#version 430 core
#extension GL_ARB_shader_draw_parameters : require

layout(location = 0) in vec3 aPosition;
layout(location = 1) in vec3 aNormal;
layout(location = 2) in vec2 aTexCoord;

struct InstanceData {
    mat4 transform;
    // Reserved for Phase B.4 follow-up (selection-blink retail-faithful
    // highlight): vec4 highlightColor; — extend stride here, increase the
    // _instanceSsbo upload size in WbDrawDispatcher, add a flat varying out,
    // and consume in mesh_modern.frag.
};

struct BatchData {
    uvec2 textureHandle;   // bindless handle for sampler2DArray
    uint  textureLayer;    // layer index (always 0 for per-instance composites)
    uint  flags;           // reserved — N.5 dispatcher owns all blend state
                           // (glBlendFunc per pass). If a future phase wants
                           // shader-side per-batch additive flag (Decision 2
                           // fallback), encode it here as bit 0.
};

layout(std430, binding = 0) readonly buffer InstanceBuffer {
    InstanceData Instances[];
};

// binding=1 here is the SSBO namespace — distinct from the UBO namespace.
// SceneLighting UBO also uses binding=1 in the fragment shader; GL keeps
// GL_SHADER_STORAGE_BUFFER and GL_UNIFORM_BUFFER binding tables separate.
// Task 10 dispatcher binds:
//   glBindBufferBase(GL_SHADER_STORAGE_BUFFER, 0, instanceSsbo)
//   glBindBufferBase(GL_SHADER_STORAGE_BUFFER, 1, batchSsbo)
// Existing SceneLightingUboBinding handles the UBO side.
layout(std430, binding = 1) readonly buffer BatchBuffer {
    BatchData Batches[];
};

uniform mat4 uViewProjection;

// Phase Post-A.5 (ISSUE #52, 2026-05-10): per-pass offset into Batches[].
// gl_DrawIDARB resets to 0 at the start of each glMultiDrawElementsIndirect
// call, so the transparent pass — which begins later in the indirect buffer
// — was fetching Batches[0..transparentCount) instead of its actual section
// at Batches[opaqueCount..end). The lifestone crystal (a transparent draw)
// ended up reading the FIRST OPAQUE batch's TextureHandle every frame. As
// the camera moved and the opaque front-to-back sort reordered which group
// landed at BatchData[0], the lifestone's apparent texture flickered to
// whatever was first — frequently the player character's body parts.
//
// WbDrawDispatcher.Draw sets this to 0 before the opaque MDI call and to
// _opaqueDrawCount before the transparent MDI call, matching WorldBuilder's
// uDrawIDOffset pattern in BaseObjectRenderManager.cs line 845.
uniform int uDrawIDOffset;

out vec3 vNormal;
out vec2 vTexCoord;
out vec3 vWorldPos;
out flat uvec2 vTextureHandle;
out flat uint  vTextureLayer;

void main() {
    int instanceIndex = gl_BaseInstanceARB + gl_InstanceID;
    mat4 model = Instances[instanceIndex].transform;

    vec4 worldPos = model * vec4(aPosition, 1.0);
    gl_Position = uViewProjection * worldPos;

    vWorldPos = worldPos.xyz;
    vNormal = normalize(mat3(model) * aNormal);
    vTexCoord = aTexCoord;

    BatchData b = Batches[uDrawIDOffset + gl_DrawIDARB];
    vTextureHandle = b.textureHandle;
    vTextureLayer = b.textureLayer;
}