langs a-z

Task/Huffman-coding/Objective-C/huffman-coding.m

@@ -0,0 +1,156 @@
+#import <Foundation/Foundation.h>
+
+
+@interface HuffmanTree : NSObject {
+	int freq;
+}
+-(id)initWithFreq:(int)f;
+@property (readonly) int freq;
+@end
+
+@implementation HuffmanTree
+@synthesize freq; // the frequency of this tree
+-(id)initWithFreq:(int)f {
+	if (self = [super init]) {
+		freq = f;
+	}
+	return self;
+}
+@end
+
+
+const void *HuffmanRetain(CFAllocatorRef allocator, const void *ptr) {
+	return [(id)ptr retain];
+}
+void HuffmanRelease(CFAllocatorRef allocator, const void *ptr) {
+	[(id)ptr release];
+}
+CFComparisonResult HuffmanCompare(const void *ptr1, const void *ptr2, void *unused) {
+	int f1 = ((HuffmanTree *)ptr1).freq;
+	int f2 = ((HuffmanTree *)ptr2).freq;
+	if (f1 == f2)
+		return kCFCompareEqualTo;
+	else if (f1 > f2)
+		return kCFCompareGreaterThan;
+	else
+		return kCFCompareLessThan;
+}
+
+
+@interface HuffmanLeaf : HuffmanTree {
+	char value; // the character this leaf represents
+}
+@property (readonly) char value;
+-(id)initWithFreq:(int)f character:(char)c;
+@end
+
+@implementation HuffmanLeaf
+@synthesize value;
+-(id)initWithFreq:(int)f character:(char)c {
+	if (self = [super initWithFreq:f]) {
+		value = c;
+	}
+	return self;
+}
+@end
+
+
+@interface HuffmanNode : HuffmanTree {
+	HuffmanTree *left, *right; // subtrees
+}
+@property (readonly) HuffmanTree *left, *right;
+-(id)initWithLeft:(HuffmanTree *)l right:(HuffmanTree *)r;
+@end
+
+@implementation HuffmanNode
+@synthesize left, right;
+-(id)initWithLeft:(HuffmanTree *)l right:(HuffmanTree *)r {
+	if (self = [super initWithFreq:l.freq+r.freq]) {
+		left = [l retain];
+		right = [r retain];
+	}
+	return self;
+}
+-(void)dealloc {
+	[left release];
+	[right release];
+	[super dealloc];
+}
+@end
+
+
+HuffmanTree *buildTree(NSCountedSet *chars) {
+	
+	CFBinaryHeapCallBacks callBacks = {0, HuffmanRetain, HuffmanRelease, NULL, HuffmanCompare};
+	CFBinaryHeapRef trees = CFBinaryHeapCreate(NULL, 0, &callBacks, NULL);
+
+	// initially, we have a forest of leaves
+	// one for each non-empty character
+	for (NSNumber *ch in chars) {
+		int freq = [chars countForObject:ch];
+		if (freq > 0)
+			CFBinaryHeapAddValue(trees, [[[HuffmanLeaf alloc] initWithFreq:freq character:(char)[ch intValue]] autorelease]);
+	}
+	
+	NSCAssert(CFBinaryHeapGetCount(trees) > 0, @"String must have at least one character");
+	// loop until there is only one tree left
+	while (CFBinaryHeapGetCount(trees) > 1) {
+		// two trees with least frequency
+		HuffmanTree *a = (HuffmanTree *)CFBinaryHeapGetMinimum(trees);
+		CFBinaryHeapRemoveMinimumValue(trees);
+		HuffmanTree *b = (HuffmanTree *)CFBinaryHeapGetMinimum(trees);
+		CFBinaryHeapRemoveMinimumValue(trees);
+		
+		// put into new node and re-insert into queue
+		CFBinaryHeapAddValue(trees, [[[HuffmanNode alloc] initWithLeft:a right:b] autorelease]);
+	}
+	HuffmanTree *result = [(HuffmanTree *)CFBinaryHeapGetMinimum(trees) retain];
+	CFRelease(trees);
+	return [result autorelease];
+}
+
+void printCodes(HuffmanTree *tree, NSMutableString *prefix) {
+	NSCAssert(tree != nil, @"tree must not be nil");
+	if ([tree isKindOfClass:[HuffmanLeaf class]]) {
+		HuffmanLeaf *leaf = (HuffmanLeaf *)tree;
+		
+		// print out character, frequency, and code for this leaf (which is just the prefix)
+		NSLog(@"%c\t%d\t%@", leaf.value, leaf.freq, prefix);
+		
+	} else if ([tree isKindOfClass:[HuffmanNode class]]) {
+		HuffmanNode *node = (HuffmanNode *)tree;
+		
+		// traverse left
+		[prefix appendString:@"0"];
+		printCodes(node.left, prefix);
+		[prefix deleteCharactersInRange:NSMakeRange([prefix length]-1, 1)];
+		
+		// traverse right
+		[prefix appendString:@"1"];
+		printCodes(node.right, prefix);
+		[prefix deleteCharactersInRange:NSMakeRange([prefix length]-1, 1)];
+	}
+}
+
+int main(int argc, const char * argv[]) {
+    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
+
+	NSString *test = @"this is an example for huffman encoding";
+	
+	// read each character and record the frequencies
+	NSCountedSet *chars = [[NSCountedSet alloc] init];
+	int n = [test length];
+	for (int i = 0; i < n; i++)
+		[chars addObject:[NSNumber numberWithInt:[test characterAtIndex:i]]];
+	
+	// build tree
+	HuffmanTree *tree = buildTree(chars);
+	[chars release];
+	
+	// print out results
+	NSLog(@"SYMBOL\tWEIGHT\tHUFFMAN CODE");
+	printCodes(tree, [NSMutableString string]);
+	
+    [pool drain];
+    return 0;
+}