lanework/Kanban/UI/Board/BoardMetrics.swift

import AppKit
import CoreGraphics

/// The board strip's geometry — **derived from font metrics, never written down in points**
/// (10-accessibility.md ▸ Text scaling & visual accommodations: "relative text styles everywhere, no
/// fixed point sizes. Card face, lane header, and masonry metrics derive from font metrics, so
/// layout survives the largest system text sizes").
///
/// This is `CardWindowMetrics`' twin on the board side, and deliberately the same shape: a pure
/// arithmetic surface parameterised on the body font's point size, plus one impure read of what that
/// point size currently is. Everything the strip draws that is not a piece of text — the inter-lane
/// gap, a lane's plate inset, a card's corner radius and stripe, the masonry's card spacing, the
/// trash column's header — is a multiple of that size, so the board grows with the system text size
/// instead of squeezing text into chrome sized for 13pt.
///
/// ### Why a point size and not a `Font`
///
/// Because layout takes numbers. SwiftUI's relative text styles handle the *text* (and every `Text`
/// on the board wears one — `.body`, `.headline`, `.caption`); what they cannot do is tell an
/// `.padding()` how much room the text will need. So the two halves of "full relative scaling" are
/// split by mechanism: type styles for glyphs, this surface for everything between them, both keyed
/// to the same system font.
///
/// ### The em, and what the multiples mean
///
/// Every figure below is a multiple of the body point size — an *em*, roughly — chosen so that at
/// the standard 13pt system body font it reproduces the numbers the board already drew. That is
/// deliberate: this milestone is meant to make the board *scale*, not to redesign it, so the default
/// text size must render pixel-for-pixel what it rendered before. The multiples are what carries the
/// design to 18pt, 24pt and beyond.
///
/// Results are rounded to whole points (SwiftUI will happily lay out on halves, but a hairline
/// border on a half-point boundary blurs) and floored at 1 for anything that is a width or a height,
/// so no proposal is ever zero or negative.
///
/// ### The no-horizontal-scroll invariant is untouched
///
/// 03-board-ui.md § Layout — full visibility divides the window's width across the lanes' width
/// units, and `LaneLayoutMath` takes the gap as a parameter. A larger text size therefore means a
/// larger gap and *narrower* lanes, never a wider strip: "the degenerate case is accepted, not
/// floored … titles and cards truncate gracefully". The truncation rules are the views' own
/// (`lineLimit(1)` + `.tail` on a lane title, `lineLimit(4)` on a card title), and they hold at every
/// scale because they are stated in lines rather than in points.
enum BoardMetrics {

    // MARK: - The unit

    /// `multiple` ems of the body font, rounded to a whole point and floored at one.
    ///
    /// Floored rather than clamped to zero because every caller is a length: a spacing of zero is a
    /// legitimate design choice, but none of the figures below is one, and a rounding that produced
    /// zero would silently collapse a stripe or a band rather than shrink it.
    static func em(_ multiple: CGFloat, bodyPointSize: CGFloat) -> CGFloat {
        max(1, (bodyPointSize * multiple).rounded())
    }

    // MARK: - The strip

    /// The inter-lane gap **and** the strip's outer margin — one number, because the standard-width
    /// formula counts `units + 1` of them (`LaneLayoutMath.standardWidth`).
    ///
    /// 0.9 em: 12pt at the standard 13pt body, which is what the strip has always drawn.
    static func stripGap(bodyPointSize: CGFloat) -> CGFloat {
        em(0.9, bodyPointSize: bodyPointSize)
    }

    // MARK: - The lane

    /// The lane plate's corner radius — shared by the selection treatment and the accent band, whose
    /// top corners round to exactly this so the band reads as the lane's own edge.
    static func laneCornerRadius(bodyPointSize: CGFloat) -> CGFloat {
        em(0.75, bodyPointSize: bodyPointSize)
    }

    /// The lane plate's inset around its header and its masonry.
    static func lanePlatePadding(bodyPointSize: CGFloat) -> CGFloat {
        em(0.45, bodyPointSize: bodyPointSize)
    }

    /// Between the lane's header and its card stack.
    static func laneStackSpacing(bodyPointSize: CGFloat) -> CGFloat {
        em(0.6, bodyPointSize: bodyPointSize)
    }

    /// Between the header's glyph, its title and its count badge.
    static func laneHeaderSpacing(bodyPointSize: CGFloat) -> CGFloat {
        em(0.45, bodyPointSize: bodyPointSize)
    }

    /// The header row's own horizontal inset inside the plate.
    static func laneHeaderInset(bodyPointSize: CGFloat) -> CGFloat {
        em(0.3, bodyPointSize: bodyPointSize)
    }

    /// C7 · full-column top edge (03-board-ui.md § Styling ▸ Capabilities) — the lane accent band's
    /// height.
    static func laneAccentBandHeight(bodyPointSize: CGFloat) -> CGFloat {
        em(0.4, bodyPointSize: bodyPointSize)
    }

    /// The trailing room the header reserves for the new-card button, so **a long title truncates
    /// before it collides with the button** rather than running under it.
    ///
    /// This is the one figure that is not decoration: the button is an `Image` at
    /// `.imageScale(.small)`, which *is* a relative size, so a reserve fixed at 22pt would be
    /// overrun by the glyph itself at a large system text size and the truncation rule would stop
    /// being true. 1.7 em is the button plus its breathing room, measured in the same unit the glyph
    /// grows in.
    static func newCardButtonReserve(bodyPointSize: CGFloat) -> CGFloat {
        em(1.7, bodyPointSize: bodyPointSize)
    }

    /// The count badge's capsule inset — horizontal and vertical, which are deliberately different:
    /// a capsule around a single digit wants width, not height.
    static func badgeHorizontalPadding(bodyPointSize: CGFloat) -> CGFloat {
        em(0.45, bodyPointSize: bodyPointSize)
    }

    static func badgeVerticalPadding(bodyPointSize: CGFloat) -> CGFloat {
        em(0.08, bodyPointSize: bodyPointSize)
    }

    // MARK: - The card face

    /// Shared by the plate, the accent stripe and the selection stroke, so the stripe reads as part
    /// of the card's edge rather than a bar laid over it.
    static func cardCornerRadius(bodyPointSize: CGFloat) -> CGFloat {
        em(0.6, bodyPointSize: bodyPointSize)
    }

    /// K1 · left edge stripe (03-board-ui.md § Styling ▸ Capabilities). Reserved as padding whether
    /// or not a stripe paints, so colouring a card never shifts its title.
    static func cardStripeWidth(bodyPointSize: CGFloat) -> CGFloat {
        em(0.3, bodyPointSize: bodyPointSize)
    }

    /// The card plate's inset around its content.
    static func cardContentPadding(bodyPointSize: CGFloat) -> CGFloat {
        em(0.75, bodyPointSize: bodyPointSize)
    }

    /// Between the icon, the title and the attachments chip.
    static func cardRowSpacing(bodyPointSize: CGFloat) -> CGFloat {
        em(0.45, bodyPointSize: bodyPointSize)
    }

    /// The masonry's spacing — between interior columns and between stacked cards within a column.
    ///
    /// The lane registers this into `LaneDropRegistry.Grid`, so the drop model's analytic resting
    /// grid replays the same number the layout drew with and no second derivation exists to drift
    /// (DRAG-REORDER.md § The card masonry).
    static func cardSpacing(bodyPointSize: CGFloat) -> CGFloat {
        em(0.6, bodyPointSize: bodyPointSize)
    }

    /// The height a card with no registered measurement is assumed to have — a lane whose faces have
    /// not laid out yet, and the shadow a Finder file drop opens for a card that does not exist.
    ///
    /// 3.4 em: one body line of title inside two content paddings, plus the plate's own rhythm.
    /// Nominal rather than zero, so the resting rows still tile.
    static func nominalCardHeight(bodyPointSize: CGFloat) -> CGFloat {
        em(3.4, bodyPointSize: bodyPointSize)
    }

    // MARK: - The drag replica

    /// The width the card drag's replica is drawn at — a card at a representative lane width, since
    /// the image under the cursor has no lane to measure itself against.
    static func cardReplicaWidth(bodyPointSize: CGFloat) -> CGFloat {
        em(17, bodyPointSize: bodyPointSize)
    }

    /// The transparent margin around a drag replica, which is what keeps its shadow from being
    /// clipped by the drag image's bounds.
    static func replicaPadding(bodyPointSize: CGFloat) -> CGFloat {
        em(0.9, bodyPointSize: bodyPointSize)
    }

    /// The lane replica's floor dimensions, for the frame a lane that has not measured itself yet
    /// would otherwise be drawn at.
    static func laneReplicaMinimumWidth(bodyPointSize: CGFloat) -> CGFloat {
        em(6, bodyPointSize: bodyPointSize)
    }

    static func laneReplicaMinimumHeight(bodyPointSize: CGFloat) -> CGFloat {
        em(9, bodyPointSize: bodyPointSize)
    }

    // MARK: - The lane resize handle

    /// The invisible grab strip at a lane's trailing edge, and how far right it is shifted so most
    /// of it hangs into the inter-lane gap rather than sitting over the lane's own scrollbar
    /// (`LaneResizeHandle`). Both scale, because the gap they live in does.
    static func resizeHandleWidth(bodyPointSize: CGFloat) -> CGFloat {
        em(0.9, bodyPointSize: bodyPointSize)
    }

    static func resizeHandleOverhang(bodyPointSize: CGFloat) -> CGFloat {
        em(0.6, bodyPointSize: bodyPointSize)
    }

    // MARK: - The trash column

    /// The trash header's inset — horizontal and vertical (03-board-ui.md § Trash ▸ Rendering).
    static func trashHeaderHorizontalPadding(bodyPointSize: CGFloat) -> CGFloat {
        em(0.75, bodyPointSize: bodyPointSize)
    }

    static func trashHeaderVerticalPadding(bodyPointSize: CGFloat) -> CGFloat {
        em(0.6, bodyPointSize: bodyPointSize)
    }

    /// The gap between the trash header's diagonal hatch strokes.
    ///
    /// It scales for a reason the other figures do not share: the hatch is **the trash's non-colour
    /// distinction** (10-accessibility.md's never-colour-alone rule — "the trash header is hatched
    /// plus labeled"), and a fixed 7pt pitch behind text twice its usual size reads as a texture
    /// rather than as hatching.
    static func trashHatchSpacing(bodyPointSize: CGFloat) -> CGFloat {
        em(0.55, bodyPointSize: bodyPointSize)
    }

    // MARK: - The board window

    /// The board window's minimum content size — two standard lanes' worth of width and enough
    /// height for a header and a few cards. Derived so a large system text size cannot leave the
    /// window smaller than one lane's own header.
    static func windowMinimumSize(bodyPointSize: CGFloat) -> CGSize {
        CGSize(
            width: em(49, bodyPointSize: bodyPointSize),
            height: em(31, bodyPointSize: bodyPointSize)
        )
    }

    // MARK: - The live metric

    /// The body font's point size as the system currently reports it.
    ///
    /// **The app asks the system exactly once, in `CardWindowMetrics.bodyPointSize`**, and this
    /// forwards to it: the board and the card window must agree about what "the body font" is, or a
    /// card face and the window it opens into would scale on two different rulers.
    @MainActor
    static var bodyPointSize: CGFloat {
        CardWindowMetrics.bodyPointSize
    }
}