new UI system

interfaces/Crafter.Graphics-Font.cppm

@@ -26,14 +26,45 @@ export module Crafter.Graphics:Font;
 import std;
 
 namespace Crafter {
+    // Decode the UTF-8 codepoint at `text[i]` and advance `i` past it.
+    // Returns 0 once `i` reaches the end. Malformed sequences yield U+FFFD
+    // and the index is moved past one byte to keep iteration finite.
+    export inline std::uint32_t DecodeUtf8(std::string_view text, std::size_t& i) {
+        if (i >= text.size()) return 0;
+        std::uint8_t b0 = static_cast<std::uint8_t>(text[i]);
+
+        // Single-byte ASCII is the common path.
+        if (b0 < 0x80) { ++i; return b0; }
+
+        int extra;
+        std::uint32_t cp;
+        if      ((b0 & 0xE0) == 0xC0) { extra = 1; cp = b0 & 0x1F; }
+        else if ((b0 & 0xF0) == 0xE0) { extra = 2; cp = b0 & 0x0F; }
+        else if ((b0 & 0xF8) == 0xF0) { extra = 3; cp = b0 & 0x07; }
+        else { ++i; return 0xFFFD; }   // continuation byte at start, or 5+-byte leader
+
+        ++i;
+        for (int k = 0; k < extra; ++k) {
+            if (i >= text.size()) return 0xFFFD;
+            std::uint8_t b = static_cast<std::uint8_t>(text[i]);
+            if ((b & 0xC0) != 0x80) return 0xFFFD;       // missing continuation
+            cp = (cp << 6) | (b & 0x3Fu);
+            ++i;
+        }
+        return cp;
+    }
+
     export class Font {
     public:
         std::vector<unsigned char> fontBuffer;
-        std::int_fast32_t ascent; 
-        std::int_fast32_t descent; 
+        std::int_fast32_t ascent;
+        std::int_fast32_t descent;
         std::int_fast32_t lineGap;
         stbtt_fontinfo font;
         Font(const std::filesystem::path& font);
         std::uint32_t GetLineWidth(const std::string_view text, float size);
+        float LineHeight(float size);
+        float AscentPx(float size);     // baseline offset from line-top
+        float ScaleForSize(float size); // stb's pixel-units-per-em factor
     };
 }