Add intergalactic-transmission (#1062)

* Add intergalactic-transmission

* remove redundant #define

Author: ahans

config.json

@@ -1142,6 +1142,20 @@
           "functions",
           "memory_management"
         ]
+      },
+      {
+        "slug": "intergalactic-transmission",
+        "name": "Intergalactic Transmission",
+        "uuid": "aa30f9ef-014e-4abb-aee3-fc0613fe20ae",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 6,
+        "topics": [
+          "arrays",
+          "bitwise_operations",
+          "control_flow_if_statements",
+          "control_flow_loops"
+        ]
       }
     ]
   },

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 1s in each transmission and 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 right most 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": [],
+  "files": {
+    "solution": [
+      "intergalactic_transmission.c",
+      "intergalactic_transmission.h"
+    ],
+    "test": [
+      "test_intergalactic_transmission.c"
+    ],
+    "example": [
+      ".meta/example.c",
+      ".meta/example.h"
+    ]
+  },
+  "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.c

@@ -0,0 +1,73 @@
+#include "intergalactic_transmission.h"
+
+int transmit_sequence(uint8_t *buffer, const uint8_t *message,
+                      int message_length)
+{
+   uint8_t cur = 0;
+   int parity = 0;
+   int byte_count = 0;
+   int bit_count = 0;
+   // go over each input byte
+   for (int i = 0; i < message_length; ++i) {
+      // read bits of input byte, starting from most significant bit
+      for (int j = 0; j < 8; ++j) {
+         uint8_t const bit = (message[i] >> (7 - j)) & 1;
+         // keep track of number of set bits
+         parity += bit;
+         // set bit in output byte and shift to make room for next bit
+         cur = (cur | bit) << 1;
+         // if 7 bits have been written, output byte is ready
+         if (++bit_count == 7) {
+            // set parity bit
+            cur |= parity & 1;
+            // and write to output buffer
+            buffer[byte_count++] = cur;
+            // reset state for next output byte
+            parity = 0;
+            bit_count = 0;
+            cur = 0;
+         }
+      }
+   }
+   // make sure to write remaining state
+   if (bit_count > 0) {
+      // shift to the left to fill up with zero bits
+      cur <<= 7 - bit_count;
+      // set parity bit
+      cur |= parity & 1;
+      // and write to output buffer
+      buffer[byte_count++] = cur;
+   }
+   return byte_count;
+}
+
+int decode_message(uint8_t *buffer, const uint8_t *message, int message_length)
+{
+   uint8_t cur_dec = 0;
+   int num_bits_left = 8;
+   int num_decoded = 0;
+   // iterate over input message: each byte has 7 bits payload and parity bit
+   for (int i = 0; i < message_length; ++i) {
+      uint8_t const payload = message[i] >> 1;
+      uint8_t const parity = message[i] & 1;
+      // check parity bit and return early if mismatch found
+      if ((__builtin_popcount(payload) & 1) != parity)
+         return WRONG_PARITY;
+      // write payload bits to current output byte, starting from MSB
+      for (int j = 6; j >= 0; --j) {
+         // read bit
+         uint8_t const bit = (payload >> j) & 1;
+         // write bit to currently decoded byte
+         cur_dec |= bit << (num_bits_left - 1);
+         // bits left?
+         if (--num_bits_left == 0) {
+            // no bits left, write decoded byte to buffer
+            buffer[num_decoded++] = cur_dec;
+            // reset state
+            cur_dec = 0;
+            num_bits_left = 8;
+         }
+      }
+   }
+   return num_decoded;
+}

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

@@ -0,0 +1,13 @@
+#ifndef INTERGALACTIC_TRANSMISSION_H
+#define INTERGALACTIC_TRANSMISSION_H
+
+#include <stdint.h>
+
+#define WRONG_PARITY -1
+
+int transmit_sequence(uint8_t *buffer, const uint8_t *message,
+                      int message_length);
+
+int decode_message(uint8_t *buffer, const uint8_t *message, int message_length);
+
+#endif

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"

exercises/practice/intergalactic-transmission/intergalactic_transmission.c

@@ -0,0 +1 @@
+#include "intergalactic_transmission.h"

exercises/practice/intergalactic-transmission/intergalactic_transmission.h

@@ -0,0 +1,13 @@
+#ifndef INTERGALACTIC_TRANSMISSION_H
+#define INTERGALACTIC_TRANSMISSION_H
+
+#include <stdint.h>
+
+#define WRONG_PARITY -1
+
+int transmit_sequence(uint8_t *buffer, const uint8_t *message,
+                      int message_length);
+
+int decode_message(uint8_t *buffer, const uint8_t *message, int message_length);
+
+#endif

exercises/practice/intergalactic-transmission/makefile

@@ -0,0 +1,37 @@
+### If you wish to use extra libraries (math.h for instance),
+### add their flags here (-lm in our case) in the "LIBS" variable.
+
+LIBS = -lm
+
+###
+CFLAGS  = -std=c99
+CFLAGS += -g
+CFLAGS += -Wall
+CFLAGS += -Wextra
+CFLAGS += -pedantic
+CFLAGS += -Werror
+CFLAGS += -Wmissing-declarations
+CFLAGS += -DUNITY_SUPPORT_64 -DUNITY_OUTPUT_COLOR
+
+ASANFLAGS  = -fsanitize=address
+ASANFLAGS += -fno-common
+ASANFLAGS += -fno-omit-frame-pointer
+
+.PHONY: test
+test: tests.out
+	@./tests.out
+
+.PHONY: memcheck
+memcheck: ./*.c ./*.h
+	@echo Compiling $@
+	@$(CC) $(ASANFLAGS) $(CFLAGS) test-framework/unity.c ./*.c -o memcheck.out $(LIBS)
+	@./memcheck.out
+	@echo "Memory check passed"
+
+.PHONY: clean
+clean:
+	rm -rf *.o *.out *.out.dSYM
+
+tests.out: ./*.c ./*.h
+	@echo Compiling $@
+	@$(CC) $(CFLAGS) test-framework/unity.c ./*.c -o tests.out $(LIBS)