//! Choosing a theme.
//!
//! The file half of this crate was always shared; the *selection* half was not,
//! and four apps re-rolled it four ways. GoingsOn stores a "system" sentinel in
//! localStorage, Balanced Breakfast treats an absent value as follow-the-system
//! and hardcodes two theme ids as its light/dark pair, audiofiles keeps the id
//! in a synced SQLite table, and the Alloy console parses COLORFGBG. They also
//! disagreed about what a variant string means: this crate defaults a missing
//! one to "dark" while alloy_tui parsed an unrecognized one as light.
//!
//! What cannot be shared is the store — localStorage, a synced config table and
//! a TOML file are genuinely different places. What can be shared, and is here,
//! is the *meaning*: one vocabulary for variants, one encoding for "what did the
//! user choose", and one rule for turning that into an id that exists.

use crate::{Rgb, ThemeColors, ThemeMeta, list_themes_from_dirs, load_theme, wcag_contrast};
use serde::Serialize;
use std::path::PathBuf;

// Names this module's prose links to, resolved for rustdoc.
#[allow(unused_imports)]
use crate::parse_meta;

/// A theme's kind, as declared by `meta.variant`.
///
/// Three, not two: one shipped theme is `high-contrast`, and an app that
/// matched on light-or-dark alone would quietly file it under the wrong one.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, Serialize)]
#[serde(rename_all = "kebab-case")]
pub enum Variant {
    Light,
    Dark,
    HighContrast,
}

impl Variant {
    /// The spelling used in a theme file and in [`ThemeMeta::variant`].
    #[must_use]
    pub const fn as_str(self) -> &'static str {
        match self {
            Variant::Light => "light",
            Variant::Dark => "dark",
            Variant::HighContrast => "high-contrast",
        }
    }

    /// Read a variant string, or `None` if it names none of them.
    #[must_use]
    pub fn parse(raw: &str) -> Option<Self> {
        match raw {
            "light" => Some(Variant::Light),
            "dark" => Some(Variant::Dark),
            "high-contrast" => Some(Variant::HighContrast),
            _ => None,
        }
    }
}

impl std::fmt::Display for Variant {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        f.write_str(self.as_str())
    }
}

/// Anything unrecognized reads as dark, which is what [`parse_meta`] already
/// does with a missing one. Consumers that guessed light for an unknown string
/// were disagreeing with the crate that produced it.
impl From<&str> for Variant {
    fn from(raw: &str) -> Self {
        Variant::parse(raw).unwrap_or(Variant::Dark)
    }
}

impl ThemeMeta {
    /// This theme's variant as a value rather than a string.
    #[must_use]
    pub fn kind(&self) -> Variant {
        Variant::from(self.variant.as_str())
    }
}

/// The spelling of "follow whatever the system is doing", in every store.
pub const FOLLOW: &str = "system";

/// What the user chose, as opposed to what is being rendered.
///
/// The distinction is the whole point: `Follow` is a standing instruction that
/// resolves differently as the ambient mode changes, and a `Fixed` id is an
/// answer that does not. An app that stored only the rendered id could not tell
/// the two apart the next time the system flipped to dark.
#[derive(Debug, Clone, PartialEq, Eq, Default)]
pub enum ThemeSelection {
    /// Track the ambient light/dark mode.
    #[default]
    Follow,
    /// Always this theme.
    Fixed(String),
}

impl ThemeSelection {
    /// Read a stored selection. An empty or absent value is [`Follow`], which
    /// is what an app with nothing saved yet should do.
    ///
    /// [`Follow`]: ThemeSelection::Follow
    #[must_use]
    pub fn parse(raw: Option<&str>) -> Self {
        match raw.map(str::trim) {
            None | Some("" | FOLLOW) => ThemeSelection::Follow,
            Some(id) => ThemeSelection::Fixed(id.to_string()),
        }
    }

    /// The string to persist, whatever the store is.
    #[must_use]
    pub fn as_str(&self) -> &str {
        match self {
            ThemeSelection::Follow => FOLLOW,
            ThemeSelection::Fixed(id) => id,
        }
    }

    /// Turn a selection into a theme id that exists.
    ///
    /// `ambient` is the light/dark mode the app learned however it can: a
    /// `prefers-color-scheme` media query, an OS appearance API, `COLORFGBG`
    /// from a terminal. `available` is what [`list_themes_from_dirs`] found.
    ///
    /// A `Fixed` id that is no longer on disk falls through to the same path as
    /// `Follow` rather than being returned anyway. Themes are deletable in
    /// three of the four apps, and handing back an id that will fail to load
    /// only moves the error somewhere less helpful.
    ///
    /// The fallback chain is: the app's own default for the ambient mode if it
    /// is installed, then any installed theme of that variant, then the app's
    /// default regardless. The last step means this always returns something,
    /// and an app with no theme directory at all gets the id it ships with and
    /// the load error it would have had anyway.
    #[must_use]
    pub fn resolve(
        &self,
        ambient: Variant,
        defaults: &ThemeDefaults,
        available: &[ThemeMeta],
    ) -> String {
        let installed = |id: &str| available.iter().any(|meta| meta.id == id);

        if let ThemeSelection::Fixed(id) = self
            && installed(id)
        {
            return id.clone();
        }

        let preferred = defaults.for_variant(ambient);
        if installed(preferred) {
            return preferred.to_string();
        }
        available
            .iter()
            .find(|meta| meta.kind() == ambient)
            .map_or_else(|| preferred.to_string(), |meta| meta.id.clone())
    }
}

impl std::fmt::Display for ThemeSelection {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        f.write_str(self.as_str())
    }
}

/// The themes an app falls back to, one per ambient mode.
///
/// App-specific on purpose: which theme is "the app's own" is the app's
/// identity, not this crate's business. What is shared is everything around it.
#[derive(Debug, Clone)]
pub struct ThemeDefaults {
    light: String,
    dark: String,
    high_contrast: Option<String>,
}

impl ThemeDefaults {
    pub fn new(light: impl Into<String>, dark: impl Into<String>) -> Self {
        Self {
            light: light.into(),
            dark: dark.into(),
            high_contrast: None,
        }
    }

    /// Name a theme for a high-contrast ambient mode. Without one, that mode
    /// falls back to the dark default, which is the safer of the two to read.
    #[must_use]
    pub fn high_contrast(mut self, id: impl Into<String>) -> Self {
        self.high_contrast = Some(id.into());
        self
    }

    /// Whether a high-contrast default was named.
    ///
    /// [`for_variant`] answers for every mode by falling back to the dark
    /// theme, which is right for resolving a selection and wrong for emitting
    /// a `prefers-contrast: more` block: that block would then answer the
    /// preference with a theme that does not honour it. A caller that renders
    /// per ambient mode asks this first.
    ///
    /// [`for_variant`]: ThemeDefaults::for_variant
    #[must_use]
    pub const fn names_high_contrast(&self) -> bool {
        self.high_contrast.is_some()
    }

    #[must_use]
    pub fn for_variant(&self, variant: Variant) -> &str {
        match variant {
            Variant::Light => &self.light,
            Variant::Dark => &self.dark,
            Variant::HighContrast => self.high_contrast.as_ref().unwrap_or(&self.dark),
        }
    }
}

/// How legible a theme's muted text is, measured rather than declared.
///
/// The worst WCAG contrast ratio of `content.muted` against the two panel
/// grounds a reader actually meets it on, `surface.page` and `surface.sunken`,
/// bucketed at the two thresholds WCAG 2.x draws. Worst rather than average,
/// because a theme that is legible on one panel and not the other is a theme
/// with an illegible panel.
///
/// It is measured here rather than authored in the theme file for the reason
/// the whole crate exists: a curated palette keeps its identity and the reader
/// still gets told what it costs them. An author cannot mis-declare it, and a
/// theme edited on disk re-measures on the next scan.
///
/// Ordered worst-first, so `sort` puts the most legible theme last and
/// [`theme_options`] reverses it into what a picker wants at the top.
#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash, Serialize)]
#[serde(rename_all = "kebab-case")]
pub enum ContrastTier {
    /// Muted text below the 3:1 floor WCAG sets for large text and UI parts.
    Low,
    /// Muted text meets 3:1 but not the 4.5:1 bar for normal text.
    Standard,
    /// Muted text meets WCAG AA on every panel ground, 4.5:1 or better.
    High,
}

impl ContrastTier {
    /// The machine spelling, for a data attribute or a stored value.
    #[must_use]
    pub const fn as_str(self) -> &'static str {
        match self {
            ContrastTier::Low => "low",
            ContrastTier::Standard => "standard",
            ContrastTier::High => "high",
        }
    }

    /// Measure a loaded theme.
    ///
    /// A theme missing either ground or the muted content colour reads as
    /// [`Standard`](Self::Standard): the measurement did not happen, and
    /// claiming `Low` would badge a theme for the scan's failure rather than
    /// its own.
    #[must_use]
    pub fn of(theme: &ThemeColors) -> Self {
        let colour = |key: &str| theme.colors.get(key).and_then(|v| Rgb::from_hex(v));
        let (Some(muted), Some(page), Some(sunken)) = (
            colour("content.muted"),
            colour("surface.page"),
            colour("surface.sunken"),
        ) else {
            return ContrastTier::Standard;
        };

        let worst = wcag_contrast(muted, page).min(wcag_contrast(muted, sunken));
        if worst >= 4.5 {
            ContrastTier::High
        } else if worst >= 3.0 {
            ContrastTier::Standard
        } else {
            ContrastTier::Low
        }
    }
}

/// One theme, as a picker offers it.
///
/// [`ThemeMeta`] plus the two facts a picker needs and a scan is what supplies:
/// the variant as a value rather than a string, and the measured contrast tier.
/// Owned, because it outlives the directory scan that produced it and is held
/// by an app across the frames or requests that draw the control.
///
/// It carries no `is_custom`. A picker that sorted the user's own themes apart
/// from the shipped ones would be answering a different question, and
/// [`ThemeMeta`] is still there for a screen that wants it.
#[derive(Debug, Clone, PartialEq, Eq, Serialize)]
#[serde(rename_all = "camelCase")]
pub struct ThemeOption {
    /// The id stored, and the value the picker submits.
    pub id: String,
    /// What the picker reads.
    pub name: String,
    /// Which group it belongs to.
    pub variant: Variant,
    /// How legible its muted text measured.
    pub contrast: ContrastTier,
}

/// Every installed theme, in the order a picker should offer them.
///
/// This is the half of a theme picker that is not the control: which themes
/// exist, which group each is in, how legible each one is, and what order that
/// puts them in. Three apps derived it three ways and two of them lost it
/// entirely when their pickers were described, which is what makes it the
/// crate's job rather than each app's.
///
/// # The order
///
/// By variant in [`Variant`]'s own order — light, dark, high contrast — then
/// by measured contrast **best first**, then by name. The middle key is the one
/// no app can supply without redoing the work this crate has already done: the
/// tier comes off the resolved colours, and an app sorting a `Vec<ThemeMeta>`
/// has only the names.
///
/// Grouping is left implicit in the order rather than returned as groups. A
/// renderer that draws headings walks the run of one variant; one that cannot
/// draw headings still gets the useful order. Handing back
/// `Vec<(Variant, Vec<ThemeOption>)>` would force the second renderer to
/// flatten what the first wanted, and neither shape is more true.
///
/// # What it costs
///
/// Every theme file is parsed twice: once by [`list_themes_from_dirs`] for its
/// metadata, once here for the colours the tier is measured from. Measured
/// rather than assumed to be cheap: a picker is drawn on a settings screen, the
/// shipped set is around twenty files, and the alternative is caching a
/// derived value that a theme edited on disk would then be wrong about.
/// A theme whose colours will not load keeps its metadata and reads as
/// [`ContrastTier::Standard`], on the same footing as one missing a ground.
///
/// A host whose themes are not all on disk builds its own [`ThemeOption`]s and
/// calls [`order_theme_options`], which is this function's second half.
#[must_use]
pub fn theme_options(dirs: &[(PathBuf, bool)]) -> Vec<ThemeOption> {
    let mut options: Vec<ThemeOption> = list_themes_from_dirs(dirs)
        .into_iter()
        .map(|meta| {
            let contrast = load_theme(dirs, &meta.id)
                .map_or(ContrastTier::Standard, |theme| ContrastTier::of(&theme));
            ThemeOption {
                variant: meta.kind(),
                contrast,
                id: meta.id,
                name: meta.name,
            }
        })
        .collect();

    order_theme_options(&mut options);
    options
}

/// Put an already-collected set into the order a picker offers them in.
///
/// [`theme_options`]' second half, reachable on its own because not every host
/// resolves its themes by scanning a directory. audiofiles embeds its shipped
/// set at compile time and reads only its custom themes off disk, so a
/// directory scan cannot see most of what it offers, and the alternative to
/// this being public was that app re-deriving the sort — which is exactly the
/// three-apps-three-orders state the picker was described to end.
///
/// The order is by variant in [`Variant`]'s own order, then by measured
/// contrast **best first**, then by name.
pub fn order_theme_options(options: &mut [ThemeOption]) {
    options.sort_by(|a, b| {
        variant_order(a.variant)
            .cmp(&variant_order(b.variant))
            .then(b.contrast.cmp(&a.contrast))
            .then_with(|| a.name.cmp(&b.name))
    });
}

/// Where a variant sits in a picker, light first.
///
/// Not `Variant as usize`: the declaration order of an enum is not a promise
/// about how it reads, and a member inserted for a fourth variant would
/// silently reorder every picker in the tree.
const fn variant_order(variant: Variant) -> u8 {
    match variant {
        Variant::Light => 0,
        Variant::Dark => 1,
        Variant::HighContrast => 2,
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::bundled_themes_dir;
    use std::collections::HashMap;

    fn meta(id: &str, variant: &str) -> ThemeMeta {
        ThemeMeta {
            id: id.to_string(),
            name: id.to_string(),
            variant: variant.to_string(),
            is_custom: false,
        }
    }

    fn defaults() -> ThemeDefaults {
        ThemeDefaults::new("flatwhite", "nord")
    }

    // The three the shipped themes actually declare.
    #[test]
    fn every_shipped_variant_parses() {
        assert_eq!(Variant::parse("light"), Some(Variant::Light));
        assert_eq!(Variant::parse("dark"), Some(Variant::Dark));
        assert_eq!(Variant::parse("high-contrast"), Some(Variant::HighContrast));
        assert_eq!(Variant::parse("sepia"), None);
    }

    // parse_meta already defaults a *missing* variant to dark, so an
    // unrecognized one reading as light would have the crate disagreeing with
    // itself. alloy_tui did exactly that before this existed.
    #[test]
    fn an_unrecognized_variant_reads_the_way_a_missing_one_does() {
        assert_eq!(Variant::from("sepia"), Variant::Dark);
        assert_eq!(Variant::from(""), Variant::Dark);

        let missing: toml::Table = "[meta]\nname = \"X\"\n".parse().unwrap();
        assert_eq!(parse_meta("x", &missing, false).kind(), Variant::Dark);
    }

    #[test]
    fn a_selection_round_trips_through_any_store() {
        for (stored, expect) in [
            (Some("system"), ThemeSelection::Follow),
            (None, ThemeSelection::Follow),
            (Some(""), ThemeSelection::Follow),
            (Some("  "), ThemeSelection::Follow),
            (Some("nord"), ThemeSelection::Fixed("nord".into())),
        ] {
            let parsed = ThemeSelection::parse(stored);
            assert_eq!(parsed, expect, "{stored:?}");
            assert_eq!(
                ThemeSelection::parse(Some(parsed.as_str())),
                expect,
                "what is written reads back as what was meant",
            );
        }
    }

    // Nothing saved is follow-the-system, which is what Balanced Breakfast
    // expressed as an absent value and GoingsOn as a sentinel. Both are now the
    // same thing.
    #[test]
    fn nothing_chosen_yet_is_follow() {
        assert_eq!(ThemeSelection::default(), ThemeSelection::Follow);
    }

    #[test]
    fn a_fixed_selection_wins_when_its_theme_is_installed() {
        let available = [meta("nord", "dark"), meta("flatwhite", "light")];
        let fixed = ThemeSelection::Fixed("nord".into());
        assert_eq!(
            fixed.resolve(Variant::Light, &defaults(), &available),
            "nord",
            "a chosen theme is not overridden by the ambient mode",
        );
    }

    // Themes are deletable in three of the four apps. Handing back an id that
    // will fail to load only moves the error somewhere less helpful.
    #[test]
    fn a_fixed_selection_whose_theme_is_gone_falls_back() {
        let available = [meta("nord", "dark"), meta("flatwhite", "light")];
        let fixed = ThemeSelection::Fixed("deleted".into());
        assert_eq!(
            fixed.resolve(Variant::Light, &defaults(), &available),
            "flatwhite",
        );
    }

    #[test]
    fn follow_picks_the_apps_default_for_the_ambient_mode() {
        let available = [meta("nord", "dark"), meta("flatwhite", "light")];
        let follow = ThemeSelection::Follow;
        assert_eq!(
            follow.resolve(Variant::Dark, &defaults(), &available),
            "nord",
        );
        assert_eq!(
            follow.resolve(Variant::Light, &defaults(), &available),
            "flatwhite",
        );
    }

    // The behaviour Balanced Breakfast could not have: following the system
    // into a theme the user installed, when the app's own default is absent.
    #[test]
    fn follow_uses_any_installed_theme_of_the_right_variant() {
        let available = [meta("solarized-light", "light"), meta("mine", "dark")];
        assert_eq!(
            ThemeSelection::Follow.resolve(Variant::Dark, &defaults(), &available),
            "mine",
            "the app's `nord` is not installed, but a dark theme is",
        );
    }

    // Always returns something: an app with no theme directory gets the id it
    // ships with, and the load error it would have had anyway.
    #[test]
    fn an_empty_catalog_still_names_the_apps_default() {
        assert_eq!(
            ThemeSelection::Follow.resolve(Variant::Dark, &defaults(), &[]),
            "nord",
        );
    }

    #[test]
    fn high_contrast_falls_back_to_dark_unless_named() {
        let plain = defaults();
        assert_eq!(plain.for_variant(Variant::HighContrast), "nord");

        let named = defaults().high_contrast("sharp");
        assert_eq!(named.for_variant(Variant::HighContrast), "sharp");
    }

    #[test]
    fn theme_options_groups_by_variant_light_first() {
        let dirs = vec![(bundled_themes_dir().unwrap(), false)];
        let options = theme_options(&dirs);
        assert!(!options.is_empty(), "the shipped set is not empty");

        let order: Vec<u8> = options.iter().map(|o| variant_order(o.variant)).collect();
        let mut sorted = order.clone();
        sorted.sort_unstable();
        assert_eq!(
            order, sorted,
            "every variant should occupy one run, light first"
        );
    }

    #[test]
    fn theme_options_puts_the_most_legible_theme_first_in_its_group() {
        let dirs = vec![(bundled_themes_dir().unwrap(), false)];
        let options = theme_options(&dirs);

        for pair in options.windows(2) {
            let (a, b) = (&pair[0], &pair[1]);
            if a.variant != b.variant {
                continue;
            }
            assert!(
                a.contrast >= b.contrast,
                "within {}, {} ({:?}) should not follow {} ({:?})",
                a.variant,
                b.id,
                b.contrast,
                a.id,
                a.contrast
            );
            if a.contrast == b.contrast {
                assert!(
                    a.name <= b.name,
                    "ties break by name: {} then {}",
                    a.name,
                    b.name
                );
            }
        }
    }

    #[test]
    fn theme_options_carries_every_theme_the_scan_found() {
        let dirs = vec![(bundled_themes_dir().unwrap(), false)];
        let mut scanned: Vec<String> = list_themes_from_dirs(&dirs)
            .into_iter()
            .map(|meta| meta.id)
            .collect();
        let mut offered: Vec<String> = theme_options(&dirs).into_iter().map(|o| o.id).collect();
        scanned.sort();
        offered.sort();
        assert_eq!(scanned, offered, "ordering must not drop a theme");
    }

    #[test]
    fn a_theme_that_cannot_be_measured_reads_as_standard() {
        // Not Low: a missing ground is the scan failing, and badging the theme
        // for that would tell the reader something untrue about the theme.
        let theme = ThemeColors {
            meta: ThemeMeta {
                id: "unmeasurable".to_string(),
                name: "Unmeasurable".to_string(),
                variant: "dark".to_string(),
                is_custom: false,
            },
            colors: HashMap::new(),
        };
        assert_eq!(ContrastTier::of(&theme), ContrastTier::Standard);
    }

    #[test]
    fn the_house_themes_measure_high() {
        // The two we author. A change that drops either below AA is a
        // regression in a theme we control.
        //
        // `high-contrast` is deliberately not in this list. It measures
        // 4.89/3.53 and therefore reads as Standard: its muted text misses AA
        // on its own sunken panel. That is a finding about the theme file, not
        // about the measurement, and it is filed rather than asserted away.
        let dirs = vec![(bundled_themes_dir().unwrap(), false)];
        for id in ["goingson", "audiofiles"] {
            let theme = load_theme(&dirs, id).expect("shipped");
            assert_eq!(
                ContrastTier::of(&theme),
                ContrastTier::High,
                "{id} is one of ours and should meet AA on both grounds"
            );
        }
    }

    #[test]
    fn contrast_tiers_order_worst_first() {
        assert!(ContrastTier::Low < ContrastTier::Standard);
        assert!(ContrastTier::Standard < ContrastTier::High);
    }
}