Merge pull request #107 from ikostan/exercism-sync/112f341f33c21714

[Sync Iteration] python/isbn-verifier/1

Author: ikostan

solutions/python/isbn-verifier/1/isbn_verifier.py

@@ -0,0 +1,50 @@
+"""
+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.
+"""
+
+
+def calc_formula(digits: str) -> int:
+    """
+    Calculate the weighted sum for ISBN-10 validation formula.
+
+    :param digits: List of integer digits to calculate the formula for
+    :return: The weighted sum result used in ISBN-10 validation
+    """
+    result: int = 0
+    for i, digit in enumerate(digits):
+        if digit.isdigit():
+            result += int(digit) * (10 - i)
+        elif digit.lower() == "x":
+            result += 10 * 1
+        else:
+            # Invalid char, calc failed
+            return 1
+    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
+    """
+    formated_isbn: str = isbn.replace("-", "")
+    if len(formated_isbn) != 10:
+        return False
+
+    return calc_formula(formated_isbn) % 11 == 0