// Shared declarations, prepended to every kernel.

struct Params {
  nx: u32,
  ny: u32,
  n: u32,
  histLen: u32,
  hx: f32,
  hy: f32,
  dt: f32,
  nu: f32,
  D: f32,      // bending stiffness [N m]
  C: f32,      // E h / (1 - nu^2) [N/m]
  G: f32,      // E h / (2 (1 + nu)) [N/m]
  rhoh: f32,   // mass per unit area [kg/m^2]
  pushX: f32,  // center of the pushed patch [m]
  pushY: f32,
  pushR2: f32, // squared patch radius [m^2]
  mass: f32,   // total mass [kg]
  Ix: f32,     // sum m y^2
  Iy: f32,     // sum m x^2
  Iz: f32,     // sum m (x^2 + y^2)
  pad0: f32,
};

// Per-step parameters, addressed with a dynamic uniform offset.
struct StepParams {
  step: u32,
  flags: u32,      // bit 0: push constraint active; bit 1: record the constraint reaction; bit 2: apply the recorded reaction
  audioSlot: i32,  // ring-buffer slot to record acceleration into (-1: none)
  audioOut: i32,   // output sample index within the batch
  pushW: f32,      // prescribed patch displacement [m]
  pushV: f32,      // prescribed patch velocity [m/s]
  sigma0: f32,
  sigma1: f32,
  releaseGain: f32, // fraction of the recorded reaction still applied (finger letting go)
  pad0: f32,
  pad1: f32,
  pad2: f32,
};

fn nodeX(i: i32) -> f32 { return (f32(i) - 0.5 * f32(P.nx - 1u)) * P.hx; }
fn nodeY(j: i32) -> f32 { return (f32(j) - 0.5 * f32(P.ny - 1u)) * P.hy; }
fn trapWeight(i: i32, j: i32) -> f32 {
  var t = 1.0;
  if (i == 0 || i == i32(P.nx) - 1) { t = t * 0.5; }
  if (j == 0 || j == i32(P.ny) - 1) { t = t * 0.5; }
  return t;
}