gnu: Add scn.

* gnu/packages/patches/scn-fast-float-compat.patch: New file.
* gnu/local.mk (dist_patch_DATA): Register it here.
* gnu/packages/cpp.scm (scn): New variable.

gnu/local.mk

@@ -2045,6 +2045,7 @@ dist_patch_DATA =						\
   %D%/packages/patches/quodlibet-fix-mtime-tests.patch		\
   %D%/packages/patches/qucs-s-qucsator-rf-search.patch		\
   %D%/packages/patches/qxlsx-fix-include-directory.patch	\
+  %D%/packages/patches/scn-fast-float-compat.patch		\
   %D%/packages/patches/sdcc-disable-non-free-code.patch		\
   %D%/packages/patches/sdl-pango-api_additions.patch		\
   %D%/packages/patches/sdl-pango-blit_overflow.patch		\

gnu/packages/cpp.scm

@@ -1719,6 +1719,34 @@ code and retrieving their output.")
    (home-page "https://github.com/DaanDeMeyer/reproc")
    (license license:expat)))
 
+(define-public scn
+  (package
+    (name "scn")
+    (version "4.0.1")
+    (source (origin
+              (method git-fetch)
+              (uri (git-reference
+                    (url "https://github.com/eliaskosunen/scnlib")
+                    (commit (string-append "v" version))))
+              (file-name (git-file-name name version))
+              (patches (search-patches "scn-fast-float-compat.patch"))
+              (sha256
+               (base32 "0lnb9r004y75n4s4pd3k58cdcjpcylhdgr5phwja713g3dd40im8"))))
+    (build-system cmake-build-system)
+    (arguments
+     (list #:configure-flags #~(list "-DSCN_USE_EXTERNAL_GTEST=yes"
+                                     "-DSCN_USE_EXTERNAL_BENCHMARK=yes"
+                                     "-DSCN_USE_EXTERNAL_FAST_FLOAT=yes"
+                                     "-DBUILD_SHARED_LIBS=yes")))
+    (propagated-inputs (list fast-float))
+    (native-inputs (list googletest googlebenchmark))
+    (home-page "https://scnlib.dev/")
+    (synopsis "Type-safe text parsing library")
+    (description "@code{scn} is a text parsing library for C++.  It can
+be used as a safe alternative to @code{scanf} or as a fast alternative to
+@code{IOStreams}, analogous to @code{fmt}.")
+    (license license:asl2.0)))
+
 (define-public sobjectizer
   (package
     (name "sobjectizer")

gnu/packages/patches/scn-fast-float-compat.patch

@@ -0,0 +1,48 @@
+From 144a590f6c7861101579069e89dfb1db0ddfec25 Mon Sep 17 00:00:00 2001
+From: Elias Kosunen <elias.kosunen@gmail.com>
+Date: Wed, 21 May 2025 23:59:59 +0300
+Subject: [PATCH] Fix incompatibility with newer fast_float
+
+---
+ benchmark/runtime/float/repeated.cpp | 1 -
+ benchmark/runtime/float/single.cpp   | 1 -
+ src/scn/impl.cpp                     | 6 ++++--
+ 3 files changed, 4 insertions(+), 4 deletions(-)
+
+diff --git a/benchmark/runtime/float/repeated.cpp b/benchmark/runtime/float/repeated.cpp
+index 0aa0c39a..8a4de0a1 100644
+--- a/benchmark/runtime/float/repeated.cpp
++++ b/benchmark/runtime/float/repeated.cpp
+@@ -210,4 +210,3 @@ static void scan_float_repeated_fastfloat(benchmark::State& state)
+ }
+ BENCHMARK_TEMPLATE(scan_float_repeated_fastfloat, float);
+ BENCHMARK_TEMPLATE(scan_float_repeated_fastfloat, double);
+-BENCHMARK_TEMPLATE(scan_float_repeated_fastfloat, long double);
+diff --git a/benchmark/runtime/float/single.cpp b/benchmark/runtime/float/single.cpp
+index e06cd138..6819621b 100644
+--- a/benchmark/runtime/float/single.cpp
++++ b/benchmark/runtime/float/single.cpp
+@@ -185,4 +185,3 @@ static void scan_float_single_fastfloat(benchmark::State& state)
+ }
+ BENCHMARK_TEMPLATE(scan_float_single_fastfloat, float);
+ BENCHMARK_TEMPLATE(scan_float_single_fastfloat, double);
+-BENCHMARK_TEMPLATE(scan_float_single_fastfloat, long double);
+diff --git a/src/scn/impl.cpp b/src/scn/impl.cpp
+index a36117d8..1e38f1f4 100644
+--- a/src/scn/impl.cpp
++++ b/src/scn/impl.cpp
+@@ -723,10 +723,12 @@ struct fast_float_impl_base : impl_base {
+     {
+         unsigned format_flags{};
+         if ((m_options & float_reader_base::allow_fixed) != 0) {
+-            format_flags |= fast_float::fixed;
++            format_flags |=
++                static_cast<unsigned>(fast_float::chars_format::fixed);
+         }
+         if ((m_options & float_reader_base::allow_scientific) != 0) {
+-            format_flags |= fast_float::scientific;
++            format_flags |=
++                static_cast<unsigned>(fast_float::chars_format::scientific);
+         }
+ 
+         return static_cast<fast_float::chars_format>(format_flags);