diff --git a/.clangd b/.clangd
new file mode 100644
index 0000000..ef516ad
--- /dev/null
+++ b/.clangd
@@ -0,0 +1,2 @@
+CompileFlags:
+  Add: -ferror-limit=0
diff --git a/.woodpecker.yml b/.woodpecker.yml
new file mode 100644
index 0000000..a7e8fe3
--- /dev/null
+++ b/.woodpecker.yml
@@ -0,0 +1,36 @@
+variables:
+  &image 'git.rustybever.be/chewing_bever/c-devop:alpine3.17'
+
+matrix:
+  PLATFORM:
+    - 'linux/amd64'
+    - 'linux/arm64'
+
+branches:
+  exclude: [ main ]
+platform: ${PLATFORM}
+
+pipeline:
+  lint:
+    image: *image
+    pull: true
+    commands:
+      - make lint
+    when:
+      event: [push, pull_request]
+
+  build:
+    image: *image
+    commands:
+      - make
+      - make clean
+      - CFLAGS='-O3' make
+    when:
+      event: [push, pull_request]
+
+  test:
+    image: *image
+    commands:
+      - make test
+    when:
+      event: [push, pull_request]
diff --git a/Makefile b/Makefile
index 238d4d3..30b6a31 100644
--- a/Makefile
+++ b/Makefile
@@ -25,7 +25,7 @@ INC_FLAGS := $(addprefix -I,$(INC_DIRS))
 #  object file is also recompiled if only a header is changed.
 # -MP: generate a dummy target for every header file (according to the  docs it
 #  prevents some errors when removing header files)
-CFLAGS ?= -MMD -MP -Wall -Werror -Wextra
+CFLAGS ?= -MMD -MP -Wall -Wextra
 CFLAGS += $(INC_FLAGS)
 
 .PHONY: all
@@ -56,10 +56,15 @@ build-test: $(BINS_TEST)
 $(BINS_TEST): %: %.c.o $(OBJS)
 	$(CC) $^ -o $@
 
-# Each test includes the test directory, which contains the acutest header file
+# Allow with the include directory, each test includes $(TEST_DIR) (which
+# contains the acutest.h header file), and the src directory of the module it's
+# testing. This allows tests to access internal methods, which aren't publicly
+# exposed.
 $(BUILD_DIR)/$(TEST_DIR)/%.c.o: $(TEST_DIR)/%.c
 	mkdir -p $(dir $@)
-	$(CC) $(CFLAGS) -I$(TEST_DIR) -c $< -o $@
+	$(CC) $(CFLAGS) -I$(TEST_DIR) \
+		-I$(dir $(@:$(BUILD_DIR)/$(TEST_DIR)/%=$(SRC_DIR)/%)) \
+		-c $< -o $@
 
 # =====MAINTENANCE=====
 .PHONY: lint
diff --git a/README.md b/README.md
index 724ba54..9381fb1 100644
--- a/README.md
+++ b/README.md
@@ -1,15 +1,12 @@
 # libvieter
 
-This library powers part of Vieter, most noteably the sections that can easily
+This library powers part of Vieter, most notably the sections that can easily
 be implemented in C (or just parts I want to implement in C because it's fun).
 
-The goal of this library is to be completely self-contained, meaning any
-required data structures have to be implemented as well. It can only depend on
-the C standard libraries.
+The goal of this library is to be as self-contained as possible; data
+structures should be implemented manually if possible.
 
-Currently it contains the following:
-
-* Cron expression parser & next time calculator
+See the [source code](/src) for the list of modules.
 
 ## Development
 
@@ -18,6 +15,27 @@ Currently it contains the following:
 Everything is handled by the provided Makefile. To compile the static library,
 simply run `make`.
 
+### Project structure
+
+Each module has its own subdirectory inside `src`, e.g. `src/cron`. This
+directory contains the actual implementation of a module, along with any
+internally used header files. Each internal function should be defined in a
+header file, as to make testing these possible.
+
+Each module should also have its own header file inside the `include`
+directory. This header file defines the public API that the library exposes for
+this specific module.
+
+Any code in a module may only import internal headers from that module, along
+with any of the public API header files. Modules should not depend on each
+other's internal implementationns.
+
+Each module should contain a README describing its contents.
+
+All file names, function names... (even internals) should follow snake case
+convention and have a prefix unique to that module, starting with `vieter_`.
+For example, the `cron` modules uses the `vieter_cron_` prefix for everything.
+
 ### Testing
 
 This library uses [Acutest](https://github.com/mity/acutest) for its tests.
@@ -25,6 +43,10 @@ Tests should be placed in the `test` subdirectory, further divided into
 directories that correspond those in `src`. Test files should begin with
 `test_`, and their format should follow the expected format for Acutest.
 
+Each `test_` is compiled separately into a binary, linked with libvieter. A
+test file can import any of the public API header files, along with any header
+files defined in its respective module. This allows testing internal functions.
+
 To run the tests, simply run `make test`. If you wish to only run a specific
 test binary, you can find them in `build/test`.
 
diff --git a/include/vieter_heap.h b/include/vieter_heap.h
new file mode 100644
index 0000000..c7a9706
--- /dev/null
+++ b/include/vieter_heap.h
@@ -0,0 +1,27 @@
+#ifndef VIETER_HEAP
+#define VIETER_HEAP
+
+#include <stddef.h>
+#include <stdint.h>
+
+typedef struct vieter_heap vieter_heap;
+
+typedef enum vieter_heap_error {
+  vieter_heap_ok = 0,
+  vieter_heap_empty = 1
+} vieter_heap_error;
+
+vieter_heap *vieter_heap_init();
+
+void vieter_heap_free(vieter_heap *heap);
+
+uint64_t vieter_heap_size(vieter_heap *heap);
+
+vieter_heap_error vieter_heap_insert(vieter_heap *heap, uint64_t key,
+                                     void *data);
+
+vieter_heap_error vieter_heap_pop(void **out, vieter_heap *heap);
+
+vieter_heap_error vieter_heap_peek(void **out, vieter_heap *heap);
+
+#endif
diff --git a/src/cron/expression.c b/src/cron/vieter_cron_expression.c
similarity index 100%
rename from src/cron/expression.c
rename to src/cron/vieter_cron_expression.c
diff --git a/src/cron/parse.c b/src/cron/vieter_cron_parse.c
similarity index 100%
rename from src/cron/parse.c
rename to src/cron/vieter_cron_parse.c
diff --git a/src/heap/vieter_heap.c b/src/heap/vieter_heap.c
new file mode 100644
index 0000000..6d543df
--- /dev/null
+++ b/src/heap/vieter_heap.c
@@ -0,0 +1,38 @@
+#include "vieter_heap.h"
+#include "vieter_heap_tree.h"
+
+#include <stdlib.h>
+
+struct vieter_heap {
+  vieter_heap_tree *tree;
+};
+
+vieter_heap *vieter_heap_init() { return calloc(1, sizeof(vieter_heap)); }
+
+uint64_t vieter_heap_size(vieter_heap *heap) { return 0; }
+
+void vieter_heap_free(vieter_heap *heap) {}
+
+vieter_heap_error vieter_heap_insert(vieter_heap *heap, uint64_t key,
+                                     void *data) {
+  vieter_heap_node *new_node = vieter_heap_node_init();
+  new_node->key = key;
+  new_node->data = data;
+  vieter_heap_tree *new_tree = vieter_heap_tree_init(new_node, NULL, 1);
+
+  if (heap->tree == NULL) {
+    heap->tree = new_tree;
+  } else {
+    heap->tree = vieter_heap_tree_merge(heap->tree, new_tree);
+  }
+
+  return vieter_heap_ok;
+}
+
+vieter_heap_error vieter_heap_pop(void **out, vieter_heap *heap) {
+  return vieter_heap_ok;
+}
+
+vieter_heap_error vieter_heap_peek(void **out, vieter_heap *heap) {
+  return vieter_heap_ok;
+}
diff --git a/src/heap/vieter_heap_tree.c b/src/heap/vieter_heap_tree.c
new file mode 100644
index 0000000..e65fb56
--- /dev/null
+++ b/src/heap/vieter_heap_tree.c
@@ -0,0 +1,38 @@
+#include "vieter_heap_tree.h"
+
+vieter_heap_node *vieter_heap_node_init() {
+  return calloc(1, sizeof(vieter_heap_node));
+}
+
+vieter_heap_node *vieter_heap_node_merge_same_order(vieter_heap_node *root_a,
+                                                    vieter_heap_node *root_b) {
+  vieter_heap_node *new_root, *new_child;
+
+  if (root_a->key <= root_b->key) {
+    new_root = root_a;
+    new_child = root_b;
+  } else {
+    new_root = root_b;
+    new_child = root_a;
+  }
+
+  new_root->next_largest_order = new_root->largest_order;
+  new_root->largest_order = new_child;
+
+  return new_root;
+}
+
+vieter_heap_tree *vieter_heap_tree_init(vieter_heap_node *root,
+                                        vieter_heap_tree *next,
+                                        uint64_t order) {
+  vieter_heap_tree *tree = malloc(sizeof(vieter_heap_tree));
+
+  tree->root = root;
+  tree->next = next;
+  tree->order = order;
+
+  return tree;
+}
+
+vieter_heap_tree *vieter_heap_tree_merge(vieter_heap_tree *tree_a,
+                                         vieter_heap_tree *tree_b) {}
diff --git a/src/heap/vieter_heap_tree.h b/src/heap/vieter_heap_tree.h
new file mode 100644
index 0000000..18808e1
--- /dev/null
+++ b/src/heap/vieter_heap_tree.h
@@ -0,0 +1,30 @@
+#ifndef VIETER_HEAP_TREE
+#define VIETER_HEAP_TREE
+
+#include <stdint.h>
+#include <stdlib.h>
+
+typedef struct vieter_heap_node {
+    uint64_t key;
+    void *data;
+    struct vieter_heap_node *largest_order;
+    struct vieter_heap_node *next_largest_order;
+} vieter_heap_node;
+
+vieter_heap_node *vieter_heap_node_init();
+
+void vieter_heap_node_free(vieter_heap_node *node);
+
+vieter_heap_node *vieter_heap_tree_merge_same_order(vieter_heap_node *root_a, vieter_heap_node *root_b);
+
+typedef struct vieter_heap_tree {
+    uint64_t order;
+    vieter_heap_node *root;
+    struct vieter_heap_tree *next;
+} vieter_heap_tree;
+
+vieter_heap_tree *vieter_heap_tree_init(vieter_heap_node *root, vieter_heap_tree *next, uint64_t order);
+
+vieter_heap_tree *vieter_heap_tree_merge(vieter_heap_tree *tree_a, vieter_heap_tree *tree_b);
+
+#endif
diff --git a/test/heap/test_heap.c b/test/heap/test_heap.c
new file mode 100644
index 0000000..832404d
--- /dev/null
+++ b/test/heap/test_heap.c
@@ -0,0 +1,19 @@
+#include "acutest.h"
+#include "vieter_heap.h"
+#include "vieter_heap_tree.h"
+
+#define TEST_SIZE(heap, size) \
+    TEST_CHECK(vieter_heap_size(heap) == size); \
+    TEST_MSG("Size: %zu", vieter_heap_size(heap))
+
+void test_init() {
+    vieter_heap *heap = vieter_heap_init();
+    TEST_CHECK(heap != NULL);
+    TEST_SIZE(heap, 0);
+    vieter_heap_free(heap);
+}
+
+TEST_LIST = {
+    {"test_init", test_init},
+    {NULL, NULL}
+};