From 62d620e2abaedbb28996670ec8d1627fd10dc1e1 Mon Sep 17 00:00:00 2001
From: Yiheng Wu <36156959+jingkaimori@users.noreply.github.com>
Date: Mon, 10 Jun 2024 14:45:31 +0800
Subject: [PATCH] [8_11] optimize performance of as_hexadecimal

---
 bench/lolly/data/numeral_bench.cpp |  2 +-
 lolly/data/numeral.cpp             | 62 +++++++++++++++++++++++++++---
 2 files changed, 58 insertions(+), 6 deletions(-)

diff --git a/bench/lolly/data/numeral_bench.cpp b/bench/lolly/data/numeral_bench.cpp
index 09ff71be..c6a33a4b 100644
--- a/bench/lolly/data/numeral_bench.cpp
+++ b/bench/lolly/data/numeral_bench.cpp
@@ -31,7 +31,7 @@ main () {
     bench.complexityN (d);
     bench.run ("convert hexadecimal string to int",
                [&] { from_hex (hex_string); });
-    int hex_number= (0x1 << (d + 1)) - 1;
+    int hex_number= (0x1 << ((d + 1) * 4)) - 1;
     bench.run ("convert signed int to hexadecimal string",
                [&] { to_Hex (hex_number); });
     bench.run ("convert unsigned int to hexadecimal string",
diff --git a/lolly/data/numeral.cpp b/lolly/data/numeral.cpp
index d50fd59b..38a8eaf6 100644
--- a/lolly/data/numeral.cpp
+++ b/lolly/data/numeral.cpp
@@ -233,12 +233,12 @@ from_hex (string s) {
  * to the tail.
  * @tparam T unsigned integral type is expected.
  */
-template <typename T>
+template <unsigned int cur, typename T>
 std::enable_if_t<std::conjunction_v<std::is_integral<T>, std::is_unsigned<T>>,
                  void>
-as_hexadecimal_sub (T i, unsigned int cur, string& s) {
-  if (cur > 0) {
-    as_hexadecimal_sub (i >> 4, cur - 1, s);
+as_hexadecimal_sub (T i, string& s) {
+  if constexpr (cur > 0) {
+    as_hexadecimal_sub<cur - 1> (i >> 4, s);
   }
   s[cur]= hex_string[i & 15];
 }
@@ -246,7 +246,59 @@ as_hexadecimal_sub (T i, unsigned int cur, string& s) {
 string
 as_hexadecimal (int i, int len) {
   string result (len);
-  as_hexadecimal_sub ((unsigned) i, len - 1, result);
+  switch (len) {
+  case 1:
+    as_hexadecimal_sub<0> ((unsigned int) i, result);
+    break;
+  case 2:
+    as_hexadecimal_sub<1> ((unsigned int) i, result);
+    break;
+  case 3:
+    as_hexadecimal_sub<2> ((unsigned int) i, result);
+    break;
+  case 4:
+    as_hexadecimal_sub<3> ((unsigned int) i, result);
+    break;
+  case 5:
+    as_hexadecimal_sub<4> ((unsigned int) i, result);
+    break;
+  case 6:
+    as_hexadecimal_sub<5> ((unsigned int) i, result);
+    break;
+  case 7:
+    as_hexadecimal_sub<6> ((unsigned int) i, result);
+    break;
+  case 8:
+    as_hexadecimal_sub<7> ((unsigned int) i, result);
+    break;
+  case 9:
+    as_hexadecimal_sub<8> ((unsigned int) i, result);
+    break;
+  case 10:
+    as_hexadecimal_sub<9> ((unsigned int) i, result);
+    break;
+  case 11:
+    as_hexadecimal_sub<10> ((unsigned int) i, result);
+    break;
+  case 12:
+    as_hexadecimal_sub<11> ((unsigned int) i, result);
+    break;
+  case 13:
+    as_hexadecimal_sub<12> ((unsigned int) i, result);
+    break;
+  case 14:
+    as_hexadecimal_sub<13> ((unsigned int) i, result);
+    break;
+  case 15:
+    as_hexadecimal_sub<14> ((unsigned int) i, result);
+    break;
+  case 16:
+    as_hexadecimal_sub<15> ((unsigned int) i, result);
+    break;
+  default:
+    TM_FAILED ("len is too large");
+    break;
+  }
   return result;
 }