frust-engine 0.6.2

//! The `Config` uniform every strip shader binds.
//!
//! Named [`GpuConfig`] rather than `Config` because the crate root already
//! owns a `config` module of process-wide kill switches ([`crate::config`]);
//! this one is the GPU-side uniform buffer's layout and nothing else.
//!
//! Field order and size are part of the shader contract — see the module
//! header of [`super`].
//!
//! ## The width shifts are the source of truth
//!
//! The strip program addresses its resource textures by shift and mask alone:
//! a flat texel index `i` in a texture `1 << bits` wide is the texel
//! `(i & ((1 << bits) - 1), i >> bits)`. It never asks a texture for its own
//! width (`textureDimensions` per fragment was measurably not free on a mobile
//! GPU), so the two shifts here are the only statement of those widths the GPU
//! sees, and three textures depend on them:
//!
//! - the alpha texture, by [`GpuConfig::alphas_tex_width_bits`];
//! - the encoded-paint texture, by [`GpuConfig::encoded_paints_tex_width_bits`];
//! - the gradient LUT, by the *same* `encoded_paints_tex_width_bits` — the
//!   uniform has no field of its own for it, and needs none, because the
//!   renderer sizes every resource texture `resource_texture_dim` wide.
//!
//! [`resource_widths_agree`] is that rule as a check: a `debug_assert!` at the
//! one site that builds the surface config from the resource textures
//! (`renderer`'s config upload); the pooled-page configs are built from the
//! same `dim` in the same frame and are not re-checked, and a release build
//! checks nothing. The one pass with a narrower stand-in — the
//! glyph-atlas replay, whose encoded-paint texture is a one-texel placeholder —
//! draws solid instances only, which read neither the paint texture nor the
//! LUT, so its config's paint shift addresses nothing.

use bytemuck::{Pod, Zeroable};
use vello_common::tile::Tile;

/// The uniform block the strip shaders read once per draw.
///
/// Laid out to match the reference renderer's `Config` byte for byte: eight
/// 4-byte scalars in a 16-byte-aligned block.
#[repr(C, align(16))]
#[derive(Debug, Clone, Copy, Pod, Zeroable)]
pub struct GpuConfig {
    /// Width of the render target in pixels.
    pub width: u32,
    /// Height of the render target in pixels.
    pub height: u32,
    /// Height of one strip in pixels — always `Tile::HEIGHT`.
    pub strip_height: u32,
    /// `log2` of the alpha texture's width in texels.
    ///
    /// Pre-computed on the CPU because a downlevel (GLES 3.0 / WebGL2)
    /// target has no `firstTrailingBit`; the shader shifts by this instead of
    /// dividing.
    pub alphas_tex_width_bits: u32,
    /// `log2` of the encoded-paint texture's width in texels, pre-computed
    /// for the same reason as [`Self::alphas_tex_width_bits`] — and, because
    /// the gradient LUT is sized to the same width, `log2` of the LUT's
    /// width too (see the module doc).
    pub encoded_paints_tex_width_bits: u32,
    /// Horizontal offset applied to every strip, in pixels.
    pub strip_offset_x: i32,
    /// Vertical offset applied to every strip, in pixels.
    pub strip_offset_y: i32,
    /// Whether the shader negates the y component of its NDC position
    /// (non-zero) or leaves it alone (zero).
    ///
    /// Kept for a future GLSL/WebGL backend: transpiling WGSL to GLSL applies
    /// a y-flip to reconcile WebGPU's y-down clip space with a WebGL
    /// framebuffer's y-up one. Rendering straight into a caller-provided
    /// framebuffer wants that flip undone rather than a second framebuffer
    /// allocated and blitted, and the rest of the strip pipeline (slot
    /// textures included) assumes y-down throughout. Every backend this crate
    /// drives today leaves it zero.
    pub negate_ndc: u32,
}

const _: () = assert!(
    size_of::<GpuConfig>() == 32,
    "`GpuConfig` must stay 32 bytes — the shaders' `Config` uniform layout",
);
const _: () = assert!(
    align_of::<GpuConfig>() == 16,
    "`GpuConfig` must stay 16-byte aligned — uniform-buffer layout rules",
);

impl GpuConfig {
    /// The uniform's size in bytes, for a buffer binding's `min_binding_size`.
    pub const SIZE: u64 = size_of::<Self>() as u64;

    /// A config for a `width` x `height` target sampling resource textures
    /// `alphas_tex_width` and `encoded_paints_tex_width` texels wide, with no
    /// strip offset and no NDC negation.
    ///
    /// Both texture widths must be powers of two: the shader reconstructs
    /// them as `1 << bits` (see [`tex_width_bits`]).
    #[must_use]
    pub fn new(
        width: u32,
        height: u32,
        alphas_tex_width: u32,
        encoded_paints_tex_width: u32,
    ) -> Self {
        Self {
            width,
            height,
            strip_height: u32::from(Tile::HEIGHT),
            alphas_tex_width_bits: tex_width_bits(alphas_tex_width),
            encoded_paints_tex_width_bits: tex_width_bits(encoded_paints_tex_width),
            strip_offset_x: 0,
            strip_offset_y: 0,
            negate_ndc: 0,
        }
    }

    /// The same config with strips offset by `(x, y)` pixels.
    #[must_use]
    pub const fn with_strip_offset(mut self, x: i32, y: i32) -> Self {
        self.strip_offset_x = x;
        self.strip_offset_y = y;
        self
    }

    /// The same config with NDC y negation switched on or off
    /// ([`Self::negate_ndc`]).
    #[must_use]
    pub const fn with_negate_ndc(mut self, negate: bool) -> Self {
        self.negate_ndc = negate as u32;
        self
    }
}

/// Whether a frame's three strip resource textures are exactly as wide as
/// the config built for `dim` says — the precondition of the strip program's
/// shift-and-mask addressing (see the module doc).
///
/// `dim` is `TierCaps::resource_texture_dim`; the alpha and encoded-paint
/// textures must be `dim` wide because their config shifts are taken from it,
/// and the gradient LUT must be too because the shader addresses it by the
/// encoded-paint shift.
#[must_use]
pub fn resource_widths_agree(dim: u32, alphas: u32, paints: u32, lut: u32) -> bool {
    dim.is_power_of_two() && alphas == dim && paints == dim && lut == dim
}

/// `log2` of a resource texture's width in texels.
///
/// `width` must be a power of two — every resource texture is sized from
/// `frust_gpu::TierCaps::resource_texture_dim`, which is. A width that is not
/// makes the shader's `1 << bits` reconstruction disagree with the real
/// texture, so the precondition is checked in debug builds; release builds
/// take the trailing-zero count as-is rather than panicking on a frame path.
#[must_use]
pub fn tex_width_bits(width: u32) -> u32 {
    debug_assert!(
        width.is_power_of_two(),
        "resource texture width must be a power of two, got {width}",
    );
    width.trailing_zeros()
}

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

    #[test]
    fn the_lut_and_paint_textures_share_one_width_shift() {
        let config = GpuConfig::new(800, 600, 2048, 2048);
        // The LUT is addressed by the paint shift: a ramp texel at flat
        // index `i` lands at `(i & mask, i >> bits)` of a 2048-wide LUT.
        let bits = config.encoded_paints_tex_width_bits;
        let flat = 2048 * 3 + 17;
        assert_eq!((flat & ((1 << bits) - 1), flat >> bits), (17, 3));
        assert!(resource_widths_agree(2048, 2048, 2048, 2048));
    }

    #[test]
    fn a_resource_texture_of_another_width_breaks_the_guard() {
        assert!(!resource_widths_agree(2048, 1024, 2048, 2048));
        assert!(!resource_widths_agree(2048, 2048, 4096, 2048));
        assert!(!resource_widths_agree(2048, 2048, 2048, 1024));
        assert!(!resource_widths_agree(1000, 1000, 1000, 1000));
    }

    /// Shift-and-mask addressing equals `%`/`/` for every power-of-two width —
    /// the arithmetic the LUT and alpha lookups moved from — over indices past
    /// several rows.
    #[test]
    fn shift_and_mask_addressing_equals_division_for_power_of_two_widths() {
        for bits in 0..=13_u32 {
            let width = 1_u32 << bits;
            for index in (0..width * 5).step_by(((width / 7) as usize).max(1)) {
                let mask = (1_u32 << bits) - 1;
                assert_eq!(
                    (index & mask, index >> bits),
                    (index % width, index / width)
                );
            }
        }
    }
}