struct Settings {
  eraserRadiusSquared: f32,
  lineDistanceEpsilon: f32,
  clearRed: f32,
  clearGreen: f32,
  clearBlue: f32,
  clearAlpha: f32,
  eraserRadius: f32,
  padding1: f32,
};

@group(1) @binding(0) var<uniform> settings: Settings;

struct VertexOutput {
  @builtin(position) position: vec4<f32>,
  @location(0) screenPosition: vec2<f32>,
  @location(1) @interpolate(flat) start: vec2<f32>,
  @location(2) @interpolate(flat) direction: vec2<f32>,
  @location(3) @interpolate(flat) inverseLengthSquared: f32,
}

struct EraserTextureTargets {
  @location(0) source: vec4<f32>,
  @location(1) influence: vec4<f32>,
  @location(2) trail: vec4<f32>,
}

struct EraserCombinedTargets {
  @location(0) mask: vec4<f32>,
  @location(1) source: vec4<f32>,
  @location(2) influence: vec4<f32>,
  @location(3) trail: vec4<f32>,
}

@vertex
fn vertex(
  @builtin(vertex_index) vertexIndex: u32,
  @location(0) start: vec2<f32>,
  @location(1) end: vec2<f32>
) -> VertexOutput {
  let direction = end - start;
  let denominator = dot(direction, direction);
  var inverseLengthSquared = 0.0;
  if denominator > settings.lineDistanceEpsilon {
    inverseLengthSquared = 1.0 / denominator;
  }
  let screenPosition = segment_vertex_position(vertexIndex, start, end, settings.eraserRadius);
  let uv = screenPosition / state.size;
  let position = vec2(uv.x * 2.0 - 1.0, 1.0 - uv.y * 2.0);
  return VertexOutput(vec4(position, 0.0, 1.0), screenPosition, start, direction, inverseLengthSquared);
}

@fragment
fn fragment(
  @location(0) screenPosition: vec2<f32>,
  @location(1) @interpolate(flat) start: vec2<f32>,
  @location(2) @interpolate(flat) direction: vec2<f32>,
  @location(3) @interpolate(flat) inverseLengthSquared: f32
) -> @location(0) vec4<f32> {
  if shouldDiscardEraserFragment(screenPosition, start, direction, inverseLengthSquared) {
    discard;
  }

  return getEraserClearValue();
}

@fragment
fn fragmentMrt(
  @location(0) screenPosition: vec2<f32>,
  @location(1) @interpolate(flat) start: vec2<f32>,
  @location(2) @interpolate(flat) direction: vec2<f32>,
  @location(3) @interpolate(flat) inverseLengthSquared: f32
) -> EraserTextureTargets {
  if shouldDiscardEraserFragment(screenPosition, start, direction, inverseLengthSquared) {
    discard;
  }

  let cleared = getEraserClearValue();
  return EraserTextureTargets(cleared, cleared, cleared);
}

@fragment
fn fragmentCombined(
  @location(0) screenPosition: vec2<f32>,
  @location(1) @interpolate(flat) start: vec2<f32>,
  @location(2) @interpolate(flat) direction: vec2<f32>,
  @location(3) @interpolate(flat) inverseLengthSquared: f32
) -> EraserCombinedTargets {
  if shouldDiscardEraserFragment(screenPosition, start, direction, inverseLengthSquared) {
    discard;
  }

  let cleared = getEraserClearValue();
  return EraserCombinedTargets(getEraserMaskValue(), cleared, cleared, cleared);
}

fn getEraserMaskValue() -> vec4<f32> {
  return vec4<f32>(settings.clearAlpha, 0.0, 0.0, 1.0);
}

fn getEraserClearValue() -> vec4<f32> {
  return vec4<f32>(
    settings.clearRed,
    settings.clearGreen,
    settings.clearBlue,
    settings.clearAlpha
  );
}

fn shouldDiscardEraserFragment(
  screenPosition: vec2<f32>,
  start: vec2<f32>,
  direction: vec2<f32>,
  inverseLengthSquared: f32
) -> bool {
  return distanceSquaredFromLine(screenPosition, start, direction, inverseLengthSquared) > settings.eraserRadiusSquared;
}

fn distanceSquaredFromLine(
  position: vec2<f32>,
  start: vec2<f32>,
  direction: vec2<f32>,
  inverseLengthSquared: f32
) -> f32 {
  let pa = position - start;

  if inverseLengthSquared <= 0.0 {
    return dot(pa, pa);
  }

  let q = clamp(dot(pa, direction) * inverseLengthSquared, 0.0, 1.0);
  let nearestOffset = pa - direction * q;
  return dot(nearestOffset, nearestOffset);
}

fn segment_vertex_position(
  vertexIndex: u32,
  start: vec2<f32>,
  end: vec2<f32>,
  radius: f32
) -> vec2<f32> {
  let directionVector = end - start;
  let segmentLength = length(directionVector);
  var direction = vec2<f32>(1.0, 0.0);
  if segmentLength > 0.0 {
    direction = directionVector / segmentLength;
  }
  let perpendicular = vec2<f32>(direction.y, -direction.x);
  let corner = segment_vertex_corner(vertexIndex % 6u);
  let center = mix(start, end, (corner.x + 1.0) * 0.5);
  return center + direction * corner.x * radius + perpendicular * corner.y * radius;
}

fn segment_vertex_corner(index: u32) -> vec2<f32> {
  if index == 0u {
    return vec2<f32>(-1.0, 1.0);
  }
  if index == 1u || index == 3u {
    return vec2<f32>(-1.0, -1.0);
  }
  if index == 2u || index == 4u {
    return vec2<f32>(1.0, 1.0);
  }
  return vec2<f32>(1.0, -1.0);
}