use crate::Nesting;

// Names this module's prose links to, resolved for rustdoc.
#[allow(unused_imports)]
use crate::{Awaiting, Choice, Column, Field, FieldKind};

/// A named dimension a set can be narrowed by.
///
/// One word for six things that were six mechanisms. MNW's discover page filters
/// by free text, a flat any-of over item types, a tree of tags, a numeric range
/// over price, a nested one-of over AI tier, and a browse position in the tag
/// tree held separately from the tag selection — and the last two being separate
/// is the whole reason a filter row there needs a tick box *and* a chevron. The
/// panel is a mixed bag of hand-written controls because nothing named the thing
/// they all are.
///
/// Deliberately wider than that one page. audiofiles' library browser and
/// goingson's filters are the same shape, and a word that only fitted discover
/// would be discover's markup with a neutral name on it.
///
/// # What it does not say
///
/// **What picking a value calls.** This crate names no address, so a facet is
/// paired with routes the way a column's [`sortable`](Column::sortable) flag is
/// paired with what reordering calls.
///
/// **How a tree is drawn.** Indented rows, a column of panes, a breadcrumb and a
/// list: all four are honest renderings of the same described facet, and a
/// terminal will not pick the same one a browser does. [`FacetValue::depth`] is
/// what a renderer needs to draw any of them; the choice is not described.
///
/// **Which values to show.** A tag tree has thousands of nodes and a panel shows
/// a handful. Deciding which handful is the app's — it is the same question as
/// which rows go in a table, and no table member answers it either.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
#[non_exhaustive]
pub struct Facet<'a> {
    /// What the dimension is called, as the user reads it.
    pub name: &'a str,
    /// How many of its values may be in force, and in what shape.
    pub mode: Selecting,
    /// The values on offer, in the order they are drawn.
    ///
    /// A [`Selecting::Text`] facet has none: the value is whatever was typed,
    /// and a description that listed the possible strings would be listing the
    /// corpus. A [`Selecting::Range`] facet has none either, for the reason
    /// [`FieldKind::Range`] takes bounds rather than options — the ends are the
    /// question and the values between them are not enumerable.
    pub values: &'a [FacetValue<'a>],
}

impl<'a> Facet<'a> {
    /// A dimension with values to pick from.
    #[must_use]
    pub const fn new(name: &'a str, mode: Selecting, values: &'a [FacetValue<'a>]) -> Self {
        Self { name, mode, values }
    }

    /// Whether the facet is narrowing the set right now.
    ///
    /// The question a renderer asks to decide whether to offer a way out of it,
    /// and the reason it is derived rather than carried: a facet with nothing
    /// standing is unengaged by construction, so a member saying so could
    /// disagree with the values beside it. [`Standing::Inherited`] does not
    /// count — something further up is what is doing the narrowing, and clearing
    /// a child that was never picked clears nothing.
    ///
    /// Always false for [`Selecting::Text`] and [`Selecting::Range`], which
    /// carry no values. A host that wants a clear affordance on those knows
    /// whether its own box is empty; the description does not hold the typed
    /// string.
    #[must_use]
    pub fn engaged(&self) -> bool {
        self.values.iter().any(|value| value.standing.is_picked())
    }

    /// The deepest value in the facet, or zero when it is flat.
    ///
    /// What an indenting renderer needs to reserve a gutter before it draws the
    /// first row, which is "First paint is final paint" applied to a tree: a
    /// gutter widened as deeper values arrive is the reflow that rule forbids.
    #[must_use]
    pub fn reach(&self) -> u8 {
        self.values
            .iter()
            .map(|value| value.depth.level)
            .max()
            .unwrap_or(0)
    }
}

/// How many of a [`Facet`]'s values may be in force, and in what shape.
///
/// Five, and the fifth is what made this an enum rather than a bool. `one-of`,
/// `any-of`, a range and free text are the four a form vocabulary already has in
/// [`FieldKind`]; a tree's selection is none of them, and describing tags as
/// any-of was what forced browsing to be a second mechanism beside filtering.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
#[non_exhaustive]
pub enum Selecting {
    /// Exactly one value, and picking another replaces it.
    ///
    /// MNW's AI tier, whose three options are nested ranges rather than
    /// independent values, so two of them at once means nothing.
    OneOf,
    /// Any number of values, each independent of the others.
    AnyOf,
    /// A low end, a high end, or both.
    ///
    /// Carries no values for [`FieldKind::Range`]'s reason: the ends are the
    /// question.
    Range,
    /// Whatever the user types.
    Text,
    /// A position in a tree, edited by taking branches in and pruning branches
    /// out.
    ///
    /// The one mode that is not reducible to the others, and the one gesture
    /// that replaced two. Picking a value narrows the set to it *and* reveals
    /// its children, so browsing a tree and filtering by it stop being separate
    /// mechanisms with separate state. What a selection then is: a set of
    /// branches taken and a set pruned, resolved nearest-ancestor-first, so
    /// `music` in and `music/synths` out is sayable and no flat mode can say it.
    ///
    /// Resolution happens in the app, and what reaches a renderer is the
    /// [`Standing`] each drawn value ended up with. A renderer walking ancestors
    /// itself would be a renderer that can disagree with the results beside it.
    Subtree,
}

impl Selecting {
    /// Whether the mode picks from values the description lists.
    ///
    /// False for [`Text`](Self::Text) and [`Range`](Self::Range), which are the
    /// two whose answer is not one of a set. A renderer asks this before it
    /// looks at [`Facet::values`], the way it asks
    /// [`FieldKind::offers_options`] before it looks at [`Field::options`].
    #[must_use]
    pub const fn offers_values(self) -> bool {
        matches!(self, Self::OneOf | Self::AnyOf | Self::Subtree)
    }

    /// Whether a value can be pruned as well as picked.
    ///
    /// [`Subtree`](Self::Subtree) alone. Excluding a value from a flat facet is
    /// the same fact as not picking it, so an exclude affordance there would be
    /// a second control for a state the first one already holds.
    #[must_use]
    pub const fn prunes(self) -> bool {
        matches!(self, Self::Subtree)
    }

    /// Whether picking a second value keeps the first.
    #[must_use]
    pub const fn accumulates(self) -> bool {
        matches!(self, Self::AnyOf | Self::Subtree)
    }
}

/// One value a [`Facet`] offers, as it currently stands.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
#[non_exhaustive]
pub struct FacetValue<'a> {
    /// What identifies it, and what a host keys its route on.
    ///
    /// [`Choice::value`]'s split, and a tree is why it is not optional: two
    /// leaves under different parents are legitimately both called "Ambient",
    /// and the path is the only thing telling them apart. It is also what
    /// nearest-ancestor-wins resolves over, so an app that carried only labels
    /// could not compute the [`standing`](Self::standing) it hands back here.
    pub value: &'a str,
    /// What it is called, as the user reads it.
    ///
    /// The leaf's own name rather than its path: a facet drawn as an indented
    /// tree repeats every ancestor on every line otherwise, and one drawn as a
    /// breadcrumb has the ancestors already.
    pub label: &'a str,
    /// How many members of the set carry it.
    ///
    /// Optional, and settled that way rather than made mandatory: a count is a
    /// measured fact the app may not have. Counting a tag subtree under an
    /// active text search is a second query, and an app that will not pay for it
    /// should be able to describe the facet anyway rather than write a zero that
    /// reads as "none of them". That is [`Awaiting::amount`]'s rule in a second
    /// place — state a number when it was measured, and nothing when it was not.
    pub count: Option<u64>,
    /// Whether it is narrowing the set, and how it came to be.
    pub standing: Standing,
    /// How far down the tree it sits, counting from zero at the root.
    ///
    /// Always zero for a flat facet, which is what makes an indenting renderer
    /// one code path rather than two. A renderer that draws no tree at all still
    /// reads this, since a value's depth is what distinguishes two same-named
    /// leaves under different parents.
    pub depth: Nesting,
    /// Whether taking it reveals values under it.
    ///
    /// Distinct from having a nonzero [`depth`](Self::depth): a leaf deep in the
    /// tree branches no further, and a root with children does. Both facts are
    /// needed and neither implies the other, which is why the pair is two
    /// members rather than one count.
    pub branching: bool,
}

impl<'a> FacetValue<'a> {
    /// An unpicked value at the root of the facet.
    #[must_use]
    pub const fn new(value: &'a str, label: &'a str) -> Self {
        Self {
            value,
            label,
            count: None,
            standing: Standing::Open,
            depth: Nesting::top(),
            branching: false,
        }
    }

    /// A value whose identifier is also what the user reads.
    ///
    /// [`Choice::plain`]'s convenience, and it is the flat case: a type or a tier
    /// is its own name, and only a tree needs a path that is not one.
    #[must_use]
    pub const fn of(value: &'a str) -> Self {
        Self::new(value, value)
    }

    /// How many members carry it, when that was measured.
    #[must_use]
    pub const fn counted(mut self, count: u64) -> Self {
        self.count = Some(count);
        self
    }

    /// How it stands in the current selection.
    #[must_use]
    pub const fn standing(mut self, standing: Standing) -> Self {
        self.standing = standing;
        self
    }

    /// Where it sits in the tree, and whether anything hangs off it.
    #[must_use]
    pub const fn at(mut self, depth: Nesting, branching: bool) -> Self {
        self.depth = depth;
        self.branching = branching;
        self
    }
}

/// Whether a [`FacetValue`] is narrowing the set, and how it came to be.
///
/// Four rather than a bool, and the two extra members are what a tree costs. A
/// pruned branch and an untaken one are not the same state — one was decided
/// against and the other was never reached — and a child under a taken parent is
/// in force without anybody having picked it. A renderer given a bool either
/// marks every descendant of a taken branch, which reads as forty deliberate
/// choices, or marks none of them, which reads as unfiltered.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, Default)]
#[non_exhaustive]
pub enum Standing {
    /// Not picked, and nothing above it is either.
    #[default]
    Open,
    /// Picked here. The set is narrowed to it and whatever hangs off it.
    Taken,
    /// In force because something above it was taken.
    Inherited,
    /// Pruned out, though something above it was taken.
    ///
    /// The state that only [`Selecting::Subtree`] can reach, and the reason
    /// exclusion is drawn as a visible affordance beside each label rather than
    /// as a modifier on the ordinary one: a gesture a terminal cannot express is
    /// a gesture half the renderers would have to leave out, and an affordance
    /// nothing teaches is one users do not find.
    Pruned,
}

impl Standing {
    /// Whether the user decided this value, either way.
    ///
    /// True for [`Taken`](Self::Taken) and [`Pruned`](Self::Pruned) — both are
    /// choices, and both are things a "clear this" affordance has to clear.
    /// [`Inherited`](Self::Inherited) is not: clearing it clears nothing,
    /// because the decision is further up.
    #[must_use]
    pub const fn is_picked(self) -> bool {
        matches!(self, Self::Taken | Self::Pruned)
    }

    /// Whether the value narrows the set in.
    ///
    /// [`Taken`](Self::Taken) and [`Inherited`](Self::Inherited): one was picked
    /// and one came down from above, and to the set they mean the same thing.
    /// The pair is named here so a renderer colouring in-force values does not
    /// have to know which is which.
    #[must_use]
    pub const fn in_force(self) -> bool {
        matches!(self, Self::Taken | Self::Inherited)
    }

    /// The content intent the value takes.
    ///
    /// [`Pruned`](Self::Pruned) reads back a step, which is the three-tone rule
    /// above rather than a new decision: a pruned branch is still a live control
    /// — pressing it takes the prune off — so it may not wear `content-muted`,
    /// and it is not the thing itself either.
    #[must_use]
    pub const fn intent(self) -> &'static str {
        match self {
            Self::Taken | Self::Inherited | Self::Open => "content",
            Self::Pruned => "content-secondary",
        }
    }
}