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When OCR reads extra characters (e.g. sticker border as 'C', spurious 'Z' insertion), the raw text exceeds 17 chars and the old first-17 trim produced wrong VINs. New strategy tries all 17-char sliding windows and single/double character deletions, validating each via check digit. For 'CWVGGNPE2Z4NP069500', this finds the correct VIN 'WVGGNPE24NP069500' (valid check digit) instead of 'CWVGGNPE2Z4NP0695' (invalid). Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
339 lines
11 KiB
Python
339 lines
11 KiB
Python
"""VIN validation with check digit verification and OCR error correction."""
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import re
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from dataclasses import dataclass
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from typing import Optional
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@dataclass
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class VinValidationResult:
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"""Result of VIN validation."""
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is_valid: bool
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vin: str
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confidence_adjustment: float
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error: Optional[str] = None
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class VinValidator:
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"""Validates and corrects VIN strings."""
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# VIN character set (excludes I, O, Q)
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VALID_CHARS = set("ABCDEFGHJKLMNPRSTUVWXYZ0123456789")
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# Common OCR misreads and their corrections.
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# Only map characters that are INVALID in VINs to their likely correct values.
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# B and S are valid VIN characters and must NOT be transliterated.
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TRANSLITERATION = {
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"I": "1",
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"O": "0",
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"Q": "0",
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"i": "1",
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"o": "0",
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"q": "0",
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"l": "1",
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}
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# Weights for check digit calculation (positions 1-17)
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CHECK_WEIGHTS = [8, 7, 6, 5, 4, 3, 2, 10, 0, 9, 8, 7, 6, 5, 4, 3, 2]
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# Character to value mapping for check digit
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CHAR_VALUES = {
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"A": 1,
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"B": 2,
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"C": 3,
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"D": 4,
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"E": 5,
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"F": 6,
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"G": 7,
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"H": 8,
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"J": 1,
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"K": 2,
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"L": 3,
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"M": 4,
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"N": 5,
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"P": 7,
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"R": 9,
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"S": 2,
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"T": 3,
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"U": 4,
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"V": 5,
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"W": 6,
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"X": 7,
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"Y": 8,
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"Z": 9,
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"0": 0,
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"1": 1,
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"2": 2,
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"3": 3,
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"4": 4,
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"5": 5,
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"6": 6,
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"7": 7,
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"8": 8,
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"9": 9,
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}
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# Modern VIN pattern (1981+): exactly 17 alphanumeric, no I/O/Q
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MODERN_VIN_PATTERN = re.compile(r"^[A-HJ-NPR-Z0-9]{17}$")
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# Pre-1981 VIN pattern: 11-17 characters
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LEGACY_VIN_PATTERN = re.compile(r"^[A-HJ-NPR-Z0-9]{11,17}$")
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def correct_ocr_errors(self, vin: str) -> str:
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"""
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Apply common OCR error corrections to a VIN string.
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Args:
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vin: Raw VIN string from OCR
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Returns:
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Corrected VIN string
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"""
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corrected = vin.upper().strip()
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# Remove any spaces or dashes (common in formatted VINs)
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corrected = corrected.replace(" ", "").replace("-", "")
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# Apply transliteration for common OCR errors
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result = []
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for char in corrected:
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if char in self.TRANSLITERATION:
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result.append(self.TRANSLITERATION[char])
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else:
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result.append(char)
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return "".join(result)
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def calculate_check_digit(self, vin: str) -> Optional[str]:
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"""
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Calculate the check digit (position 9) for a VIN.
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Args:
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vin: 17-character VIN string
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Returns:
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Expected check digit character, or None if calculation fails
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"""
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if len(vin) != 17:
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return None
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try:
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total = 0
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for i, char in enumerate(vin.upper()):
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if i == 8: # Skip check digit position
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continue
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value = self.CHAR_VALUES.get(char)
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if value is None:
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return None
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total += value * self.CHECK_WEIGHTS[i]
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remainder = total % 11
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if remainder == 10:
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return "X"
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return str(remainder)
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except (KeyError, ValueError):
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return None
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def validate_check_digit(self, vin: str) -> bool:
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"""
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Validate the check digit of a VIN.
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Args:
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vin: 17-character VIN string
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Returns:
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True if check digit is valid
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"""
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if len(vin) != 17:
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return False
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expected = self.calculate_check_digit(vin)
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if expected is None:
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return False
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return vin[8].upper() == expected
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def validate(
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self, vin: str, correct_errors: bool = True, allow_legacy: bool = False
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) -> VinValidationResult:
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"""
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Validate a VIN string and optionally correct OCR errors.
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Args:
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vin: VIN string to validate
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correct_errors: Whether to apply OCR error corrections
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allow_legacy: Whether to allow pre-1981 VINs (11-17 chars)
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Returns:
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VinValidationResult with validation status and corrected VIN
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"""
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if not vin:
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return VinValidationResult(
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is_valid=False, vin="", confidence_adjustment=-1.0, error="Empty VIN"
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)
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# Apply error corrections if enabled
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corrected_vin = self.correct_ocr_errors(vin) if correct_errors else vin.upper()
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# Check length
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if len(corrected_vin) != 17:
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if allow_legacy and 11 <= len(corrected_vin) <= 17:
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# Legacy VIN - reduced confidence
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if self.LEGACY_VIN_PATTERN.match(corrected_vin):
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return VinValidationResult(
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is_valid=True,
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vin=corrected_vin,
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confidence_adjustment=-0.2,
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)
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return VinValidationResult(
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is_valid=False,
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vin=corrected_vin,
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confidence_adjustment=-0.5,
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error=f"Invalid length: {len(corrected_vin)} (expected 17)",
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)
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# Check character set
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if not self.MODERN_VIN_PATTERN.match(corrected_vin):
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invalid_chars = [c for c in corrected_vin if c not in self.VALID_CHARS]
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return VinValidationResult(
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is_valid=False,
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vin=corrected_vin,
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confidence_adjustment=-0.3,
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error=f"Invalid characters: {invalid_chars}",
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)
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# Validate check digit
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if self.validate_check_digit(corrected_vin):
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# Valid check digit - boost confidence
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return VinValidationResult(
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is_valid=True, vin=corrected_vin, confidence_adjustment=0.1
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)
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else:
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# Invalid check digit - could be OCR error or old VIN
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return VinValidationResult(
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is_valid=True, # Still return as valid but with reduced confidence
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vin=corrected_vin,
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confidence_adjustment=-0.15,
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error="Check digit validation failed",
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)
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def extract_candidates(
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self, text: str, max_candidates: int = 5
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) -> list[tuple[str, int, int]]:
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"""
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Extract VIN candidates from raw OCR text.
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Uses two strategies:
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1. Find continuous 11-20 char alphanumeric runs (handles intact VINs)
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2. Concatenate adjacent short fragments separated by spaces/dashes
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(handles Tesseract fragmenting VINs into multiple words)
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Args:
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text: Raw OCR text
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max_candidates: Maximum number of candidates to return
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Returns:
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List of (vin, start_pos, end_pos) tuples
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"""
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candidates = []
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seen_vins: set[str] = set()
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upper_text = text.upper()
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# Strategy 1: Find continuous runs of VIN-like characters
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continuous_pattern = re.compile(r"[A-Z0-9IOQ]{11,20}", re.IGNORECASE)
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for match in continuous_pattern.finditer(upper_text):
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self._try_add_candidate(
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match.group(), match.start(), match.end(), candidates, seen_vins
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)
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# Strategy 2: Concatenate adjacent alphanumeric fragments
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# This handles OCR fragmentation like "1HGBH 41JXMN 109186"
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# Only consider fragments >= 3 chars (filters out noise/short words)
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fragment_pattern = re.compile(r"[A-Z0-9IOQ]{3,}", re.IGNORECASE)
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fragments = [
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(m.group(), m.start(), m.end())
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for m in fragment_pattern.finditer(upper_text)
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]
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# Try sliding windows of 2-4 adjacent fragments
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for window_size in range(2, min(5, len(fragments) + 1)):
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for i in range(len(fragments) - window_size + 1):
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window = fragments[i : i + window_size]
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combined = "".join(f[0] for f in window)
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# Combined length must be close to 17 (allow +/- 2 for OCR noise)
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# Must contain at least 2 digit characters (VINs always have digits;
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# pure-alphabetic text is almost certainly not a VIN)
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if 15 <= len(combined) <= 19 and sum(c.isdigit() for c in combined) >= 2:
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self._try_add_candidate(
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combined, window[0][1], window[-1][2], candidates, seen_vins
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)
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# Sort by likelihood of being valid (check digit first, then position)
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def score_candidate(c: tuple[str, int, int]) -> int:
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vin = c[0]
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if self.validate_check_digit(vin):
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return 0
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return 1
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candidates.sort(key=score_candidate)
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return candidates[:max_candidates]
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def _try_add_candidate(
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self,
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raw: str,
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start: int,
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end: int,
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candidates: list[tuple[str, int, int]],
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seen_vins: set[str],
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) -> None:
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"""Try to add a corrected VIN candidate if it passes validation."""
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corrected = self.correct_ocr_errors(raw)
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if len(corrected) == 17:
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self._add_if_valid(corrected, start, end, candidates, seen_vins)
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return
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if len(corrected) > 17:
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# Strategy A: try every 17-char sliding window
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for i in range(len(corrected) - 16):
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window = corrected[i : i + 17]
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self._add_if_valid(window, start, end, candidates, seen_vins)
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# Strategy B: for 18-19 char strings, try deleting each
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# character one at a time. OCR often inserts a spurious
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# character (e.g. sticker border read as 'C') that breaks
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# the VIN. Check-digit validation filters out false hits.
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if len(corrected) <= 19:
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for i in range(len(corrected)):
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reduced = corrected[:i] + corrected[i + 1 :]
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if len(reduced) == 17:
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self._add_if_valid(
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reduced, start, end, candidates, seen_vins
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)
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elif len(reduced) == 18:
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# Two deletions needed — try removing one more
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for j in range(len(reduced)):
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reduced2 = reduced[:j] + reduced[j + 1 :]
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self._add_if_valid(
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reduced2, start, end, candidates, seen_vins
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)
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def _add_if_valid(
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self,
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vin: str,
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start: int,
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end: int,
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candidates: list[tuple[str, int, int]],
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seen_vins: set[str],
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) -> None:
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"""Add a 17-char VIN to candidates if it matches the pattern."""
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if len(vin) == 17 and self.MODERN_VIN_PATTERN.match(vin):
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if vin not in seen_vins:
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seen_vins.add(vin)
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candidates.append((vin, start, end))
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# Singleton instance
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vin_validator = VinValidator()
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