# ============================================================================
# grass_gpu.ludic — the meadow's blades culled on the GPU (Vulkan), drawn from what survived.
#
# grass.vert decided every blade per VERTEX: its hashes, the ground under it, whether anything
# grows there, its colour field - ten times for a five-row blade, and in full for every index a
# tile asked for and threw away. That was 3.5 ms of a MoltenVK frame. Here the CPU still walks the
# visible tiles (as grass_tiles does), but grass_cull.comp decides each candidate once and writes
# the survivors into three bands by distance, one indirect draw each; grass_inst.vert only bends
# and places the vertices. OpenGL keeps the per-vertex path. R3D_GRASS_GPU=0 does too, to compare.
# ============================================================================

const GG_TILES: int = 4096
const GG_BANDS: int = 3
const GG_NEAR: float = 6.0     # band 0, five rows
const GG_MID: float = 16.0     # band 1, three rows; band 2 is one quad

function gg_cap(b: int) -> int {
  if b == 0 { return 16384 }
  if b == 1 { return 49152 }
  return 163840
}
function gg_base(b: int) -> int {
  if b == 0 { return 0 }
  if b == 1 { return 16384 }
  return 65536
}

# once, after grass_init: the programs, the buffers and the three instanced blades
function gg_init(render3d_st: mut Render3dState) -> void {
  if not gpu_has_compute(render3d_st) or render3d_st.grass_prog == 0 { return }
  if r3d_env_has(render3d_st, "R3D_GRASS_GPU") and r3d_env(render3d_st, "R3D_GRASS_GPU") == "0" { return }
  render3d_st.gg_reset = gpu_compute(render3d_st, "grass_reset", 1)
  render3d_st.gg_cull = gpu_compute_tex(render3d_st, "grass_cull", 3, 3)
  render3d_st.gg_prog = r3d_program(render3d_st, "grass_inst.vert", "model.frag", "#define FOLIAGE\n#define BLADE\n#define GBLADE\n#define GINST\n")
  if render3d_st.gg_reset == 0 or render3d_st.gg_cull == 0 or render3d_st.gg_prog == 0 { return }
  render3d_st.gg_tiles_buf = new []int
  for k in 0 .. 2 { push(render3d_st.gg_tiles_buf, gpu_buffer_new(render3d_st)) }
  render3d_st.gg_tv = floats(GG_TILES * 4)
  render3d_st.gg_out = gpu_buffer_new(render3d_st)
  gpu_buffer_upload(render3d_st, render3d_st.gg_out, (gg_base(2) + gg_cap(2)) * 64, null, GPU_DYNAMIC)
  gpu_buffer_gpu_owned(render3d_st, render3d_st.gg_out)
  render3d_st.gg_mesh = new []Mesh
  push(render3d_st.gg_mesh, gg_blade(render3d_st, 5))
  push(render3d_st.gg_mesh, gg_blade(render3d_st, 3))
  push(render3d_st.gg_mesh, gg_blade(render3d_st, 2))
  let rec = words(GG_BANDS * 5)
  for b in 0 .. GG_BANDS {
    rec[b * 5] = render3d_st.gg_mesh[b].count;  rec[b * 5 + 1] = 0;  rec[b * 5 + 2] = 0;  rec[b * 5 + 3] = 0;  rec[b * 5 + 4] = gg_base(b)
  }
  render3d_st.gg_cmds = gpu_buffer_new(render3d_st)
  gpu_buffer_upload(render3d_st, render3d_st.gg_cmds, GG_BANDS * 20, data_of(rec), GPU_DYNAMIC)
  gpu_buffer_gpu_owned(render3d_st, render3d_st.gg_cmds)
  render3d_st.gg_on = true
  print("r3d: grass: blades are culled on the GPU")
}

# a blade mesh reading its instance record (four vec4s) from the cull's output
function gg_blade(render3d_st: mut Render3dState, rows: int) -> Mesh {
  let m = grass_blade_mesh(render3d_st, rows)
  gpu_mesh_bind_instances(render3d_st, m, render3d_st.gg_out)
  for k in 0 .. 4 { gpu_mesh_attr_inst(render3d_st, m, 3 + k, 4, GPU_F32, 64, k * 16) }
  gpu_mesh_done(render3d_st, m)
  return m
}

# one tile size over one distance band: its visible tiles into gg_tv (corner, indices per cell, cells)
function gg_tiles(render3d_st: mut Render3dState, size: int, d_min: float, d_max: float) -> void {
  if d_min >= render3d_st.grass_radius { return }
  let cells = size / GRASS_CELL
  let sz = float(size)
  let half = sz * 0.5
  let reach = d_max + half * 1.5
  let corner_r = half * 1.42
  let tx0 = int(Math.floor((render3d_st.cam_pos[0] - reach) / sz))
  let tx1 = int(Math.floor((render3d_st.cam_pos[0] + reach) / sz))
  let tz0 = int(Math.floor((render3d_st.cam_pos[2] - reach) / sz))
  let tz1 = int(Math.floor((render3d_st.cam_pos[2] + reach) / sz))
  for tz in tz0 .. tz1 + 1 {
    for tx in tx0 .. tx1 + 1 {
      let ox = float(tx) * sz;  let oz = float(tz) * sz
      let dx = ox + half - render3d_st.cam_pos[0];  let dz = oz + half - render3d_st.cam_pos[2]
      let dc = Math.sqrt(dx * dx + dz * dz)
      let dnear = Math.max(dc - corner_r, 0.0)
      if dc < d_min or not (dnear < d_max) or render3d_st.gg_n >= GG_TILES { continue }
      if not cam_sphere_visible(render3d_st, ox + half, terrain_height(render3d_st, ox + half, oz + half), oz + half, corner_r + 6.0) { continue }
      let per = grass_count_at(render3d_st, dnear)
      var total = per * cells * cells
      if total > 65535 { total = 65535 }
      let t = render3d_st.gg_n * 4
      render3d_st.gg_tv[t] = ox;  render3d_st.gg_tv[t + 1] = oz;  render3d_st.gg_tv[t + 2] = float(total / (cells * cells));  render3d_st.gg_tv[t + 3] = float(cells)
      if total > render3d_st.gg_max { render3d_st.gg_max = total }
      render3d_st.gg_n += 1
    }
  }
}

# before the frame's first pass: the tiles, then the reset and the cull
function gg_cull_frame(render3d_st: mut Render3dState) -> void {
  if not render3d_st.gg_on or not grass_live(render3d_st) { return }
  render3d_st.gg_n = 0
  render3d_st.gg_max = 0
  gg_tiles(render3d_st, 4, 0.0, 16.0)
  gg_tiles(render3d_st, 8, 16.0, 40.0)
  gg_tiles(render3d_st, 16, 40.0, render3d_st.grass_radius)
  # made once: a words() a frame is a calloc nothing frees
  if render3d_st.gg_rb == null {
    render3d_st.gg_rb = words(4);  render3d_st.gg_cb = words(1);  render3d_st.gg_bufs = words(3)
    render3d_st.gg_texs = words(3);  render3d_st.gg_pr = words(48)
  }
  let rb = render3d_st.gg_rb
  rb[0] = GG_BANDS;  rb[1] = 0;  rb[2] = 0;  rb[3] = 0
  let cb = render3d_st.gg_cb
  cb[0] = render3d_st.gg_cmds
  gpu_dispatch(render3d_st, render3d_st.gg_reset, data_of(rb), 16, cb, 1)
  if render3d_st.gg_n == 0 { return }
  # the tile list alternates buffers by frame, so the frame still on the GPU keeps its own
  let tb = render3d_st.gg_tiles_buf[render3d_st.r3d_test_frame % 2]
  gpu_buffer_upload(render3d_st, tb, render3d_st.gg_n * 16, data_of(render3d_st.gg_tv), GPU_DYNAMIC)
  let pr = gg_params(render3d_st)
  let bufs = render3d_st.gg_bufs
  bufs[0] = tb;  bufs[1] = render3d_st.gg_out;  bufs[2] = render3d_st.gg_cmds
  let texs = render3d_st.gg_texs
  texs[0] = render3d_st.ter_height_tex;  texs[1] = render3d_st.ter_ortho_tex;  texs[2] = render3d_st.ter_normal_tex
  gpu_dispatch_tex(render3d_st, render3d_st.gg_cull, data_of(pr), 192, bufs, texs, (render3d_st.gg_max + 63) / 64, render3d_st.gg_n)
}

# grass_cull.comp's Params, std140: 12 vec4s, into the block made once in gg_cull_frame
function gg_params(render3d_st: mut Render3dState) -> words {
  let pr = render3d_st.gg_pr
  for i in 0 .. 48 { pr[i] = 0 }
  if render3d_st.cam_planes != null { for i in 0 .. 16 { pr[i] = float_bits(render3d_st.cam_planes[i]) } }
  pr[16] = float_bits(render3d_st.cam_pos[0]);  pr[17] = float_bits(render3d_st.cam_pos[1]);  pr[18] = float_bits(render3d_st.cam_pos[2]);  pr[19] = float_bits(render3d_st.grass_radius)
  var ph = render3d_st.post_h
  if ph <= 0 { ph = gl_height() }
  pr[20] = float_bits(render3d_st.grass_s0);  pr[21] = float_bits(render3d_st.grass_d0);  pr[22] = float_bits(2.0 * Math.tan(render3d_st.cam_fov * 0.5) / float(ph));  pr[23] = float_bits(render3d_st.ter_snow_line)
  pr[24] = float_bits(render3d_st.ter_lake_cx);  pr[25] = float_bits(render3d_st.ter_lake_cz);  pr[26] = float_bits(render3d_st.ter_lake_ex);  pr[27] = float_bits(render3d_st.ter_lake_ez)
  var lake = -100000.0
  if render3d_st.ter_lake_ex != 0.0 { lake = render3d_st.ter_lake_level }
  var sea = lake
  if render3d_st.ter_sea_set { sea = render3d_st.ter_sea_level }
  var oon = 0.0
  if render3d_st.ter_ortho_tex != 0 { oon = 1.0 }
  pr[28] = float_bits(lake);  pr[29] = float_bits(sea);  pr[30] = float_bits(oon)
  pr[32] = float_bits(render3d_st.ter_ox);  pr[33] = float_bits(render3d_st.ter_oz);  pr[34] = float_bits(float(render3d_st.TERRAIN_HALF))
  pr[36] = float_bits(GG_NEAR);  pr[37] = float_bits(GG_MID)
  for b in 0 .. GG_BANDS { pr[40 + b] = gg_base(b);  pr[44 + b] = gg_cap(b) }
  return pr
}

# the survivors: one indirect draw per band, from the records grass_cull.comp counted into
function gg_draw(render3d_st: mut Render3dState) -> void {
  let p = render3d_st.gg_prog
  gpu_use_program(render3d_st, p)
  grass_bind(render3d_st, p)
  u_f(render3d_st, gpu_uniform(render3d_st, p, "u_time"), render3d_st.r3d_time)
  gpu_cull(render3d_st, false)
  for b in 0 .. GG_BANDS { gpu_draw_mesh_indirect(render3d_st, render3d_st.gg_mesh[b], render3d_st.gg_cmds, b * 20, 1, 0, 0) }
  gpu_cull(render3d_st, true)
}