mesa/src/poly/prim.h

/*
 * Copyright 2023 Alyssa Rosenzweig
 * Copyright 2023 Valve Corporation
 * Copyright 2025 Collabora Ltd.
 * SPDX-License-Identifier: MIT
 */

#include "compiler/libcl/libcl.h"
#include "compiler/shader_enums.h"

#pragma once

/* Swap the two non-provoking vertices in odd triangles. This generates a vertex
 * ID list with a consistent winding order.
 *
 * Holding prim and flatshade_first constant, the map : [0, 1, 2] -> [0, 1, 2]
 * is its own inverse. It is hence used both vertex fetch and transform
 * feedback.
 */
static inline uint32_t
poly_map_vertex_in_tri_strip(uint32_t prim, uint32_t vert, bool flatshade_first)
{
   unsigned pv = flatshade_first ? 0 : 2;

   bool even = (prim & 1) == 0;
   bool provoking = vert == pv;

   return (provoking || even) ? vert : ((3 - pv) - vert);
}

static inline uint32_t
poly_vertex_id_for_line_loop(uint32_t prim, uint32_t vert, uint32_t num_prims)
{
   /* (0, 1), (1, 2), (2, 0) */
   if (prim == (num_prims - 1) && vert == 1)
      return 0;
   else
      return prim + vert;
}

static inline uint32_t
poly_vertex_id_for_tri_fan(uint32_t prim, uint32_t vert, bool flatshade_first)
{
   /* Vulkan spec section 20.1.7 gives (i + 1, i + 2, 0) for a provoking
    * first. OpenGL instead wants (0, i + 1, i + 2) with a provoking last.
    * Piglit clipflat expects us to switch between these orders depending on
    * provoking vertex, to avoid trivializing the fan.
    *
    * Rotate accordingly.
    */
   if (flatshade_first) {
      vert = (vert == 2) ? 0 : (vert + 1);
   }

   /* The simpler form assuming last is provoking. */
   return (vert == 0) ? 0 : prim + vert;
}

static inline uint32_t
poly_vertex_id_for_tri_strip_adj(uint32_t prim, uint32_t vert,
                                 uint32_t num_prims, bool flatshade_first)
{
   /* See Vulkan spec section 20.1.11 "Triangle Strips With Adjancency".
    *
    * There are different cases for first/middle/last/only primitives and for
    * odd/even primitives.  Determine which case we're in.
    */
   bool last = prim == (num_prims - 1);
   bool first = prim == 0;
   bool even = (prim & 1) == 0;
   bool even_or_first = even || first;

   /* When the last vertex is provoking, we rotate the primitives
    * accordingly. This seems required for OpenGL.
    */
   if (!flatshade_first && !even_or_first) {
      vert = (vert + 4u) % 6u;
   }

   /* Offsets per the spec. The spec lists 6 cases with 6 offsets. Luckily,
    * there are lots of patterns we can exploit, avoiding a full 6x6 LUT.
    *
    * Here we assume the first vertex is provoking, the Vulkan default.
    */
   uint32_t offsets[6] = {
      0,
      first ? 1 : (even ? -2 : 3),
      even_or_first ? 2 : 4,
      last ? 5 : 6,
      even_or_first ? 4 : 2,
      even_or_first ? 3 : -2,
   };

   /* Ensure NIR can see thru the local array */
   uint32_t offset = 0;
   for (uint32_t i = 1; i < 6; ++i) {
      if (i == vert)
         offset = offsets[i];
   }

   /* Finally add to the base of the primitive */
   return (prim * 2) + offset;
}

static inline uint32_t
poly_vertex_id_for_topology(enum mesa_prim mode, bool flatshade_first,
                            uint32_t prim, uint32_t vert, uint32_t num_prims)
{
   switch (mode) {
   case MESA_PRIM_POINTS:
   case MESA_PRIM_LINES:
   case MESA_PRIM_TRIANGLES:
   case MESA_PRIM_LINES_ADJACENCY:
   case MESA_PRIM_TRIANGLES_ADJACENCY:
      /* Regular primitive: every N vertices defines a primitive */
      return (prim * mesa_vertices_per_prim(mode)) + vert;

   case MESA_PRIM_LINE_LOOP:
      return poly_vertex_id_for_line_loop(prim, vert, num_prims);

   case MESA_PRIM_LINE_STRIP:
   case MESA_PRIM_LINE_STRIP_ADJACENCY:
      /* (i, i + 1) or (i, ..., i + 3) */
      return prim + vert;

   case MESA_PRIM_TRIANGLE_STRIP: {
      /* Order depends on the provoking vert.
       *
       * First: (0, 1, 2), (1, 3, 2), (2, 3, 4).
       * Last:  (0, 1, 2), (2, 1, 3), (2, 3, 4).
       *
       * Pull the (maybe swapped) vert from the corresponding primitive
       */
      return prim + poly_map_vertex_in_tri_strip(prim, vert, flatshade_first);
   }

   case MESA_PRIM_TRIANGLE_FAN:
      return poly_vertex_id_for_tri_fan(prim, vert, flatshade_first);

   case MESA_PRIM_TRIANGLE_STRIP_ADJACENCY:
      return poly_vertex_id_for_tri_strip_adj(prim, vert, num_prims,
                                              flatshade_first);

   default:
      return 0;
   }
}