// Copyright (C) 2009-present, Panagiotis Christopoulos Charitos and contributors.
// All rights reserved.
// Code licensed under the BSD License.
// http://www.anki3d.org/LICENSE

#define TEX_SIZE_X 4096u
#define TEX_SIZE_Y 4096u
#define TILE_SIZE_X 32u
#define TILE_SIZE_Y 32u
#define TILE_COUNT_X (TEX_SIZE_X / TILE_SIZE_X)
#define TILE_COUNT_Y (TEX_SIZE_Y / TILE_SIZE_Y)
#define TILE_COUNT (TILE_COUNT_X * TILE_COUNT_Y)

Texture2D g_inputTex : register(t0);
RWStructuredBuffer<float4> g_tileMaxColors : register(u0);
RWStructuredBuffer<float4> g_result : register(u1);

#if defined(FIRST)

groupshared float4 g_tileMax[TILE_SIZE_X * TILE_SIZE_Y];

[numthreads(TILE_SIZE_X, TILE_SIZE_Y, 1)] void main(uint2 svDispatchThreadId : SV_DISPATCHTHREADID, uint svGroupIndex : SV_GROUPINDEX,
													uint2 svGroupId : SV_GROUPID)
{
	g_tileMax[svGroupIndex] = g_inputTex[svDispatchThreadId];

	GroupMemoryBarrierWithGroupSync();

	[loop] for(uint s = TILE_SIZE_X * TILE_SIZE_Y / 2u; s > 0u; s >>= 1u)
	{
		if(svGroupIndex < s)
		{
			g_tileMax[svGroupIndex] = max(g_tileMax[svGroupIndex], g_tileMax[svGroupIndex + s]);
		}

		GroupMemoryBarrierWithGroupSync();
	}

	const uint tileIdx = svGroupId.y * TILE_COUNT_X + svGroupId.x;
	g_tileMaxColors[tileIdx] = g_tileMax[0];
}

#else

groupshared float4 g_maxColor[64];

[numthreads(64, 1, 1)] void main(uint svGroupIndex : SV_GROUPINDEX)
{
	const uint tilesPerThread = TILE_COUNT / 64;

	const uint start = svGroupIndex * tilesPerThread;
	const uint end = start + tilesPerThread;

	float4 localMax = 0.0;
	for(uint tileIdx = start; tileIdx < end; ++tileIdx)
	{
		localMax = max(localMax, g_tileMaxColors[tileIdx]);
	}

	g_maxColor[svGroupIndex] = localMax;

	GroupMemoryBarrierWithGroupSync();

	[loop] for(uint s = 64 / 2u; s > 0u; s >>= 1u)
	{
		if(svGroupIndex < s)
		{
			g_maxColor[svGroupIndex] = max(g_maxColor[svGroupIndex], g_maxColor[svGroupIndex + s]);
		}

		GroupMemoryBarrierWithGroupSync();
	}

	g_result[0] = g_maxColor[0];
}

#endif