pyWebLayout/tests/layout/test_backward_navigation.py

"""
Regression tests for backward page navigation (spec S16).

The previous page of P is the position q for which laying out forward from q ends
exactly at P. The old implementation searched for q by guessing a block index and
bisecting, with word_index pinned to 0 - so a page starting mid-paragraph was not
in the search space at all. It exhausted its ten iterations and fell back to a
position that was not the previous page, typically the start of the document.
"""

import pytest

from pyWebLayout.abstract.block import Paragraph
from pyWebLayout.abstract.inline import Word
from pyWebLayout.layout.ereader_layout import BidirectionalLayouter, RenderingPosition
from pyWebLayout.style import Font
from pyWebLayout.style.page_style import PageStyle


PAGE_SIZE = (800, 600)


@pytest.fixture
def font():
    return Font(font_size=16)


def paragraph(font, count, tag):
    block = Paragraph(font)
    for i in range(count):
        block.add_word(Word(f"{tag}{i}", font))
    return block


@pytest.fixture
def long_document(font):
    """Short paragraphs around one that spans several pages."""
    return [
        paragraph(font, 60, "a"),
        paragraph(font, 80, "b"),
        paragraph(font, 1200, "long"),
        paragraph(font, 70, "c"),
        paragraph(font, 90, "d"),
    ]


@pytest.fixture
def block_document(font):
    """Many small blocks, so every page starts on a block boundary."""
    return [paragraph(font, 40, f"p{i}") for i in range(40)]


def forward_chain(layouter, limit=30):
    """The page start positions a reader would visit going forward."""
    starts = []
    pos = RenderingPosition()
    for _ in range(limit):
        starts.append(pos)
        _, nxt = layouter.render_page_forward(pos, 1.0)
        if nxt.block_index >= len(layouter.blocks):
            break
        if (nxt.block_index, nxt.word_index) == (pos.block_index, pos.word_index):
            pytest.fail("forward pagination made no progress")
        pos = nxt
    return starts


def key(position):
    return (position.chapter_index, position.block_index, position.word_index)


class TestBackwardMatchesForward:
    """The defining invariant: forward from the answer lands exactly on P."""

    @pytest.mark.parametrize("document", ["long_document", "block_document"])
    def test_previous_page_is_the_forward_predecessor(self, document, request):
        blocks = request.getfixturevalue(document)
        layouter = BidirectionalLayouter(blocks, PageStyle(), PAGE_SIZE)
        starts = forward_chain(layouter)
        assert len(starts) > 2, "need a few pages to test against"

        for i in range(1, len(starts)):
            _, got = layouter.render_page_backward(starts[i], 1.0)
            assert key(got) == key(starts[i - 1]), (
                f"page {i}: expected to land on page {i - 1} "
                f"{key(starts[i - 1])}, got {key(got)}")

    def test_result_lays_out_to_the_target(self, long_document):
        """Independent of the recorded chain: replaying the answer must reach P."""
        layouter = BidirectionalLayouter(long_document, PageStyle(), PAGE_SIZE)
        starts = forward_chain(layouter)

        for target in starts[1:]:
            _, start = layouter.render_page_backward(target, 1.0)
            _, end = layouter.render_page_forward(start, 1.0)
            assert key(end) == key(target), (
                f"a page starting at {key(start)} ends at {key(end)}, "
                f"not at the requested {key(target)}")

    def test_mid_paragraph_targets_are_reachable(self, long_document):
        """The specific regression: starts inside a block, not on its boundary."""
        layouter = BidirectionalLayouter(long_document, PageStyle(), PAGE_SIZE)
        starts = forward_chain(layouter)
        mid = [s for s in starts if s.word_index > 0]
        assert mid, "this document should paginate mid-paragraph"

        for target in mid:
            _, got = layouter.render_page_backward(target, 1.0)
            assert key(got) != (0, 0, 0) or key(target) == key(starts[1]), \
                "backward navigation fell back to the document start"


class TestRoundTrip:

    def test_forward_then_back_returns_to_the_same_place(self, long_document):
        layouter = BidirectionalLayouter(long_document, PageStyle(), PAGE_SIZE)
        pos = RenderingPosition()

        for _ in range(4):
            _, nxt = layouter.render_page_forward(pos, 1.0)
            _, back = layouter.render_page_backward(nxt, 1.0)
            assert key(back) == key(pos), \
                f"round trip drifted: {key(pos)} -> {key(nxt)} -> {key(back)}"
            pos = nxt


class TestEdges:

    def test_at_document_start_stays_there(self, long_document):
        layouter = BidirectionalLayouter(long_document, PageStyle(), PAGE_SIZE)
        _, got = layouter.render_page_backward(RenderingPosition(), 1.0)
        assert key(got) == (0, 0, 0)

    def test_second_page_goes_back_to_the_first(self, long_document):
        layouter = BidirectionalLayouter(long_document, PageStyle(), PAGE_SIZE)
        _, second = layouter.render_page_forward(RenderingPosition(), 1.0)
        _, got = layouter.render_page_backward(second, 1.0)
        assert key(got) == (0, 0, 0)

    def test_empty_document_is_safe(self):
        layouter = BidirectionalLayouter([], PageStyle(), PAGE_SIZE)
        page, got = layouter.render_page_backward(RenderingPosition(), 1.0)
        assert page is not None
        assert key(got) == (0, 0, 0)


class TestCost:

    def test_backward_is_not_wildly_more_expensive_than_forward(self, long_document):
        """The old path burned ten full layouts per call and still got it wrong."""
        layouter = BidirectionalLayouter(long_document, PageStyle(), PAGE_SIZE)
        starts = forward_chain(layouter)

        calls = {"n": 0}
        original = BidirectionalLayouter.render_page_forward

        def counting(self, position, font_scale=1.0):
            calls["n"] += 1
            return original(self, position, font_scale)

        BidirectionalLayouter.render_page_forward = counting
        try:
            worst = 0
            for target in starts[1:]:
                calls["n"] = 0
                layouter.render_page_backward(target, 1.0)
                worst = max(worst, calls["n"])
        finally:
            BidirectionalLayouter.render_page_forward = original

        assert worst <= 10, f"backward navigation cost {worst} forward layouts"