"""Parsing of the source document, heading extraction and table of contents."""

from __future__ import annotations

import logging
import re
from dataclasses import dataclass

from lxml import html as lxml_html

logger = logging.getLogger(__name__)

HEADING_TAGS = ("h1", "h2", "h3", "h4", "h5", "h6")
ID_SAFE = re.compile(r"[^a-zA-Z0-9_-]+")

TOC_PLACEHOLDER = "\u2007\u2007\u2007"  # figure spaces, keeps the pass 1 layout stable


@dataclass
class Heading:
    level: int
    text: str
    anchor: str
    page: int | None = None


class SourceDocument:
    """Thin wrapper over the parsed document with the operations we need."""

    def __init__(self, html: str) -> None:
        self.tree = lxml_html.document_fromstring(html)
        self.head = self.tree.find("head")
        if self.head is None:
            self.head = lxml_html.Element("head")
            self.tree.insert(0, self.head)
        self.body = self.tree.find("body")
        if self.body is None:
            raise ValueError("Dokument neobsahuje element body.")
        self.container = self._find_container()
        self._toc_css_added = False

    def _find_container(self):
        """Element whose children are the natural split points.

        Many documents wrap everything in a single <main> or <div>. Splitting the
        body would then produce a single chunk, so we descend through up to two
        single child wrappers.
        """
        container = self.body
        for _ in range(2):
            children = [child for child in container if isinstance(child.tag, str)]
            if len(children) == 1 and len(list(children[0])) > 1:
                container = children[0]
                continue
            break
        return container

    # -- styles ---------------------------------------------------------
    def append_stylesheet(self, css: str) -> None:
        """Append a stylesheet as the last element of head so it wins on ties."""
        style = lxml_html.Element("style")
        style.set("type", "text/css")
        style.text = css
        self.head.append(style)

    def set_base_url(self, base_url: str | None) -> None:
        if not base_url or self.head.find("base") is not None:
            return
        base = lxml_html.Element("base")
        base.set("href", base_url)
        self.head.insert(0, base)

    # -- headings -------------------------------------------------------
    def collect_headings(self, max_depth: int) -> list[Heading]:
        """Assign ids to headings that lack one and return them in document order."""
        headings: list[Heading] = []
        used: set[str] = {el.get("id") for el in self.tree.iter() if el.get("id")}

        for element in self.body.iter(*HEADING_TAGS):
            level = int(element.tag[1])
            if level > max_depth:
                continue

            text = " ".join(element.text_content().split())
            if not text:
                continue

            anchor = element.get("id")
            if not anchor:
                anchor = self._unique_anchor(text, used)
                element.set("id", anchor)
                used.add(anchor)

            headings.append(Heading(level=level, text=text, anchor=anchor))

        return headings

    @staticmethod
    def _unique_anchor(text: str, used: set[str]) -> str:
        base = ID_SAFE.sub("-", text.strip().lower()).strip("-") or "nadpis"
        base = f"htp-{base[:60]}"
        candidate = base
        counter = 2
        while candidate in used:
            candidate = f"{base}-{counter}"
            counter += 1
        return candidate

    # -- table of contents ----------------------------------------------
    def insert_toc(self, headings: list[Heading], title: str, pages: dict[str, int] | None) -> None:
        """Insert the table of contents as the first block of the body.

        Pass 1 uses a fixed width placeholder instead of the page number so the
        table keeps exactly the same layout in pass 2.
        """
        container = lxml_html.Element("nav")
        container.set("id", "htp-toc")
        container.set("class", "htp-toc")

        heading = lxml_html.Element("h1")
        heading.set("class", "htp-toc-title")
        heading.text = title
        container.append(heading)

        table = lxml_html.Element("table")
        table.set("class", "htp-toc-table")
        tbody = lxml_html.Element("tbody")

        for item in headings:
            if item.anchor == "htp-toc-title":
                continue
            row = lxml_html.Element("tr")
            row.set("class", f"htp-toc-level-{item.level}")

            label_cell = lxml_html.Element("td")
            label_cell.set("class", "htp-toc-label")
            link = lxml_html.Element("a")
            link.set("href", f"#{item.anchor}")
            link.text = item.text
            label_cell.append(link)

            page_cell = lxml_html.Element("td")
            page_cell.set("class", "htp-toc-page")
            if pages is None:
                page_cell.text = TOC_PLACEHOLDER
            else:
                page_cell.text = str(pages.get(item.anchor, "")) or TOC_PLACEHOLDER

            row.append(label_cell)
            row.append(page_cell)
            tbody.append(row)

        table.append(tbody)
        container.append(table)

        spacer = lxml_html.Element("div")
        spacer.set("class", "htp-toc-break")
        container.append(spacer)

        self.container.insert(0, container)
        if not self._toc_css_added:
            self.append_stylesheet(TOC_CSS)
            self._toc_css_added = True

    def remove_toc(self) -> None:
        existing = self.tree.find(".//nav[@id='htp-toc']")
        if existing is not None:
            existing.getparent().remove(existing)

    # -- serialization ---------------------------------------------------
    def to_html(self) -> str:
        return "<!DOCTYPE html>\n" + lxml_html.tostring(self.tree, encoding="unicode")


TOC_CSS = """
.htp-toc { break-after: page; }
.htp-toc-table { width: 100%; border-collapse: collapse; }
.htp-toc-table td { padding: 2pt 0; vertical-align: bottom; }
.htp-toc-label a { text-decoration: none; color: inherit; }
.htp-toc-page { text-align: right; width: 4em; font-variant-numeric: tabular-nums; white-space: nowrap; }
.htp-toc-level-2 .htp-toc-label { padding-left: 1.2em; }
.htp-toc-level-3 .htp-toc-label { padding-left: 2.4em; }
.htp-toc-level-4 .htp-toc-label { padding-left: 3.6em; }
.htp-toc-level-5 .htp-toc-label { padding-left: 4.8em; }
.htp-toc-level-6 .htp-toc-label { padding-left: 6em; }
"""