feat: add normalisation helpers for book matching

Reduce a title, an author and an identifier to a single comparison key, so
two copies of one book can be recognised by equality rather than by a
similarity score.

backend/src/chitai/services/matching.py

@@ -0,0 +1,163 @@
+# src/chitai/services/matching.py
+
+"""
+Normalisation for book-level duplicate detection.
+
+Two copies of one book rarely agree on how it is written down. One says
+`The Metamorphosis`, the other `Metamorphosis`; one credits `Kafka, Franz`, the other
+`Franz Kafka`; one carries the ISBN-10 and the other the ISBN-13 of the same edition.
+These functions reduce each of those to a single key, so the comparison is an equality
+test the database can index rather than a similarity score nobody can explain.
+
+Everything here is pure: the keys are computed once and stored on the row (see
+`Book.normalized_title`, `Author.normalized_name`, `Identifier.normalized_value`), so
+no Postgres extension is needed at query time.
+"""
+
+from __future__ import annotations
+
+import re
+import unicodedata
+
+from chitai.services.utils import is_valid_isbn, isbn10_to_isbn13
+
+
+# Asides a title carries that say nothing about which book it is:
+# "Frankenstein (Illustrated)", "Dune [Deluxe]".
+_BRACKETED = re.compile(r"[(\[{][^)\]}]*[)\]}]")
+
+# Edition and format qualifiers, matched only as a *trailing* run of words. Anchoring
+# to the end is what keeps "The Illustrated Man" a book and "Moby Dick Illustrated" a
+# format note — a qualifier trails the title, it is never the thing the title is about.
+_EDITION_NOISE = re.compile(
+    r"\s+(?:"
+    r"\d+(?:st|nd|rd|th)?\s+ed(?:ition|n)?"
+    r"|(?:first|second|third|fourth|fifth|sixth|new|revised|expanded|updated|"
+    r"annotated|illustrated|unabridged|abridged|complete|definitive|deluxe|"
+    r"anniversary|collectors|international|kindle|paperback|hardcover|hardback|"
+    r"ebook|audiobook)"
+    r"(?:\s+(?:and|&)\s+\w+)*"
+    r"(?:\s+ed(?:ition|n)?)?"
+    r")$"
+)
+
+_LEADING_ARTICLE = re.compile(r"^(?:the|a|an)\s+")
+
+# Anything that is not a letter, a digit or a space, once accents are gone.
+_PUNCTUATION = re.compile(r"[^0-9a-z ]+")
+
+_WHITESPACE = re.compile(r"\s+")
+
+# `J. R. R.` survives punctuation stripping as three one-letter words; `J.R.R.` as one.
+# Joining any run of them makes both `jrr`.
+_INITIAL_RUN = re.compile(r"\b(?:[a-z] )+[a-z]\b")
+
+# ISBNs are the same number under several names; everything else keeps its own.
+_ISBN_NAMES = {"isbn", "isbn-10", "isbn10", "isbn-13", "isbn13"}
+
+# Generated fresh for every build of a file, so two copies of one book never share one.
+# Matching on them would only re-find files the hash check already catches.
+_PER_BUILD_NAMES = {"uuid", "urn:uuid"}
+
+# Below this an identifier is not specific enough to be evidence: a Calibre `id` of
+# "42" would otherwise pair two unrelated books.
+_MIN_IDENTIFIER_LENGTH = 4
+
+
+def _fold(text: str) -> str:
+    """Casefolded, accent-free, punctuation-free, single-spaced."""
+    decomposed = unicodedata.normalize("NFKD", text.casefold())
+    unaccented = "".join(c for c in decomposed if not unicodedata.combining(c))
+
+    return _WHITESPACE.sub(" ", _PUNCTUATION.sub(" ", unaccented)).strip()
+
+
+def normalize_title(title: str | None) -> str:
+    """
+    Reduce a title to the key two copies of one book should share.
+
+    `Book.subtitle` is already split off by `Extractor.format_book_title`, so only what
+    is left in the title column is considered here.
+
+    Args:
+        title: The title as it was stored.
+
+    Returns:
+        The comparison key, or an empty string if nothing survives normalisation —
+        which is the signal not to match on the title at all.
+    """
+    if not title:
+        return ""
+
+    folded = _fold(_BRACKETED.sub(" ", title).replace("&", " and "))
+
+    # Repeated because qualifiers stack: "Dune Deluxe Edition Illustrated".
+    while (trimmed := _EDITION_NOISE.sub("", folded)) != folded:
+        folded = trimmed
+
+    # An article says nothing, but a title that is only an article is not improved by
+    # having none, and neither is one that noise removal emptied out.
+    return _LEADING_ARTICLE.sub("", folded, count=1) or folded
+
+
+def normalize_author(name: str | None) -> str:
+    """
+    Reduce an author's name to the key their other books should share.
+
+    Deliberately not reduced to surname plus initial: that collides unrelated people,
+    and a wrong match here is a book pointed at a stranger's shelf.
+
+    Args:
+        name: The name as it was stored, in either `Franz Kafka` or `Kafka, Franz` form.
+
+    Returns:
+        The comparison key, or an empty string if nothing survives normalisation.
+    """
+    if not name:
+        return ""
+
+    # `Kafka, Franz` is one name written backwards. More than one comma is a list, or a
+    # suffix, and guessing at either does more harm than leaving it alone.
+    if name.count(",") == 1:
+        surname, forename = name.split(",")
+        name = f"{forename.strip()} {surname.strip()}"
+
+    folded = _fold(name)
+
+    return _INITIAL_RUN.sub(lambda run: run.group().replace(" ", ""), folded)
+
+
+def normalize_identifier(name: str, value: str) -> str | None:
+    """
+    Reduce one identifier to a `scheme:value` key, if it can carry a match at all.
+
+    Args:
+        name: What kind of identifier it is, as stored.
+        value: The identifier itself.
+
+    Returns:
+        The key, or None when the identifier is no use for matching: a per-build UUID,
+        something too short to be evidence, or an ISBN that fails its own checksum.
+    """
+    name = (name or "").strip().casefold()
+    value = (value or "").strip()
+
+    if not name or not value or name in _PER_BUILD_NAMES:
+        return None
+
+    if name in _ISBN_NAMES:
+        digits = re.sub(r"[^0-9Xx]", "", value).upper()
+
+        if not is_valid_isbn(digits):
+            return None
+
+        # One scheme for both forms: a publisher prints whichever it likes, and the
+        # ISBN-10 and ISBN-13 of an edition are the same number written twice.
+        isbn = digits if len(digits) == 13 else isbn10_to_isbn13(digits)
+        return f"isbn:{isbn}" if isbn else None
+
+    folded = _fold(value) or value.casefold()
+    if len(folded) < _MIN_IDENTIFIER_LENGTH:
+        return None
+
+    return f"{name}:{folded}"

backend/src/chitai/services/utils.py

@@ -512,6 +512,29 @@ def is_valid_isbn10(isbn: str) -> bool:
     return str(check_digit) == isbn[-1] or (check_digit == 10 and isbn[-1] in "Xx")
 
 
+def isbn10_to_isbn13(isbn: str) -> str | None:
+    """
+    Convert an ISBN-10 to the ISBN-13 naming the same edition.
+
+    The two are the same number written twice: prefix `978`, drop the ISBN-10 check
+    digit, recompute the check digit under the ISBN-13 rule. Matching only works if
+    both forms collapse onto one, since a publisher may print either.
+
+    Args:
+        isbn: A 10-character ISBN, digits and an optional trailing `X` only.
+
+    Returns:
+        The equivalent ISBN-13, or None if the input is not a valid ISBN-10.
+    """
+    if not is_valid_isbn(isbn) or len(isbn) != 10:
+        return None
+
+    digits = f"978{isbn[:9]}"
+    total = sum(int(digit) * (1 if i % 2 == 0 else 3) for i, digit in enumerate(digits))
+
+    return f"{digits}{(10 - total % 10) % 10}"
+
+
 def is_valid_isbn13(isbn: str) -> bool:
     """
     Validate an ISBN-13 number using its check digit.

backend/tests/unit/test_matching.py

@@ -0,0 +1,125 @@
+"""Tests for the normalization behind book-level duplicate detection."""
+
+import pytest
+
+from chitai.services.matching import (
+    normalize_author,
+    normalize_identifier,
+    normalize_title,
+)
+from chitai.services.utils import isbn10_to_isbn13
+
+
+class TestNormalizeTitle:
+    """Two copies of one book rarely agree on how the title is written."""
+
+    @pytest.mark.parametrize(
+        ("title", "expected"),
+        [
+            ("The Metamorphosis", "metamorphosis"),
+            ("Metamorphosis", "metamorphosis"),
+            ("METAMORPHOSIS", "metamorphosis"),
+            ("A Tale of Two Cities", "tale of two cities"),
+            ("An Enquiry", "enquiry"),
+            # Accents, punctuation and ampersands are spelling, not identity.
+            ("Les Misérables", "les miserables"),
+            ("Moby Dick; Or, The Whale", "moby dick or the whale"),
+            ("Sense & Sensibility", "sense and sensibility"),
+            # Bracketed asides and trailing edition noise say nothing about the book.
+            ("Frankenstein (Illustrated)", "frankenstein"),
+            ("Frankenstein [Kindle Edition]", "frankenstein"),
+            ("Frankenstein, 2nd Edition", "frankenstein"),
+            ("Frankenstein Revised Edition", "frankenstein"),
+            ("Dune Deluxe Edition Illustrated", "dune"),
+            ("", ""),
+            (None, ""),
+        ],
+    )
+    def test_titles_that_should_agree(self, title: str | None, expected: str) -> None:
+        assert normalize_title(title) == expected
+
+    def test_a_qualifier_that_is_the_title_survives(self) -> None:
+        """A trailing qualifier is noise; the same word at the front is the book."""
+        assert normalize_title("The Illustrated Man") == "illustrated man"
+
+    def test_normalization_never_empties_a_title(self) -> None:
+        """An article-only title is not improved by having no article left."""
+        assert normalize_title("The") == "the"
+
+
+
+class TestNormalizeAuthor:
+    """One person, written down several ways."""
+
+    @pytest.mark.parametrize(
+        ("name", "expected"),
+        [
+            ("Franz Kafka", "franz kafka"),
+            ("Kafka, Franz", "franz kafka"),
+            ("KAFKA, FRANZ", "franz kafka"),
+            ("Émile Zola", "emile zola"),
+            ("Doyle, Arthur Conan", "arthur conan doyle"),
+            # Runs of initials are joined, so spacing them out changes nothing.
+            ("J.R.R. Tolkien", "jrr tolkien"),
+            ("J. R. R. Tolkien", "jrr tolkien"),
+            ("JRR Tolkien", "jrr tolkien"),
+            ("", ""),
+            (None, ""),
+        ],
+    )
+    def test_names_that_should_agree(self, name: str | None, expected: str) -> None:
+        assert normalize_author(name) == expected
+
+    def test_two_people_are_not_reduced_together(self) -> None:
+        """Surname plus initial would collide unrelated writers; it is not used."""
+        assert normalize_author("Charles Dickens") != normalize_author("Colin Dexter")
+
+    def test_a_list_is_left_alone(self) -> None:
+        """More than one comma is a list or a suffix, and guessing does more harm."""
+        assert normalize_author("Smith, John, Jr.") == "smith john jr"
+
+
+class TestNormalizeIdentifier:
+    """Identifiers only help if the same edition produces the same key."""
+
+    def test_isbn_10_and_isbn_13_are_one_key(self) -> None:
+        assert normalize_identifier("isbn-10", "0486282112") == "isbn:9780486282114"
+        assert normalize_identifier("isbn-13", "9780486282114") == "isbn:9780486282114"
+
+    @pytest.mark.parametrize(
+        "written", ["978-0-486-28211-4", "978 0 486 28211 4", "9780486282114"]
+    )
+    def test_formatting_is_not_part_of_an_isbn(self, written: str) -> None:
+        assert normalize_identifier("isbn", written) == "isbn:9780486282114"
+
+    def test_an_isbn_that_fails_its_checksum_is_no_evidence(self) -> None:
+        assert normalize_identifier("isbn-13", "9780486282115") is None
+
+    def test_uuids_are_refused(self) -> None:
+        """Generated per build, so they only re-find what the hash check catches."""
+        assert normalize_identifier("uuid", "3f2b1c4e-1111-2222-3333-444455556666") is None
+        assert normalize_identifier("urn:uuid", "3f2b1c4e-1111-2222-3333-444455556666") is None
+
+    def test_other_schemes_keep_their_own_key(self) -> None:
+        assert normalize_identifier("asin", "B000FC0PDA") == "asin:b000fc0pda"
+        assert normalize_identifier("ASIN", "b000fc0pda") == "asin:b000fc0pda"
+
+    def test_something_too_short_is_not_evidence(self) -> None:
+        """A Calibre id of "42" would otherwise pair two unrelated books."""
+        assert normalize_identifier("calibre", "42") is None
+
+    @pytest.mark.parametrize(("name", "value"), [("", "1234567"), ("asin", "")])
+    def test_half_an_identifier_is_no_identifier(self, name: str, value: str) -> None:
+        assert normalize_identifier(name, value) is None
+
+
+class TestIsbnConversion:
+    def test_isbn_10_converts_to_its_isbn_13(self) -> None:
+        assert isbn10_to_isbn13("0486282112") == "9780486282114"
+
+    def test_a_trailing_x_is_a_digit(self) -> None:
+        assert isbn10_to_isbn13("043942089X") == "9780439420891"
+
+    @pytest.mark.parametrize("isbn", ["0486282113", "9780486282114", "nonsense"])
+    def test_anything_that_is_not_an_isbn_10_converts_to_nothing(self, isbn: str) -> None:
+        assert isbn10_to_isbn13(isbn) is None