//! Backend-neutral vocabulary and math for pane transitions.
//!
//! The core publishes plain transition data. GUI shells evaluate that data in
//! shaders; the TTY shell evaluates the same progress over cells in its grid.
//! There are deliberately no callbacks or backend objects here.
const std = @import("std");

pub const Easing = enum(u8) {
    linear,
    smooth,
    out_cubic,
    out_back,
};

pub const Transition = enum(u8) {
    // These values cross both GUI shader ABIs. GLSL receives the enum in the
    // instance uvec4 and the Core Image kernel receives it as a float, so
    // spelling the numbers here keeps a source reorder from changing pixels.
    off = 0,
    slide = 1,
    zoom = 2,
    dissolve = 3,
    ascii = 4,
    vertical = 5,

    pub fn easing(effect: Transition) Easing {
        return switch (effect) {
            .off, .dissolve => .smooth,
            .slide, .vertical => .out_cubic,
            .zoom => .out_back,
            .ascii => .linear,
        };
    }

    pub fn frames(effect: Transition) u16 {
        return switch (effect) {
            .off => 0,
            .slide => 12,
            .zoom => 14,
            .dissolve => 10,
            .ascii => 16,
            .vertical => 12,
        };
    }

    pub fn needsPreviousGrid(effect: Transition) bool {
        return effect == .dissolve or effect == .ascii or effect == .vertical;
    }

    pub fn lifecycleOnly(effect: Transition) bool {
        return effect == .vertical;
    }
};

/// Full-scene shader effects. CRT is one effect in this vocabulary rather
/// than a separate renderer switch; only one scene pass is needed even when
/// more than one bit is enabled.
pub const SceneEffect = struct {
    crt: bool = false,
    ripple: bool = false,
    glitch: bool = false,
};

pub const Phase = enum(u8) {
    opening = 0,
    moving = 1,
    /// Presentation-only content whose pane lifetime has already ended.
    /// It is never a valid input target.
    closing = 2,
};

pub const Box = extern struct {
    x: f32 = 0,
    y: f32 = 0,
    w: f32 = 0,
    h: f32 = 0,

    pub fn eql(a: Box, b: Box) bool {
        return a.x == b.x and a.y == b.y and a.w == b.w and a.h == b.h;
    }
};

/// One POD record is enough for every backend. `from` and `to` are logical
/// cell boxes; frontends convert them to pixels only at their render edge.
pub const Track = extern struct {
    serial: u32 = 0,
    pane: u8 = 0,
    phase: Phase = .moving,
    effect: Transition = .off,
    _padding: u8 = 0,
    frame: u16 = 0,
    _frame_padding: u16 = 0,
    from: Box = .{},
    to: Box = .{},

    pub fn active(track: Track) bool {
        return track.effect != .off and track.frame < track.effect.frames();
    }

    pub fn amount(track: Track) f32 {
        return progress(track.effect, track.frame);
    }

    pub fn presented(track: Track) Box {
        return lerpBox(track.from, track.to, track.amount());
    }

    /// Geometry actually painted by every backend. Content transitions reveal
    /// cells in place; slide, zoom, and vertical transform the panel rectangle.
    pub fn visualBox(track: Track) Box {
        return switch (track.effect) {
            .slide, .zoom, .vertical => track.presented(),
            .off, .dissolve, .ascii => track.to,
        };
    }

    /// The canonical box whose cells a backend samples. Opening and moving
    /// tracks sample the new frame at `to`; a closing tombstone samples the
    /// frozen old frame at `from` because its pane no longer exists.
    pub fn contentBox(track: Track) Box {
        return if (track.phase == .closing) track.from else track.to;
    }
};

/// Preset starting geometry for a newly-visible panel. Dissolve and ASCII
/// animate content in place; slide, zoom, and vertical animate its rectangle.
pub fn openingBox(effect: Transition, target: Box, screen_width: u16) Box {
    return switch (effect) {
        .slide => blk: {
            var from = target;
            const middle = target.x + target.w * 0.5;
            from.x = if (middle < @as(f32, @floatFromInt(screen_width)) * 0.5)
                -target.w
            else
                @floatFromInt(screen_width);
            break :blk from;
        },
        .zoom => .{
            .x = target.x + target.w * 0.5,
            .y = target.y + target.h * 0.5,
            .w = 0,
            .h = 0,
        },
        // Slide upward into a fixed clip equal to the new pane's own box.
        // Starting one panel-height below that box keeps the translated
        // content from travelling across any surviving pane.
        .vertical => blk: {
            var from = target;
            from.y += target.h;
            break :blk from;
        },
        .off, .dissolve, .ascii => target,
    };
}

/// Destination for a lifecycle-only closing track. The old panel rises out
/// through its own fixed clip; surviving panes are already at canonical
/// geometry underneath it and never receive tracks for this effect.
pub fn closingBox(effect: Transition, source: Box) Box {
    return switch (effect) {
        .vertical => blk: {
            var to = source;
            to.y -= source.h;
            break :blk to;
        },
        else => source,
    };
}

pub fn sample(easing: Easing, raw: f32) f32 {
    const t = std.math.clamp(raw, 0.0, 1.0);
    return switch (easing) {
        .linear => t,
        .smooth => t * t * (3.0 - 2.0 * t),
        .out_cubic => 1.0 - (1.0 - t) * (1.0 - t) * (1.0 - t),
        // Robert Penner's ease-out-back polynomial. It intentionally travels
        // a little past one before settling exactly on the endpoint.
        .out_back => blk: {
            const c1: f32 = 1.70158;
            const c3 = c1 + 1.0;
            const u = t - 1.0;
            break :blk 1.0 + c3 * u * u * u + c1 * u * u;
        },
    };
}

pub fn progress(effect: Transition, frame: u16) f32 {
    const frames = effect.frames();
    if (frames <= 1 or frame >= frames - 1) return 1.0;
    // `frames` is the number of presented samples, including both exact
    // endpoints. This makes the last active frame the real final image rather
    // than 15/16 followed by an unrendered snap to canonical content.
    return sample(effect.easing(), @as(f32, @floatFromInt(frame)) / @as(f32, @floatFromInt(frames - 1)));
}

pub fn lerpBox(from: Box, to: Box, t: f32) Box {
    // Keep easing overshoot for opening/moving geometry—the whole visual
    // distinction of out-back—while preventing any shrinking dimension from
    // becoming negative and flipping its quad.
    const u = @max(0.0, t);
    return .{
        .x = from.x + (to.x - from.x) * u,
        .y = from.y + (to.y - from.y) * u,
        .w = @max(0.0, from.w + (to.w - from.w) * u),
        .h = @max(0.0, from.h + (to.h - from.h) * u),
    };
}

/// Stable cell noise shared by the TTY reveal and shader ports. Integer-only
/// hashing means resizing or repainting a frame does not make cells flicker.
pub fn cellNoise(serial: u32, col: u16, row: u16) f32 {
    var x = serial ^ (@as(u32, col) *% 0x9e37_79b9) ^ (@as(u32, row) *% 0x85eb_ca6b);
    x ^= x >> 16;
    x *%= 0x7feb_352d;
    x ^= x >> 15;
    x *%= 0x846c_a68b;
    x ^= x >> 16;
    return @as(f32, @floatFromInt(x & 0xffff)) / 65535.0;
}

/// Whether a changed dissolve cell has crossed from the frozen old grid to
/// the new one. Exact endpoints are part of the presentation contract.
pub fn dissolveRevealed(serial: u32, col: u16, row: u16, raw_progress: f32) bool {
    const t = std.math.clamp(raw_progress, 0.0, 1.0);
    if (t <= 0) return false;
    if (t >= 1) return true;
    return cellNoise(serial, col, row) < t;
}

pub const AsciiCell = struct {
    /// When true the pane's real cell has arrived.
    reveal: bool,
    /// A transient one-byte glyph while `reveal` is false. Space means the
    /// cell has not entered the animation yet.
    glyph: u8,
};

/// ASCII-materialization preset: a diagonal wave perturbed by stable noise.
/// It is content animation rather than geometry; TTY replaces the cell glyph,
/// while GUI shaders use the same threshold to dissolve glyph coverage.
pub fn asciiCell(serial: u32, col: u16, row: u16, raw_progress: f32) AsciiCell {
    const t = std.math.clamp(raw_progress, 0.0, 1.0);
    // These endpoints are real presented frames: even a cell whose stable
    // threshold is zero starts as old data, and every cell ends as new data.
    if (t <= 0) return .{ .reveal = false, .glyph = ' ' };
    if (t >= 1) return .{ .reveal = true, .glyph = ' ' };
    const noise = cellNoise(serial, col, row);
    const diagonal = @as(f32, @floatFromInt((@as(u32, col) + @as(u32, row) * 2) % 17)) / 17.0;
    const threshold = noise * 0.72 + diagonal * 0.28;
    if (t >= threshold) return .{ .reveal = true, .glyph = ' ' };
    if (t + 0.22 < threshold) return .{ .reveal = false, .glyph = ' ' };
    const glyphs = ".:+*#%@";
    const index: usize = @intFromFloat(@floor(noise * @as(f32, @floatFromInt(glyphs.len))));
    return .{ .reveal = false, .glyph = glyphs[@min(index, glyphs.len - 1)] };
}

test "easing presets have exact endpoints and intended shapes" {
    inline for (std.enums.values(Easing)) |easing| {
        try std.testing.expectEqual(@as(f32, 0), sample(easing, 0));
        try std.testing.expectEqual(@as(f32, 1), sample(easing, 1));
    }
    try std.testing.expectEqual(@as(f32, 0.5), sample(.linear, 0.5));
    try std.testing.expect(sample(.out_cubic, 0.5) > sample(.linear, 0.5));
    try std.testing.expect(sample(.out_back, 0.8) > 1.0);
}

test "transition progress completes exactly" {
    inline for (std.enums.values(Transition)) |effect| {
        try std.testing.expectEqual(@as(f32, 1), progress(effect, effect.frames()));
        if (effect.frames() > 0)
            try std.testing.expectEqual(@as(f32, 1), progress(effect, effect.frames() - 1));
        try std.testing.expectEqual(@as(f32, 1), progress(effect, std.math.maxInt(u16)));
    }
    try std.testing.expectEqual(@as(f32, 1), progress(.off, 0));

    const shrinking = lerpBox(.{ .w = 100, .h = 40 }, .{}, sample(.out_back, 0.8));
    try std.testing.expectEqual(@as(f32, 0), shrinking.w);
    try std.testing.expectEqual(@as(f32, 0), shrinking.h);
    const opening = lerpBox(.{}, .{ .w = 100, .h = 40 }, sample(.out_back, 0.8));
    try std.testing.expect(opening.w > 100);
    try std.testing.expect(opening.h > 40);
}

test "opening presets separate geometry and content transitions" {
    const target: Box = .{ .x = 30, .y = 2, .w = 20, .h = 8 };
    try std.testing.expectEqual(target, openingBox(.ascii, target, 80));
    try std.testing.expectEqual(@as(f32, 0), openingBox(.zoom, target, 80).w);
    try std.testing.expectEqual(@as(f32, 80), openingBox(.slide, target, 80).x);
    try std.testing.expectEqual(@as(f32, target.y + target.h), openingBox(.vertical, target, 80).y);
    try std.testing.expectEqual(@as(f32, target.y - target.h), closingBox(.vertical, target).y);

    var track: Track = .{ .effect = .slide, .from = target, .to = target };
    try std.testing.expect(track.active());
    track.frame = track.effect.frames();
    try std.testing.expect(!track.active());

    track = .{ .effect = .dissolve, .frame = 3, .from = .{}, .to = target };
    try std.testing.expectEqual(target, track.visualBox());

    const closing: Track = .{
        .phase = .closing,
        .effect = .vertical,
        .from = target,
        .to = closingBox(.vertical, target),
    };
    try std.testing.expectEqual(target, closing.contentBox());
}

test "ASCII materialization is stable and finishes as real content" {
    const a = asciiCell(42, 11, 7, 0.4);
    const b = asciiCell(42, 11, 7, 0.4);
    try std.testing.expectEqual(a, b);
    try std.testing.expect(!asciiCell(42, 11, 7, 0).reveal);
    try std.testing.expectEqual(@as(u8, ' '), asciiCell(42, 11, 7, 0).glyph);
    try std.testing.expect(asciiCell(42, 11, 7, 1).reveal);

    var changed = false;
    for (0..32) |col| {
        if (!std.meta.eql(asciiCell(42, @intCast(col), 3, 0.35), asciiCell(43, @intCast(col), 3, 0.35))) {
            changed = true;
            break;
        }
    }
    try std.testing.expect(changed);
}

test "dissolve has exact stable endpoints" {
    for (0..64) |col| {
        const x: u16 = @intCast(col);
        try std.testing.expect(!dissolveRevealed(42, x, 7, 0));
        try std.testing.expect(dissolveRevealed(42, x, 7, 1));
        if (dissolveRevealed(42, x, 7, 0.25))
            try std.testing.expect(dissolveRevealed(42, x, 7, 0.75));
    }
}