Roman Numeral Conversion Method

HardPython

Problem

Global Healthcare Exchange systems may receive compact Roman numeral identifiers from legacy healthcare documents. Implement a parser that converts a Roman numeral to its integer value only when the numeral follows canonical Roman numeral rules.

Return the decoded integer for a valid numeral. Return -1 for an invalid numeral.

Formal Specification

Implement roman_to_integer(s), where s is a non-empty uppercase string containing only the characters I, V, X, L, C, D, and M. Valid values range from 1 through 3999 and must use canonical notation:

I, X, and C may precede only their valid subtractive partners: IV, IX, XL, XC, CD, or CM.
V, L, and D may not repeat.
No symbol may repeat more than three times consecutively.
Symbols must appear in descending place-value order, except for valid subtractive pairs.

The function returns an integer for valid input and -1 otherwise. Aim for a single left-to-right parse and explain how canonical validation prevents malformed strings from being accepted.

Constraints

1 <= len(s) <= 15
s contains only uppercase Roman numeral letters
Valid values are between 1 and 3999
Return -1 for any non-canonical representation

Function Signature

def roman_to_integer(s):

Return the decoded integer for a valid numeral. Return -1 for an invalid numeral.

Formal Specification

I, X, and C may precede only their valid subtractive partners: IV, IX, XL, XC, CD, or CM.
V, L, and D may not repeat.
No symbol may repeat more than three times consecutively.
Symbols must appear in descending place-value order, except for valid subtractive pairs.

The function returns an integer for valid input and -1 otherwise. Aim for a single left-to-right parse and explain how canonical validation prevents malformed strings from being accepted.