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re-implement HammingEncode/Decode added HammingEncodeProperly
to generate valid hamming codes for "HammingCodes: Encoded Binary to Integer"
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s2ks
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// by Discord: H3draut3r#6722, feel free to ask me any questions. i probably don't know the answer 🤣
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export function HammingEncode(data: number): string {
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export function HammingEncode(value: number): string {
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// encoding following Hammings rule
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function HammingSumOfParity(_lengthOfDBits: number): number {
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// will calculate the needed amount of parityBits 'without' the "overall"-Parity (that math took me 4 Days to get it working)
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return _lengthOfDBits < 3 || _lengthOfDBits == 0 // oh and of course using ternary operators, it's a pretty neat function
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? _lengthOfDBits == 0
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? 0
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: _lengthOfDBits + 1
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: // the following math will only work, if the length is greater equal 3, otherwise it's "kind of" broken :D
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Math.ceil(Math.log2(_lengthOfDBits * 2)) <=
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Math.ceil(Math.log2(1 + _lengthOfDBits + Math.ceil(Math.log2(_lengthOfDBits))))
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? Math.ceil(Math.log2(_lengthOfDBits) + 1)
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: Math.ceil(Math.log2(_lengthOfDBits));
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}
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const _data = value.toString(2).split(""); // first, change into binary string, then create array with 1 bit per index
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const _sumParity: number = HammingSumOfParity(_data.length); // get the sum of needed parity bits (for later use in encoding)
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const count = (arr: Array<string>, val: string): number =>
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arr.reduce((a: number, v: string) => (v === val ? a + 1 : a), 0);
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// function count for specific entries in the array, for later use
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const _build = ["x", "x", ..._data.splice(0, 1)]; // init the "pre-build"
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const enc: Array<number> = [0];
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for (let i = 2; i < _sumParity; i++) {
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const data_bits: Array<any> = data.toString(2).split("").reverse();
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// add new paritybits and the corresponding data bits (pre-building array)
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_build.push("x", ..._data.splice(0, Math.pow(2, i) - 1));
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data_bits.forEach((e, i, a) => {
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a[i] = parseInt(e);
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});
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let k = data_bits.length;
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/* NOTE: writing the data like this flips the endianness, this is what the
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* original implementation by Hedrauta did so I'm keeping it like it was. */
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for(let i = 1; k > 0; i++) {
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if((i & (i - 1)) != 0) {
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enc[i] = data_bits[--k];
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} else {
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enc[i] = 0;
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}
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}
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// now the "calculation"... get the paritybits ('x') working
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for (const index of _build.reduce(function (a: Array<number>, e: string, i: number) {
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if (e == "x") a.push(i);
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return a;
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}, [])) {
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// that reduce will result in an array of index numbers where the "x" is placed
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const _tempcount = index + 1; // set the "stepsize" for the parityBit
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const _temparray = []; // temporary array to store the extracted bits
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const _tempdata = [..._build]; // only work with a copy of the _build
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while (_tempdata[index] !== undefined) {
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// as long as there are bits on the starting index, do "cut"
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const _temp: Array<string> = _tempdata.splice(index, _tempcount * 2); // cut stepsize*2 bits, then...
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_temparray.push(..._temp.splice(0, _tempcount)); // ... cut the result again and keep the first half
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}
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_temparray.splice(0, 1); // remove first bit, which is the parity one
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_build[index] = (count(_temparray, "1") % 2).toString(); // count with remainder of 2 and"toString" to store the parityBit
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} // parity done, now the "overall"-parity is set
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_build.unshift((count(_build, "1") % 2).toString()); // has to be done as last element
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return _build.join(""); // return the _build as string
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}
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}
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export function HammingDecode(_data: string): number {
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let parity: any = 0;
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//check for altered bit and decode
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const _build = _data.split(""); // ye, an array for working, again
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const _testArray = []; //for the "truthtable". if any is false, the data has an altered bit, will check for and fix it
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const _sumParity = Math.ceil(Math.log2(_data.length)); // sum of parity for later use
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const count = (arr: Array<string>, val: string): number =>
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arr.reduce((a: number, v: string) => (v === val ? a + 1 : a), 0);
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// the count.... again ;)
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let _overallParity = _build.splice(0, 1).join(""); // store first index, for checking in next step and fix the _build properly later on
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/* Figure out the subsection parities */
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_testArray.push(_overallParity == (count(_build, "1") % 2).toString() ? true : false); // first check with the overall parity bit
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for(let i = 0; i < enc.length; i++) {
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for (let i = 0; i < _sumParity; i++) {
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if(enc[i]) {
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// for the rest of the remaining parity bits we also "check"
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parity ^= i;
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const _tempIndex = Math.pow(2, i) - 1; // get the parityBits Index
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const _tempStep = _tempIndex + 1; // set the stepsize
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const _tempData = [..._build]; // get a "copy" of the build-data for working
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const _tempArray = []; // init empty array for "testing"
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while (_tempData[_tempIndex] != undefined) {
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// extract from the copied data until the "starting" index is undefined
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const _temp = [..._tempData.splice(_tempIndex, _tempStep * 2)]; // extract 2*stepsize
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_tempArray.push(..._temp.splice(0, _tempStep)); // and cut again for keeping first half
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}
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}
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const _tempParity = _tempArray.shift(); // and again save the first index separated for checking with the rest of the data
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_testArray.push(_tempParity == (count(_tempArray, "1") % 2).toString() ? true : false);
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// is the _tempParity the calculated data? push answer into the 'truthtable'
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}
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}
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let _fixIndex = 0; // init the "fixing" index and start with 0
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for (let i = 1; i < _sumParity + 1; i++) {
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parity = parity.toString(2).split("").reverse();
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// simple binary adding for every boolean in the _testArray, starting from 2nd index of it
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parity.forEach((e: any, i: any , a: any) => {
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_fixIndex += _testArray[i] ? 0 : Math.pow(2, i) / 2;
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a[i] = parseInt(e);
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});
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/* Set the parity bits accordingly */
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for(let i = 0; i < parity.length; i++) {
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enc[2 ** i] = parity[i] ? 1 : 0;
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}
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}
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_build.unshift(_overallParity); // now we need the "overall" parity back in it's place
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// try fix the actual encoded binary string if there is an error
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parity = 0;
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if (_fixIndex > 0 && _testArray[0] == false) {
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/* Figure out the overall parity for the entire block */
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// if the overall is false and the sum of calculated values is greater equal 0, fix the corresponding hamming-bit
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for(let i = 0; i < enc.length; i++) {
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_build[_fixIndex] = _build[_fixIndex] == "0" ? "1" : "0";
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if(enc[i]) {
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} else if (_testArray[0] == false) {
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parity++;
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// otherwise, if the the overall_parity is the only wrong, fix that one
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_overallParity = _overallParity == "0" ? "1" : "0";
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} else if (_testArray[0] == true && _testArray.some((truth) => truth == false)) {
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return 0; // uhm, there's some strange going on... 2 bits are altered? How? This should not happen 👀
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}
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}
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// oof.. halfway through... we fixed an possible altered bit, now "extract" the parity-bits from the _build
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for (let i = _sumParity; i >= 0; i--) {
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// start from the last parity down the 2nd index one
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_build.splice(Math.pow(2, i), 1);
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}
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}
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_build.splice(0, 1); // remove the overall parity bit and we have our binary value
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return parseInt(_build.join(""), 2); // parse the integer with redux 2 and we're done!
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/* Finally set the overall parity bit */
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enc[0] = parity % 2 == 0 ? 0 : 1;
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return enc.join("");
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}
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export function HammingEncodeProperly(data: number): string {
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/* How many bits do we need?
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* n = 2^m
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* k = 2^m - m - 1
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* where k is the number of data bits, m the number
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* of parity bits and n the number of total bits. */
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let m = 1;
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while((2 ** ((2 ** m) - m - 1)) < data) {
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m++;
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}
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const n: number = (2 ** m);
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const k: number = (2 ** m) - m - 1;
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const enc: Array<number> = [0];
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const data_bits: Array<any> = data.toString(2).split("").reverse();
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data_bits.forEach((e, i, a) => {
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a[i] = parseInt(e);
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});
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/* Flip endianness as in the original implementation by Hedrauta
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* and write the data back to front
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* XXX why do we do this? */
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for(let i = 1, j = k; i < n; i++) {
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if((i & (i - 1)) != 0) {
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enc[i] = data_bits[--j] ? data_bits[j] : 0;
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}
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}
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let parity: any = 0;
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/* Figure out the subsection parities */
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for(let i = 0; i < n; i++) {
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if(enc[i]) {
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parity ^= i;
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}
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}
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parity = parity.toString(2).split("").reverse();
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parity.forEach((e: any, i: any , a: any) => {
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a[i] = parseInt(e);
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});
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/* Set the parity bits accordingly */
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for(let i = 0; i < m; i++) {
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enc[2 ** i] = parity[i] ? 1 : 0;
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}
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parity = 0;
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/* Figure out the overall parity for the entire block */
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for(let i = 0; i < n; i++) {
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if(enc[i]) {
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parity++;
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}
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}
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/* Finally set the overall parity bit */
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enc[0] = parity % 2 == 0 ? 0 : 1;
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return enc.join("");
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}
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export function HammingDecode(data: string): number {
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let err = 0;
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const bits: Array<number> = [];
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/* TODO why not just work with an array of digits from the start? */
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for(const i in data.split("")) {
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const bit = parseInt(data[i]);
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bits[i] = bit;
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if(bit) {
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err ^= +i;
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}
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}
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/* If err != 0 then it spells out the index of the bit that was flipped */
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if(err) {
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/* Flip to correct */
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bits[err] = bits[err] ? 0 : 1;
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}
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/* Now we have to read the message, bit 0 is unused (it's the overall parity bit
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* which we don't care about). Each bit at an index that is a power of 2 is
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* a parity bit and not part of the actual message. */
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let ans = '';
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for(let i = 1; i < bits.length; i++) {
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/* i is not a power of two so it's not a parity bit */
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if((i & (i - 1)) != 0) {
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ans += bits[i];
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}
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}
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/* TODO to avoid ambiguity about endianness why not let the player return the extracted (and corrected)
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* data bits, rather than guessing at how to convert it to a decimal string? */
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return parseInt(ans, 2);
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}
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}
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