h_x + h_y - h_xy
# Example: Entropy of English alphabet
english_freqs = {
'E': 0.127, 'T': 0.091, 'A': 0.082, 'O': 0.075, 'I': 0.070,
'N': 0.067, 'S': 0.063, 'R': 0.060, 'H': 0.059, 'L': 0.040
}
entropy = InformationTheory.entropy(list(english_freqs.values()))
# ~3.1 bits per character
Information Content
class InformationContent:
"""Information content and surprise"""
@staticmethod
def self_information(p: float) -> float:
"""Self-information: I(x) = -log₂ p(x)"""
return -math.log2(p) if p > 0 else 0
@staticmethod
def bits_needed(probability: float) -> int:
"""Minimum bits to encode event"""
return math.ceil(-math.log2(probability))
Coding
Huffman Coding
import heapq
from collections import Counter
class HuffmanCoding:
"""Huffman coding for compression"""
class Node:
def __init__(self, char: str, freq: int):
self.char = char
self.freq = freq
self.left = None
self.right = None
def __lt__(self, other):
return self.freq < other.freq
def __init__(self):
self.codes = {}
def build_tree(self, data: str) -> 'HuffmanCoding.Node':
"""Build Huffman tree"""
freq = Counter(data)
heap = [self.Node(char, f) for char, f in freq.items()]
heapq.heapify(heap)
while len(heap) > 1:
left = heapq.heappop(heap)
right = heapq.heappop(heap)
merged = self.Node(None, left.freq + right.freq)
merged.left = left
merged.right = right
heapq.heappush(heap, merged)
return heap[0]
def build_codes(self, node: 'HuffmanCoding.Node',
prefix: str = ""):
"""Build codes by traversing tree"""
if node is None:
return
if node.char is not None:
self.codes[node.char] = prefix
return
self.build_codes(node.left, prefix + "0")
self.build_codes(node.right, prefix + "1")
def encode(self, data: str) -> tuple:
"""Encode data using Huffman coding"""
tree = self.build_tree(data)
self.build_codes(tree)
encoded = "".join(self.codes[char] for char in data)
return encoded, tree
def decode(self, encoded: str, tree: 'HuffmanCoding.Node') -> str:
"""Decode Huffman-encoded data"""
result = []
node = tree
for bit in encoded:
if bit == "0":
node = node.left
else:
node = node.right
if node.char is not None:
result.append(node.char)
node = tree
return "".join(result)
Arithmetic Coding
class ArithmeticCoding:
"""Arithmetic coding"""
def __init__(self):
self.range_start = 0.0
self.range_end = 1.0
def encode(self, data: str, frequencies: dict) -> float:
"""Arithmetic encoding"""
total = sum(frequencies.values())
cumulative = {}
start = 0.0
for char, freq in frequencies.items():
end = start + freq / total
cumulative[char] = (start, end)
start = end
low = 0.0
high = 1.0
for char in data:
range_size = high - low
char_range = cumulative[char]
low = low + range_size * char_range[0]
high = low + range_size * char_range[1]
return (low + high) / 2
Channel Capacity
Channel Capacity (Shannon)
class ChannelCapacity:
"""Calculate channel capacity"""
@staticmethod
def binary_symmetric_channel(p: float) -> float:
"""Capacity of BSC with crossover probability p
C = 1 + p*log₂(p) + (1-p)*log₂(1-p)
"""
if p == 0 or p == 1:
return 0
return 1 + p * math.log2(p) + (1 - p) * math.log2(1 - p)
@staticmethod
def awgn_channel(snr_db: float) -> float:
"""Capacity of AWGN channel
C = B * log₂(1 + SNR)
"""
snr = 10 ** (snr_db / 10)
return math.log2(1 + snr)
@staticmethod
def shannon_hartley(snr_db: float, bandwidth: float) -> float:
"""Shannon-Hartley theorem
C = B * log₂(1 + S/N)
"""
snr = 10 ** (snr_db / 10)
return bandwidth * math.log2(1 + snr)
Compression
LZW Compression
class LZWCompression:
"""LZW compression algorithm"""
def compress(self, data: str) -> list:
"""Compress data using LZW"""
dictionary = {chr(i): i for i in range(256)}
result = []
current = ""
for char in data:
combined = current + char
if combined in dictionary:
current = combined
else:
result.append(dictionary[current])
dictionary[combined] = len(dictionary)
current = char
if current:
result.append(dictionary[current])
return result
def decompress(self, compressed: list) -> str:
"""Decompress LZW data"""
dictionary = {i: chr(i) for i in range(256)}
result = []
current = chr(compressed[0])
result.append(current)
for code in compressed[1:]:
if code in dictionary:
entry = dictionary[code]
elif code == len(dictionary):
entry = current + current[0]
result.append(entry)
dictionary[len(dictionary)] = current + entry[0]
current = entry
return "".join(result)