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104 lines
2.7 KiB
Markdown
104 lines
2.7 KiB
Markdown
---
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id: 69bc6cb30c1d112a2e110a07
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title: "Challenge 250: Hidden Key"
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challengeType: 29
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dashedName: challenge-250
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---
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# --description--
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Welcome to the 250th daily challenge!
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Given an encoded string, decode it using an encryption key and return it.
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To find the key:
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- Look at all daily challenges up to today whose challenge number is a multiple of 25 (including this one).
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- Take the first letter from each of those challenge titles and combine them into a string. If the title starts with a non-letter, find its first letter.
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To decode the message, go over each letter in the encoded message and:
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1. Look at the corresponding letter in the key (repeat the key if the message is longer than the key).
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2. Convert the key letter to its corresponding number: `"A"` = 1, `"B"` = 2, ..., `"Z"` = 26.
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3. Shift the encoded letter backward in the alphabet by that number.
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4. If the shift goes before `"A"`, wrap around to `"Z"`.
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For example, if the encoded message starts with `"Y"` and the first key letter is `"V"` (22), shift `"Y"` back 22 places to get `"C"`. Repeat this process for each letter to decode the full message.
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- Only letters are shifted, spaces are returned as-is.
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- All given and returned letters are uppercase.
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# --hints--
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`decode("YAVJYNXE")` should return `"CONGRATS"`.
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```js
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({test: () => { runPython(`
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from unittest import TestCase
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TestCase().assertEqual(decode("YAVJYNXE"), "CONGRATS")`)
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}})
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```
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`decode("YALLUT PQUMJP")` should return `"CODING LEGEND"`.
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```js
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({test: () => { runPython(`
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from unittest import TestCase
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TestCase().assertEqual(decode("YALLUT PQUMJP"), "CODING LEGEND")`)
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}})
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```
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`decode("UAC DYR EISAKYM")` should return `"YOU ARE AWESOME"`.
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```js
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({test: () => { runPython(`
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from unittest import TestCase
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TestCase().assertEqual(decode("UAC DYR EISAKYM"), "YOU ARE AWESOME")`)
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}})
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```
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`decode("GQMS NBMZU")` should return `"KEEP GOING"`.
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```js
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({test: () => { runPython(`
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from unittest import TestCase
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TestCase().assertEqual(decode("GQMS NBMZU"), "KEEP GOING")`)
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}})
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```
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`decode("W IQQURV UG I ZDMDTRV IVW JQDHY TMHSA QB")` should return `"A WINNER IS A DREAMER WHO NEVER GIVES UP"`.
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```js
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({test: () => { runPython(`
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from unittest import TestCase
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TestCase().assertEqual(decode("W IQQURV UG I ZDMDTRV IVW JQDHY TMHSA QB"), "A WINNER IS A DREAMER WHO NEVER GIVES UP")`)
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}})
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```
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# --seed--
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## --seed-contents--
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```py
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def decode(message):
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return message
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```
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# --solutions--
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```py
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def decode(message):
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key = "VLHCGMDLNH"
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result = []
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key_index = 0
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for char in message:
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if char == ' ':
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result.append(char)
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else:
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shift = ord(key[key_index % len(key)]) - 64
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key_index += 1
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result.append(chr((ord(char) - 65 - shift) % 26 + 65))
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return ''.join(result)
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```
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