//! The colour theme for a request. A signed-in user's stored preference wins. Anonymous
//! visitors use the `forge_theme` cookie, which the footer form sets. The default is light: a
//! visitor with no explicit choice gets light whatever their operating system uses. Only an
//! explicit `auto` renders no `data-theme`, so the stylesheet follows the operating system.

use std::time::Duration;

use forge_core::User;
use topcoat::context::Cx;
use topcoat::cookie::{Cookie, Cookies, SameSite, cookies, time};

/// Name of the anonymous theme cookie.
pub const THEME_COOKIE: &str = "forge_theme";

/// Lifetime of the anonymous theme cookie.
pub const THEME_COOKIE_TTL: Duration = Duration::from_secs(365 * 24 * 60 * 60);

/// The stored colour preference for this request: `"auto"`, `"light"` or `"dark"`. A signed-in
/// user's stored preference wins. Anonymous visitors use the cookie. A missing, empty or
/// unknown value reads as `"light"`, the default. Only an explicit `"auto"` follows the system.
pub fn preference(cx: &Cx, viewer: Option<&User>) -> &'static str {
    let stored = match viewer {
        Some(user) => user.theme.clone(),
        None => cookies(cx)
            .get(THEME_COOKIE)
            .map(|cookie| cookie.value().to_owned())
            .unwrap_or_default(),
    };
    resolve_preference(&stored)
}

/// Maps a stored value to a preference. Only `"auto"` and `"dark"` are kept as they are. Every
/// other value, including the empty string, means the default, `"light"`.
pub fn resolve_preference(stored: &str) -> &'static str {
    match stored {
        "auto" => "auto",
        "dark" => "dark",
        _ => "light",
    }
}

/// The `data-theme` value for this request: `Some("light")` or `Some("dark")`, or `None` when the
/// page should follow the operating system.
pub fn data_theme(cx: &Cx, viewer: Option<&User>) -> Option<&'static str> {
    match preference(cx, viewer) {
        "auto" => None,
        theme => Some(theme),
    }
}

/// The preference the top-bar button switches to from `current`: light, then dark, then auto,
/// then back to light. Anything that is not `"light"` or `"dark"` (including `"auto"`) moves on
/// to light, the first step of the cycle.
pub fn next_preference(current: &str) -> &'static str {
    match current {
        "light" => "dark",
        "dark" => "auto",
        _ => "light",
    }
}

/// Sets the anonymous theme cookie. The caller has already checked that `theme` is valid.
pub fn set_theme_cookie(cx: &Cx, theme: &str) {
    let max_age =
        time::Duration::seconds(i64::try_from(THEME_COOKIE_TTL.as_secs()).unwrap_or(i64::MAX));
    cookies(cx).add(
        Cookie::build((THEME_COOKIE, theme.to_owned()))
            .path("/")
            .secure(true)
            .http_only(true)
            .same_site(SameSite::Lax)
            .max_age(max_age)
            .build(),
    );
}

#[cfg(test)]
mod tests {
    use super::{next_preference, resolve_preference};

    #[test]
    fn the_toggle_cycles_light_dark_auto_light() {
        assert_eq!(next_preference("light"), "dark");
        assert_eq!(next_preference("dark"), "auto");
        assert_eq!(next_preference("auto"), "light");
    }

    #[test]
    fn unknown_preferences_move_on_to_light() {
        assert_eq!(next_preference(""), "light");
        assert_eq!(next_preference("neon"), "light");
    }

    #[test]
    fn no_choice_means_light() {
        assert_eq!(resolve_preference(""), "light");
        assert_eq!(resolve_preference("neon"), "light");
        assert_eq!(resolve_preference("light"), "light");
    }

    #[test]
    fn explicit_choices_are_kept() {
        assert_eq!(resolve_preference("dark"), "dark");
        assert_eq!(resolve_preference("auto"), "auto");
    }

    #[test]
    fn the_default_cycle_starts_on_light_and_reaches_every_choice() {
        let start = resolve_preference("");
        assert_eq!(start, "light");
        assert_eq!(next_preference(start), "dark");
        assert_eq!(next_preference(next_preference(start)), "auto");
        assert_eq!(
            next_preference(next_preference(next_preference(start))),
            "light"
        );
    }
}