cpponsea2024

cpponsea2024/index.html

@@ -1609,42 +1609,80 @@
 
 #### string_to_enum - Minimal perfect hashing
 
-#### bit mixer - magic
+```cpp
+enum uri { ftp = 0, file = 1, http = 2, ws = 3, wss = 4, };
+```
 <!-- .element: class="fragment" style="float:left" data-fragment-index="0" -->
+
+---
+
+#### bit mixer
+<!-- .element: class="fragment" style="float:left" data-fragment-index="1" -->
 <br />
 
 ```cpp
 (value * magic) >> shift
 ```
-<!-- .element: class="fragment" data-fragment-index="1" -->
+<!-- .element: class="fragment" data-fragment-index="2" -->
+
+```cpp
+// shift
+static_assert(
+  (sizeof(u32) * CHAR_BIT) - bits_required(max(enumerator<E>.second))
+  ==
+  32 - bits_required(4) == 32 - 3 = 29
+);
+```
+<!-- .element: class="fragment" data-fragment-index="3" -->
 
-#### lookup table - lut
+---
+
+```cpp
+// magic
+assert((to<u32>("ftp")  * 2893426669) >> 29) == 1);
+assert((to<u32>("file") * 2893426669) >> 29) == 4);
+assert((to<u32>("http") * 2893426669) >> 29) == 0);
+assert((to<u32>("ws")   * 2893426669) >> 29) == 2);
+assert((to<u32>("wss")  * 2893426669) >> 29) == 3);
+```
+<!-- .element: class="fragment" data-fragment-index="4" -->
+
+----
+
+#### string_to_enum - Minimal perfect hashing
+
+#### lookup table
 <!-- .element: class="fragment" style="float:left" data-fragment-index="2" -->
 <br />
 
 ```cpp
-{7, 42, 1, 10}[n]
+{1, 4, 0, 2, 3}[n]
 ```
 <!-- .element: class="fragment" data-fragment-index="3" -->
 
 ```cpp
-{0b000111, 0b101010, 0b000001, 0b001010}[n]
+{0b001, 0b100, 0b000, 0b010, 0b011}[n]
 ```
 <!-- .element: class="fragment" data-fragment-index="4" -->
 
 ```cpp
-(0b'000111'101010'000001'001010 >> 6*n) & 0b111111
+(0b'001'100'000'010'011 >> 3*n) & 0b111
 ```
 <!-- .element: class="fragment" data-fragment-index="5" -->
 
 ---
 
+#### lookup
+<!-- .element: class="fragment" style="float:left" data-fragment-index="6" -->
+<br />
+
 ```cpp
-// shift: how many bits are required to represent all values (6 bits)
-// mask:  0b111111 (6 bits)
-// magic: ???
-// lut:   ???
 ((lut >> (value * magic)) >> shift) & mask
+
+// shift: 29
+// mask:  0b111
+// magic: 2893426669 (???)
+// lut:   0b001'100'000'010'011 = 6163 (???)
 ```
 <!-- .element: class="fragment" data-fragment-index="6" -->
 
@@ -1659,10 +1697,11 @@
 <!-- .element: class="fragment" data-fragment-index="0" -->
 
 ```cpp
-  constexpr auto size  = sizeof(T) * CHAR_BIT; // T = uint32_t/...
+  using T = uint32_t;
+  constexpr auto size  = sizeof(T) * CHAR_BIT;
   constexpr auto nbits = size - countr_zero(max(enumerators<E>));
-  constexpr auto shift = size - nbits;
-  constexpr auto mask  = (1u << nbits) - 1u; // 0b111111...
+  constexpr auto shift = size - nbits;       // 29
+  constexpr auto mask  = (1u << nbits) - 1u; // 0b111
   constexpr auto [magic, lut] = magic_lut(mask, shift, MaxAttempts);
 ```
 <!-- .element: class="fragment" data-fragment-index="1" -->
@@ -1725,7 +1764,7 @@
 #### string_to_enum - Minimal perfect hashing - https://godbolt.org/z/3j8qcabsj
 
 ```cpp
-enum uri { ftp = 1, file = 2, http = 3, ws = 4, wss = 5, };
+enum uri { ftp = 0, file = 1, http = 2, ws = 3, wss = 4, };
 ```
 <!-- .element: class="fragment" data-fragment-index="0" -->
 
@@ -1741,11 +1780,11 @@
 [1] [2] [3]  [4] [5] [6]   Instructions:
  1   1  0.50               shl edi, 3
  2   5  0.50  *            bzhi eax, dword ptr [rsi], edi
- 1   3  1.00               imul eax, eax, 385596323 /*magic*/
- 1   1  0.50               shr eax, 29              /*shift*/
- 1   1  0.50               mov ecx, 358             /*lut*/
+ 1   3  1.00               imul eax, eax, 2893426669 /*magic*/
+ 1   1  0.50               shr eax, 29               /*shift*/
+ 1   1  0.50               mov ecx, 6163             /*lut*/
  1   1  0.50               shrx eax, ecx, eax
- 1   1  0.25               and eax, 7               /*mask*/
+ 1   1  0.25               and eax, 0b111            /*mask*/
  1   5  0.50          U    ret
 </code></pre>