AK: Support long doubles in format strings

Author: https://github.com/Hendiadyoin1
Commit: https://github.com/SerenityOS/serenity/commit/8ed3315dec1
Pull-request: https://github.com/SerenityOS/serenity/pull/8361
Reviewed-by: https://github.com/awesomekling

AK/Format.cpp

@@ -380,6 +380,53 @@ void FormatBuilder::put_f64(
 
     put_string(string_builder.string_view(), align, min_width, NumericLimits<size_t>::max(), fill);
 }
+
+void FormatBuilder::put_f80(
+    long double value,
+    u8 base,
+    bool upper_case,
+    Align align,
+    size_t min_width,
+    size_t precision,
+    char fill,
+    SignMode sign_mode)
+{
+    StringBuilder string_builder;
+    FormatBuilder format_builder { string_builder };
+
+    bool is_negative = value < 0.0;
+    if (is_negative)
+        value = -value;
+
+    format_builder.put_u64(static_cast<u64>(value), base, false, upper_case, false, Align::Right, 0, ' ', sign_mode, is_negative);
+
+    if (precision > 0) {
+        // FIXME: This is a terrible approximation but doing it properly would be a lot of work. If someone is up for that, a good
+        // place to start would be the following video from CppCon 2019:
+        // https://youtu.be/4P_kbF0EbZM (Stephan T. Lavavej “Floating-Point <charconv>: Making Your Code 10x Faster With C++17's Final Boss”)
+        value -= static_cast<i64>(value);
+
+        long double epsilon = 0.5;
+        for (size_t i = 0; i < precision; ++i)
+            epsilon /= 10.0;
+
+        size_t visible_precision = 0;
+        for (; visible_precision < precision; ++visible_precision) {
+            if (value - static_cast<i64>(value) < epsilon)
+                break;
+            value *= 10.0;
+            epsilon *= 10.0;
+        }
+
+        if (visible_precision > 0) {
+            string_builder.append('.');
+            format_builder.put_u64(static_cast<u64>(value), base, false, upper_case, true, Align::Right, visible_precision);
+        }
+    }
+
+    put_string(string_builder.string_view(), align, min_width, NumericLimits<size_t>::max(), fill);
+}
+
 #endif
 
 void FormatBuilder::put_hexdump(ReadonlyBytes bytes, size_t width, char fill)
@@ -608,6 +655,29 @@ void Formatter<bool>::format(FormatBuilder& builder, bool value)
     }
 }
 #ifndef KERNEL
+void Formatter<long double>::format(FormatBuilder& builder, long double value)
+{
+    u8 base;
+    bool upper_case;
+    if (m_mode == Mode::Default || m_mode == Mode::Float) {
+        base = 10;
+        upper_case = false;
+    } else if (m_mode == Mode::Hexfloat) {
+        base = 16;
+        upper_case = false;
+    } else if (m_mode == Mode::HexfloatUppercase) {
+        base = 16;
+        upper_case = true;
+    } else {
+        VERIFY_NOT_REACHED();
+    }
+
+    m_width = m_width.value_or(0);
+    m_precision = m_precision.value_or(6);
+
+    builder.put_f80(value, base, upper_case, m_align, m_width.value(), m_precision.value(), m_fill, m_sign_mode);
+}
+
 void Formatter<double>::format(FormatBuilder& builder, double value)
 {
     u8 base;

AK/Format.h

@@ -185,6 +185,16 @@ public:
         SignMode sign_mode = SignMode::OnlyIfNeeded);
 
 #ifndef KERNEL
+    void put_f80(
+        long double value,
+        u8 base = 10,
+        bool upper_case = false,
+        Align align = Align::Right,
+        size_t min_width = 0,
+        size_t precision = 6,
+        char fill = ' ',
+        SignMode sign_mode = SignMode::OnlyIfNeeded);
+
     void put_f64(
         double value,
         u8 base = 10,
@@ -397,6 +407,17 @@ struct Formatter<double> : StandardFormatter {
 
     void format(FormatBuilder&, double value);
 };
+
+template<>
+struct Formatter<long double> : StandardFormatter {
+    Formatter() = default;
+    explicit Formatter(StandardFormatter formatter)
+        : StandardFormatter(formatter)
+    {
+    }
+
+    void format(FormatBuilder&, long double value);
+};
 #endif
 
 template<>