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hufftree.js
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hufftree.js
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/**
* Binary tree to hold Huffman codes.
*
* Leaves hold code values. Non-leaves cannot hold values.
*
* Values are added in level-order traversal, with a maximum of (2^i - 1)
* values at each level (except for the last level, which could have 2^i).
*
* Each branch represents a '0' or a '1' in the bit string:
* Left: 0
* Right: 1
*
* We can construct the bitstream for a given value by keeping track
* of the branches as we descend the tree, appending the value of the
* branch in order of descent.
*
*/
class HuffTree {
/**
* Create a new node in the Huffman tree
*
* @param {Number} level Optional level of the tree. Defaults to 0 (for the root)
*/
constructor(level = 0) {
this.level = level;
}
/**
* Initialize the decoder with the 'BITS' array containing the number of huffman codes
* of each length.
*
* @param {Array} bits List of the number of Huffman codes of each length (0-NUM_HUFFMAN_LENGTHS)
*/
initDecoder(bits) {
// NOOP, since this implementation supports arbitrary tree sizes
}
/**
* Insert a value into the tree at the given depth. Uses a level-order
* traversal to fill the tree from left to right at a given level.
*
* Returns 'true' if inserting the value was successful, otherwise false
*
* @param {*} codeLength Depth of the tree at which to insert (length of huffman code)
* @param {*} value Value to insert into the tree
*/
insertCode(codeLength, value) {
if (this.data != undefined) {
return false; // Invariant: leaf nodes can't have children
}
if (codeLength === this.level) {
if (this.data === undefined) {
this.data = value;
return true;
} else {
return false; // We're full, try the node to the right
}
} else {
if (this.left === undefined) {
// Invariant: nodes always created in pairs.
// Saves us from lots of checking for 'undefined' elsewhere
this.left = new HuffTree(this.level + 1);
this.right = new HuffTree(this.level + 1);
return this.left.insertCode(codeLength, value);
} else {
if (this.left.insertCode(codeLength, value)) {
return true;
} else {
return this.right.insertCode(codeLength, value);
}
}
}
}
/**
* Read the next huffman code from the data source and return the
* value represented by the code.
*
* F.2.2.3
*
* @param {DataViewReader} reader Data source
* @param {*} img Struct containing information about the image
*/
decodeHuffman(reader, img) {
let bitString = ""; // For debugging
let node = this;
while (true) {
let bit = reader.nextBit(img);
// bitString += bit;
node = node.descendNode(bit);
if (node === undefined) {
console.error("Error: Couldn't find huffman value for huffman code " + bitString);
break;
} else if (node.data != undefined) {
// Found a value, so we're done!
// console.log("Found value " + node.data + " for huffman code " + bitString);
return node.data;
}
}
return undefined;
}
/**
* Print the entire tree to the console. Not recommended for large trees
*/
dumpTree() {
let str = "";
for (let i = 0; i < this.level; i++) {
str += " ";
}
str += this.level + ":";
if (this.data != undefined) {
str += this.data;
}
console.log(str);
if (this.left != undefined) {
this.left.dumpTree();
this.right.dumpTree();
}
}
/**
* Return the next node in the tree for the given bit.
* Returns the left node for '0' and the right node for '1'.
* All other inputs are invalid.
*
* @param {*} bit
*/
descendNode(bit) {
if (bit === 0) {
return this.left;
} else if (bit === 1) {
return this.right;
} else {
console.error("Error: invalid bit string (expected 0 or 1, got " + bit + ")");
}
return undefined;
}
/**
* Return the value in the node represented by the given bitstring,
* or undefined if the sequence is not found in the tree.
*
* @param {String} bitString String representation of a sequence of bits
*/
getValue(bitString) {
if (bitString === "") {
return this.data;
}
let c = bitString[0];
if (c === '0') { // '0' is left
return this.left.getValue(bitString.slice(1));
} else if (c === '1') { // '1' is right
return this.right.getValue(bitString.slice(1));
} else {
console.error("Invalid character in bit string: " + c);
}
}
/**
* Additional tests for HuffTree's getValue(). getValue() isn't currently used by the
* JPEG decoder, but it's a good way to experiement with the tree.
*/
static testGetValue() {
let tree = new HuffTree(0);
let testCnt = 0;
let errCnt = 0;
function runInsertTest(level, value, expected) {
let val = tree.insertCode(level, value);
testCnt++;
if (val != expected) {
console.error("Test " + testCnt + " Failed (expected " + expected + ", got " + val + ")");
errCnt++;
}
}
runInsertTest(3, 4, true);
runInsertTest(3, 5, true);
runInsertTest(3, 3, true);
runInsertTest(3, 2, true);
runInsertTest(3, 6, true);
runInsertTest(3, 1, true);
runInsertTest(3, 0, true);
// We shouldn't be able to insert an 8th node at L3,
// but the tree currently has no way of knowing this
// since nodes don't know the state of other nodes.
// For this to be a problem in practice, the JPEG
// file would have to be out-of-spec/corrupt
//runInsertTest(3, 12, false); // Too many nodes in tree
runInsertTest(4, 7, true);
runInsertTest(5, 8, true);
runInsertTest(6, 9, true);
runInsertTest(7, 10, true);
runInsertTest(8, 11, true);
function runGetTest(input, expected) {
let val = tree.getValue(input);
testCnt++;
if (val != expected) {
console.error("Test " + testCnt + " Failed (expected " + expected + ", got " + val + ")");
errCnt++;
}
}
runGetTest("000", 4);
runGetTest("001", 5);
runGetTest("010", 3);
runGetTest("011", 2);
runGetTest("100", 6);
runGetTest("101", 1);
runGetTest("110", 0);
runGetTest("1110", 7);
runGetTest("11110", 8);
runGetTest("111110", 9);
runGetTest("1111110", 10);
runGetTest("11111110", 11);
// Sequence not in the table
runGetTest("11111111", undefined);
const testName = "HuffTree.getValue";
if (errCnt === 0) {
console.log(testName + "tests passed!");
} else {
console.error(testName + "tests failed: " + errCnt + " errors");
}
}
}
HuffTree.testGetValue();