From 1bb566fcdaafc23d1f7cbf88d8906f36d4c6c626 Mon Sep 17 00:00:00 2001
From: Danny Robson <danny@nerdcruft.net>
Date: Sun, 16 Dec 2018 16:47:08 +1100
Subject: [PATCH] encode/number: add base36 encode

---
 encode/number.hpp      | 32 ++++++++++++++++++++++++++++++++
 test/encode/number.cpp | 14 ++++++++++++++
 2 files changed, 46 insertions(+)

diff --git a/encode/number.hpp b/encode/number.hpp
index e99d90d6..9ee674fe 100644
--- a/encode/number.hpp
+++ b/encode/number.hpp
@@ -9,6 +9,11 @@
 #pragma once
 
 #include "../view.hpp"
+#include "../maths.hpp"
+
+#include <array>
+#include <limits>
+
 
 ///////////////////////////////////////////////////////////////////////////////
 namespace cruft::encode {
@@ -31,4 +36,31 @@ namespace cruft::encode {
 
         return accum;
     }
+
+
+    template <typename ValueT, typename OutputT>
+    OutputT
+    encode36 (OutputT out, ValueT val)
+    {
+        constexpr auto count = digits(std::numeric_limits<ValueT>::max (), 36u);
+        std::array<char, count> res;
+
+        auto cursor = res.begin ();
+        // Ensure we do at least one iteration so we output a zero.
+        do {
+            switch (auto const d = val % 36; d) {
+            case  0 ... 9: *cursor = d      + '0'; break;
+            case 10 ...35: *cursor = d - 10 + 'a'; break;
+            }
+
+            val /= 36;
+            ++cursor;
+        } while (val);
+
+        return std::copy (
+            std::make_reverse_iterator (cursor),
+            res.rend (),
+            out
+        );
+    }
 }
diff --git a/test/encode/number.cpp b/test/encode/number.cpp
index 56c49f2d..6984615f 100644
--- a/test/encode/number.cpp
+++ b/test/encode/number.cpp
@@ -20,8 +20,22 @@ int main ()
     cruft::TAP::logger tap;
 
     for (auto const &t: TESTS) {
+        // decode the truth string and test against the truth number
         cruft::view src (t.str);
         tap.expect_eq (cruft::encode::decode36<unsigned> (src), t.value, "base36 decode '%!'", t.str);
+
+        // encode the truth number
+        std::ostringstream dst;
+        cruft::encode::encode36<unsigned> (std::ostream_iterator<char> (dst), t.value);
+
+        // covert computed and truth strings to lower case
+        std::string encoded_computed = dst.str ();
+        std::string encoded_truth    = t.str;
+        std::transform (encoded_computed.begin (), encoded_computed.end (), encoded_computed.begin (), ::tolower);
+        std::transform (encoded_truth.begin (), encoded_truth.end (), encoded_truth.begin (), ::tolower);
+
+        // compare lower case encoded strings
+        tap.expect_eq (encoded_computed, encoded_truth, "base36 encode '%!'", t.value);
     }
 
     return tap.status ();