diff --git a/src/fpk.c b/src/fpk.c
new file mode 100644
index 0000000..55ad015
--- /dev/null
+++ b/src/fpk.c
@@ -0,0 +1,354 @@
+#include "fpk.h"
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+#define FPK_HEADER_FIELD_ENTRY_COUNT 4
+#define FPK_HEADER_FIELD_DIR_OFFSET 12
+#define FPK_HEADER_FIELD_STRTAB_OFFSET 20
+
+#define FPK_DIR_ENTRY_FIELD_PATH_OFFSET 0
+#define FPK_DIR_ENTRY_FIELD_DATA_OFFSET 8
+#define FPK_DIR_ENTRY_FIELD_DATA_SIZE 16
+#define FPK_DIR_ENTRY_FIELD_METHOD 24
+
+struct fpk {
+	FILE *file;
+	uint64_t entry_count;
+	fpk_entry_t *entries;
+	char *strtab;
+};
+
+struct fpk_writer_entry {
+	char *path;
+	uint64_t data_offset;
+	uint64_t data_size;
+	uint8_t method;
+};
+
+struct fpk_writer {
+	FILE *file;
+	struct fpk_writer_entry *entries;
+	uint64_t count;
+	uint64_t capacity;
+	uint64_t cursor;
+};
+
+static uint32_t get_u32le(const unsigned char *p)
+{
+	return (uint32_t)p[0] | (uint32_t)p[1] << 8 | (uint32_t)p[2] << 16 |
+	       (uint32_t)p[3] << 24;
+}
+
+static uint64_t get_u64le(const unsigned char *p)
+{
+	return (uint64_t)get_u32le(p) | (uint64_t)get_u32le(p + 4) << 32;
+}
+
+static void put_u32le(unsigned char *p, uint32_t v)
+{
+	p[0] = (unsigned char)(v);
+	p[1] = (unsigned char)(v >> 8);
+	p[2] = (unsigned char)(v >> 16);
+	p[3] = (unsigned char)(v >> 24);
+}
+
+static void put_u64le(unsigned char *p, uint64_t v)
+{
+	put_u32le(p, (uint32_t)v);
+	put_u32le(p + 4, (uint32_t)(v >> 32));
+}
+
+static char *dup_path(const char *path)
+{
+	size_t len = strlen(path) + 1;
+	char *copy = malloc(len);
+
+	if (copy != NULL)
+		memcpy(copy, path, len);
+
+	return copy;
+}
+
+struct fpk *fpk_open(const char *path)
+{
+	struct fpk *fpk;
+	unsigned char header[FPK_HEADER_SIZE];
+	uint64_t dir_offset, strtab_offset, strtab_size;
+	uint64_t i;
+
+	fpk = malloc(sizeof(*fpk));
+	if (fpk == NULL)
+		goto out;
+	fpk->file = NULL;
+	fpk->entries = NULL;
+	fpk->strtab = NULL;
+
+	fpk->file = fopen(path, "rb");
+	if (fpk->file == NULL)
+		goto fail;
+
+	if (fread(header, 1, FPK_HEADER_SIZE, fpk->file) != FPK_HEADER_SIZE)
+		goto fail;
+	if (memcmp(header, FPK_MAGIC, FPK_MAGIC_SIZE) != 0)
+		goto fail;
+
+	fpk->entry_count = get_u64le(header + FPK_HEADER_FIELD_ENTRY_COUNT);
+	dir_offset = get_u64le(header + FPK_HEADER_FIELD_DIR_OFFSET);
+	strtab_offset = get_u64le(header + FPK_HEADER_FIELD_STRTAB_OFFSET);
+	if (dir_offset < strtab_offset)
+		goto fail;
+	strtab_size = dir_offset - strtab_offset;
+
+	fpk->strtab = malloc((size_t)strtab_size);
+	if (fpk->strtab == NULL)
+		goto fail;
+	if (fseek(fpk->file, (long)strtab_offset, SEEK_SET) != 0)
+		goto fail;
+	if (fread(fpk->strtab, 1, (size_t)strtab_size, fpk->file) !=
+	    strtab_size)
+		goto fail;
+
+	fpk->entries = malloc((size_t)fpk->entry_count * sizeof(fpk_entry_t));
+	if (fpk->entries == NULL)
+		goto fail;
+	if (fseek(fpk->file, (long)dir_offset, SEEK_SET) != 0)
+		goto fail;
+	for (i = 0; i < fpk->entry_count; i++) {
+		unsigned char rec[FPK_DIR_ENTRY_SIZE];
+		uint64_t path_offset;
+
+		if (fread(rec, 1, FPK_DIR_ENTRY_SIZE, fpk->file) !=
+		    FPK_DIR_ENTRY_SIZE)
+			goto fail;
+
+		path_offset = get_u64le(rec + FPK_DIR_ENTRY_FIELD_PATH_OFFSET);
+		if (path_offset >= strtab_size)
+			goto fail;
+
+		fpk->entries[i].path = fpk->strtab + path_offset;
+		fpk->entries[i].data_offset =
+			get_u64le(rec + FPK_DIR_ENTRY_FIELD_DATA_OFFSET);
+		fpk->entries[i].data_size =
+			get_u64le(rec + FPK_DIR_ENTRY_FIELD_DATA_SIZE);
+		fpk->entries[i].method = rec[FPK_DIR_ENTRY_FIELD_METHOD];
+	}
+	goto out;
+
+fail:
+	fpk_close(fpk);
+	fpk = NULL;
+out:
+	return fpk;
+}
+
+void fpk_close(struct fpk *fpk)
+{
+	if (fpk == NULL)
+		return;
+	if (fpk->file != NULL)
+		fclose(fpk->file);
+	free(fpk->entries);
+	free(fpk->strtab);
+	free(fpk);
+}
+
+fpk_entry_t *fpk_find(struct fpk *fpk, const char *path)
+{
+	uint64_t i;
+
+	for (i = 0; i < fpk->entry_count; i++) {
+		if (strcmp(fpk->entries[i].path, path) == 0)
+			return &fpk->entries[i];
+	}
+	return NULL;
+}
+
+void *fpk_read(struct fpk *fpk, const fpk_entry_t *entry, size_t *out_size)
+{
+	void *buf;
+
+	buf = malloc((size_t)entry->data_size);
+	if (buf == NULL)
+		goto out;
+
+	if (fseek(fpk->file, (long)entry->data_offset, SEEK_SET) != 0 ||
+	    fread(buf, 1, (size_t)entry->data_size, fpk->file) !=
+		    entry->data_size) {
+		free(buf);
+		buf = NULL;
+		goto out;
+	}
+
+	*out_size = (size_t)entry->data_size;
+out:
+	return buf;
+}
+
+uint64_t fpk_entry_count(const struct fpk *fpk)
+{
+	return fpk->entry_count;
+}
+
+const fpk_entry_t *fpk_entry_at(const struct fpk *fpk, uint64_t index)
+{
+	if (index >= fpk->entry_count)
+		return NULL;
+	return &fpk->entries[index];
+}
+
+#define FPK_WRITER_INITIAL_CAPACITY 16
+
+struct fpk_writer *fpk_writer_open(const char *path)
+{
+	struct fpk_writer *w;
+	unsigned char placeholder[FPK_HEADER_SIZE];
+
+	w = malloc(sizeof(*w));
+	if (w == NULL)
+		goto out;
+	w->entries = NULL;
+	w->count = 0;
+	w->capacity = 0;
+
+	w->file = fopen(path, "wb");
+	if (w->file == NULL)
+		goto fail_free;
+
+	memset(placeholder, 0, sizeof(placeholder));
+	if (fwrite(placeholder, 1, FPK_HEADER_SIZE, w->file) !=
+	    FPK_HEADER_SIZE)
+		goto fail_close;
+	w->cursor = FPK_HEADER_SIZE;
+	goto out;
+
+fail_close:
+	fclose(w->file);
+fail_free:
+	free(w);
+	w = NULL;
+out:
+	return w;
+}
+
+int fpk_writer_add(struct fpk_writer *w, const char *path, const void *data,
+		    size_t size)
+{
+	struct fpk_writer_entry *entry;
+	char *path_copy = NULL;
+	int ret = 0;
+
+	if (w->count == w->capacity) {
+		uint64_t new_capacity = w->capacity == 0
+						 ? FPK_WRITER_INITIAL_CAPACITY
+						 : w->capacity * 2;
+		struct fpk_writer_entry *new_entries = realloc(
+			w->entries, (size_t)new_capacity * sizeof(*new_entries));
+
+		if (new_entries == NULL) {
+			ret = -1;
+			goto out;
+		}
+		w->entries = new_entries;
+		w->capacity = new_capacity;
+	}
+
+	path_copy = dup_path(path);
+	if (path_copy == NULL) {
+		ret = -1;
+		goto out;
+	}
+
+	if (size > 0 && fwrite(data, 1, size, w->file) != size) {
+		ret = -1;
+		goto out;
+	}
+
+	entry = &w->entries[w->count];
+	entry->path = path_copy;
+	entry->data_offset = w->cursor;
+	entry->data_size = size;
+	entry->method = FPK_METHOD_STORE;
+
+	w->cursor += size;
+	w->count++;
+
+out:
+	if (ret != 0)
+		free(path_copy);
+	return ret;
+}
+
+int fpk_writer_finalize(struct fpk_writer *w)
+{
+	uint64_t strtab_offset, dir_offset, path_offset;
+	unsigned char header[FPK_HEADER_SIZE];
+	unsigned char rec[FPK_DIR_ENTRY_SIZE];
+	uint64_t i;
+	int ret = 0;
+
+	strtab_offset = w->cursor;
+	if (fseek(w->file, (long)strtab_offset, SEEK_SET) != 0) {
+		ret = -1;
+		goto out;
+	}
+
+	path_offset = 0;
+	for (i = 0; i < w->count && ret == 0; i++) {
+		size_t len = strlen(w->entries[i].path) + 1;
+
+		if (fwrite(w->entries[i].path, 1, len, w->file) != len)
+			ret = -1;
+		else
+			path_offset += len;
+	}
+	if (ret != 0)
+		goto out;
+	dir_offset = strtab_offset + path_offset;
+
+	path_offset = 0;
+	for (i = 0; i < w->count && ret == 0; i++) {
+		put_u64le(rec + FPK_DIR_ENTRY_FIELD_PATH_OFFSET, path_offset);
+		put_u64le(rec + FPK_DIR_ENTRY_FIELD_DATA_OFFSET,
+			  w->entries[i].data_offset);
+		put_u64le(rec + FPK_DIR_ENTRY_FIELD_DATA_SIZE,
+			  w->entries[i].data_size);
+		rec[FPK_DIR_ENTRY_FIELD_METHOD] = w->entries[i].method;
+
+		if (fwrite(rec, 1, FPK_DIR_ENTRY_SIZE, w->file) !=
+		    FPK_DIR_ENTRY_SIZE)
+			ret = -1;
+		else
+			path_offset += strlen(w->entries[i].path) + 1;
+	}
+	if (ret != 0)
+		goto out;
+
+	memcpy(header, FPK_MAGIC, FPK_MAGIC_SIZE);
+	put_u64le(header + FPK_HEADER_FIELD_ENTRY_COUNT, w->count);
+	put_u64le(header + FPK_HEADER_FIELD_DIR_OFFSET, dir_offset);
+	put_u64le(header + FPK_HEADER_FIELD_STRTAB_OFFSET, strtab_offset);
+
+	if (fseek(w->file, 0, SEEK_SET) != 0 ||
+	    fwrite(header, 1, FPK_HEADER_SIZE, w->file) != FPK_HEADER_SIZE ||
+	    fflush(w->file) != 0)
+		ret = -1;
+
+out:
+	return ret;
+}
+
+void fpk_writer_close(struct fpk_writer *w)
+{
+	uint64_t i;
+
+	if (w == NULL)
+		return;
+	if (w->file != NULL)
+		fclose(w->file);
+	for (i = 0; i < w->count; i++)
+		free(w->entries[i].path);
+	free(w->entries);
+	free(w);
+}
diff --git a/test/bmake.mk b/test/bmake.mk
new file mode 100644
index 0000000..0a99ef3
--- /dev/null
+++ b/test/bmake.mk
@@ -0,0 +1,16 @@
+CC ?= cc
+CFLAGS = -Wall -Wpedantic -I../include -std=c89
+LDFLAGS = -L../bin/debug -lfpk
+
+.PHONY: all test clean
+
+all: test
+
+test_program: test.c
+	${CC} ${CFLAGS} -o ${.TARGET} test.c ${LDFLAGS}
+
+test: test_program
+	@./test_program
+
+clean:
+	rm -f test_program
diff --git a/test/gmake.mk b/test/gmake.mk
new file mode 100644
index 0000000..ac03a6c
--- /dev/null
+++ b/test/gmake.mk
@@ -0,0 +1,28 @@
+CC := cc
+CFLAGS = -Wall -Wpedantic -I../include -std=c89
+LDFLAGS = -L../bin/debug -lfpk
+
+ifeq ($(OS),Windows_NT)
+    SHELL := cmd.exe
+    .SHELLFLAGS := /c
+    EXE_EXT = .exe
+else
+    EXE_EXT =
+endif
+
+.PHONY: all test clean
+
+all: test
+
+test_program$(EXE_EXT): test.c
+	$(CC) $(CFLAGS) -o $@ test.c $(LDFLAGS)
+
+test: test_program$(EXE_EXT)
+	@./test_program$(EXE_EXT)
+
+clean:
+ifeq ($(OS),Windows_NT)
+	@del /Q test_program.exe 2>nul || true
+else
+	@rm -f test_program
+endif
diff --git a/test/test.c b/test/test.c
new file mode 100644
index 0000000..7deff34
--- /dev/null
+++ b/test/test.c
@@ -0,0 +1,71 @@
+#include <fpk.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+#define CHECK(cond, msg)                                                     \
+	do {                                                                  \
+		if (!(cond)) {                                                \
+			fprintf(stderr, "FAIL: %s\n", msg);                  \
+			return EXIT_FAILURE;                                  \
+		}                                                             \
+	} while (0)
+
+static const char *PAK_PATH = "roundtrip.fpk";
+
+int main(void)
+{
+	struct fpk_writer *w;
+	struct fpk *r;
+	fpk_entry_t *entry;
+	void *data;
+	size_t out_size;
+
+	static const char hello[] = "hello, fpk";
+	static const char world[] = "world";
+
+	w = fpk_writer_open(PAK_PATH);
+	CHECK(w != NULL, "fpk_writer_open failed");
+	CHECK(fpk_writer_add(w, "a/hello.txt", hello, sizeof(hello)) == 0,
+	      "fpk_writer_add(hello) failed");
+	CHECK(fpk_writer_add(w, "b/world.txt", world, sizeof(world)) == 0,
+	      "fpk_writer_add(world) failed");
+	CHECK(fpk_writer_add(w, "c/empty.bin", NULL, 0) == 0,
+	      "fpk_writer_add(empty) failed");
+	CHECK(fpk_writer_finalize(w) == 0, "fpk_writer_finalize failed");
+	fpk_writer_close(w);
+
+	r = fpk_open(PAK_PATH);
+	CHECK(r != NULL, "fpk_open failed");
+	CHECK(fpk_entry_count(r) == 3, "unexpected entry_count");
+
+	entry = fpk_find(r, "a/hello.txt");
+	CHECK(entry != NULL, "fpk_find(hello) failed");
+	CHECK(entry->data_size == sizeof(hello), "hello size mismatch");
+	data = fpk_read(r, entry, &out_size);
+	CHECK(data != NULL, "fpk_read(hello) failed");
+	CHECK(out_size == sizeof(hello), "hello read size mismatch");
+	CHECK(memcmp(data, hello, sizeof(hello)) == 0, "hello content mismatch");
+	free(data);
+
+	entry = fpk_find(r, "b/world.txt");
+	CHECK(entry != NULL, "fpk_find(world) failed");
+	data = fpk_read(r, entry, &out_size);
+	CHECK(data != NULL, "fpk_read(world) failed");
+	CHECK(out_size == sizeof(world), "world read size mismatch");
+	CHECK(memcmp(data, world, sizeof(world)) == 0, "world content mismatch");
+	free(data);
+
+	entry = fpk_find(r, "c/empty.bin");
+	CHECK(entry != NULL, "fpk_find(empty) failed");
+	CHECK(entry->data_size == 0, "empty size mismatch");
+
+	CHECK(fpk_find(r, "does/not/exist") == NULL,
+	      "fpk_find(missing) should return NULL");
+
+	fpk_close(r);
+	remove(PAK_PATH);
+
+	printf("PASS\n");
+	return EXIT_SUCCESS;
+}