Merge pull request #111 from ikostan/exercism-sync/6a8574709146f471

[Sync Iteration] python/isbn-verifier/4

Author: ikostan

solutions/python/isbn-verifier/4/isbn_verifier.py

@@ -0,0 +1,65 @@
+"""
+ISBN Verifier.
+
+The ISBN-10 verification process is used to validate book identification
+numbers. These normally contain dashes and look like: 3-598-21508-8
+
+ISBN
+The ISBN-10 format is 9 digits (0 to 9) plus one check character
+(either a digit or an X only). In the case the check character is an X,
+this represents the value '10'. These may be communicated with or without
+hyphens, and can be checked for their validity by the following formula:
+
+(d₁ * 10 + d₂ * 9 + d₃ * 8 + d₄ * 7 + d₅ * 6 +
+d₆ * 5 + d₇ * 4 + d₈ * 3 + d₉ * 2 + d₁₀ * 1) mod 11 == 0
+
+If the result is 0, then it is a valid ISBN-10, otherwise it is invalid.
+"""
+
+ISBN: str = "0123456789X"
+
+
+def formatted_isbn(isbn: str) -> list[int]:
+    """
+    Extract and format ISBN digits from input string.
+
+    Converts ISBN string to list of integers, treating 'X' as 10.
+    Ignores non-digit characters except for trailing 'X'.
+
+    :param isbn: ISBN string that may contain digits, hyphens, and trailing 'X'
+    :return: List of integers representing ISBN digits, 'X' converted to 10
+    """
+    result: list[int] = []
+    for char in isbn:
+        if char.isdigit():
+            result.append(int(char))
+    if isbn[-1].lower() == "x":
+        result.append(10)
+
+    print(f"result: {result}, len: {len(result)}")
+    return result
+
+
+def is_valid(isbn: str) -> bool:
+    """
+    Verify ISBN.
+
+    :param isbn: 9 digits ISBN 10 format
+    :return: Tru if isbn is valid, False otherwise
+    """
+    # Non ISBN chars or empty strings not allowed
+    if not all(char in ISBN for char in isbn.replace("-", "")) or not isbn:
+        return False
+    # In case X is present, it should be at the end of the string only
+    if 'X' in isbn and isbn.index('X') != len(isbn) - 1:
+        return False
+
+    # Convert all isbn chars to digits
+    isbn_digits: list[int] = formatted_isbn(isbn)
+    # ISBN total length should be = 10
+    if len(isbn_digits) != 10:
+        return False
+
+    return (
+        sum(digit * (10 - i) for i, digit in enumerate(isbn_digits)) % 11 == 0
+    )