From 18cdd4d9b50e82941afa1f9af0eb014dc3bdb497 Mon Sep 17 00:00:00 2001
From: King_DuckZ <king_duckz@gmx.com>
Date: Mon, 7 Jun 2021 18:58:28 +0200
Subject: [PATCH] Trying with a smaller lookup, still slower than
 std::to_string

---
 include/duckhandy/implem/int_conv.hpp | 35 +++++++++++++--------------
 1 file changed, 17 insertions(+), 18 deletions(-)

diff --git a/include/duckhandy/implem/int_conv.hpp b/include/duckhandy/implem/int_conv.hpp
index ca59424..ded9174 100644
--- a/include/duckhandy/implem/int_conv.hpp
+++ b/include/duckhandy/implem/int_conv.hpp
@@ -155,22 +155,18 @@ namespace dhandy {
 
 		template <typename I, typename Tr>
 		struct IntConversion<I, 10, Tr, typename std::enable_if<std::is_integral<I>::value and not std::is_same<I, bool>::value>::type> {
-		private:
-			static const constexpr char lookup[200] = {
-				0,0,0,1,0,2,0,3,0,4,0,5,0,6,0,7,0,8,0,9,1,0,1,1,1,2,1,3,1,4,
-				1,5,1,6,1,7,1,8,1,9,2,0,2,1,2,2,2,3,2,4,2,5,2,6,2,7,2,8,2,9,
-				3,0,3,1,3,2,3,3,3,4,3,5,3,6,3,7,3,8,3,9,4,0,4,1,4,2,4,3,4,4,
-				4,5,4,6,4,7,4,8,4,9,5,0,5,1,5,2,5,3,5,4,5,5,5,6,5,7,5,8,5,9,
-				6,0,6,1,6,2,6,3,6,4,6,5,6,6,6,7,6,8,6,9,7,0,7,1,7,2,7,3,7,4,
-				7,5,7,6,7,7,7,8,7,9,8,0,8,1,8,2,8,3,8,4,8,5,8,6,8,7,8,8,8,9,
-				9,0,9,1,9,2,9,3,9,4,9,5,9,6,9,7,9,8,9,9
-			};
-
-		public:
 			static constexpr RevArray<I, 10, Tr> to_ary (I in) {
 				using RetType = RevArray<I, 10, Tr>;
 				using Num = implem::NumberAdaptation<I, 10>;
 
+				//How to build this lookup: 1) write numbers 0, 10, 20..., 90
+				//in binary; 2) take the lowest 5 bits, div by 2 and write as
+				//decimal; 3) divide by 2; 4) the result is the position in the
+				//below array that should be filled in with the leading digit
+				//from the original 0-90 number
+				constexpr const char lookup[16] = {
+					 0, 0, 0, 7, 4, 1, 0, 0, 8, 5, 2, 0, 0, 9, 6, 3
+				};
 				const bool was_negative = implem::is_negative<I, 10>(in);
 
 				RetType arr;
@@ -179,18 +175,21 @@ namespace dhandy {
 				arr.push_front(Tr::NullChar);
 				in = Num::abs(in);
 				while (in >= static_cast<I>(100)) {
-					const auto index = (in % 100) * 2;
-					arr.push_front(Tr::to_digit(static_cast<int>(lookup[index + 1])));
-					arr.push_front(Tr::to_digit(static_cast<int>(lookup[index + 0])));
+					const auto last_two = in % 100;
+					const auto digit_low = in % 10;
 					in = static_cast<I>(in / static_cast<I>(100));
+					const auto digit_high = lookup[((last_two - digit_low) >> 1) & 0xF];
+					arr.push_front(Tr::to_digit(static_cast<int>(digit_low)));
+					arr.push_front(Tr::to_digit(static_cast<int>(digit_high)));
 				};
 				if (in < static_cast<I>(10)) {
 					arr.push_front(Tr::to_digit(static_cast<int>(in)));
 				}
 				else {
-					const auto index = in * 2;
-					arr.push_front(Tr::to_digit(static_cast<int>(lookup[index + 1])));
-					arr.push_front(Tr::to_digit(static_cast<int>(lookup[index + 0])));
+					const auto digit_low = in % 10;
+					const auto digit_high = lookup[((in - digit_low) >> 1) & 0xF];
+					arr.push_front(Tr::to_digit(static_cast<int>(digit_low)));
+					arr.push_front(Tr::to_digit(static_cast<int>(digit_high)));
 				}
 				if (was_negative)
 					arr.push_front(Tr::Minus);