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+                    GNU GENERAL PUBLIC LICENSE
+                       Version 3, 29 June 2007
+
+ Copyright (C) 2007 Free Software Foundation, Inc. <https://fsf.org/>
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+  IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING
+WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MODIFIES AND/OR CONVEYS
+THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES, INCLUDING ANY
+GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE
+USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED TO LOSS OF
+DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD
+PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER PROGRAMS),
+EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF
+SUCH DAMAGES.
+
+  17. Interpretation of Sections 15 and 16.
+
+  If the disclaimer of warranty and limitation of liability provided
+above cannot be given local legal effect according to their terms,
+reviewing courts shall apply local law that most closely approximates
+an absolute waiver of all civil liability in connection with the
+Program, unless a warranty or assumption of liability accompanies a
+copy of the Program in return for a fee.
+
+                     END OF TERMS AND CONDITIONS
+
+            How to Apply These Terms to Your New Programs
+
+  If you develop a new program, and you want it to be of the greatest
+possible use to the public, the best way to achieve this is to make it
+free software which everyone can redistribute and change under these terms.
+
+  To do so, attach the following notices to the program.  It is safest
+to attach them to the start of each source file to most effectively
+state the exclusion of warranty; and each file should have at least
+the "copyright" line and a pointer to where the full notice is found.
+
+    <one line to give the program's name and a brief idea of what it does.>
+    Copyright (C) <year>  <name of author>
+
+    This program is free software: you can redistribute it and/or modify
+    it under the terms of the GNU General Public License as published by
+    the Free Software Foundation, either version 3 of the License, or
+    (at your option) any later version.
+
+    This program is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU General Public License for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+Also add information on how to contact you by electronic and paper mail.
+
+  If the program does terminal interaction, make it output a short
+notice like this when it starts in an interactive mode:
+
+    <program>  Copyright (C) <year>  <name of author>
+    This program comes with ABSOLUTELY NO WARRANTY; for details type `show w'.
+    This is free software, and you are welcome to redistribute it
+    under certain conditions; type `show c' for details.
+
+The hypothetical commands `show w' and `show c' should show the appropriate
+parts of the General Public License.  Of course, your program's commands
+might be different; for a GUI interface, you would use an "about box".
+
+  You should also get your employer (if you work as a programmer) or school,
+if any, to sign a "copyright disclaimer" for the program, if necessary.
+For more information on this, and how to apply and follow the GNU GPL, see
+<https://www.gnu.org/licenses/>.
+
+  The GNU General Public License does not permit incorporating your program
+into proprietary programs.  If your program is a subroutine library, you
+may consider it more useful to permit linking proprietary applications with
+the library.  If this is what you want to do, use the GNU Lesser General
+Public License instead of this License.  But first, please read
+<https://www.gnu.org/licenses/why-not-lgpl.html>.
diff --git a/README.md b/README.md
new file mode 100644
index 0000000..4eca2e5
--- /dev/null
+++ b/README.md
@@ -0,0 +1,32 @@
+# PNG to QOI conversion utility
+
+A simple utility which recursively converts PNG images to the [QOI](https://qoiformat.org/) image format. I used this project to learn about writing multi-threaded non-blocking io code in the [Odin](https://odin-lang.org/) programming language.
+
+## Features
+
+* Converts PNG data to match QOI data layout automatically.
+* Very fast multi-threaded IO (250mb of 16bit PNG data converts in less than 2s on my 2015 Dell XPS 15).
+
+## Usage
+
+Run the executable and it will find every .png file in its own directory plus sub-directories and save a .qoi copy.
+
+Alternatively you can pass a specific directory as input either by the command line or by clicking and dragging a directory onto the executable itself and it will only convert the .png files within that directory/subdirectories. 
+
+## Building
+Requires an odin compiler and core packages.
+
+To build, simply run:
+```sh
+odin build .
+```
+
+Or for a highly optimised build that executes orders of magnitude faster you could run this:
+```sh
+odin build . -o:aggressive -linker:lld -lto:thin-files -microarch:native -out:png_to_qoi_util.exe
+```
+
+## Notes
+* I didn't bother adding a replace file argument or support for other image formats, it just does exactly what I need for now.
+* Clicking and dragging a directory onto the executable works on windows but might not work on other platforms/file managers.
+* GPLv3 Licensed.
diff --git a/png_to_qoi_util.odin b/png_to_qoi_util.odin
new file mode 100644
index 0000000..e590297
--- /dev/null
+++ b/png_to_qoi_util.odin
@@ -0,0 +1,357 @@
+package png_to_qoi_util
+
+import "core:os"
+import "core:fmt"
+import "core:mem"
+import "core:nbio"
+import "core:time"
+import "core:bytes"
+import "core:thread"
+import "core:strings"
+import "core:image"
+import "core:image/png"
+import "core:image/qoi"
+import si "core:sys/info"
+
+FileDesc :: struct {
+	handle: nbio.Handle,
+	src_path, dst_path: string,
+	png_data, qoi_data: []u8,
+	qoi_bfr: bytes.Buffer,
+	read_ok, write_ok: bool,
+}
+
+files : [dynamic]FileDesc
+
+thread_pool: thread.Pool
+io_loop: ^nbio.Event_Loop
+io_pending, io_succeeded, io_failed: int
+
+open_dir :: proc(name: string) {
+	fi, err := os.read_all_directory_by_path(name, context.allocator)
+	defer os.file_info_slice_delete(fi, context.allocator)
+
+	if err != nil {
+		fmt.printf("err %v: failed to read dir %v\n", err, name)
+		os.exit(1)
+	}
+
+	for file in fi {
+		if file.type == .Directory {
+			open_dir(file.fullpath)
+			continue
+		}
+
+		if os.ext(file.name) == ".png" {
+			src := strings.clone(file.fullpath)
+			dst, _:= strings.replace(file.fullpath,
+					".png", ".qoi", 1)
+			append(&files, FileDesc{
+				src_path = src,
+				dst_path = dst,
+			})
+		}
+	}
+}
+
+main :: proc() {
+	sw: time.Stopwatch
+	time.stopwatch_start(&sw)
+
+	root := "."
+	if len(os.args) > 1 {
+		assert(os.is_dir(os.args[1]), "input must be dir")
+		root = os.args[1]
+	}
+
+	open_dir(root)
+	defer delete(files)
+
+	fmt.printfln("found %d png files", len(files))
+
+	if len(files) == 0 {
+		fmt.println("no png files found")
+		return
+	}
+
+	core_cnt, thread_cnt, ok := si.cpu_core_count()
+	if !ok {
+		fmt.printf("err: Could not retrieve CPU info.\n")
+		os.exit(1)
+	}
+
+	if thread_cnt < 1 {
+		thread_cnt = 1
+	}
+
+	if thread_cnt > len(files) {
+		thread_cnt = len(files)
+	}
+
+	thread.pool_init(&thread_pool, context.allocator, thread_cnt)
+	thread.pool_start(&thread_pool)
+
+	assert(nbio.acquire_thread_event_loop() == nil)
+	defer nbio.release_thread_event_loop()
+	io_loop = nbio.current_thread_event_loop()
+	io_pending = len(files)
+
+	for &file in files {
+		nbio.read_entire_file(
+			file.src_path,
+			&file,
+			read_cb,
+			context.allocator,
+			l = io_loop
+		)
+	}
+
+	for io_pending > 0 {
+		job_err := nbio.tick(50 * time.Millisecond)
+		if job_err != .None {
+			fmt.println("nbio tick failed:", job_err)
+			break
+		}
+	}
+
+	thread.pool_finish(&thread_pool)
+	thread.pool_destroy(&thread_pool)
+
+	time.stopwatch_stop(&sw)
+	fmt.println(io_succeeded, "succeeded")
+	fmt.println(io_failed, "failed")
+	fmt.printfln("Total time: %.3fs",
+		time.duration_seconds(time.stopwatch_duration(sw)))
+}
+
+read_cb :: proc(
+	user_data: rawptr, data: []byte, err: nbio.Read_Entire_File_Error) {
+	file := cast(^FileDesc)user_data
+
+	if err.value != nil {
+		fmt.printf("io read err: %v\n", err)
+		file.read_ok = false
+		file_load_done(file, false)
+		return
+	}
+
+	file.png_data = data
+	file.read_ok = true
+	fmt.printfln("read complete: %q bytes=%d err=%v",
+		file.src_path, len(data), err.value)
+
+	thread.pool_add_task(
+		&thread_pool,
+		context.allocator,
+		png_to_qoi_task,
+		file
+	)
+}
+
+png_to_qoi_task :: proc(task: thread.Task) {
+	file := cast(^FileDesc)task.data
+
+	png_img, png_err := 
+			png.load_from_bytes(
+				file.png_data,
+				{.alpha_add_if_missing,},
+				context.allocator
+			)
+
+	if png_err != nil {
+		fmt.eprintln("png", file.src_path, "failed:", png_err)
+		nbio.next_tick_poly(file, on_encode_failed, l = io_loop)
+		return
+	}
+	defer png.destroy(png_img)
+
+	if !normalize_for_qoi(png_img) {
+		fmt.eprintfln(
+			"qoi normalize failed:" + 
+			"%q width=%d height=%d channels=%d depth=%d bytes=%d",
+			file.src_path,
+			png_img.width,
+			png_img.height,
+			png_img.channels,
+			png_img.depth,
+			len(png_img.pixels.buf),
+		)
+		nbio.next_tick_poly(file, on_encode_failed, l = io_loop)
+		return
+	}
+
+	qoi_err := qoi.save_to_buffer(&file.qoi_bfr, png_img)
+	if qoi_err != nil {
+		fmt.eprintln("qoi encode failed:", file.dst_path, qoi_err)
+		nbio.next_tick_poly(file, on_encode_failed, l = io_loop)
+		return
+	}
+
+	delete(file.png_data)
+	file.png_data = nil
+
+	file.qoi_data = bytes.buffer_to_bytes(&file.qoi_bfr)
+	if len(file.qoi_data) == 0 {
+		fmt.eprintln("qoi encode empty output:", file.dst_path)
+		nbio.next_tick_poly(file, on_encode_failed, l = io_loop)
+		return
+	}
+
+	nbio.open_poly(
+		file.dst_path,
+		file,
+		on_open_qoi,
+		mode = {.Write, .Create, .Trunc},
+		l = io_loop
+	)
+}
+
+normalize_for_qoi :: proc(img: ^image.Image) -> bool {
+	if img == nil {
+		return false
+	}
+
+	if img.channels < 3 {
+		if !image.expand_grayscale(img) {
+			return false
+		}
+	}
+
+	if img.channels == 3 {
+		if !image.alpha_add_if_missing(img) {
+			return false
+		}
+	}
+
+	if img.channels != 4 {
+		return false
+	}
+
+	pixel_count := img.width * img.height
+	channel_count := pixel_count * img.channels
+
+	if pixel_count <= 0 {
+		return false
+	}
+
+	if img.depth == 8 {
+		return len(img.pixels.buf) == channel_count
+	}
+
+	if img.depth != 16 {
+		return false
+	}
+
+	if len(img.pixels.buf) != channel_count * 2 {
+		return false
+	}
+
+	old_pixels := img.pixels
+	new_pixels := bytes.Buffer{}
+
+	if resize(&new_pixels.buf, channel_count) != nil {
+		return false
+	}
+
+	src := mem.slice_data_cast([]u16, old_pixels.buf[:])
+	dst := new_pixels.buf[:]
+
+	for i in 0..<channel_count {
+		v := u32(src[i])
+
+		// 16-bit 0..65535 -> 8-bit 0..255, rounded.
+		dst[i] = u8((v * 255 + 32767) / 65535)
+	}
+
+	bytes.buffer_destroy(&old_pixels)
+
+	img.pixels = new_pixels
+	img.depth = 8
+
+	return true
+}
+
+on_encode_failed :: proc(_: ^nbio.Operation, file: ^FileDesc) {
+	file_load_done(file, false)
+}
+
+on_open_qoi :: proc(op: ^nbio.Operation, file: ^FileDesc) {
+	if op.open.err != .None {
+		fmt.eprintfln("open failed %q: %v", file.dst_path, op.open.err)
+		file_load_done(file, false)
+		return
+	}
+
+	file.handle = op.open.handle
+
+	nbio.write_poly(
+		file.handle,
+		0,
+		file.qoi_data,
+		file,
+		on_write_qoi,
+		all = true,
+		l = io_loop,
+	)
+}
+
+on_write_qoi :: proc(op: ^nbio.Operation, file: ^FileDesc) {
+	file.write_ok = 
+		op.write.err == .None && op.write.written == len(file.qoi_data)
+
+	if !file.write_ok {
+		fmt.eprintfln(
+			"write failed %q: err=%v written=%d expected=%d",
+			file.dst_path,
+			op.write.err,
+			op.write.written,
+			len(file.qoi_data),
+		)
+	}
+
+	nbio.close_poly(file.handle, file, on_close_qoi, l = io_loop)
+}
+
+on_close_qoi :: proc(op: ^nbio.Operation, file: ^FileDesc) {
+	close_ok := op.close.err == .None
+
+	if !close_ok {
+		fmt.eprintln("close failed:", op.close.err, file.dst_path)
+	}
+
+	if file.write_ok && close_ok {
+		fmt.printfln("converted %q -> %q", file.src_path, file.dst_path)
+		file_load_done(file, true)
+	} else {
+		file_load_done(file, false)
+	}
+}
+
+file_load_done :: proc(file: ^FileDesc, ok: bool) {
+	if file.png_data != nil {
+		delete(file.png_data)
+		file.png_data = nil
+	}
+
+	bytes.buffer_destroy(&file.qoi_bfr)
+
+	if len(file.src_path) > 0 {
+		delete(file.src_path)
+		file.src_path = ""
+	}
+
+	if len(file.dst_path) > 0 {
+		delete(file.dst_path)
+		file.dst_path = ""
+	}
+
+	file.qoi_data = nil
+
+	if ok {
+		io_succeeded += 1
+	} else {
+		io_failed += 1
+	}
+
+	io_pending -= 1
+}