//! Parameter descriptors — operations described as data (ARCH §3.3).
//!
//! The core never builds a control. It publishes what its parameters *are*,
//! and `dr-ui` maps each `ParamKind` to a widget appropriate to the current
//! input modality (ARCH §4.3). Adding an operation therefore needs no UI
//! change (FR-DEV-3c).
//!
//! Labels are keys, not strings: resolving them needs a localiser, and
//! `core/` must not depend on one (NFR-A11Y-1).

use std::fmt;

/// Identifies a parameter within an operation.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, PartialOrd, Ord)]
pub struct ParamId(pub &'static str);

impl fmt::Display for ParamId {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.write_str(self.0)
    }
}

/// Identifies an operation kind.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, PartialOrd, Ord)]
pub struct OpId(pub &'static str);

impl fmt::Display for OpId {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.write_str(self.0)
    }
}

/// A localisation key. The UI resolves it; the core never sees the string.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub struct LocalizedKey(pub &'static str);

/// What a slider's travel means.
///
/// Photographic controls are rarely linear in their underlying quantity:
/// exposure is linear in stops but exponential in light, and a temperature
/// slider that is linear in kelvin feels wrong at both ends.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum Scale {
    Linear,
    /// Even *perceptual* steps across the range, for controls whose effect
    /// concentrates near one end.
    Perceptual,
}

/// The unit a value carries, for display.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum Unit {
    None,
    /// Exposure value — photographers think in stops, not multipliers.
    Stops,
    Kelvin,
    Percent,
}

/// A control that does not reduce to a slider or a switch.
///
/// The core names the *kind* of widget; `dr-widgets` owns what it looks like
/// and how it behaves (ARCH §3.3). This is deliberately a small closed
/// enum rather than an open string: a UI must be able to match exhaustively
/// and know it has covered everything the core can ask for.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum WidgetKind {
    /// A tone curve, edited by dragging points on a grid.
    ///
    /// The underlying parameters are ordinary [`ParamKind::Scalar`]s — the
    /// point coordinates — so a UI that does not implement the curve widget
    /// can still present them as sliders and remain fully functional. That
    /// fallback is the reason the points are scalars rather than an opaque
    /// blob.
    Curve,
}

/// The shape of a parameter's value.
#[derive(Debug, Clone, PartialEq)]
pub enum ParamKind {
    Scalar {
        min: f32,
        max: f32,
        scale: Scale,
        unit: Unit,
        precision: u8,
    },
    Bool,
}

/// TRACES: FR-DEV-3a | FR-DEV-3b
/// How an operation would like its parameters presented.
///
/// A *hint*, never a requirement. An operation's parameters are always
/// individually addressable scalars; this only says that several of them
/// form one conceptual control, and which widget draws it best. A UI is free
/// to ignore it entirely and render plain sliders — the edit still works, it
/// is merely more tedious.
///
/// Sitting on the operation rather than on a parameter is what allows a
/// widget to span several parameters, which a curve necessarily does.
///
/// Declared through [`crate::Operation::presentation`] — a defaulted trait
/// method rather than a field on [`OpDescriptor`], so the great majority of
/// operations, which want plain sliders, say nothing at all.
#[derive(Debug, Clone, PartialEq)]
pub struct Presentation {
    pub widget: WidgetKind,
    /// The parameters this widget owns, in the order it expects them.
    ///
    /// Parameters absent from this list are presented normally, so an
    /// operation can pair a curve with an ordinary strength slider.
    pub params: &'static [ParamId],
}

/// One parameter of an operation.
#[derive(Debug, Clone, PartialEq)]
pub struct ParamDescriptor {
    pub id: ParamId,
    pub label: LocalizedKey,
    pub kind: ParamKind,
    pub default: f32,
}

impl ParamDescriptor {
    /// A scalar in stops — the exposure-like controls.
    pub const fn stops(id: &'static str, label: &'static str, min: f32, max: f32) -> Self {
        Self {
            id: ParamId(id),
            label: LocalizedKey(label),
            kind: ParamKind::Scalar {
                min,
                max,
                scale: Scale::Linear,
                unit: Unit::Stops,
                precision: 2,
            },
            default: 0.0,
        }
    }

    /// A symmetric −100…+100 control, the familiar shape for tone and colour
    /// adjustments. Neutral at zero, so a double-tap reset is meaningful.
    pub const fn amount(id: &'static str, label: &'static str) -> Self {
        Self {
            id: ParamId(id),
            label: LocalizedKey(label),
            kind: ParamKind::Scalar {
                min: -100.0,
                max: 100.0,
                scale: Scale::Linear,
                unit: Unit::None,
                precision: 0,
            },
            default: 0.0,
        }
    }

    /// A toggle. Neutral when off, so the reset contract still holds.
    pub const fn switch(id: &'static str, label: &'static str) -> Self {
        Self {
            id: ParamId(id),
            label: LocalizedKey(label),
            kind: ParamKind::Bool,
            default: 0.0,
        }
    }

    /// A 0…1 fraction — a proportion of something, rather than an amount.
    ///
    /// Its own constructor because the crop rect needs four of them and the
    /// default differs per edge: an origin starts at 0 and an extent at 1.
    /// Precision of 4 because at 6000px a step of 0.0001 is under a pixel,
    /// and a coarser one would make a crop edge unplaceable.
    pub const fn fraction(id: &'static str, label: &'static str, default: f32) -> Self {
        Self {
            id: ParamId(id),
            label: LocalizedKey(label),
            kind: ParamKind::Scalar {
                min: 0.0,
                max: 1.0,
                scale: Scale::Linear,
                unit: Unit::None,
                precision: 4,
            },
            default,
        }
    }

    /// A general scalar with an explicit range and default.
    //
    // Eight arguments, and a builder would be the usual answer — but this has
    // to be `const` so descriptors can be `static`, and `const` functions
    // cannot use a builder's method chain. The two common shapes have their
    // own constructors above; this is the escape hatch for the rest.
    #[allow(clippy::too_many_arguments)]
    pub const fn scalar(
        id: &'static str,
        label: &'static str,
        min: f32,
        max: f32,
        default: f32,
        unit: Unit,
        scale: Scale,
        precision: u8,
    ) -> Self {
        Self {
            id: ParamId(id),
            label: LocalizedKey(label),
            kind: ParamKind::Scalar {
                min,
                max,
                scale,
                unit,
                precision,
            },
            default,
        }
    }

    /// Clamp a value into this parameter's declared range.
    ///
    /// Applied before the value reaches a shader: a slider dragged past its
    /// bounds, or a sidecar written by a newer version with a wider range,
    /// must not produce out-of-range uniforms.
    pub fn clamp(&self, value: f32) -> f32 {
        match self.kind {
            ParamKind::Scalar { min, max, .. } => {
                if value.is_finite() {
                    value.clamp(min, max)
                } else {
                    // A NaN from a corrupt sidecar would otherwise poison the
                    // uniform block and blank the image.
                    self.default
                }
            }
            ParamKind::Bool => {
                if value != 0.0 {
                    1.0
                } else {
                    0.0
                }
            }
        }
    }
}

/// The static description of an operation.
#[derive(Debug, Clone, PartialEq)]
pub struct OpDescriptor {
    pub id: OpId,
    pub label: LocalizedKey,
    pub params: &'static [ParamDescriptor],
}

impl OpDescriptor {
    pub fn param(&self, id: ParamId) -> Option<&ParamDescriptor> {
        self.params.iter().find(|p| p.id == id)
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    const P: ParamDescriptor = ParamDescriptor::amount("test", "test.label");

    #[test]
    fn values_clamp_into_range() {
        assert_eq!(P.clamp(150.0), 100.0);
        assert_eq!(P.clamp(-150.0), -100.0);
        assert_eq!(P.clamp(42.0), 42.0);
    }

    #[test]
    fn a_nan_falls_back_to_the_default_rather_than_poisoning_the_uniform() {
        // A corrupt sidecar must not blank the image: one NaN in a uniform
        // block propagates through every pixel.
        assert_eq!(P.clamp(f32::NAN), P.default);
        assert_eq!(P.clamp(f32::INFINITY), P.default);
    }

    #[test]
    fn amount_controls_are_neutral_at_zero() {
        // Double-tap-to-reset and "is this op doing anything" both depend on
        // neutral being zero.
        assert_eq!(P.default, 0.0);
    }
}