Add intergalactic-transmission exercise (#2944)

Author: kahgoh

config.json

@@ -1882,6 +1882,18 @@
           "searching"
         ]
       },
+      {
+        "slug": "intergalactic-transmission",
+        "name": "Intergalactic Transmission",
+        "uuid": "70388d18-427b-4421-831c-b5bc987e7890",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 6,
+        "topics": [
+          "bitwise_operations",
+          "arrays"
+        ]
+      },
       {
         "slug": "knapsack",
         "name": "Knapsack",

exercises/practice/intergalactic-transmission/.docs/instructions.md

@@ -0,0 +1,54 @@
+# Instructions
+
+Your job is to help implement
+
+- the transmitter, which calculates the transmission sequence, and
+- the receiver, which decodes it.
+
+A parity bit is simple way of detecting transmission errors.
+The transmitters and receivers can only transmit and receive _exactly_ eight bits at a time (including the parity bit).
+The parity bit is set so that there is an _even_ number of 1 bits in each transmission, and the parity bit is always the first bit from the right.
+So if the receiver receives `11000001`, `01110101` or `01000000` (i.e. a transmission with an odd number of 1 bits), it knows there is an error.
+
+However, messages are rarely this short, and need to be transmitted in a sequence when they are longer.
+
+For example, consider the message `11000000 00000001 11000000 11011110` (or `C0 01 C0 DE` in hex).
+
+Since each transmission contains exactly eight bits, it can only contain seven bits of data and the parity bit.
+A parity bit must then be inserted after every seven bits of data:
+
+```text
+11000000 00000001 11000000 11011110
+      ↑       ↑       ↑       ↑          (7th bits)
+```
+
+The transmission sequence for this message looks like this:
+
+```text
+1100000_ 0000000_ 0111000_ 0001101_ 1110
+       ↑        ↑        ↑        ↑      (parity bits)
+```
+
+The data in the first transmission in the sequence (`1100000`) has two 1 bits (an even number), so the parity bit is 0.
+The first transmission becomes `11000000` (or `C0` in hex).
+
+The data in the next transmission (`0000000`) has zero 1 bits (an even number again), so the parity bit is 0 again.
+The second transmission thus becomes `00000000` (or `00` in hex).
+
+The data for the next two transmissions (`0111000` and `0001101`) have three 1 bits.
+Their parity bits are set to 1 so that they have an even number of 1 bits in the transmission.
+They are transmitted as `01110001` and `00011011` (or `71` and `1B` in hex).
+
+The last transmission (`1110`) has only four bits of data.
+Since exactly eight bits are transmitted at a time and the parity bit is the rightmost bit, three 0 bits and then the parity bit are added to make up eight bits.
+It now looks like this (where `_` is the parity bit):
+
+```text
+1110 000_
+     ↑↑↑   (added 0 bits)
+```
+
+There is an odd number of 1 bits again, so the parity bit is 1.
+The last transmission in the sequence becomes `11100001` (or `E1` in hex).
+
+The entire transmission sequence for this message is `11000000 00000000 01110001 00011011 11100001` (or `C0 00 71 1B E1` in hex).

exercises/practice/intergalactic-transmission/.docs/introduction.md

@@ -0,0 +1,23 @@
+# Introduction
+
+Trillions upon trillions of messages zip between Earth and neighboring galaxies every millisecond.
+But transmitting over such long distances is tricky.
+Pesky solar flares, temporal distortions, stray forces, and even the flap of a space butterfly's wing can cause a random bit to change during transmission.
+
+Now imagine the consequences:
+
+- Crashing the Intergalactic Share Market when "buy low" turns to "sell now".
+- Losing contact with the Kepler Whirl system when "save new worm hole" becomes "cave new worm hole".
+- Or plunging the universe into existential horror by replacing a cowboy emoji 🤠 with a clown emoji 🤡.
+
+Detecting corrupted messages isn't just important — it's critical.
+The receiver _must_ know when something has gone wrong before disaster strikes.
+
+But how?
+Scientists and engineers from across the universe have been battling this problem for eons.
+Entire cosmic AI superclusters churn through the data.
+And then, one day, a legend resurfaces — an ancient, powerful method, whispered in debugging forums, muttered by engineers who've seen too much...
+
+The Parity Bit!
+
+A method so simple, so powerful, that it might just save interstellar communication.

exercises/practice/intergalactic-transmission/.meta/config.json

@@ -0,0 +1,19 @@
+{
+  "authors": [
+    "kahgoh"
+  ],
+  "files": {
+    "solution": [
+      "intergalactic_transmission.go"
+    ],
+    "test": [
+      "intergalactic_transmission_test.go"
+    ],
+    "example": [
+      ".meta/example.go"
+    ]
+  },
+  "blurb": "Add parity bits to a message for transmission",
+  "source": "Kah Goh",
+  "source_url": "https://github.com/exercism/problem-specifications/pull/2543"
+}

exercises/practice/intergalactic-transmission/.meta/example.go

@@ -0,0 +1,72 @@
+package intergalactictransmission
+
+import (
+	"errors"
+	"math/bits"
+)
+
+func Transmit(message []byte) []byte {
+	encoded := make([]byte, 0)
+	var dataCarry byte = 0x00
+	var dataMask byte = 0xFE
+
+	for _, dataByte := range message {
+		if dataMask == 0 {
+			encoded = append(encoded, addParity(dataCarry))
+			dataCarry = 0
+			dataMask = 0xFE
+		}
+
+		dataOffset := bits.TrailingZeros8(dataMask)
+		currentByte := (dataCarry << (8 - dataOffset)) | (dataByte >> dataOffset)
+		encoded = append(encoded, addParity(currentByte))
+
+		dataCarry = dataByte & (^dataMask)
+		dataMask = dataMask << 1
+	}
+
+	if dataMask != 0xFE {
+		lastByte := dataCarry << byte(bits.OnesCount8(dataMask))
+		encoded = append(encoded, addParity(lastByte))
+	}
+	return encoded
+}
+
+func addParity(dataByte byte) byte {
+	if bits.OnesCount8(dataByte)%2 == 0 {
+		return dataByte << 1
+	}
+	return (dataByte << 1) | 0x01
+}
+
+func Decode(message []byte) ([]byte, error) {
+	if len(message) == 0 {
+		return []byte{}, nil
+	}
+
+	decoded := make([]byte, 0)
+	var carryBits byte = 0x00
+	var dataMask byte = 0xFF
+
+	for _, byteToDecode := range message {
+		if bits.OnesCount8(byteToDecode)%2 != 0 {
+			return []byte{}, errors.New("wrong parity")
+		}
+
+		if dataMask == 0xFF {
+			// This means we are at the first byte or
+			// some multiple where there will be only
+			//  7 bits of data.
+			carryBits = byteToDecode & 0xFE
+			dataMask = 0x80
+		} else {
+			currentByteData := byteToDecode & 0xFE
+			contribution := currentByteData >> bits.TrailingZeros8(dataMask)
+			decoded = append(decoded, carryBits|contribution)
+
+			carryBits = (byteToDecode & ^(dataMask | 0x01)) << bits.OnesCount8(dataMask)
+			dataMask = (dataMask >> 1) | 0x80
+		}
+	}
+	return decoded, nil
+}

exercises/practice/intergalactic-transmission/.meta/gen.go

@@ -0,0 +1,95 @@
+package main
+
+import (
+	"fmt"
+	"log"
+	"strings"
+	"text/template"
+
+	"../../../../gen"
+)
+
+func main() {
+	t, err := template.New("").Parse(tmpl)
+	if err != nil {
+		log.Fatal(err)
+	}
+	j := map[string]any{
+		"transmitSequence": &[]testCase{},
+		"decodeMessage":    &[]testCase{},
+	}
+	if err := gen.Gen("intergalactic-transmission", j, t); err != nil {
+		log.Fatal(err)
+	}
+}
+
+type testCase struct {
+	Description string `json:"description"`
+	Input       struct {
+		Message []string `json:"message"`
+	} `json:"input"`
+	Expected any `json:"expected"`
+}
+
+func toCommaSeparatedString(values []interface{}) string {
+	asStrings := make([]string, len(values))
+	for i, value := range values {
+		asStrings[i] = fmt.Sprintf("%v", value)
+	}
+
+	return strings.Join(asStrings, ",")
+}
+
+func (t testCase) FormatInput() string {
+	return fmt.Sprintf("[]byte{%s}", strings.Join(t.Input.Message, ", "))
+}
+
+func (t testCase) FormatExpectedValue() string {
+	if expected, ok := t.Expected.([]interface{}); ok {
+		return fmt.Sprintf("[]byte{%s}", toCommaSeparatedString(expected))
+	}
+	return "[]byte{}"
+}
+
+func (t testCase) FormatExpectedError() string {
+	if asMap, ok := t.Expected.(map[string]any); ok {
+		var errString string
+		if errString, ok = asMap["error"].(string); ok {
+			return errString
+		}
+	}
+	return ""
+}
+
+var tmpl = `package transmission
+
+{{.Header}}
+
+var transmitCases = []struct {
+	description string
+	input       []byte
+	expected    []byte
+}{
+{{range .J.transmitSequence}}{
+	description: 	{{printf "%q" .Description}},
+	input: 			{{.FormatInput}},
+	expected: 	    {{.FormatExpectedValue}},
+},
+{{end}}
+}
+
+var decodeCases = []struct {
+	description string
+	input       []byte
+	expectedValue []byte
+	expectedError string
+}{
+{{range .J.decodeMessage}}{
+	description: 	{{printf "%q" .Description}},
+	input: 			{{.FormatInput}},
+	expectedValue: 	{{.FormatExpectedValue}},
+	expectedError:  {{printf "%q" .FormatExpectedError}},
+},
+{{end}}
+}
+`

exercises/practice/intergalactic-transmission/.meta/tests.toml

@@ -0,0 +1,88 @@
+# This is an auto-generated file.
+#
+# Regenerating this file via `configlet sync` will:
+# - Recreate every `description` key/value pair
+# - Recreate every `reimplements` key/value pair, where they exist in problem-specifications
+# - Remove any `include = true` key/value pair (an omitted `include` key implies inclusion)
+# - Preserve any other key/value pair
+#
+# As user-added comments (using the # character) will be removed when this file
+# is regenerated, comments can be added via a `comment` key.
+
+[f99d4046-b429-4582-9324-f0bcac7ab51c]
+description = "calculate transmit sequences -> empty message"
+
+[ee27ea2d-8999-4f23-9275-8f6879545f86]
+description = "calculate transmit sequences -> 0x00 is transmitted as 0x0000"
+
+[97f27f98-8020-402d-be85-f21ba54a6df0]
+description = "calculate transmit sequences -> 0x02 is transmitted as 0x0300"
+
+[24712fb9-0336-4e2f-835e-d2350f29c420]
+description = "calculate transmit sequences -> 0x06 is transmitted as 0x0600"
+
+[7630b5a9-dba1-4178-b2a0-4a376f7414e0]
+description = "calculate transmit sequences -> 0x05 is transmitted as 0x0581"
+
+[ab4fe80b-ef8e-4a99-b4fb-001937af415d]
+description = "calculate transmit sequences -> 0x29 is transmitted as 0x2881"
+
+[4e200d84-593b-4449-b7c0-4de1b6a0955e]
+description = "calculate transmit sequences -> 0xc001c0de is transmitted as 0xc000711be1"
+
+[fbc537e9-6b21-4f4a-8c2b-9cf9b702a9b7]
+description = "calculate transmit sequences -> six byte message"
+
+[d5b75adf-b5fc-4f77-b4ab-77653e30f07c]
+description = "calculate transmit sequences -> seven byte message"
+
+[6d8b297b-da1d-435e-bcd7-55fbb1400e73]
+description = "calculate transmit sequences -> eight byte message"
+
+[54a0642a-d5aa-490c-be89-8e171a0cab6f]
+description = "calculate transmit sequences -> twenty byte message"
+
+[9a8084dd-3336-474c-90cb-8a852524604d]
+description = "decode received messages -> empty message"
+
+[879af739-0094-4736-9127-bd441b1ddbbf]
+description = "decode received messages -> zero message"
+
+[7a89eeef-96c5-4329-a246-ec181a8e959a]
+description = "decode received messages -> 0x0300 is decoded to 0x02"
+
+[3e515af7-8b62-417f-960c-3454bca7f806]
+description = "decode received messages -> 0x0581 is decoded to 0x05"
+
+[a1b4a3f7-9f05-4b7a-b86e-d7c6fc3f16a9]
+description = "decode received messages -> 0x2881 is decoded to 0x29"
+
+[2e99d617-4c91-4ad5-9217-e4b2447d6e4a]
+description = "decode received messages -> first byte has wrong parity"
+
+[507e212d-3dae-42e8-88b4-2223838ff8d2]
+description = "decode received messages -> second byte has wrong parity"
+
+[b985692e-6338-46c7-8cea-bc38996d4dfd]
+description = "decode received messages -> 0xcf4b00 is decoded to 0xce94"
+
+[7a1f4d48-696d-4679-917c-21b7da3ff3fd]
+description = "decode received messages -> 0xe2566500 is decoded to 0xe2ad90"
+
+[467549dc-a558-443b-80c5-ff3d4eb305d4]
+description = "decode received messages -> six byte message"
+
+[1f3be5fb-093a-4661-9951-c1c4781c71ea]
+description = "decode received messages -> seven byte message"
+
+[6065b8b3-9dcd-45c9-918c-b427cfdb28c1]
+description = "decode received messages -> last byte has wrong parity"
+
+[98af97b7-9cca-4c4c-9de3-f70e227a4cb1]
+description = "decode received messages -> eight byte message"
+
+[aa7d4785-2bb9-43a4-a38a-203325c464fb]
+description = "decode received messages -> twenty byte message"
+
+[4c86e034-b066-42ac-8497-48f9bc1723c1]
+description = "decode received messages -> wrong parity on 16th byte"