//
//  PointRenderer.metal
//  Metrika
//
//  Author:  Simon-Pierre Boucher
//  Contact: contact@spboucher.ai
//  Copyright © 2026 Simon-Pierre Boucher. All rights reserved.
//
//  Instanced point-sprite renderer for large scatter plots (CLAUDE.md §7:
//  custom MTKView renderer past the Swift Charts threshold). One packed
//  12-byte vertex per observation; the fragment stage rounds sprites.
//

#include <metal_stdlib>
using namespace metal;

struct PackedPoint {
    packed_float2 position;   // data coordinates
    uchar4 color;
};

struct Uniforms {
    float2 center;            // data-space center of the viewport
    float2 scale;             // data → clip scaling (zoom × aspect)
    float pointSize;
};

struct VertexOut {
    float4 position [[position]];
    float pointSize [[point_size]];
    float4 color;
};

vertex VertexOut scatterVertex(
    uint vertexID [[vertex_id]],
    const device PackedPoint *points [[buffer(0)]],
    constant Uniforms &uniforms [[buffer(1)]]
) {
    PackedPoint point = points[vertexID];
    float2 clip = (float2(point.position) - uniforms.center) * uniforms.scale;

    VertexOut out;
    out.position = float4(clip, 0.0, 1.0);
    out.pointSize = uniforms.pointSize;
    out.color = float4(point.color) / 255.0;
    return out;
}

fragment float4 scatterFragment(
    VertexOut in [[stage_in]],
    float2 pointCoord [[point_coord]]
) {
    float2 offset = pointCoord - 0.5;
    if (dot(offset, offset) > 0.25) {
        discard_fragment();
    }
    return in.color;
}