colour: add html-style colour parsing

colour.cpp

@@ -16,19 +16,56 @@
 
 #include "./colour.hpp"
 
+#include "./ascii.hpp"
 #include "./debug.hpp"
+#include "./log.hpp"
 #include "./range.hpp"
 
 #include <array>
 #include <map>
 
-//-----------------------------------------------------------------------------
+///////////////////////////////////////////////////////////////////////////////
 using util::colour;
 using util::colour3f;
 using util::colour4f;
 
 
-//-----------------------------------------------------------------------------
+///////////////////////////////////////////////////////////////////////////////
+template <size_t S, typename T>
+colour<S,T>
+colour<S,T>::parse_html (const char *fmt)
+{
+    // ensure the format is the correct length
+    auto len = strlen (fmt);
+
+    switch (len) {
+    case 1 + 2 * S:
+        if (*fmt != '#')
+            throw std::invalid_argument ("missing leading hash");
+        ++fmt;
+        break;
+
+    case 2 * S:
+        break;
+
+    default:
+        throw std::invalid_argument ("format is the wrong length");
+    }
+
+    // parse the octets
+    util::colour<S,uint8_t> res;
+    for (size_t i = 0; i < res.size (); ++i) {
+        auto a = util::ascii::from_hex (fmt[i*2+0]);
+        auto b = util::ascii::from_hex (fmt[i*2+1]);
+
+        res[i] = (a << 4u) | b;
+    }
+
+    return res.template cast<T> ();
+}
+
+
+///////////////////////////////////////////////////////////////////////////////
 static const std::map<std::string, colour<4,uint8_t>>
 HTML_COLOURS { {
     { "white",      { 0xff, 0xff, 0xff, 0xff } },

colour.hpp

@@ -34,9 +34,22 @@ namespace util {
         colour<S,U>
         cast (void) const;
 
-        static colour from_html (const std::string&);
-        static colour from_x11  (const std::string&);
-        static colour from_string (const std::string&);
+        /// parse colours specified as "#AABBCCDD".
+        ///
+        /// * the leading hash is optional.
+        /// * all components must be 2 hex digits.
+        static colour parse_html (const char*);
+        static colour parse_html (const std::string&);
+
+        /// look up the name of a colour from those specified in
+        /// html/x11/etc specifications.
+        static colour from_html (const std::string &name);
+        static colour from_x11  (const std::string &name);
+
+        /// look up all the specifications and returns the colour from one
+        /// that matches. the search order is unspecified, so if you want a
+        /// known colour then try them first yourself.
+        static colour from_string (const std::string &name);
     };
 
     // Convenience types

test/colour.cpp

@@ -16,6 +16,47 @@ main (int, char**)
         tap.expect_eq (u.cast<float> (), f, "cast u8 to float");
     }
 
+    // Check parsing is working
+    tap.expect_eq (
+        util::colour4u::parse_html ("#11223344"),
+        util::colour4u (0x11, 0x22, 0x33, 0x44),
+        "4-component html parsing with hash"
+    );
+
+    tap.expect_eq (
+        util::colour4u::parse_html ("11223344"),
+        util::colour4u (0x11, 0x22, 0x33, 0x44),
+        "4-component html parsing without hash"
+    );
+
+    tap.expect_eq (
+        util::colour1f::parse_html ("ff"),
+        util::colour1f (1.f),
+        "1-component html parsing"
+    );
+
+    tap.expect_eq (
+        util::colour3u::parse_html ("3399ff"),
+        util::colour3u (0x33, 0x99, 0xff),
+        "3-component html parsing"
+    );
+
+    tap.expect_throw<std::invalid_argument> (
+        [] () { util::colour1f::parse_html ("00112233"); },
+        "1-component parsing with 4-component format"
+    );
+
+    tap.expect_throw<std::invalid_argument> (
+        [] () { util::colour4f::parse_html ("00"); },
+        "4-component parsing with 1-component format"
+    );
+
+    tap.expect_throw<std::invalid_argument> (
+        [] () { util::colour4f::parse_html ("0011223"); },
+        "4-component parsing with one too few digits"
+    );
+
+
     // Check lookups are working
     tap.expect_eq (util::colour4f::from_html  ("white"), util::colour4f {1}, "HTML lookup");
     tap.expect_eq ( util::colour4f::from_x11  ("white"), util::colour4f {1}, "X11 lookup");