Corrected the "how many bits do i need for n colors"-algorithm.

CMakeLists.txt

@@ -1,6 +1,6 @@
 cmake_minimum_required (VERSION 3.2)
 project(identiconpp
-        VERSION 0.3.1
+        VERSION 0.3.2
         LANGUAGES CXX
 )
 

README.md

@@ -1,4 +1,4 @@
-**identiconpp** is a library to generate identicons. Written in C++.
+**identiconpp** is a library to generate identicons for C++.
 
 You get the images as `Magick::Image`. This allows you to make all kinds of
 modifications.
@@ -119,6 +119,7 @@ and you are welcome to redistribute it under certain conditions.
 * Use bits from digest to determine if a pixel is painted(1) or not(0).
 * Mirror the pixels vertically.
 * Use the following bits to pick the foreground color.
+  * You need `log2(n_colors) + 1` bits.
 * Scale image proportionally to requested width.
 
 ```PLAIN
@@ -139,6 +140,7 @@ Implemented in [ltr_symmetric.cpp]
 * Pixels are drawn from left to right, top to bottom.
 * Use bits from digest to determine if a pixel is painted(1) or not(0).
 * Use the following bits to pick the foreground color.
+  * * You need `log2(n_colors) + 1` bits.
 * Scale image proportionally to requested width.
 
 ```PLAIN

src/checks.cpp

@@ -17,6 +17,7 @@
 #include <exception>
 #include <stdexcept>
 #include <algorithm>
+#include <cmath>
 #include "identiconpp.hpp"
 #include "debug.hpp"
 
@@ -41,14 +42,14 @@ void Identiconpp::check_entropy(const string &digest, algorithm type)
             // We need bits for each field in half of the columns, +1 column if
             // they are uneven. Then we need enough bits to pick a color.
             entropy_required = (_columns / 2 + _columns % 2) * _rows
-                + (_foreground.size() / 2 + _foreground.size() % 2);
+                + std::log2(_foreground.size()) + 1;
             break;
         }
         case algorithm::ltr_asymmetric:
         {
             entropy_provided = digest.length() * 4;
             entropy_required = _columns * _rows
-                + (_foreground.size() / 2 + _foreground.size() % 2);
+                + std::log2(_foreground.size()) + 1;
             break;
         }
         case algorithm::sigil:

src/identiconpp.cpp

@@ -17,6 +17,7 @@
 #include <exception>
 #include <stdexcept>
 #include <sstream>
+#include <cmath>
 #include "identiconpp.hpp"
 #include "debug.hpp"
 
@@ -100,7 +101,7 @@ Magick::Color Identiconpp::get_color(const uint16_t firstbit,
                                      const string &digest)
 {
     // Number of bits to use
-    const uint16_t colorbits = _foreground.size() / 2 + _foreground.size() % 2;
+    const uint16_t colorbits = std::log2(_foreground.size()) + 1;
 
     // Extract approximation
     std::stringstream ss;