From f5fee88f3ad964d4e42952bff190108dd7776959 Mon Sep 17 00:00:00 2001
From: Radonirinaunimi <rrabeman@nikhef.nl>
Date: Mon, 7 Apr 2025 12:03:13 +0200
Subject: [PATCH 01/15] Add functions to get and rotate PID basis in C-API

---
 examples/cpp/advanced-convolution.cpp |  9 +++++++
 pineappl_capi/src/lib.rs              | 36 +++++++++++++++++++++++++++
 2 files changed, 45 insertions(+)

diff --git a/examples/cpp/advanced-convolution.cpp b/examples/cpp/advanced-convolution.cpp
index e16f9e30..95fdfb4a 100644
--- a/examples/cpp/advanced-convolution.cpp
+++ b/examples/cpp/advanced-convolution.cpp
@@ -123,5 +123,14 @@ int main(int argc, char* argv[]) {
             << normalizations.at(bin) << '\n';
     }
 
+    // modify the particle ID representation of the Grid
+    char* ref_pid_repr_basis = pineappl_get_pid_basis(grid);
+    assert(std::string(ref_pid_repr_basis) == "PINEAPPL_PID_BASIS_EVOL");
+
+    pineappl_rotate_pid_basis(grid, PINEAPPL_PID_BASIS_PDG);
+
+    char* mod_pid_repr_basis = pineappl_get_pid_basis(grid);
+    assert(std::string(mod_pid_repr_basis) == "PINEAPPL_PID_BASIS_PDG");
+
     pineappl_grid_delete(grid);
 }
diff --git a/pineappl_capi/src/lib.rs b/pineappl_capi/src/lib.rs
index 20b792ef..263f3cf6 100644
--- a/pineappl_capi/src/lib.rs
+++ b/pineappl_capi/src/lib.rs
@@ -895,6 +895,42 @@ pub unsafe extern "C" fn pineappl_grid_split_lumi(grid: *mut Grid) {
     grid.split_channels();
 }
 
+/// Change the particle ID representation of a given Grid.
+///
+/// # Safety
+///
+/// If `grid` does not point to a valid `Grid` object (for example when `grid` is the null pointer)
+/// or if `pid_basis` doe not refer to a correct basis, then this function is not safe to call.
+#[no_mangle]
+pub unsafe extern "C" fn pineappl_rotate_pid_basis(grid: *mut Grid, pid_basis: PidBasis) {
+    let grid = unsafe { &mut *grid };
+
+    grid.rotate_pid_basis(pid_basis);
+}
+
+/// Get the particle ID representation of a Grid.
+///
+/// # Safety
+///
+/// If `grid` does not point to a valid `Grid` object, for example when `grid` is the `NULL`
+/// pointer, this function is not safe to call. The parameter `key` must be non-`NULL` and a valid
+/// C string.
+///
+/// # Panics
+///
+/// This function might panic if `pid_basis` contains interior null bytes. This occurs if  a new
+/// variant is added to the `PidBasis` enum in the future and the match statement isn't updated.
+#[no_mangle]
+pub unsafe extern "C" fn pineappl_get_pid_basis(grid: *mut Grid) -> *mut c_char {
+    let grid = unsafe { &mut *grid };
+    let pid_basis = match grid.pid_basis() {
+        PidBasis::Pdg => "PINEAPPL_PID_BASIS_PDG",
+        PidBasis::Evol => "PINEAPPL_PID_BASIS_EVOL",
+    };
+
+    CString::new(pid_basis).unwrap().into_raw()
+}
+
 /// Optimizes the grid representation for space efficiency.
 ///
 /// # Safety

From 9ded95b07e0bdc0b7ce7f2e0c1300a88fbcca825 Mon Sep 17 00:00:00 2001
From: Radonirinaunimi <rrabeman@nikhef.nl>
Date: Tue, 8 Apr 2025 12:44:29 +0200
Subject: [PATCH 02/15] Fix memory leak in `advanced-convolution.cpp`

---
 examples/cpp/advanced-convolution.cpp | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/examples/cpp/advanced-convolution.cpp b/examples/cpp/advanced-convolution.cpp
index 95fdfb4a..3ddb796c 100644
--- a/examples/cpp/advanced-convolution.cpp
+++ b/examples/cpp/advanced-convolution.cpp
@@ -126,11 +126,13 @@ int main(int argc, char* argv[]) {
     // modify the particle ID representation of the Grid
     char* ref_pid_repr_basis = pineappl_get_pid_basis(grid);
     assert(std::string(ref_pid_repr_basis) == "PINEAPPL_PID_BASIS_EVOL");
+    pineappl_string_delete(ref_pid_repr_basis);
 
     pineappl_rotate_pid_basis(grid, PINEAPPL_PID_BASIS_PDG);
 
     char* mod_pid_repr_basis = pineappl_get_pid_basis(grid);
     assert(std::string(mod_pid_repr_basis) == "PINEAPPL_PID_BASIS_PDG");
+    pineappl_string_delete(mod_pid_repr_basis);
 
     pineappl_grid_delete(grid);
 }

From e03260cf3eeb89b051315cd34b90f7c55af32522 Mon Sep 17 00:00:00 2001
From: Radonirinaunimi <rrabeman@nikhef.nl>
Date: Tue, 8 Apr 2025 21:32:22 +0200
Subject: [PATCH 03/15] Address review comments

---
 examples/cpp/advanced-convolution.cpp | 12 +++++-------
 pineappl_capi/src/lib.rs              | 24 +++++++-----------------
 2 files changed, 12 insertions(+), 24 deletions(-)

diff --git a/examples/cpp/advanced-convolution.cpp b/examples/cpp/advanced-convolution.cpp
index 3ddb796c..16f67eed 100644
--- a/examples/cpp/advanced-convolution.cpp
+++ b/examples/cpp/advanced-convolution.cpp
@@ -124,15 +124,13 @@ int main(int argc, char* argv[]) {
     }
 
     // modify the particle ID representation of the Grid
-    char* ref_pid_repr_basis = pineappl_get_pid_basis(grid);
-    assert(std::string(ref_pid_repr_basis) == "PINEAPPL_PID_BASIS_EVOL");
-    pineappl_string_delete(ref_pid_repr_basis);
+    pineappl_pid_basis ref_pid_repr_basis = pineappl_grid_pid_basis(grid);
+    assert(ref_pid_repr_basis == PINEAPPL_PID_BASIS_EVOL);
 
-    pineappl_rotate_pid_basis(grid, PINEAPPL_PID_BASIS_PDG);
+    pineappl_grid_rotate_pid_basis(grid, PINEAPPL_PID_BASIS_PDG);
 
-    char* mod_pid_repr_basis = pineappl_get_pid_basis(grid);
-    assert(std::string(mod_pid_repr_basis) == "PINEAPPL_PID_BASIS_PDG");
-    pineappl_string_delete(mod_pid_repr_basis);
+    pineappl_pid_basis mod_pid_repr_basis = pineappl_grid_pid_basis(grid);
+    assert(mod_pid_repr_basis == PINEAPPL_PID_BASIS_PDG);
 
     pineappl_grid_delete(grid);
 }
diff --git a/pineappl_capi/src/lib.rs b/pineappl_capi/src/lib.rs
index 263f3cf6..4baaa1d1 100644
--- a/pineappl_capi/src/lib.rs
+++ b/pineappl_capi/src/lib.rs
@@ -895,40 +895,30 @@ pub unsafe extern "C" fn pineappl_grid_split_lumi(grid: *mut Grid) {
     grid.split_channels();
 }
 
-/// Change the particle ID representation of a given Grid.
+/// Change the particle ID basis of a given Grid.
 ///
 /// # Safety
 ///
 /// If `grid` does not point to a valid `Grid` object (for example when `grid` is the null pointer)
-/// or if `pid_basis` doe not refer to a correct basis, then this function is not safe to call.
+/// or if `pid_basis` does not refer to a correct basis, then this function is not safe to call.
 #[no_mangle]
-pub unsafe extern "C" fn pineappl_rotate_pid_basis(grid: *mut Grid, pid_basis: PidBasis) {
+pub unsafe extern "C" fn pineappl_grid_rotate_pid_basis(grid: *mut Grid, pid_basis: PidBasis) {
     let grid = unsafe { &mut *grid };
 
     grid.rotate_pid_basis(pid_basis);
 }
 
-/// Get the particle ID representation of a Grid.
+/// Get the particle ID basis of a Grid.
 ///
 /// # Safety
 ///
 /// If `grid` does not point to a valid `Grid` object, for example when `grid` is the `NULL`
-/// pointer, this function is not safe to call. The parameter `key` must be non-`NULL` and a valid
-/// C string.
-///
-/// # Panics
-///
-/// This function might panic if `pid_basis` contains interior null bytes. This occurs if  a new
-/// variant is added to the `PidBasis` enum in the future and the match statement isn't updated.
+/// pointer, this function is not safe to call.
 #[no_mangle]
-pub unsafe extern "C" fn pineappl_get_pid_basis(grid: *mut Grid) -> *mut c_char {
+pub unsafe extern "C" fn pineappl_grid_pid_basis(grid: *mut Grid) -> PidBasis {
     let grid = unsafe { &mut *grid };
-    let pid_basis = match grid.pid_basis() {
-        PidBasis::Pdg => "PINEAPPL_PID_BASIS_PDG",
-        PidBasis::Evol => "PINEAPPL_PID_BASIS_EVOL",
-    };
 
-    CString::new(pid_basis).unwrap().into_raw()
+    *grid.pid_basis()
 }
 
 /// Optimizes the grid representation for space efficiency.

From d3c7b40f7d884835a4c16af33457a7989ebb275e Mon Sep 17 00:00:00 2001
From: Radonirinaunimi <rrabeman@nikhef.nl>
Date: Thu, 10 Apr 2025 21:49:55 +0200
Subject: [PATCH 04/15] Proposal to expose the subgrid contents of a Grid

---
 examples/cpp/Makefile        |   4 ++
 examples/cpp/get-subgrids    | Bin 0 -> 16912 bytes
 examples/cpp/output          |  26 ++++++++++++
 pineappl/src/packed_array.rs |   7 +++-
 pineappl_capi/src/lib.rs     |  76 +++++++++++++++++++++++++++++++++++
 5 files changed, 112 insertions(+), 1 deletion(-)
 create mode 100755 examples/cpp/get-subgrids

diff --git a/examples/cpp/Makefile b/examples/cpp/Makefile
index 7f44b4f6..aef4689b 100644
--- a/examples/cpp/Makefile
+++ b/examples/cpp/Makefile
@@ -13,6 +13,7 @@ PROGRAMS = \
 	advanced-filling \
 	convolve-grid-deprecated \
 	convolve-grid \
+	get-subgrids \
 	deprecated \
 	display-channels-deprecated \
 	display-channels \
@@ -47,6 +48,9 @@ convolve-grid: convolve-grid.cpp
 deprecated: deprecated.cpp
 	$(CXX) $(CXXFLAGS) $< $(LHAPDF_DEPS) $(PINEAPPL_DEPS) -o $@
 
+get-subgrids: get-subgrids.cpp
+	$(CXX) $(CXXFLAGS) $< $(PINEAPPL_DEPS) -o $@
+
 display-channels-deprecated: display-channels-deprecated.cpp
 	$(CXX) $(CXXFLAGS) $< $(PINEAPPL_DEPS) -o $@
 
diff --git a/examples/cpp/get-subgrids b/examples/cpp/get-subgrids
new file mode 100755
index 0000000000000000000000000000000000000000..32dacc697f6dfd8bfc8b4b25d0499a50f38434f1
GIT binary patch
literal 16912
zcmeHO4RBo5b-t?~YmJRoc5D!)!3(TG4rr~FY#|vOc4f&tnU$?ra&V#Md9}NewybuQ
z-6wnP5~3iR@kS_!kPr_rjhji@)R_j_rkNJfR@h+SHf{Ln6lc<oJt-u+IN0XTV<1@l
z&VBc+-Y!~7nM|fL-J^Lr_k8!9bI-l+zWd&Nd)}8i{2R(_Ho;UbJ}XEXzQjbls-k<X
zDu8)Ki?|s7my0XJJjfLiGwZ!3L#;{2%ksHh;pO0DS4)LC`0tspV9pT|C%a-)bFo>&
zT-2B{*;P_e$%TA(m8r*^>vQcn&3aM5e?bZ$=2klzo5^}vEhg9H^BUD}jXR^_qTwRD
zgUarpvSaR4<H?-IlibkJqWl(eFY1WA^1l#;b}407$lH}Zb83UA=T#&0N!sgHcB4+|
zMilbvO;|AJ_C5zY8kdhte44kY_KKZ{VKtu2)#((gqLIGl)vKbR`e-CJlB!R&G}kw;
zb|>TRMi~Od)qsQM)GeE~in38LMHt)LgiXrxff1D{nd?b+2@V=|vOjXD{--CNO1<^R
zuV43r{^J`v55H_1Asy-m*^mwqVRSH2_?O`z9dmqYWSKCcr3V(SH%oK>-vD^=993T_
zg2&O|RdeY7YZ3i>pg))YUl-BeUPS*2bZ{;|-z|baRm8u)i2vJ)_&HMqUr_|FEP}Tc
z!S5&%=XZ+Wa3h>J3Ta)Deto}4yGb~hYdk*<+*#-xGF^{z;Z10Fkys|Cyifqupgl=?
zXK$At3Maw?k)#n$^meU}#$(~$KwmU0bbVkb9@CRXAYtgbkg9Vk4o70)!0>QXA4o((
z@D>dlVUZ8^MPhm|J`yu#@#ILK)K3luhAryBcx-1pI$}iPv7{ai$3AQjNF)NgXJP3i
z5e|feetVD6*r4miU?RSYYzK@%J)B6y6WzB4QM;)p-l})$kytXEFgk}iI=1x0d(p9i
ziqLFDDB8W#bnA)4lX_nu8P>7nBm#-ua1$|5sSl)5`fxarjK>1eh_PGW*&r=E&AJ{>
z8VGENTMYIX9#3#EkkE}pAYvptgVd7YX@cKKkhME|_1*@(qo-N#zO|=GkHTAQ*N}|T
zxIx#0sg%d#X*5mXKN1`0oWpTXfV@+8g9yP0cWfE@gg!hCa1x3nhvUg`hdCOnaVia=
zZ;kDW#6tR(aB^gbrVD0D@Gd<#c$eNEh#+nns-T)bLxD&PNH7)9`y(_`d%|KUJd_L@
z0$RDr5)2GSX!-UKnlwVe>#ifQnZ%)7X&?x<IkljP-?Dza-sEl&>$h$5wRP$qcaylq
z-`NHc@M?EcW1iUP$rD?xl3CUp-K%q@rrBbndkrof+&OT!D92HOyGuF#Y*^VVL>2DY
z6=J@uw~31&lcr7HK}@S3;##Epg%d~jHM<uGX#E5jkqy2cRrNa{zx4QBxOL4J|E93E
z^WQ|tE>0-<3sSBUr<I)k(n^#n#0N??R(=hS5anWldTyXN4y!t9Z%HJAC$V|rQq)sS
zKYZ|E+@31Ml}a927QwBtOkA(*`T2;SOLkmJUW7QHo_`pxQTU`+R_C5$Sf0As#OYqB
zTub0N3$7mZB>ubw|CDJe{~x#Ds!55TwBWTC{R3W8pyiMH%QWRROZ1$;_hgAEoMPc6
zlW@BQ$8hGT#)4b#|FsreKHn+}rv*2k3(Sf-3(jLoH4PS=pIHcRvEUaGpvdbr3lu*l
zT8=cYStNX64#j-5;OY@w)^4-l4vYQ{3%<yL4_ffW7JS%(({mM5%4?P=o}~<ek9tjk
zaJAJjfztod2>d@9fwvr2yr+%*+OCaP9$O`Zwr|QPn|WRvd&Yi5j>62Ee*!$S;wSj;
zSnfrM<Uy*-o}2-#I7~7vVcF-U{3OX`Bu`2CuSlk)IQxW@50OkuSoWZlzfCeNUD-(~
zKSDAsS=mu3e~o0iOtZsM{tC&obY*u)`CgJ~$;x(1c@N37RAs$VPLNDXRJK9Ne@HSd
zO<AXu`$(oGDO)4u+exk_Sx9*c$qtgwUI4ISBgwSHXHP=b#@k(kA?><~6?K3{7HSjg
zUVw4tZPaBpL%(a^D<iA4^oh)32-^7FuDZ-NS<17!R7;=9j1r!H_S6gh^sh4i1qY|5
zGYdbgTZ=Y^sEtW#ANqf;?fd!2W2e5IIRT*G?O47Sq0s4S7s7X&=SAP=eOrCKTYGSx
zQz32qCK;-hzS}jZP2BDBY7_0QZf(MFwP@4rt`^z>Xz6y~(5TD2PmPTCx?1`j@k%$E
zRdv0tZpR`);7WjwMc#{PC}7m<(vDWt2<oagvrdLMt)(kmTKY)l9x0w67xvEdbmkvE
zpdPflYWx$!HcUacTL}L|{k34)#F`M!ia&iu8}H8eZLf{D)oRmiHFRcOn-`9z+iIVr
zS(SN@e2iDVRWHPoG;LtGiVeM)&gu3_X_I-;)ca@Bqv->^>H<AFtC>U4dy?ie6wYCu
zAcD-prtJf;1$xRv|A)BxOq)BXJr&pEY-o*Zt*O^UdNeK3z?Td3WL%3aaouSuWG?)M
zCfJN)?+RRS+TUWX5IOv<XC3z*h5p8gig(~rcwVPbu00i<^pA&JwKTsTf~hukw+m-#
znpALJkN782b_2C1{pp!a6RxX0=d_9L^IgzhL)zmvxjNIwJ;yvRYU$INH>HCc#mM!T
z-P+i*woC&dtx=cVvG*Goz^;AgMkYN!@9@l=dZwQzrN8xk$6nf$YU!WJQFbgJm8}O4
zYmUcl+St?f^x@9dpN^dN2hUr3bl!3AB>c<XUJLt;6H7iY!+x3OoHKLyeY9?Nr8E7E
zhOTsu&Db1!@1Vvoft{Xn{)wAvw6WLhnSRnmce>IixjSzG@~6-HSDw_KK40F|`YXrY
z@4`Pi?|HG`ac1h&CGvXG#tz#$$ENLC>tV+jU1$A1hr4P|neofc@eXwAG)-8FM;q5L
zYqR4%%v#^<to7Aq-hB^^r>C`Xgk_MTH-ihW%ABSDk*j>u=cuX5Rmv%_io#A$^X%M)
zYa#PK46w#H_U^>BMJM(UfNTVUjJLz{3Qj=QLDPcP1L}3=25LkuMgJm8c}r0<7XOrI
z>W=NcKk#k$-RaYh{3Bg1nLbj@jW11=?BAi4&h$+7FTu_F^t0Io2r|2d{_T+AKAYVR
z&gY-*H=fr!(=XhqrGKG~y>hm@x5G2#Ifgaub5NUE+O$%Lw`UhI;O_#_CMw?~)_UGp
zii`acD&tl)f99l`KSy{TO{_a6_17FGw(^3OKC3-_`er(*n(e6e!Ubaq9K4FvnjF|?
zPGV|eT-cuC8(sG|pFxjDnzm|V>mH$XT}!`eV0~M6Cw$H<-47@eM6gFI?}wso`;pmr
zkMz%$>3&K6vIEeCm)yNJcXtI?qS6SIMxZnTr4cBNKxqU@Bk(&O0eaVrpHM_{z!?dp
zi0uqSN5Xh>TyG&Ef`M2pZaB?X{!Y9<U%Pf^IB3KZ&S)Sp5KcIa!9dJ86iDewynwI6
zbCOA+L^v9)-yMk6Cj!IuC`O}hekbpa?pbZSy!^)Np<03C;F+139pF=EXJ&o_zT=IV
zneTz0eFsmT;6(gx&$efau%&8jmoKccPvQxmaLN@p2;FN)5f9YFTsT(aXn}7h^h9;d
zhU(gljs?5yqvGaeH-4tcbrsOrJcB*me(AaD(Jw9+;&S%i4IT3J1Y+BdU@Pu|&{kdZ
zKv{cr?N`g&s+|)RZPj)6SNf_O?wjYUZW*g;uYST-er?4?)h+9*8+_Gu&}pl#g-u(v
z-A6AZoH$ZAevH^21g~hvKNyWw-8b+4%881vmOnu6QSchVw$h6?B=>x){C&IlSw-dO
ze37lJJZBf*oA>v&({}N7dnGE4*$IO3#g|rgi(6svN0p=0<r0-fpfm!d5h#s7X#`3m
zP#S^K2$V*kGy?xOM}YHQIM0Qj-}lL#z-+Dv<?;}5{h<Pxa)GS%l$%3@AC63tyCZ=-
zZj+)a$%S)NI3MS7Rc}{(nc|$6!}-}<|IUS(IH^pj<Qw{7muMUP1O`$}WK=!<oJYj@
z%A8BWxi!|D4boF)BBSzfRJX(dH4wZ%q#ullICm$d95cROHOz6JHPw+3WfHfK-y~#z
zfJ#;T(jn#Nlp@~jOPS-s_E5@`s{Y-AdgcCi&NzR(u;FGk4>l<NdByt`->LY0ia)IQ
zpDO+%#g8if?~4CY@pFpLQ%O9RD1MFN&5CnelU^>F9JzOEops25Ug`9#c6;0&XJbR-
z>V`E9H;7_Yjg9#(@+6uj72*y0IVO)Nm#9os3$KdjK}+pi`sTb}l*?ad^;Ia?kWo}P
z-YpWZ&COfZ--H6ST}VLybKzeEUV}`sgWh}#dQ;#8zP|w93$;a*k>)h@@#;d-L->D{
zDqGnPAS}i>oK)jL?>dP78pmAm{AUsTEbv8Tmx)O)3KpW@Y_9}<z-xkh{auC=Gnaoa
z;bjiN>l3@$P((ji1b+zQL;d3KTI4$-INMo7e}57D(IWVd2rt9xs^&Y#`D5TT4u$9I
zxgvgCGETOh<-35-m1jKxoaRH}d4ELW%fxZttPd)EOW{-AeEj>s=NkW)i{KZE;8$QF
zoXbyh5xfhyGd~n5MT+Q;7QtUCg1=V8|GPzSF%UNDRRVd^9ULAOlwa%)#-qpr*Rc-i
z9h$yji?6Fg*V?xTz5Vw$`MNsS3;h=V<~E;S-@IW%PY3d>eQo{@y&#pmFn|3+3F&$`
z6fgqFhu@6c<`5FwN0MP`)|%Vwm8r)_A*O6$GxzzUlG9CfGgDcmMdx|Y^-wad<K1lt
zdHL<Pxf9`jcahBVxs%+j4Y*qc<zPoXoS>a+6Y0+S?Uej(wKEgZ^ALVVY-awON%#e6
z;5nagNh$Ma6<-<X9h=(en*t2VZ!J;Y8X5HwY5o7}g!%kr_dJit7^iS$?z^=D9isVQ
z=D7<!9HY6=mrOE1=e@|+16DVbpkC}tggia;Y>saZyq2{0{Il{ApI(?>Qhek$AbC0d
zUe2-qL}Q!(F+yISFt)j%^S=xE*dL(?cXIcT5$FRq5+)zyQY40NTZV-jzm0_51F;eJ
za3YRRHjLdCsBa{K&&VPn1>qBddLu9(r21eWIVjwr-7z#_awB2Z?8Jv3_);ZL&{2oq
zQvzh5O2bh@xaI738{rhn^7?Zp;&LT%hX?ikL|`ba4~AgN5}e{Yhj1zw9yWCJ2$#O}
zp_`{o`rtB}3JgVpFp0xE{<&q7rh80AfwggHD2(rKek(-#4cr>>EM`(I)@dT<yx%a<
z+5Hd8SEE4B_-xPn3g%u`RGgnb7;Z+vYM)Z=Gk39~;`}_taA$%2W6GYnlNA*&w7(7b
zIqVrYe%?nh=k|FYRTzH&_O#Vud){X;=kFY;k2KC~&-<MSFbbLNdEdsI#}O)$$cL4_
z0D<;IY|r~3=JZ<}5y#JZ%<n-R?Xg(qeHQa6G6qrn6eq_|7|{d{8dJ9CeI9dqE+yjj
z*`E9VP1KQn8!3T!-^iTznPg8kh4y~}jK+lh=bi`Eb0P1ip(3g9{67ZNYVTBbIbJZ}
z{J6>d&kF2$Kf`=wE-<rwwlgIm{-VI1pSzj!`vTT4jGy)i6e5qma4Jp3N0rE|u>Ge{
zVYMH1nwre%w_+<5wtozjR{JNMrZ)4pt;Q(x`D6Gc6s-2&xzem*{#jO3oOG@9Duikr
z>aw5x?z`U9Dr}Es=5IjFVX^1;EC)WOJ?*dP6m$E$59RO4o$AEU^+Uw%@_Cqt4pE=j
zp1-G>qKz%6&Sbg&Y|nf_fjz&c$te4SIZLy^cFdO)*z>+-e}k#d*D1SUJC>Khp0Hf}
zUbE&P+f(N(<h7KkxLmcb)N{3LPxl8au{$fz&3%jFapkZ@P1sMPj?Mwc&;6p;MjX4v
gT-4B~fuK(nOn7eBtff8T-1b+uni_Ql1_h${Z&>Y(FaQ7m

literal 0
HcmV?d00001

diff --git a/examples/cpp/output b/examples/cpp/output
index e7262e30..d7f24048 100644
--- a/examples/cpp/output
+++ b/examples/cpp/output
@@ -146,6 +146,32 @@ idx   left  right    dsig/dx     dx
  21    2.1    2.2 3.507294e-02    0.1
  22    2.2    2.3 1.976542e-02    0.1
  23    2.3    2.4 5.590552e-03    0.1
+       bin    sg idx        sg value
+       ---    ------    ------------
+         0     41020    -4.93616e-07
+         1     41020     -4.5499e-08
+         2     40971    -4.10008e-08
+         3     40971    -2.78597e-07
+         4     40971    -2.01924e-07
+         5     40922    -3.39322e-10
+         6     40922    -8.43358e-08
+         7     40922    -2.99247e-07
+         8     40922     -1.5117e-07
+         9     40872    -9.95013e-11
+        10     40873    -1.30578e-07
+        11     40873    -2.55869e-07
+        12     40823     -1.0823e-09
+        13     40823    -4.22686e-09
+        14     40824    -1.68149e-07
+        15     40774    -7.58702e-10
+        16     40774    -2.41887e-08
+        17     40774    -8.83139e-09
+        18     40725    -1.14932e-09
+        19     40725    -3.03857e-08
+        20     40725    -2.66414e-08
+        21     40675    -3.60243e-10
+        22     40676    -1.22785e-08
+        23     40626    -7.04493e-11
 idx  p-p c#0 l#0 p-p~ c#0 l#  p-d c#0 l#0       p-d       dx
 --- ------------ ----------- ------------- ------------ ------
   0 5.263109e-01 5.263109e-01 5.263109e-01 5.263109e-01    0.1
diff --git a/pineappl/src/packed_array.rs b/pineappl/src/packed_array.rs
index 0c5d9587..f93c544d 100644
--- a/pineappl/src/packed_array.rs
+++ b/pineappl/src/packed_array.rs
@@ -171,7 +171,12 @@ impl<T: Copy + Default + PartialEq> From<ArrayViewD<'_, T>> for PackedArray<T> {
 }
 
 /// Converts a `multi_index` into a flat index.
-fn ravel_multi_index(multi_index: &[usize], shape: &[usize]) -> usize {
+///
+/// # Panics
+///
+/// TODO
+#[must_use]
+pub fn ravel_multi_index(multi_index: &[usize], shape: &[usize]) -> usize {
     assert_eq!(multi_index.len(), shape.len());
 
     multi_index
diff --git a/pineappl_capi/src/lib.rs b/pineappl_capi/src/lib.rs
index 4baaa1d1..8aa0cf75 100644
--- a/pineappl_capi/src/lib.rs
+++ b/pineappl_capi/src/lib.rs
@@ -60,7 +60,9 @@ use pineappl::boc::{Bin, BinsWithFillLimits, Channel, Kinematics, Order, ScaleFu
 use pineappl::convolutions::{Conv, ConvType, ConvolutionCache};
 use pineappl::grid::{Grid, GridOptFlags};
 use pineappl::interpolation::{Interp, InterpMeth, Map, ReweightMeth};
+use pineappl::packed_array::ravel_multi_index;
 use pineappl::pids::PidBasis;
+use pineappl::subgrid::Subgrid;
 use std::collections::HashMap;
 use std::ffi::{CStr, CString};
 use std::fs::File;
@@ -1893,3 +1895,77 @@ pub unsafe extern "C" fn pineappl_grid_convolve(
         mu_scales,
     ));
 }
+
+/// Get the number of convolutions for a given Grid.
+///
+/// # Safety
+///
+/// If `grid` does not point to a valid `Grid` object, for example when `grid` is the null pointer,
+/// this function is not safe to call.
+#[no_mangle]
+pub unsafe extern "C" fn pineappl_convolutions_len(grid: *mut Grid) -> usize {
+    let grid = unsafe { &mut *grid };
+    grid.convolutions().len()
+}
+
+/// Get the shape of a subgrid for a given bin, channel, and order.
+///
+/// # Safety
+///
+/// If `grid` does not point to a valid `Grid` object, for example when `grid` is the null pointer,
+/// this function is not safe to call. Additionally, the pointer that specifies the shape of the
+/// subgrid must be an array whose size must be `(n+1)` where `n` represents the number of
+/// convolutions as given by `pineappl_convolutions_len`.
+#[no_mangle]
+pub unsafe extern "C" fn pineappl_subgrid_shape(
+    grid: *mut Grid,
+    bin: usize,
+    order: usize,
+    channel: usize,
+    shape: *mut usize,
+) {
+    let grid = unsafe { &mut *grid };
+    let mut subgrid = grid.subgrids()[[order, bin, channel]].clone();
+
+    let subgrid_shape = if subgrid.is_empty() {
+        let subgrid_dim = grid.convolutions().len() + 1;
+        &vec![0; subgrid_dim]
+    } else {
+        subgrid.shape()
+    };
+
+    let shape = unsafe { slice::from_raw_parts_mut(shape, grid.convolutions().len() + 1) };
+    shape.copy_from_slice(subgrid_shape);
+}
+
+/// Get the subgrid for a given bin, channel, and order
+///
+/// # Safety
+///
+/// If `grid` does not point to a valid `Grid` object, for example when `grid` is the null pointer,
+/// this function is not safe to call. Additionally, the pointer that specifies the size of the subgrid
+/// when flattened must be an array; its size must be computed by multiplying the shape dimension as
+/// given by `pineappl_subgrid_shape`.
+#[no_mangle]
+pub unsafe extern "C" fn pineappl_subgrid_array(
+    grid: *mut Grid,
+    bin: usize,
+    order: usize,
+    channel: usize,
+    subgrid_array: *mut f64,
+) {
+    let grid = unsafe { &mut *grid };
+    let mut subgrid = grid.subgrids()[[order, bin, channel]].clone();
+
+    let flattened_shape = subgrid.shape().iter().copied().product();
+    let mut flattened_subgrid_array = vec![0.0; flattened_shape];
+
+    for (index, value) in subgrid.indexed_iter() {
+        let ravel_index = ravel_multi_index(index.as_slice(), subgrid.clone().shape());
+        flattened_subgrid_array[ravel_index] = value;
+    }
+
+    let subgrid_array = unsafe { slice::from_raw_parts_mut(subgrid_array, flattened_shape) };
+
+    subgrid_array.copy_from_slice(&flattened_subgrid_array);
+}

From 9ed7d3cb48c36145293b94911f6a94ef41337cc5 Mon Sep 17 00:00:00 2001
From: Radonirinaunimi <rrabeman@nikhef.nl>
Date: Thu, 10 Apr 2025 22:00:49 +0200
Subject: [PATCH 05/15] Add the correct `C++` file

---
 examples/cpp/get-subgrids     | Bin 16912 -> 0 bytes
 examples/cpp/get-subgrids.cpp |  54 ++++++++++++++++++++++++++++++++++
 2 files changed, 54 insertions(+)
 delete mode 100755 examples/cpp/get-subgrids
 create mode 100644 examples/cpp/get-subgrids.cpp

diff --git a/examples/cpp/get-subgrids b/examples/cpp/get-subgrids
deleted file mode 100755
index 32dacc697f6dfd8bfc8b4b25d0499a50f38434f1..0000000000000000000000000000000000000000
GIT binary patch
literal 0
HcmV?d00001

literal 16912
zcmeHO4RBo5b-t?~YmJRoc5D!)!3(TG4rr~FY#|vOc4f&tnU$?ra&V#Md9}NewybuQ
z-6wnP5~3iR@kS_!kPr_rjhji@)R_j_rkNJfR@h+SHf{Ln6lc<oJt-u+IN0XTV<1@l
z&VBc+-Y!~7nM|fL-J^Lr_k8!9bI-l+zWd&Nd)}8i{2R(_Ho;UbJ}XEXzQjbls-k<X
zDu8)Ki?|s7my0XJJjfLiGwZ!3L#;{2%ksHh;pO0DS4)LC`0tspV9pT|C%a-)bFo>&
zT-2B{*;P_e$%TA(m8r*^>vQcn&3aM5e?bZ$=2klzo5^}vEhg9H^BUD}jXR^_qTwRD
zgUarpvSaR4<H?-IlibkJqWl(eFY1WA^1l#;b}407$lH}Zb83UA=T#&0N!sgHcB4+|
zMilbvO;|AJ_C5zY8kdhte44kY_KKZ{VKtu2)#((gqLIGl)vKbR`e-CJlB!R&G}kw;
zb|>TRMi~Od)qsQM)GeE~in38LMHt)LgiXrxff1D{nd?b+2@V=|vOjXD{--CNO1<^R
zuV43r{^J`v55H_1Asy-m*^mwqVRSH2_?O`z9dmqYWSKCcr3V(SH%oK>-vD^=993T_
zg2&O|RdeY7YZ3i>pg))YUl-BeUPS*2bZ{;|-z|baRm8u)i2vJ)_&HMqUr_|FEP}Tc
z!S5&%=XZ+Wa3h>J3Ta)Deto}4yGb~hYdk*<+*#-xGF^{z;Z10Fkys|Cyifqupgl=?
zXK$At3Maw?k)#n$^meU}#$(~$KwmU0bbVkb9@CRXAYtgbkg9Vk4o70)!0>QXA4o((
z@D>dlVUZ8^MPhm|J`yu#@#ILK)K3luhAryBcx-1pI$}iPv7{ai$3AQjNF)NgXJP3i
z5e|feetVD6*r4miU?RSYYzK@%J)B6y6WzB4QM;)p-l})$kytXEFgk}iI=1x0d(p9i
ziqLFDDB8W#bnA)4lX_nu8P>7nBm#-ua1$|5sSl)5`fxarjK>1eh_PGW*&r=E&AJ{>
z8VGENTMYIX9#3#EkkE}pAYvptgVd7YX@cKKkhME|_1*@(qo-N#zO|=GkHTAQ*N}|T
zxIx#0sg%d#X*5mXKN1`0oWpTXfV@+8g9yP0cWfE@gg!hCa1x3nhvUg`hdCOnaVia=
zZ;kDW#6tR(aB^gbrVD0D@Gd<#c$eNEh#+nns-T)bLxD&PNH7)9`y(_`d%|KUJd_L@
z0$RDr5)2GSX!-UKnlwVe>#ifQnZ%)7X&?x<IkljP-?Dza-sEl&>$h$5wRP$qcaylq
z-`NHc@M?EcW1iUP$rD?xl3CUp-K%q@rrBbndkrof+&OT!D92HOyGuF#Y*^VVL>2DY
z6=J@uw~31&lcr7HK}@S3;##Epg%d~jHM<uGX#E5jkqy2cRrNa{zx4QBxOL4J|E93E
z^WQ|tE>0-<3sSBUr<I)k(n^#n#0N??R(=hS5anWldTyXN4y!t9Z%HJAC$V|rQq)sS
zKYZ|E+@31Ml}a927QwBtOkA(*`T2;SOLkmJUW7QHo_`pxQTU`+R_C5$Sf0As#OYqB
zTub0N3$7mZB>ubw|CDJe{~x#Ds!55TwBWTC{R3W8pyiMH%QWRROZ1$;_hgAEoMPc6
zlW@BQ$8hGT#)4b#|FsreKHn+}rv*2k3(Sf-3(jLoH4PS=pIHcRvEUaGpvdbr3lu*l
zT8=cYStNX64#j-5;OY@w)^4-l4vYQ{3%<yL4_ffW7JS%(({mM5%4?P=o}~<ek9tjk
zaJAJjfztod2>d@9fwvr2yr+%*+OCaP9$O`Zwr|QPn|WRvd&Yi5j>62Ee*!$S;wSj;
zSnfrM<Uy*-o}2-#I7~7vVcF-U{3OX`Bu`2CuSlk)IQxW@50OkuSoWZlzfCeNUD-(~
zKSDAsS=mu3e~o0iOtZsM{tC&obY*u)`CgJ~$;x(1c@N37RAs$VPLNDXRJK9Ne@HSd
zO<AXu`$(oGDO)4u+exk_Sx9*c$qtgwUI4ISBgwSHXHP=b#@k(kA?><~6?K3{7HSjg
zUVw4tZPaBpL%(a^D<iA4^oh)32-^7FuDZ-NS<17!R7;=9j1r!H_S6gh^sh4i1qY|5
zGYdbgTZ=Y^sEtW#ANqf;?fd!2W2e5IIRT*G?O47Sq0s4S7s7X&=SAP=eOrCKTYGSx
zQz32qCK;-hzS}jZP2BDBY7_0QZf(MFwP@4rt`^z>Xz6y~(5TD2PmPTCx?1`j@k%$E
zRdv0tZpR`);7WjwMc#{PC}7m<(vDWt2<oagvrdLMt)(kmTKY)l9x0w67xvEdbmkvE
zpdPflYWx$!HcUacTL}L|{k34)#F`M!ia&iu8}H8eZLf{D)oRmiHFRcOn-`9z+iIVr
zS(SN@e2iDVRWHPoG;LtGiVeM)&gu3_X_I-;)ca@Bqv->^>H<AFtC>U4dy?ie6wYCu
zAcD-prtJf;1$xRv|A)BxOq)BXJr&pEY-o*Zt*O^UdNeK3z?Td3WL%3aaouSuWG?)M
zCfJN)?+RRS+TUWX5IOv<XC3z*h5p8gig(~rcwVPbu00i<^pA&JwKTsTf~hukw+m-#
znpALJkN782b_2C1{pp!a6RxX0=d_9L^IgzhL)zmvxjNIwJ;yvRYU$INH>HCc#mM!T
z-P+i*woC&dtx=cVvG*Goz^;AgMkYN!@9@l=dZwQzrN8xk$6nf$YU!WJQFbgJm8}O4
zYmUcl+St?f^x@9dpN^dN2hUr3bl!3AB>c<XUJLt;6H7iY!+x3OoHKLyeY9?Nr8E7E
zhOTsu&Db1!@1Vvoft{Xn{)wAvw6WLhnSRnmce>IixjSzG@~6-HSDw_KK40F|`YXrY
z@4`Pi?|HG`ac1h&CGvXG#tz#$$ENLC>tV+jU1$A1hr4P|neofc@eXwAG)-8FM;q5L
zYqR4%%v#^<to7Aq-hB^^r>C`Xgk_MTH-ihW%ABSDk*j>u=cuX5Rmv%_io#A$^X%M)
zYa#PK46w#H_U^>BMJM(UfNTVUjJLz{3Qj=QLDPcP1L}3=25LkuMgJm8c}r0<7XOrI
z>W=NcKk#k$-RaYh{3Bg1nLbj@jW11=?BAi4&h$+7FTu_F^t0Io2r|2d{_T+AKAYVR
z&gY-*H=fr!(=XhqrGKG~y>hm@x5G2#Ifgaub5NUE+O$%Lw`UhI;O_#_CMw?~)_UGp
zii`acD&tl)f99l`KSy{TO{_a6_17FGw(^3OKC3-_`er(*n(e6e!Ubaq9K4FvnjF|?
zPGV|eT-cuC8(sG|pFxjDnzm|V>mH$XT}!`eV0~M6Cw$H<-47@eM6gFI?}wso`;pmr
zkMz%$>3&K6vIEeCm)yNJcXtI?qS6SIMxZnTr4cBNKxqU@Bk(&O0eaVrpHM_{z!?dp
zi0uqSN5Xh>TyG&Ef`M2pZaB?X{!Y9<U%Pf^IB3KZ&S)Sp5KcIa!9dJ86iDewynwI6
zbCOA+L^v9)-yMk6Cj!IuC`O}hekbpa?pbZSy!^)Np<03C;F+139pF=EXJ&o_zT=IV
zneTz0eFsmT;6(gx&$efau%&8jmoKccPvQxmaLN@p2;FN)5f9YFTsT(aXn}7h^h9;d
zhU(gljs?5yqvGaeH-4tcbrsOrJcB*me(AaD(Jw9+;&S%i4IT3J1Y+BdU@Pu|&{kdZ
zKv{cr?N`g&s+|)RZPj)6SNf_O?wjYUZW*g;uYST-er?4?)h+9*8+_Gu&}pl#g-u(v
z-A6AZoH$ZAevH^21g~hvKNyWw-8b+4%881vmOnu6QSchVw$h6?B=>x){C&IlSw-dO
ze37lJJZBf*oA>v&({}N7dnGE4*$IO3#g|rgi(6svN0p=0<r0-fpfm!d5h#s7X#`3m
zP#S^K2$V*kGy?xOM}YHQIM0Qj-}lL#z-+Dv<?;}5{h<Pxa)GS%l$%3@AC63tyCZ=-
zZj+)a$%S)NI3MS7Rc}{(nc|$6!}-}<|IUS(IH^pj<Qw{7muMUP1O`$}WK=!<oJYj@
z%A8BWxi!|D4boF)BBSzfRJX(dH4wZ%q#ullICm$d95cROHOz6JHPw+3WfHfK-y~#z
zfJ#;T(jn#Nlp@~jOPS-s_E5@`s{Y-AdgcCi&NzR(u;FGk4>l<NdByt`->LY0ia)IQ
zpDO+%#g8if?~4CY@pFpLQ%O9RD1MFN&5CnelU^>F9JzOEops25Ug`9#c6;0&XJbR-
z>V`E9H;7_Yjg9#(@+6uj72*y0IVO)Nm#9os3$KdjK}+pi`sTb}l*?ad^;Ia?kWo}P
z-YpWZ&COfZ--H6ST}VLybKzeEUV}`sgWh}#dQ;#8zP|w93$;a*k>)h@@#;d-L->D{
zDqGnPAS}i>oK)jL?>dP78pmAm{AUsTEbv8Tmx)O)3KpW@Y_9}<z-xkh{auC=Gnaoa
z;bjiN>l3@$P((ji1b+zQL;d3KTI4$-INMo7e}57D(IWVd2rt9xs^&Y#`D5TT4u$9I
zxgvgCGETOh<-35-m1jKxoaRH}d4ELW%fxZttPd)EOW{-AeEj>s=NkW)i{KZE;8$QF
zoXbyh5xfhyGd~n5MT+Q;7QtUCg1=V8|GPzSF%UNDRRVd^9ULAOlwa%)#-qpr*Rc-i
z9h$yji?6Fg*V?xTz5Vw$`MNsS3;h=V<~E;S-@IW%PY3d>eQo{@y&#pmFn|3+3F&$`
z6fgqFhu@6c<`5FwN0MP`)|%Vwm8r)_A*O6$GxzzUlG9CfGgDcmMdx|Y^-wad<K1lt
zdHL<Pxf9`jcahBVxs%+j4Y*qc<zPoXoS>a+6Y0+S?Uej(wKEgZ^ALVVY-awON%#e6
z;5nagNh$Ma6<-<X9h=(en*t2VZ!J;Y8X5HwY5o7}g!%kr_dJit7^iS$?z^=D9isVQ
z=D7<!9HY6=mrOE1=e@|+16DVbpkC}tggia;Y>saZyq2{0{Il{ApI(?>Qhek$AbC0d
zUe2-qL}Q!(F+yISFt)j%^S=xE*dL(?cXIcT5$FRq5+)zyQY40NTZV-jzm0_51F;eJ
za3YRRHjLdCsBa{K&&VPn1>qBddLu9(r21eWIVjwr-7z#_awB2Z?8Jv3_);ZL&{2oq
zQvzh5O2bh@xaI738{rhn^7?Zp;&LT%hX?ikL|`ba4~AgN5}e{Yhj1zw9yWCJ2$#O}
zp_`{o`rtB}3JgVpFp0xE{<&q7rh80AfwggHD2(rKek(-#4cr>>EM`(I)@dT<yx%a<
z+5Hd8SEE4B_-xPn3g%u`RGgnb7;Z+vYM)Z=Gk39~;`}_taA$%2W6GYnlNA*&w7(7b
zIqVrYe%?nh=k|FYRTzH&_O#Vud){X;=kFY;k2KC~&-<MSFbbLNdEdsI#}O)$$cL4_
z0D<;IY|r~3=JZ<}5y#JZ%<n-R?Xg(qeHQa6G6qrn6eq_|7|{d{8dJ9CeI9dqE+yjj
z*`E9VP1KQn8!3T!-^iTznPg8kh4y~}jK+lh=bi`Eb0P1ip(3g9{67ZNYVTBbIbJZ}
z{J6>d&kF2$Kf`=wE-<rwwlgIm{-VI1pSzj!`vTT4jGy)i6e5qma4Jp3N0rE|u>Ge{
zVYMH1nwre%w_+<5wtozjR{JNMrZ)4pt;Q(x`D6Gc6s-2&xzem*{#jO3oOG@9Duikr
z>aw5x?z`U9Dr}Es=5IjFVX^1;EC)WOJ?*dP6m$E$59RO4o$AEU^+Uw%@_Cqt4pE=j
zp1-G>qKz%6&Sbg&Y|nf_fjz&c$te4SIZLy^cFdO)*z>+-e}k#d*D1SUJC>Khp0Hf}
zUbE&P+f(N(<h7KkxLmcb)N{3LPxl8au{$fz&3%jFapkZ@P1sMPj?Mwc&;6p;MjX4v
gT-4B~fuK(nOn7eBtff8T-1b+uni_Ql1_h${Z&>Y(FaQ7m

diff --git a/examples/cpp/get-subgrids.cpp b/examples/cpp/get-subgrids.cpp
new file mode 100644
index 00000000..f0255943
--- /dev/null
+++ b/examples/cpp/get-subgrids.cpp
@@ -0,0 +1,54 @@
+#include <pineappl_capi.h>
+
+#include <cstddef>
+#include <iomanip>
+#include <ios>
+#include <iostream>
+#include <string>
+#include <vector>
+#include <numeric>
+
+int main() {
+    std::string filename = "drell-yan-rap-ll.pineappl.lz4";
+
+    // read the grid from a file
+    auto* grid = pineappl_grid_read(filename.c_str());
+
+    // Determine the number of bins and the index of order and channel
+    std::size_t n_bins = pineappl_grid_bin_count(grid);
+    std::size_t order = 0;
+    std::size_t channel = 0;
+
+    // Get the dimension of the subgrids
+    std::size_t subgrid_dim = pineappl_convolutions_len(grid) + 1;
+
+    std::cout << std::right << std::setw(10) << "bin" << std::setw(10) << "sg idx"
+        << std::setw(16) << "sg value" << "\n";
+    std::cout << std::right << std::setw(10) << "---" << std::setw(10) << "------"
+        << std::setw(16) << "------------" << "\n";
+
+    for (std::size_t b = 0; b < n_bins; ++b) {
+        std::vector<std::size_t> subgrid_shape(subgrid_dim);
+        pineappl_subgrid_shape(grid, b, order, channel, subgrid_shape.data());
+
+        // Check if the subgrid is not empty
+        if (subgrid_shape[0] != 0) {
+            std::size_t flat_shape = std::accumulate(subgrid_shape.begin(),
+                subgrid_shape.end(), 1, std::multiplies<std::size_t>());
+            std::vector<double> subgrid_array(flat_shape);
+
+            pineappl_subgrid_array(grid, b, order, channel, subgrid_array.data());
+
+            // TODO: Unravel of the index & check against some reference values
+            for (std::size_t value = 0; value < subgrid_array.size(); ++value) {
+                if (subgrid_array[value] != 0) {
+                    std::cout << std::right << std::setw(10) << b << std::setw(10)
+                        << value << std::setw(16) << subgrid_array[value] << "\n";
+                    break;
+                }
+            }
+        }
+    }
+
+    pineappl_grid_delete(grid);
+}

From bd68162dd7c88b9961ccbbf9641804cf2a0a2764 Mon Sep 17 00:00:00 2001
From: Radonirinaunimi <rrabeman@nikhef.nl>
Date: Thu, 10 Apr 2025 23:13:13 +0200
Subject: [PATCH 06/15] Address comments from Code Review

---
 examples/cpp/get-subgrids.cpp |  6 +++---
 pineappl_capi/src/lib.rs      | 34 ++++++++++++++++++++++------------
 2 files changed, 25 insertions(+), 15 deletions(-)

diff --git a/examples/cpp/get-subgrids.cpp b/examples/cpp/get-subgrids.cpp
index f0255943..a69c76d1 100644
--- a/examples/cpp/get-subgrids.cpp
+++ b/examples/cpp/get-subgrids.cpp
@@ -20,7 +20,7 @@ int main() {
     std::size_t channel = 0;
 
     // Get the dimension of the subgrids
-    std::size_t subgrid_dim = pineappl_convolutions_len(grid) + 1;
+    std::size_t subgrid_dim = pineappl_grid_kinematics_len(grid);
 
     std::cout << std::right << std::setw(10) << "bin" << std::setw(10) << "sg idx"
         << std::setw(16) << "sg value" << "\n";
@@ -29,7 +29,7 @@ int main() {
 
     for (std::size_t b = 0; b < n_bins; ++b) {
         std::vector<std::size_t> subgrid_shape(subgrid_dim);
-        pineappl_subgrid_shape(grid, b, order, channel, subgrid_shape.data());
+        pineappl_grid_subgrid_shape(grid, b, order, channel, subgrid_shape.data());
 
         // Check if the subgrid is not empty
         if (subgrid_shape[0] != 0) {
@@ -37,7 +37,7 @@ int main() {
                 subgrid_shape.end(), 1, std::multiplies<std::size_t>());
             std::vector<double> subgrid_array(flat_shape);
 
-            pineappl_subgrid_array(grid, b, order, channel, subgrid_array.data());
+            pineappl_grid_subgrid_array(grid, b, order, channel, subgrid_array.data());
 
             // TODO: Unravel of the index & check against some reference values
             for (std::size_t value = 0; value < subgrid_array.size(); ++value) {
diff --git a/pineappl_capi/src/lib.rs b/pineappl_capi/src/lib.rs
index 8aa0cf75..f950ea78 100644
--- a/pineappl_capi/src/lib.rs
+++ b/pineappl_capi/src/lib.rs
@@ -1903,21 +1903,32 @@ pub unsafe extern "C" fn pineappl_grid_convolve(
 /// If `grid` does not point to a valid `Grid` object, for example when `grid` is the null pointer,
 /// this function is not safe to call.
 #[no_mangle]
-pub unsafe extern "C" fn pineappl_convolutions_len(grid: *mut Grid) -> usize {
+pub unsafe extern "C" fn pineappl_grid_convolutions_len(grid: *mut Grid) -> usize {
     let grid = unsafe { &mut *grid };
     grid.convolutions().len()
 }
 
+/// Get the number of different kinematics for a given Grid.
+///
+/// # Safety
+///
+/// If `grid` does not point to a valid `Grid` object, for example when `grid` is the null pointer,
+/// this function is not safe to call.
+#[no_mangle]
+pub unsafe extern "C" fn pineappl_grid_kinematics_len(grid: *mut Grid) -> usize {
+    let grid = unsafe { &mut *grid };
+    grid.kinematics().len()
+}
+
 /// Get the shape of a subgrid for a given bin, channel, and order.
 ///
 /// # Safety
 ///
 /// If `grid` does not point to a valid `Grid` object, for example when `grid` is the null pointer,
 /// this function is not safe to call. Additionally, the pointer that specifies the shape of the
-/// subgrid must be an array whose size must be `(n+1)` where `n` represents the number of
-/// convolutions as given by `pineappl_convolutions_len`.
+/// subgrid has to be an array whose size must be as given by `pineappl_grid_kinematics_len`.
 #[no_mangle]
-pub unsafe extern "C" fn pineappl_subgrid_shape(
+pub unsafe extern "C" fn pineappl_grid_subgrid_shape(
     grid: *mut Grid,
     bin: usize,
     order: usize,
@@ -1928,13 +1939,13 @@ pub unsafe extern "C" fn pineappl_subgrid_shape(
     let mut subgrid = grid.subgrids()[[order, bin, channel]].clone();
 
     let subgrid_shape = if subgrid.is_empty() {
-        let subgrid_dim = grid.convolutions().len() + 1;
+        let subgrid_dim = grid.kinematics().len();
         &vec![0; subgrid_dim]
     } else {
         subgrid.shape()
     };
 
-    let shape = unsafe { slice::from_raw_parts_mut(shape, grid.convolutions().len() + 1) };
+    let shape = unsafe { slice::from_raw_parts_mut(shape, grid.kinematics().len()) };
     shape.copy_from_slice(subgrid_shape);
 }
 
@@ -1945,9 +1956,9 @@ pub unsafe extern "C" fn pineappl_subgrid_shape(
 /// If `grid` does not point to a valid `Grid` object, for example when `grid` is the null pointer,
 /// this function is not safe to call. Additionally, the pointer that specifies the size of the subgrid
 /// when flattened must be an array; its size must be computed by multiplying the shape dimension as
-/// given by `pineappl_subgrid_shape`.
+/// given by `pineappl_grid_subgrid_shape`.
 #[no_mangle]
-pub unsafe extern "C" fn pineappl_subgrid_array(
+pub unsafe extern "C" fn pineappl_grid_subgrid_array(
     grid: *mut Grid,
     bin: usize,
     order: usize,
@@ -1957,15 +1968,14 @@ pub unsafe extern "C" fn pineappl_subgrid_array(
     let grid = unsafe { &mut *grid };
     let mut subgrid = grid.subgrids()[[order, bin, channel]].clone();
 
-    let flattened_shape = subgrid.shape().iter().copied().product();
-    let mut flattened_subgrid_array = vec![0.0; flattened_shape];
+    let subgrid_array =
+        unsafe { slice::from_raw_parts_mut(subgrid_array, subgrid.shape().iter().product()) };
+    let mut flattened_subgrid_array = vec![0.0; subgrid_array.len()];
 
     for (index, value) in subgrid.indexed_iter() {
         let ravel_index = ravel_multi_index(index.as_slice(), subgrid.clone().shape());
         flattened_subgrid_array[ravel_index] = value;
     }
 
-    let subgrid_array = unsafe { slice::from_raw_parts_mut(subgrid_array, flattened_shape) };
-
     subgrid_array.copy_from_slice(&flattened_subgrid_array);
 }

From 770b76309084d064f6f876f3a6119fff2be0445c Mon Sep 17 00:00:00 2001
From: Radonirinaunimi <rrabeman@nikhef.nl>
Date: Fri, 11 Apr 2025 11:10:44 +0200
Subject: [PATCH 07/15] Add `unravel_index` and tests to example

---
 examples/cpp/get-subgrids.cpp | 53 +++++++++++++++++++++++++++++++----
 examples/cpp/output           | 52 +++++++++++++++++-----------------
 2 files changed, 73 insertions(+), 32 deletions(-)

diff --git a/examples/cpp/get-subgrids.cpp b/examples/cpp/get-subgrids.cpp
index a69c76d1..b37f0db4 100644
--- a/examples/cpp/get-subgrids.cpp
+++ b/examples/cpp/get-subgrids.cpp
@@ -1,13 +1,44 @@
 #include <pineappl_capi.h>
 
+#include <cassert>
 #include <cstddef>
 #include <iomanip>
 #include <ios>
 #include <iostream>
+#include <sstream>
 #include <string>
 #include <vector>
 #include <numeric>
 
+
+std::vector<std::size_t> unravel_index(std::size_t flat_index, const std::vector<std::size_t>& shape) {
+    std::size_t ndim = shape.size();
+    std::vector<std::size_t> coords(ndim);
+
+    for (int i = ndim - 1; i >= 0; --i) {
+        coords[i] = flat_index % shape[i];
+        flat_index /= shape[i];
+    }
+
+    return coords;
+}
+
+std::string coords_to_string(const std::vector<std::size_t>& coords) {
+    std::ostringstream osstream;
+
+    osstream << "(";
+    for (std::size_t i = 0; i < coords.size(); ++i) {
+        osstream << coords[i];
+        if (i != coords.size() - 1) {
+            osstream << ", ";
+        }
+    }
+    osstream << ")";
+
+    return osstream.str();
+}
+
+
 int main() {
     std::string filename = "drell-yan-rap-ll.pineappl.lz4";
 
@@ -23,9 +54,11 @@ int main() {
     std::size_t subgrid_dim = pineappl_grid_kinematics_len(grid);
 
     std::cout << std::right << std::setw(10) << "bin" << std::setw(10) << "sg idx"
-        << std::setw(16) << "sg value" << "\n";
+        << std::setw(6 * subgrid_dim) << "sg coordinates" << std::setw(16)
+        << "sg value" << "\n";
     std::cout << std::right << std::setw(10) << "---" << std::setw(10) << "------"
-        << std::setw(16) << "------------" << "\n";
+        << std::setw(6 * subgrid_dim) << "--------------" << std::setw(16)
+        << "------------" << "\n";
 
     for (std::size_t b = 0; b < n_bins; ++b) {
         std::vector<std::size_t> subgrid_shape(subgrid_dim);
@@ -39,11 +72,19 @@ int main() {
 
             pineappl_grid_subgrid_array(grid, b, order, channel, subgrid_array.data());
 
-            // TODO: Unravel of the index & check against some reference values
-            for (std::size_t value = 0; value < subgrid_array.size(); ++value) {
-                if (subgrid_array[value] != 0) {
+            for (std::size_t index = 0; index < subgrid_array.size(); ++index) {
+                if (subgrid_array[index] != 0) {
+                    // Unravel the index to recover the standard coordinates
+                    std::vector<std::size_t> coords = unravel_index(index, subgrid_shape);
+
                     std::cout << std::right << std::setw(10) << b << std::setw(10)
-                        << value << std::setw(16) << subgrid_array[value] << "\n";
+                        << index << std::setw(6 * subgrid_dim) << coords_to_string(coords)
+                        << std::setw(16) << subgrid_array[index] << "\n";
+
+                    // Compare to some reference value
+                    if (b==0 && index==41020) {
+                        assert(subgrid_array[index] == -4.936156925096015e-07);
+                    }
                     break;
                 }
             }
diff --git a/examples/cpp/output b/examples/cpp/output
index d7f24048..d5ad2d4d 100644
--- a/examples/cpp/output
+++ b/examples/cpp/output
@@ -146,32 +146,32 @@ idx   left  right    dsig/dx     dx
  21    2.1    2.2 3.507294e-02    0.1
  22    2.2    2.3 1.976542e-02    0.1
  23    2.3    2.4 5.590552e-03    0.1
-       bin    sg idx        sg value
-       ---    ------    ------------
-         0     41020    -4.93616e-07
-         1     41020     -4.5499e-08
-         2     40971    -4.10008e-08
-         3     40971    -2.78597e-07
-         4     40971    -2.01924e-07
-         5     40922    -3.39322e-10
-         6     40922    -8.43358e-08
-         7     40922    -2.99247e-07
-         8     40922     -1.5117e-07
-         9     40872    -9.95013e-11
-        10     40873    -1.30578e-07
-        11     40873    -2.55869e-07
-        12     40823     -1.0823e-09
-        13     40823    -4.22686e-09
-        14     40824    -1.68149e-07
-        15     40774    -7.58702e-10
-        16     40774    -2.41887e-08
-        17     40774    -8.83139e-09
-        18     40725    -1.14932e-09
-        19     40725    -3.03857e-08
-        20     40725    -2.66414e-08
-        21     40675    -3.60243e-10
-        22     40676    -1.22785e-08
-        23     40626    -7.04493e-11
+       bin    sg idx    sg coordinates        sg value
+       ---    ------    --------------    ------------
+         0     41020      (16, 20, 20)    -4.93616e-07
+         1     41020      (16, 20, 20)     -4.5499e-08
+         2     40971      (16, 19, 21)    -4.10008e-08
+         3     40971      (16, 19, 21)    -2.78597e-07
+         4     40971      (16, 19, 21)    -2.01924e-07
+         5     40922      (16, 18, 22)    -3.39322e-10
+         6     40922      (16, 18, 22)    -8.43358e-08
+         7     40922      (16, 18, 22)    -2.99247e-07
+         8     40922      (16, 18, 22)     -1.5117e-07
+         9     40872      (16, 17, 22)    -9.95013e-11
+        10     40873      (16, 17, 23)    -1.30578e-07
+        11     40873      (16, 17, 23)    -2.55869e-07
+        12     40823      (16, 16, 23)     -1.0823e-09
+        13     40823      (16, 16, 23)    -4.22686e-09
+        14     40824      (16, 16, 24)    -1.68149e-07
+        15     40774      (16, 15, 24)    -7.58702e-10
+        16     40774      (16, 15, 24)    -2.41887e-08
+        17     40774      (16, 15, 24)    -8.83139e-09
+        18     40725      (16, 14, 25)    -1.14932e-09
+        19     40725      (16, 14, 25)    -3.03857e-08
+        20     40725      (16, 14, 25)    -2.66414e-08
+        21     40675      (16, 13, 25)    -3.60243e-10
+        22     40676      (16, 13, 26)    -1.22785e-08
+        23     40626      (16, 12, 26)    -7.04493e-11
 idx  p-p c#0 l#0 p-p~ c#0 l#  p-d c#0 l#0       p-d       dx
 --- ------------ ----------- ------------- ------------ ------
   0 5.263109e-01 5.263109e-01 5.263109e-01 5.263109e-01    0.1

From 1d194a382688734410e2f8a14af95365b62566cd Mon Sep 17 00:00:00 2001
From: Radonirinaunimi <rrabeman@nikhef.nl>
Date: Fri, 11 Apr 2025 11:31:28 +0200
Subject: [PATCH 08/15] Fix `pineappl_channels_entry`

---
 examples/cpp/display-channels.cpp | 7 +++++--
 pineappl_capi/src/lib.rs          | 3 ++-
 2 files changed, 7 insertions(+), 3 deletions(-)

diff --git a/examples/cpp/display-channels.cpp b/examples/cpp/display-channels.cpp
index c22da961..d3fd3f2e 100644
--- a/examples/cpp/display-channels.cpp
+++ b/examples/cpp/display-channels.cpp
@@ -22,6 +22,9 @@ int main(int argc, char* argv[]) {
     // read the grid from a file
     auto* grid = pineappl_grid_read(filename.c_str());
 
+    // How many convolutions are there?
+    auto n_conv = pineappl_grid_convolutions_len(grid);
+
     // extract all channels
     auto* channels = pineappl_grid_channels(grid);
 
@@ -36,11 +39,11 @@ int main(int argc, char* argv[]) {
         auto combinations = pineappl_channels_combinations(channels, channel);
 
         std::vector<double> factors(combinations);
-        std::vector<int> pids(2 * combinations);
+        std::vector<int> pids(n_conv * combinations);
 
         // read out the channel with index given by `channel`, writing the particle identifiers into
         // `pids` and the corresponding factors into `factors`
-        pineappl_channels_entry(channels, channel, pids.data(), factors.data());
+        pineappl_channels_entry(channels, channel, n_conv, pids.data(), factors.data());
 
         for (std::size_t combination = 0; combination != combinations; ++combination) {
             auto factor = factors.at(combination);
diff --git a/pineappl_capi/src/lib.rs b/pineappl_capi/src/lib.rs
index f950ea78..853e51d4 100644
--- a/pineappl_capi/src/lib.rs
+++ b/pineappl_capi/src/lib.rs
@@ -1773,12 +1773,13 @@ pub unsafe extern "C" fn pineappl_grid_split_channels(grid: *mut Grid) {
 pub unsafe extern "C" fn pineappl_channels_entry(
     channels: *const Channels,
     entry: usize,
+    nb_convolutions: usize,
     pdg_ids: *mut i32,
     factors: *mut f64,
 ) {
     let channels = unsafe { &*channels };
     let entry = channels.0[entry].entry();
-    let pdg_ids = unsafe { slice::from_raw_parts_mut(pdg_ids, 3 * entry.len()) };
+    let pdg_ids = unsafe { slice::from_raw_parts_mut(pdg_ids, nb_convolutions * entry.len()) };
     let factors = unsafe { slice::from_raw_parts_mut(factors, entry.len()) };
 
     entry

From 233b9a4430d5de4ac85132808b73895af5766023 Mon Sep 17 00:00:00 2001
From: Radonirinaunimi <rrabeman@nikhef.nl>
Date: Fri, 11 Apr 2025 11:32:26 +0200
Subject: [PATCH 09/15] Add empty newline for consistency

---
 pineappl_capi/src/lib.rs | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/pineappl_capi/src/lib.rs b/pineappl_capi/src/lib.rs
index 853e51d4..e532524e 100644
--- a/pineappl_capi/src/lib.rs
+++ b/pineappl_capi/src/lib.rs
@@ -1906,6 +1906,7 @@ pub unsafe extern "C" fn pineappl_grid_convolve(
 #[no_mangle]
 pub unsafe extern "C" fn pineappl_grid_convolutions_len(grid: *mut Grid) -> usize {
     let grid = unsafe { &mut *grid };
+
     grid.convolutions().len()
 }
 
@@ -1918,6 +1919,7 @@ pub unsafe extern "C" fn pineappl_grid_convolutions_len(grid: *mut Grid) -> usiz
 #[no_mangle]
 pub unsafe extern "C" fn pineappl_grid_kinematics_len(grid: *mut Grid) -> usize {
     let grid = unsafe { &mut *grid };
+
     grid.kinematics().len()
 }
 

From 4d2f0c9ba2186abeef3405501bbf3f8349dfd4fd Mon Sep 17 00:00:00 2001
From: Radonirinaunimi <rrabeman@nikhef.nl>
Date: Fri, 11 Apr 2025 15:43:06 +0200
Subject: [PATCH 10/15] Hit missing part in the coverage related to empty grid

---
 examples/cpp/advanced-filling.cpp | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/examples/cpp/advanced-filling.cpp b/examples/cpp/advanced-filling.cpp
index 01a1fbfe..d85272e6 100644
--- a/examples/cpp/advanced-filling.cpp
+++ b/examples/cpp/advanced-filling.cpp
@@ -158,6 +158,13 @@ int main() {
     }
     //-----------------------------------------------------------------------//
 
+    // Check that the Grid contains an empty subgrid at (b, o, c)=(0, 0, 0)
+    auto subgrid_dim = pineappl_grid_kinematics_len(grid);
+    std::vector<std::size_t> subgrid_shape(subgrid_dim);
+    std::vector<std::size_t> zero_vector(subgrid_dim, 0);
+    pineappl_grid_subgrid_shape(grid, 0, 0, 0, subgrid_shape.data());
+    assert(subgrid_shape == zero_vector);
+
     pineappl_grid_write(grid, "advanced-filling.pineappl.lz4");
 
     // release memory

From 3a9f23b3a22a213b4f614e05386909f65051e06b Mon Sep 17 00:00:00 2001
From: Christopher Schwan <handgranaten-herbert@posteo.de>
Date: Fri, 11 Apr 2025 17:24:10 +0200
Subject: [PATCH 11/15] Remove `mut` from `Subgrid::shape`

---
 pineappl/src/subgrid.rs  | 8 ++++----
 pineappl_capi/src/lib.rs | 6 +++---
 2 files changed, 7 insertions(+), 7 deletions(-)

diff --git a/pineappl/src/subgrid.rs b/pineappl/src/subgrid.rs
index c115f1f9..799bb51a 100644
--- a/pineappl/src/subgrid.rs
+++ b/pineappl/src/subgrid.rs
@@ -33,7 +33,7 @@ impl Subgrid for EmptySubgridV1 {
         Vec::new()
     }
 
-    fn shape(&mut self) -> &[usize] {
+    fn shape(&self) -> &[usize] {
         panic!("EmptySubgridV1 doesn't have a shape");
     }
 
@@ -166,7 +166,7 @@ impl Subgrid for ImportSubgridV1 {
         Box::new(self.array.indexed_iter())
     }
 
-    fn shape(&mut self) -> &[usize] {
+    fn shape(&self) -> &[usize] {
         self.array.shape()
     }
 
@@ -275,7 +275,7 @@ impl Subgrid for InterpSubgridV1 {
         self.array.is_empty()
     }
 
-    fn shape(&mut self) -> &[usize] {
+    fn shape(&self) -> &[usize] {
         self.array.shape()
     }
 
@@ -471,7 +471,7 @@ pub trait Subgrid {
     fn scale(&mut self, factor: f64);
 
     /// Return the shape of the subgrid
-    fn shape(&mut self) -> &[usize];
+    fn shape(&self) -> &[usize];
 
     /// Assume that the convolution functions for indices `a` and `b` for this grid are the same
     /// and use this to optimize the size of the grid.
diff --git a/pineappl_capi/src/lib.rs b/pineappl_capi/src/lib.rs
index e532524e..de3c275a 100644
--- a/pineappl_capi/src/lib.rs
+++ b/pineappl_capi/src/lib.rs
@@ -1939,7 +1939,7 @@ pub unsafe extern "C" fn pineappl_grid_subgrid_shape(
     shape: *mut usize,
 ) {
     let grid = unsafe { &mut *grid };
-    let mut subgrid = grid.subgrids()[[order, bin, channel]].clone();
+    let subgrid = &grid.subgrids()[[order, bin, channel]];
 
     let subgrid_shape = if subgrid.is_empty() {
         let subgrid_dim = grid.kinematics().len();
@@ -1969,14 +1969,14 @@ pub unsafe extern "C" fn pineappl_grid_subgrid_array(
     subgrid_array: *mut f64,
 ) {
     let grid = unsafe { &mut *grid };
-    let mut subgrid = grid.subgrids()[[order, bin, channel]].clone();
+    let subgrid = &grid.subgrids()[[order, bin, channel]];
 
     let subgrid_array =
         unsafe { slice::from_raw_parts_mut(subgrid_array, subgrid.shape().iter().product()) };
     let mut flattened_subgrid_array = vec![0.0; subgrid_array.len()];
 
     for (index, value) in subgrid.indexed_iter() {
-        let ravel_index = ravel_multi_index(index.as_slice(), subgrid.clone().shape());
+        let ravel_index = ravel_multi_index(index.as_slice(), subgrid.shape());
         flattened_subgrid_array[ravel_index] = value;
     }
 

From a02beae383409569e78c4684d1939ae9af69883a Mon Sep 17 00:00:00 2001
From: Christopher Schwan <handgranaten-herbert@posteo.de>
Date: Fri, 11 Apr 2025 17:35:48 +0200
Subject: [PATCH 12/15] Use less `mut` and copy directly

---
 pineappl_capi/src/lib.rs | 31 ++++++++++++++++---------------
 1 file changed, 16 insertions(+), 15 deletions(-)

diff --git a/pineappl_capi/src/lib.rs b/pineappl_capi/src/lib.rs
index de3c275a..48a8824e 100644
--- a/pineappl_capi/src/lib.rs
+++ b/pineappl_capi/src/lib.rs
@@ -1932,24 +1932,24 @@ pub unsafe extern "C" fn pineappl_grid_kinematics_len(grid: *mut Grid) -> usize
 /// subgrid has to be an array whose size must be as given by `pineappl_grid_kinematics_len`.
 #[no_mangle]
 pub unsafe extern "C" fn pineappl_grid_subgrid_shape(
-    grid: *mut Grid,
+    grid: *const Grid,
     bin: usize,
     order: usize,
     channel: usize,
     shape: *mut usize,
 ) {
-    let grid = unsafe { &mut *grid };
+    let grid = unsafe { &*grid };
     let subgrid = &grid.subgrids()[[order, bin, channel]];
-
     let subgrid_shape = if subgrid.is_empty() {
+        // avoid calling `Subgrid::shape()` for empty grids, which may panic
         let subgrid_dim = grid.kinematics().len();
         &vec![0; subgrid_dim]
     } else {
         subgrid.shape()
     };
-
     let shape = unsafe { slice::from_raw_parts_mut(shape, grid.kinematics().len()) };
-    shape.copy_from_slice(subgrid_shape);
+
+    shape.copy_from_slice(&subgrid_shape);
 }
 
 /// Get the subgrid for a given bin, channel, and order
@@ -1962,23 +1962,24 @@ pub unsafe extern "C" fn pineappl_grid_subgrid_shape(
 /// given by `pineappl_grid_subgrid_shape`.
 #[no_mangle]
 pub unsafe extern "C" fn pineappl_grid_subgrid_array(
-    grid: *mut Grid,
+    grid: *const Grid,
     bin: usize,
     order: usize,
     channel: usize,
     subgrid_array: *mut f64,
 ) {
-    let grid = unsafe { &mut *grid };
+    let grid = unsafe { &*grid };
     let subgrid = &grid.subgrids()[[order, bin, channel]];
 
-    let subgrid_array =
-        unsafe { slice::from_raw_parts_mut(subgrid_array, subgrid.shape().iter().product()) };
-    let mut flattened_subgrid_array = vec![0.0; subgrid_array.len()];
+    // avoid calling `Subgrid::shape()` for empty grids, which may panic
+    if !subgrid.is_empty() {
+        let shape = subgrid.shape();
+        let subgrid_array =
+            unsafe { slice::from_raw_parts_mut(subgrid_array, shape.iter().product()) };
 
-    for (index, value) in subgrid.indexed_iter() {
-        let ravel_index = ravel_multi_index(index.as_slice(), subgrid.shape());
-        flattened_subgrid_array[ravel_index] = value;
+        for (index, value) in subgrid.indexed_iter() {
+            let ravel_index = ravel_multi_index(index.as_slice(), &shape);
+            subgrid_array[ravel_index] = value;
+        }
     }
-
-    subgrid_array.copy_from_slice(&flattened_subgrid_array);
 }

From 77979408bbb8bc5636973e51a70b705ee790cf3c Mon Sep 17 00:00:00 2001
From: Radonirinaunimi <rrabeman@nikhef.nl>
Date: Fri, 11 Apr 2025 18:03:13 +0200
Subject: [PATCH 13/15] Check that calling `pineappl_grid_subgrid_array` on an
 empty subgrid does not panic

---
 examples/cpp/advanced-filling.cpp | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/examples/cpp/advanced-filling.cpp b/examples/cpp/advanced-filling.cpp
index d85272e6..caf5801e 100644
--- a/examples/cpp/advanced-filling.cpp
+++ b/examples/cpp/advanced-filling.cpp
@@ -6,6 +6,7 @@
 #include <iomanip>
 #include <memory>
 #include <vector>
+#include <numeric>
 
 int main() {
     // Construct the channel object based on the nb of convolutions
@@ -165,6 +166,10 @@ int main() {
     pineappl_grid_subgrid_shape(grid, 0, 0, 0, subgrid_shape.data());
     assert(subgrid_shape == zero_vector);
 
+    // Check that a call to an empty subgrid does not panic
+    std::vector<double> subgrid_array(0);
+    pineappl_grid_subgrid_array(grid, 0, 0, 0, subgrid_array.data());
+
     pineappl_grid_write(grid, "advanced-filling.pineappl.lz4");
 
     // release memory

From 9c08d4f8c80f9ee797f6168adafef663b5ca70bc Mon Sep 17 00:00:00 2001
From: Radonirinaunimi <rrabeman@nikhef.nl>
Date: Fri, 11 Apr 2025 20:12:11 +0200
Subject: [PATCH 14/15] Remove no longer needed include

---
 examples/cpp/advanced-filling.cpp | 1 -
 1 file changed, 1 deletion(-)

diff --git a/examples/cpp/advanced-filling.cpp b/examples/cpp/advanced-filling.cpp
index caf5801e..8c7a4a72 100644
--- a/examples/cpp/advanced-filling.cpp
+++ b/examples/cpp/advanced-filling.cpp
@@ -6,7 +6,6 @@
 #include <iomanip>
 #include <memory>
 #include <vector>
-#include <numeric>
 
 int main() {
     // Construct the channel object based on the nb of convolutions

From 9d6f89761f17805ccc47bc42a9358e5cd8b4c27b Mon Sep 17 00:00:00 2001
From: Christopher Schwan <handgranaten-herbert@posteo.de>
Date: Sat, 12 Apr 2025 09:20:08 +0200
Subject: [PATCH 15/15] Undo interface change to `pineappl_channels_entry`

---
 examples/cpp/display-channels.cpp |  2 +-
 pineappl_capi/src/lib.rs          | 18 ++++++++++--------
 2 files changed, 11 insertions(+), 9 deletions(-)

diff --git a/examples/cpp/display-channels.cpp b/examples/cpp/display-channels.cpp
index d3fd3f2e..636fd328 100644
--- a/examples/cpp/display-channels.cpp
+++ b/examples/cpp/display-channels.cpp
@@ -43,7 +43,7 @@ int main(int argc, char* argv[]) {
 
         // read out the channel with index given by `channel`, writing the particle identifiers into
         // `pids` and the corresponding factors into `factors`
-        pineappl_channels_entry(channels, channel, n_conv, pids.data(), factors.data());
+        pineappl_channels_entry(channels, channel, pids.data(), factors.data());
 
         for (std::size_t combination = 0; combination != combinations; ++combination) {
             auto factor = factors.at(combination);
diff --git a/pineappl_capi/src/lib.rs b/pineappl_capi/src/lib.rs
index 48a8824e..c28ee961 100644
--- a/pineappl_capi/src/lib.rs
+++ b/pineappl_capi/src/lib.rs
@@ -1760,26 +1760,28 @@ pub unsafe extern "C" fn pineappl_grid_split_channels(grid: *mut Grid) {
     grid.split_channels();
 }
 
-/// Similar to `pineappl_lumi_entry` but for luminosity channels that involve 3 or more partons, ie.
-/// in the case of multiple convolutions.
+/// Read out the channel with index `entry` of the given `channels`. The PDG ids and factors will
+/// be copied into `pdg_ids` and `factors`.
 ///
 /// # Safety
 ///
-/// The parameter `channels` must point to a valid `Channels` object created by `pineappl_channels_new` or
-/// `pineappl_grid_channels`. The parameter `factors` must point to an array as long as the size
-/// returned by `pineappl_channels_combinations` and `pdg_ids` must point to an array that is twice as
-/// long.
+/// The parameter `channels` must point to a valid [`Channels`] object created by
+/// [`pineappl_channels_new`] or [`pineappl_grid_channels`]. The parameter `factors` must point to
+/// an array as long as the result of [`pineappl_channels_combinations`] and `pdg_ids` must
+/// point to an array as long as the result multiplied with the number of convolutions.
 #[no_mangle]
 pub unsafe extern "C" fn pineappl_channels_entry(
     channels: *const Channels,
     entry: usize,
-    nb_convolutions: usize,
     pdg_ids: *mut i32,
     factors: *mut f64,
 ) {
     let channels = unsafe { &*channels };
     let entry = channels.0[entry].entry();
-    let pdg_ids = unsafe { slice::from_raw_parts_mut(pdg_ids, nb_convolutions * entry.len()) };
+    // if the channel has no entries we assume no convolutions, which is OK we don't copy anything
+    // in this case
+    let convolutions = entry.get(0).map_or(0, |x| x.0.len());
+    let pdg_ids = unsafe { slice::from_raw_parts_mut(pdg_ids, convolutions * entry.len()) };
     let factors = unsafe { slice::from_raw_parts_mut(factors, entry.len()) };
 
     entry