using a table-based convert_hex_to_binary function for better speed (#478)

* using a table-based convert_hex_to_binary function for better speed

* Using table throughout.

* Improving is_ascii_hex_digit

* Revert "Using table throughout."

This reverts commit 5050bdc.

---------

Co-authored-by: Daniel Lemire <dlemire@lemire.me>

Author: lemire

include/ada/unicode.h

@@ -154,9 +154,10 @@ ada_really_inline constexpr bool is_single_dot_path_segment(
 ada_really_inline constexpr bool is_lowercase_hex(const char c) noexcept;
 
 /**
- * @details Convert hex to binary.
+ * @details Convert hex to binary. Caller is responsible to ensure that
+ * the parameter is an hexadecimal digit (0-9, A-F, a-f).
  */
-unsigned constexpr convert_hex_to_binary(char c) noexcept;
+ada_really_inline unsigned constexpr convert_hex_to_binary(char c) noexcept;
 
 /**
  * first_percent should be  = input.find('%')

src/unicode.cpp

@@ -368,13 +368,12 @@ ada_really_inline constexpr bool is_lowercase_hex(const char c) noexcept {
   return (c >= '0' && c <= '9') || (c >= 'a' && c <= 'f');
 }
 
+constexpr static char hex_to_binary_table[] = {
+    0,  1,  2,  3,  4, 5, 6, 7, 8, 9, 0, 0,  0,  0,  0,  0,  0, 10, 11,
+    12, 13, 14, 15, 0, 0, 0, 0, 0, 0, 0, 0,  0,  0,  0,  0,  0, 0,  0,
+    0,  0,  0,  0,  0, 0, 0, 0, 0, 0, 0, 10, 11, 12, 13, 14, 15};
 unsigned constexpr convert_hex_to_binary(const char c) noexcept {
-  // this code can be optimized.
-  if (c <= '9') {
-    return c - '0';
-  }
-  char del = c >= 'a' ? 'a' : 'A';
-  return 10 + (c - del);
+  return hex_to_binary_table[c - '0'];
 }
 
 std::string percent_decode(const std::string_view input, size_t first_percent) {