"""The built-in identity palette + the effective UI-settings resolver
(phase 91).

Single source of the built-in **identity** palette. Phase 62's
custom-CSS-file theming (an env var named a drop-in ``:root`` override
stylesheet that ``brand.js`` linked AFTER the boot fetch — the "red
first, then pop" the owner saw) is retired in this phase: task 03
deleted the env var, the example-stylesheet directory, and the link
insertion, and the admin Theme tab is now the only theming surface.
The contract that directory's authoring guide carried is re-homed here
(built-in table, the five contrast pairs, the never-white-on-brand
trap — see below), and the 9 variables + built-in values are the
authoritative table (the unit drift test parses
``frontend/assets/styles.css``'s ``:root`` and asserts equality, so
the two can never silently diverge).

The **9 identity variables** (bare names, README order) and their
built-in values (from ``frontend/assets/styles.css`` ``:root``):

===================  ==========  =================================================
Variable             Built-in    Role
===================  ==========  =================================================
``bg``               ``#0f0a0a`` page background (text on it: ``ink``)
``surface``          ``#1a0f0f`` cards, panels, code blocks (text: ``ink``)
``ink``              ``#f0e6e6`` primary text
``ink_soft``         ``#b8a8a8`` secondary text (5.1:1 on ``surface``)
``line``             ``#2d1a1a`` decorative 1px borders (no contrast duty)
``grid_line``        ``#4a2626`` background grid texture — decorative,
                     no contrast duty, like ``line``
``brand``            ``#f43f5e`` brand accent — buttons, links (text ON
                     it is the DARK ``bg`` ink)
``brand_soft``       ``#2d0a0a`` brand-tinted surface (chips, hover washes)
``brand_ink``        ``#fca5a5`` brand-tinted text (9.0:1 on ``surface``)
===================  ==========  =================================================

The **semantic families are deliberately NOT identity** (B3,
owner-locked 2026-09-09): ``--accent-*`` (deflection amber), ``--ok-*``
(success green), ``--err-*`` (error red) encode *states*, are already AA
in the built-in theme, and are not configurable from the tab — a theme
that keeps them stays honest.

**The five contrast pairs** that must meet WCAG 2.1 AA (>= 4.5:1,
AGENTS.md rule 5) — the pairs the layout actually pairs: ``ink`` on
``bg``, ``ink`` on ``surface``, ``ink_soft`` on ``surface``, ``bg`` on
``brand`` (the text on brand buttons is the DARK background ink —
that is the pattern; never white on brand: white on the built-in
``#f43f5e`` is 3.7:1, it fails), and ``brand_ink`` on ``surface``. The
tab's client-side warnings (task 05) compute exactly these five ratios
against the values being saved; the built-in palette itself passes, so
the default deployment stays AA without any warning.

Effective-value resolution (:func:`effective_settings`) — the DB-over-
env / DB-over-built-in merge (B1, owner-locked 2026-09-09): the single
``ui_settings`` row (id 1, task 01) wins column-by-column when set; a
NULL/empty string column falls back to the ``BOR_`` env value, a NULL
color column to the built-in. ONE resolver is used by BOTH
``GET /api/ui-settings`` (the tab) and ``GET /api/config`` (the brand
layer), so the tab and the running UI can never disagree.
"""
from __future__ import annotations

import base64
import hashlib

from sqlalchemy import select
from sqlalchemy.orm import Session

from app.config import Settings, get_settings
from app.models import UiSettings

#: The 9 built-in identity colors, keyed by BARE variable name (no ``--``)
#: in the themes-README order. Copied from ``frontend/assets/styles.css``
#: ``:root`` — the unit drift test (``tests/unit/test_theming.py``)
#: re-parses the stylesheet and asserts equality on every run.
BUILTIN_COLORS: dict[str, str] = {
    "bg": "#0f0a0a",
    "surface": "#1a0f0f",
    "ink": "#f0e6e6",
    "ink_soft": "#b8a8a8",
    "line": "#2d1a1a",
    "grid_line": "#4a2626",  # the background grid texture (phase 92)
    "brand": "#f43f5e",
    "brand_soft": "#2d0a0a",
    "brand_ink": "#fca5a5",
}

#: The 9 color field names in the README's order (dicts preserve
#: insertion order) — used by the resolver, the API, and the
#: ``theme_style_tag`` renderer (task 02).
COLOR_FIELDS: tuple[str, ...] = tuple(BUILTIN_COLORS)

#: The 3 display strings the ``ui_settings`` row carries — env fallback
#: (B1: unlike the colors, the env vars stay the strings' default).
STRING_FIELDS: tuple[str, ...] = ("app_name", "input_placeholder", "footer_text")


def effective_settings(
    session: Session, settings: Settings | None = None
) -> dict[str, str]:
    """Resolve the EFFECTIVE UI settings — DB-over-env / DB-over-built-in.

    Reads the single ``ui_settings`` row (id 1) and merges it over the
    defaults, column by column:

    * **strings** (``app_name`` / ``input_placeholder`` / ``footer_text``)
      — the DB value when it is a non-empty string, else the env value
      (``settings.app_name`` etc. — B1: the env vars stay the fallback);
    * **colors** (the 9 :data:`COLOR_FIELDS`) — the DB value when not
      ``None``, else :data:`BUILTIN_COLORS` (B1: no env fallback for
      colors — the built-in palette IS the default).

    A missing row (``GET`` creates nothing) means "defaults" — the env
    strings + the built-in palette. The ``settings`` parameter names the
    env-fallback source explicitly (the routes pass their
    dependency-injected instance so test overrides apply); ``None`` uses
    the cached :func:`app.config.get_settings`. Returns all 12 keys.
    """
    if settings is None:
        settings = get_settings()
    row = session.execute(select(UiSettings).where(UiSettings.id == 1)).scalars().first()
    effective: dict[str, str] = {}
    for field in STRING_FIELDS:
        value = getattr(row, field, None) if row is not None else None
        effective[field] = value if isinstance(value, str) and value else getattr(settings, field)
    for key in COLOR_FIELDS:
        value = getattr(row, key, None) if row is not None else None
        effective[key] = value if value is not None else BUILTIN_COLORS[key]
    return effective


def theme_style_tag(colors: dict[str, str]) -> str:
    """The pre-paint inline theme tag (task 02's injection input).

    ``""`` when every color equals its built-in — the byte-identical
    contract: an unset (or "defaults saved") deployment must serve
    exactly the pre-phase-91 HTML, no ``<style>`` tag anywhere.
    Otherwise one ``<style id="bor-theme">`` tag with ALL 9 variables in
    :data:`COLOR_FIELDS` order (the non-overridden ones repeat their
    built-in value — the tag is a complete ``:root`` override, so the
    page never mixes partial palettes)::

        <style id="bor-theme">:root{--bg:#0f0a0a;…;--brand-ink:#fca5a5}</style>

    Pure function of its input — :func:`inject_theme` places it before
    the first ``</head>`` of every served HTML page (the phase-91
    pre-paint injection), so the themed deployment renders its palette
    on the FIRST paint (no red flash, no pop-in).
    """
    if all(colors[key] == BUILTIN_COLORS[key] for key in COLOR_FIELDS):
        return ""
    declarations = "".join(
        f"--{key.replace('_', '-')}:{colors[key]};" for key in COLOR_FIELDS
    )
    return f'<style id="bor-theme">:root{{{declarations}}}</style>'


def inject_theme(html: str, tag: str) -> str:
    """Insert ``tag`` immediately BEFORE the first ``</head>`` of
    ``html`` — the pure half of the phase-91 pre-paint injection.

    The :class:`~app.core.caching.CachingMiddleware` (task 02) calls
    this on every known HTML page's rewritten body, so the helper stays
    pure (no DB, no app) and unit-testable on its own. Identity rules —
    the byte-identical contract (B4, owner-locked 2026-09-09):

    * ``tag == ""`` (an unset or "defaults saved" deployment —
      :func:`theme_style_tag` returns exactly that) → ``html`` is
      returned EXACTLY as passed in, byte for byte;
    * no ``</head>`` occurrence → unchanged (nothing to anchor to);
    * ``id="bor-theme"`` already present → unchanged (defensive
      idempotence — the static files never contain the id, and one
      body can never reach the helper twice, but the guarantee is free
      for a pure function).

    Otherwise the tag is placed with a leading newline (readable HTML)
    immediately before the FIRST ``</head>`` — the browser meets the
    complete ``:root`` override before it applies any stylesheet, so
    the palette is live on the first paint.
    """
    if not tag or "</head>" not in html or 'id="bor-theme"' in html:
        return html
    index = html.index("</head>")
    return html[:index] + "\n" + tag + html[index:]


def theme_csp_hash(tag: str) -> str:
    """The CSP3 ``sha256-`` source expression for an inline theme tag.

    Phase 91 (task 05 defect fix): the phase-82 CSP (A1 —
    ``default-src 'self'`` with no explicit ``style-src``) BLOCKS the
    inline ``<style id="bor-theme">`` tag in every real browser
    (``style-src`` falls back to ``default-src 'self'``), so the
    pre-paint injection would be dead bytes in the served HTML. The
    fix is the strictest one that works: the hashing source expression
    of the tag's EXACT content (CSP3 §13.4 — the character data between
    the tags; the rendered content carries no leading/trailing
    whitespace, so no stripping applies). The caching middleware
    publishes it on themed HTML pages only, as ``style-src 'self'
    'sha256-…'`` appended to the A1 string — the current theme is the
    only inline style ever permitted, and a different palette (or any
    other inline style) is still blocked. No blanket
    ``'unsafe-inline'`` — the A1 posture holds everywhere else. Returns
    ``""`` for an empty tag (an unset/defaults deployment keeps the
    plain A1 policy — the byte- AND header-identical contract).
    """
    if not tag:
        return ""
    content = tag.split(">", 1)[1].rsplit("</style>", 1)[0]
    digest = hashlib.sha256(content.encode("utf-8")).digest()
    return "sha256-" + base64.b64encode(digest).decode("ascii")