MLopez-Ibanez · Sep 23, 2026 · Sep 23, 2026 · Oct 28, 2026 · Sep 12, 2026 · Sep 12, 2026
diff --git a/README.md b/README.md
@@ -19,7 +19,8 @@
     Mickaël Binois,
     Leonardo C.T. Bezerra,
     Fergus Rooney,
-    [Lennart Schäpermeier](https://schaepermeier.github.io).
+    [Lennart Schäpermeier](https://schaepermeier.github.io),
+    Jonathan Fieldsend.
 
 ---------------------------------------
 

diff --git a/TODO-archiving b/TODO-archiving
@@ -0,0 +1,14 @@
+
+# Unbounded Archive
+ - [ ] .add_many() is currently a loop in Python and it is not available in C.
+       First, it could loop in C.
+       Also, it could filter before adding unless the user specifies "nondominated=True".
+       If it is nondominated, it could do a faster merge operation.
+
+ - Benchmark against: https://github.com/alandefreitas/pareto
+
+
+# Bounded size archives.
+
+ - [ ] Hypervolume archiver
+ - [ ] MGA
diff --git a/bibkeys.txt b/bibkeys.txt
@@ -1,3 +1,4 @@
+AraSei1989rst
 AugBadBroZit2009gecco
 BeuFonLopPaqVah09:tec
 BezLopStu2017assessment
@@ -27,6 +28,7 @@ HanJas1998
 HerWer1987tabucol
 IshHeSha2019regular
 IshMasTanNoj2015igd
+JasLus2018ndtree
 Jen03
 JohAraMcGSch1991
 KnoCor2002cec

diff --git a/c/Makefile b/c/Makefile
@@ -72,6 +72,7 @@ SRCS =  avl.c                                                                \
         igd.c                                                                \
         io.c                                                                 \
         libutil.c                                                            \
+        main-archiver.o                                                      \
         main-hv.c                                                            \
         main-hvapprox.c                                                      \
 	main-nondominated.c                                                  \
@@ -81,11 +82,12 @@ SRCS =  avl.c                                                                \
         pareto.c                                                             \
         rng.c                                                                \
         timer.c                                                              \
+	treap2d_fixed_size.c                                                 \
+	treap_archive.c                                                      \
         whv.c                                                                \
         whv_hype.c
 
 HDRS =  avl.h                                                                \
-        avl_tiny.h                                                           \
         bit_array.h                                                          \
         cmdline.h                                                            \
         common.h                                                             \
@@ -112,6 +114,9 @@ HDRS =  avl.h                                                                \
         rng.h                                                                \
         sort.h                                                               \
         timer.h                                                              \
+	treap2d.h                                                            \
+	treap2d_dlnode.h                                                     \
+	treap2d_fixed_size.h                                                 \
         whv.h                                                                \
         whv_hype.h                                                           \
         ziggurat_constants.h
@@ -160,7 +165,7 @@ EXE_CFLAGS += $(SANITIZERS) $(OPT_CFLAGS) $(MARCH_FLAGS) $(WARN_CFLAGS) \
 
 
 # List of test target base names
-TEST_NAMES := eaf nondominated hv hvapprox igd epsilon dominatedsets ndsort
+TEST_NAMES := archiver eaf nondominated hv hvapprox igd epsilon dominatedsets ndsort
 ASAN_TARGETS := $(addprefix asan-,$(TEST_NAMES))
 UBSAN_TARGETS:= $(addprefix ubsan-,$(TEST_NAMES))
 TEST_TARGETS := $(addprefix test-,$(TEST_NAMES))
@@ -192,14 +197,15 @@ $(ALL_NAMES): %: $(BINDIR)/%$(EXE)
 
 include libhv.mk
 
-$(BINDIR)/dominatedsets$(EXE): dominatedsets.o nondominated.o
+$(BINDIR)/archiver$(EXE): main-archiver.o io.o treap_archive.o nondominated.o treap2d_fixed_size.o
+$(BINDIR)/dominatedsets$(EXE): dominatedsets.o nondominated.o treap2d_fixed_size.o
 $(BINDIR)/eaf$(EXE): eaf.o eaf_main.o eaf3d.o avl.o
-$(BINDIR)/epsilon$(EXE): epsilon.o nondominated.o
+$(BINDIR)/epsilon$(EXE): epsilon.o nondominated.o treap2d_fixed_size.o
 $(BINDIR)/hv$(EXE): main-hv.o timer.o $(LIBHV_OBJS)
-$(BINDIR)/hvapprox$(EXE): main-hvapprox.o timer.o hvapprox.o rng.o mt19937/mt19937.o nondominated.o
-$(BINDIR)/igd$(EXE): igd.o nondominated.o
-$(BINDIR)/ndsort$(EXE): ndsort.o pareto.o nondominated.o $(LIBHV_OBJS)
-$(BINDIR)/nondominated$(EXE): main-nondominated.o timer.o nondominated.o
+$(BINDIR)/hvapprox$(EXE): main-hvapprox.o timer.o hvapprox.o rng.o mt19937/mt19937.o nondominated.o treap2d_fixed_size.o
+$(BINDIR)/igd$(EXE): igd.o nondominated.o treap2d_fixed_size.o
+$(BINDIR)/ndsort$(EXE): ndsort.o pareto.o $(LIBHV_OBJS)
+$(BINDIR)/nondominated$(EXE): main-nondominated.o timer.o nondominated.o treap2d_fixed_size.o
 
 $(EXE_FILES): cmdline.o io.o
 	$(call MKDIR, $(BINDIR)/)
@@ -214,22 +220,25 @@ eaf3d.o: eaf.h io.h bit_array.h cvector.h avl.h
 eaf_main.o: cmdline.h io.h eaf.h bit_array.h cvector.h
 epsilon.o: cmdline.h io.h nondominated.h epsilon.h
 hv.o: hv.h hv_priv.h hvc4d_priv.h sort.h libmoocore-config.h
-hv3dplus.o: hv_priv.h sort.h hv3d_priv.h avl_tiny.h
+hv3dplus.o: hv_priv.h sort.h hv3d_priv.h treap2d.h
 hv4d.o: hv4d_priv.h hv_priv.h sort.h
 hv_contrib.o: hv.h libmoocore-config.h nondominated.h
 hvapprox.o: hvapprox.h pow_int.h rng.h mt19937/mt19937.h
-hvc3d.o: hv_priv.h sort.h hv3d_priv.h avl_tiny.h
+hvc3d.o: hv_priv.h sort.h hv3d_priv.h treap2d.h
 igd.o: cmdline.h io.h nondominated.h igd.h pow_int.h
 io.o: io.h io_priv.h
+main-archiver.o: cmdline.h io.h treap_archive.h treap2d.h nd_tree.h libmoocore-config.h
 main-hv.o: cmdline.h io.h hv.h timer.h libmoocore-config.h nondominated.h hvapprox.h
 main-hvapprox.o: cmdline.h io.h hvapprox.h
 main-nondominated.o : cmdline.h io.h nondominated.h nondominated_kung.h
 ndsort.o: cmdline.h io.h nondominated.h hv.h
-nondominated.h: avl_tiny.h nondominated_kung.h
-nondominated.o: nondominated.h nondominated_kung.h
-pareto.o: nondominated.h
+nondominated.h: nondominated_kung.h
+nondominated.o: nondominated.h nondominated_kung.h treap2d_fixed_size.h
+pareto.o: nondominated.h treap2d_fixed_size.h
 rng.o: rng.h mt19937/mt19937.h ziggurat_constants.h
 timer.o: timer.h
+treap2d_fixed_size.o: treap2d_fixed_size.h
+treap2d_fixed_size.h: treap2d.h
 $(OBJS): common.h gcc_attribs.h config.h maxminclamp.h
 
 shlibs: clean $(SHLIB) $(CXXSHLIB)

diff --git a/c/NEWS.md b/c/NEWS.md
@@ -6,6 +6,9 @@
  * main-nondominated.c: Renamed from nondominated.c.
  * hv and hvapprox executables now use -U instead of -u for --union to be
    consistent with nondominated.
+ * treap2d.h: New.
+ * pareto.c, nondominated.c, hv3d_priv.h: Use Treap instead of AVL-tree.
+ * avl_tiny.h: Delete.
 
 
 ## 0.19.2

diff --git a/c/README.md b/c/README.md
@@ -26,6 +26,7 @@ The [**moocore** project](https://github.com/multi-objective/moocore/) collects
  * Computation of the Empirical Attainment Function. The empirical attainment
    function (EAF) describes the probabilistic distribution of the outcomes
    obtained by a stochastic algorithm in the objective space.
+ * Archivers that allow efficiently storing a nondominated set from points given one-by-one as a sequence.
 
 The goal of **moocore** is to provide an implementation of these functions that is efficient, thoroughly tested, well-documented, multi-platform (Windows, Linux, MacOS), multi-language (C, R, and Python, so far), with minimal dependencies and reusable from other libraries and packages, thus providing an easy-to-use and high-performance building block.
 

diff --git a/c/archive.h b/c/archive.h
@@ -0,0 +1,110 @@
+#ifndef _ARCHIVE_H_
+#define _ARCHIVE_H_
+
+#include "treap_archive.h"
+#include "nd_tree.h"
+
+enum unbounded_archive_type_t {
+    UNBOUNDED_ARCHIVE_TREAP,
+    UNBOUNDED_ARCHIVE_NDTREE,
+};
+
+typedef struct UnboundedArchive {
+    enum unbounded_archive_type_t archive_type;
+    union {
+        TreapArchive treap;
+        NDTreeArchive ndtree;
+    };
+} UnboundedArchive;
+
+#define UNBOUNDED_ARCHIVE_DISPATCH(SELF, TREAP_CODE, NDTREE_CODE)              \
+    switch ((SELF)->archive_type) {                                            \
+      case UNBOUNDED_ARCHIVE_TREAP:  do { TREAP_CODE; } while(0); break;       \
+      case UNBOUNDED_ARCHIVE_NDTREE: do { NDTREE_CODE; } while(0); break;      \
+      default: unreachable();                                                  \
+    }
+
+
+static inline UnboundedArchive *
+archive_new(enum unbounded_archive_type_t type)
+{
+    UnboundedArchive * arch = malloc(sizeof(*arch));
+    arch->archive_type = type;
+    return arch;
+
+}
+static inline UnboundedArchive *
+archive_ndtree_new(dimension_t dim, NDTreeConfig config)
+{
+    UnboundedArchive * archive = archive_new(UNBOUNDED_ARCHIVE_NDTREE);
+    ndtree_init(&archive->ndtree, dim, config.max_children, config.max_bucket_size, config.allow_duplicates);
+    return archive;
+}
+
+static inline UnboundedArchive *
+archive_treap_new(void)
+{
+    UnboundedArchive * archive = archive_new(UNBOUNDED_ARCHIVE_TREAP);
+    treap_archive_init(&archive->treap);
+    return archive;
+}
+
+/**
+
+*/
+static inline int
+archive_add(UnboundedArchive * restrict arch, const double * restrict z, const void * restrict x)
+{
+    UNBOUNDED_ARCHIVE_DISPATCH(
+        arch,
+        return treap_archive_add_discard(&arch->treap, z, x),
+        return ndtree_add_discard(&arch->ndtree, z, x));
+}
+
+static inline size_t
+archive_total_size(const UnboundedArchive *arch)
+{
+    UNBOUNDED_ARCHIVE_DISPATCH(
+        arch,
+        return treap_archive_total_size(&arch->treap),
+        return ndtree_total_size(&arch->ndtree));
+}
+
+static inline size_t
+archive_unique_size(const UnboundedArchive *arch)
+{
+    UNBOUNDED_ARCHIVE_DISPATCH(
+        arch,
+        return treap_archive_unique_size(&arch->treap),
+        return ndtree_unique_size(&arch->ndtree));
+}
+
+static inline void
+archive_get_contents(const UnboundedArchive *arch, double * z_list, const void ** x_list)
+{
+    UNBOUNDED_ARCHIVE_DISPATCH(
+        arch,
+        treap_archive_get_contents(&arch->treap, z_list, x_list),
+        ndtree_get_contents(&arch->ndtree, z_list, x_list));
+}
+
+static inline void
+archive_free(UnboundedArchive *arch)
+{
+    UNBOUNDED_ARCHIVE_DISPATCH(
+        arch,
+        treap_archive_free_nodes(&arch->treap),
+        ndtree_free_nodes(&arch->ndtree));
+    free(arch);
+}
+
+static inline void
+archive_print_stats(FILE * stream, const UnboundedArchive *arch)
+{
+    UNBOUNDED_ARCHIVE_DISPATCH(
+        arch,
+        (void)0,
+        ndtree_print_stats(stream, &arch->ndtree));
+}
+
+#endif // _ARCHIVE_H_
diff --git a/c/archiving.h b/c/archiving.h
@@ -0,0 +1,19 @@
+/*****************************************************************************
+
+ Public definitions that are common to all types of archives.
+
+ *****************************************************************************/
+#ifndef _ARCHIVING_H_
+#define _ARCHIVING_H_
+
+enum archive_insert_result_t {
+    ARCHIVE_INSERT_UKNOWN_ERROR = -4,
+    ARCHIVE_INSERT_X_MUST_BE_NOT_NULL = -3,
+    ARCHIVE_INSERT_X_MUST_BE_NULL = -2,
+    ARCHIVE_INSERT_MEMORY_ERROR = -1,
+    ARCHIVE_INSERT_REJECTED = 0,
+    ARCHIVE_INSERT_ACCEPTED = 1,
+    ARCHIVE_INSERT_DUPLICATED = 2,
+};
+
+#endif // _ARCHIVING_H_
diff --git a/c/archiving_priv.h b/c/archiving_priv.h
@@ -0,0 +1,26 @@
+#ifndef _ARCHIVING_PRIV_H_
+#define _ARCHIVING_PRIV_H_
+
+#include <stdint.h>
+#include <stdlib.h>
+#include <assert.h>
+
+#include "archiving.h"
+
+/**
+   Returns 0 if OK, negative values if there is a problem.
+*/
+static inline int
+wrong_x_is_null(signed char * restrict x_is_null, const void * restrict x)
+{
+    if (*x_is_null == 0)
+        return x != NULL ? 0 : ARCHIVE_INSERT_X_MUST_BE_NOT_NULL;
+    if (*x_is_null == 1)
+        return x == NULL ? 0 : ARCHIVE_INSERT_X_MUST_BE_NULL;
+
+    *x_is_null = (x == NULL);
+    return 0;
+}
+
+
+#endif // _ARCHIVING_PRIV_H_