[Bf-blender-cvs] [6fcd071c7bc] master: GPU: Vertex Format: Increase number of byte per attribute name

[Clément Foucault]

Commit: 6fcd071c7bcb8c9a5e0a7d0e7a6ef6363e43627e
Author: Clément Foucault
Date:   Wed Aug 14 23:43:03 2019 +0200
Branches: master
https://developer.blender.org/rB6fcd071c7bcb8c9a5e0a7d0e7a6ef6363e43627e

GPU: Vertex Format: Increase number of byte per attribute name

This reduces the risks of hash collision while maintaining a small number
of character per attrib.

===================================================================

M	source/blender/gpu/GPU_vertex_format.h
M	source/blender/gpu/intern/gpu_vertex_format.c

===================================================================

diff --git a/source/blender/gpu/GPU_vertex_format.h b/source/blender/gpu/GPU_vertex_format.h
index 97c1828b593..8c22e3e1104 100644
--- a/source/blender/gpu/GPU_vertex_format.h
+++ b/source/blender/gpu/GPU_vertex_format.h
@@ -35,7 +35,7 @@
 #define GPU_VERT_ATTR_NAMES_BUF_LEN 256
 #define GPU_VERT_FORMAT_MAX_NAMES 63 /* More than enough, actual max is ~30. */
 /* Computed as GPU_VERT_ATTR_NAMES_BUF_LEN / 30 (actual max format name). */
-#define GPU_MAX_SAFE_ATTRIB_NAME 8
+#define GPU_MAX_SAFE_ATTRIB_NAME 12
 
 typedef enum {
   GPU_COMP_I8,
diff --git a/source/blender/gpu/intern/gpu_vertex_format.c b/source/blender/gpu/intern/gpu_vertex_format.c
index 2cfb17b1568..11df86c0b3a 100644
--- a/source/blender/gpu/intern/gpu_vertex_format.c
+++ b/source/blender/gpu/intern/gpu_vertex_format.c
@@ -220,20 +220,6 @@ int GPU_vertformat_attr_id_get(const GPUVertFormat *format, const char *name)
   return -1;
 }
 
-/* Encode 4 original bytes into 6 safe bytes. */
-static void safe_bytes(char out[6], const char data[4])
-{
-  char safe_chars[63] = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789_";
-
-  uint32_t in = *(uint32_t *)data;
-  for (int i = 0; i < 6; i++) {
-    /* Encoding in base63 */
-    out[i] = safe_chars[in % 63u];
-    in /= 63u;
-  }
-}
-
-#if 0 /* For when we can use 11chars names. */
 /* Encode 8 original bytes into 11 safe bytes. */
 static void safe_bytes(char out[11], const char data[8])
 {
@@ -246,7 +232,6 @@ static void safe_bytes(char out[11], const char data[8])
     in /= 63lu;
   }
 }
-#endif
 
 /* Warning: Always add a prefix to the result of this function as
  * the generated string can start with a number and not be a valid attribute name. */
@@ -254,22 +239,34 @@ void GPU_vertformat_safe_attrib_name(const char *attrib_name,
                                      char *r_safe_name,
                                      uint UNUSED(max_len))
 {
-  char data[4] = {0};
-  /* We use a hash to identify each data layer based on its name.
-   * NOTE: This is still prone to hash collision but the risks are very low.*/
-  *(uint *)data = BLI_ghashutil_strhash_p_murmur(attrib_name);
+  char data[8] = {0};
+  uint len = strlen(attrib_name);
+
+  if (len > 8) {
+    /* Start with the first 4 chars of the name; */
+    for (int i = 0; i < 4; i++) {
+      data[i] = attrib_name[i];
+    }
+    /* We use a hash to identify each data layer based on its name.
+     * NOTE: This is still prone to hash collision but the risks are very low.*/
+    /* Start hashing after the first 2 chars. */
+    *(uint *)&data[4] = BLI_ghashutil_strhash_p_murmur(attrib_name + 4);
+  }
+  else {
+    /* Copy the whole name. Collision is barelly possible
+     * (hash would have to be equal to the last 4 bytes). */
+    for (int i = 0; i < 8 && attrib_name[i] != '\0'; i++) {
+      data[i] = attrib_name[i];
+    }
+  }
   /* Convert to safe bytes characters. */
   safe_bytes(r_safe_name, data);
   /* End the string */
-  r_safe_name[6] = '\0';
-
-  /* TOOD(fclem) When UV and Tangent buffers will be separated, name buffer will have plenty more
-   * space. In this case, we can think of having ~13 chars for each name which would be enough to
-   * encode some of the input string along with the hash to reduce colision possibility. */
+  r_safe_name[11] = '\0';
 
-  BLI_assert(GPU_MAX_SAFE_ATTRIB_NAME >= 7);
+  BLI_assert(GPU_MAX_SAFE_ATTRIB_NAME >= 12);
 #if 0 /* For debugging */
-  printf("%s > %x > %s\n", attrib_name, *(uint32_t *)data, r_safe_name);
+  printf("%s > %lx > %s\n", attrib_name, *(uint64_t *)data, r_safe_name);
 #endif
 }