♻️ reorganize PR

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

include/nlohmann/json.hpp

@@ -5382,27 +5382,75 @@ inline void swap(nlohmann::NLOHMANN_BASIC_JSON_TPL& j1, nlohmann::NLOHMANN_BASIC
 NLOHMANN_BASIC_JSON_TPL_DECLARATION
 struct formatter<nlohmann::NLOHMANN_BASIC_JSON_TPL, char> // NOLINT(cert-dcl58-cpp)
 {
-    bool pretty = false;
+    // -1 means compact output (dump()); any value >= 0 means pretty-printed
+    // output with that many spaces (or indent_char) per level (dump(indent, indent_char)).
+    int indent = -1;
+    char indent_char = ' ';
 
     constexpr auto parse(format_parse_context& ctx) -> format_parse_context::iterator
     {
         auto it = ctx.begin();
-        if (it != ctx.end() && *it == '#')
+        const auto end = ctx.end();
+        constexpr auto is_align = [](char c)
+        {
+            return c == '<' || c == '>' || c == '^';
+        };
+
+        // [[fill] align] - repurposed here to pick a custom indent character,
+        // e.g. "{:.>#4}" pretty-prints with '.' as the indent character
+        if (it != end && it + 1 != end && is_align(it[1]))
+        {
+            indent_char = *it;
+            it += 2;
+        }
+        else if (it != end && is_align(*it))
+        {
+            ++it;
+        }
+
+        // ['#'] - "alternate form", used here to request pretty-printing
+        bool pretty = false;
+        if (it != end && *it == '#')
         {
             pretty = true;
             ++it;
         }
-        if (it != ctx.end() && *it != '}')
+
+        // [width] - repurposed here to pick the indent size for pretty-printing,
+        // e.g. "{:2}" or "{:#2}" pretty-print with an indent of 2; a width without
+        // '#' implies pretty-printing since an indent otherwise has no meaning
+        if (it != end && *it >= '1' && *it <= '9')
+        {
+            indent = 0;
+            while (it != end && *it >= '0' && *it <= '9')
+            {
+                indent = (indent * 10) + (*it - '0');
+                ++it;
+            }
+        }
+        else if (pretty)
+        {
+            indent = 4;
+        }
+
+        // sign, the '0' flag, precision, locale-specific formatting ('L'), dynamic
+        // width/precision ("{...}"), and type characters all have no meaning for
+        // JSON values; none of them are consumed above, so they all end up rejected
+        // by this single check along with any other unrecognized trailing spec.
+        if (it != end && *it != '}')
         {
             JSON_THROW(format_error("invalid format args for nlohmann::json"));
         }
+
         return it;
     }
 
     template<typename FormatContext>
     auto format(const nlohmann::NLOHMANN_BASIC_JSON_TPL& j, FormatContext& ctx) const -> decltype(ctx.out())
     {
-        const auto dumped = pretty ? j.dump(4) : j.dump();
+        // dump()'s own default (indent = -1) already means compact output, so this
+        // covers both the compact and pretty-printed cases without a branch.
+        const auto dumped = j.dump(indent, indent_char);
         return std::copy(dumped.begin(), dumped.end(), ctx.out());
     }
 };