Use std::to_chars for formatting float numbers as well

this gives us a larger representation in many cases since it tries to
represent floats to max precision, but that tradeoff is worth it for
dropping 2000 lines of C code.

setup/extensions.json

@@ -149,7 +149,7 @@
     },
     {
         "name": "fast_css_transform",
-        "headers": "calibre/utils/cpp_binding.h calibre/utils/stb_sprintf.h",
+        "headers": "calibre/utils/cpp_binding.h",
         "sources": "calibre/srv/fast_css_transform.cpp",
 		"inc_dirs": "perfect-hashing",
 		"needs_c++": "17"

src/calibre/srv/fast_css_transform.cpp

@@ -26,8 +26,6 @@
 #include <frozen/unordered_map.h>
 #include <frozen/string.h>
 #include "../utils/cpp_binding.h"
-#define STB_SPRINTF_IMPLEMENTATION
-#include "../utils/stb_sprintf.h"
 
 // character classes {{{
 static inline bool
@@ -465,9 +463,10 @@ class Token {
             double new_val = convert_font_size(val, lit->second);
             if (val == new_val) return false;
             char txt[128];
-            // stbsp_snprintf is locale independent unlike std::snprintf
-            int num = stbsp_snprintf(txt, sizeof(txt), "%grem", new_val);
-            if (num <= 0) throw std::runtime_error("Failed to format font size");
+            auto res = std::to_chars(txt, txt + sizeof(txt)-8, new_val, std::chars_format::general);
+            if (res.ec != std::errc()) throw std::runtime_error("Failed to format font size");
+            size_t num = res.ptr - txt;
+            txt[num++] = 'r'; txt[num++] = 'e'; txt[num++] = 'm';
             set_ascii_text(txt, num);
             return true;
         }

src/calibre/srv/tests/fast_css_transform.py

@@ -64,11 +64,11 @@ class TestTransform(SimpleTest):
         u('a:url(  "( )" /**/ )', 'a:url("( )")')
         u('a:url(  "(/*)"  )', 'a:url(  "(/*)"  )', url_callback=lambda x: x)
 
-        d('font-size: 197583965730245.28px', 'font-size: 1.2349e+13rem')
+        d('font-size: 197583965730245.28px', 'font-size: 1.234899785814033e+13rem')
         d('font-size: 19.28px', 'font-size: 1.205rem')
         d('font-size:+19.28px', 'font-size:1.205rem')
-        d('font-size: .28in', 'font-size: 1.68rem')
-        d('font-size: +.28in', 'font-size: 1.68rem')
+        d('font-size: .28in', 'font-size: 1.6800000000000002rem')
+        d('font-size: +.28in', 'font-size: 1.6800000000000002rem')
         d(r'f\ont-s\69z\65 : 16\px', 'font-size: 1rem')
         d('font -size: 16px', 'font -size: 16px')
         d('font-/* */size: 1/*x*/6/**/p/**/x !important', 'font-size: 1rem !important')