initial commit

.gitignore

@@ -0,0 +1,4 @@
+.#*
+build
+result
+result-*

LICENSE

@@ -0,0 +1,15 @@
+ISC License
+
+Copyright 2018 - 2023 Alain Zscheile <zseri.devel@ytrizja.de>
+
+Permission to use, copy, modify, and/or distribute this software for any purpose
+with or without fee is hereby granted, provided that the above copyright notice
+and this permission notice appear in all copies.
+
+THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH
+REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND
+FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT,
+INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS
+OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER
+TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF
+THIS SOFTWARE.

meson.build

@@ -0,0 +1,22 @@
+project('y16t 16-field quad tree library', 'c',
+  default_options : ['c_std=c17'],
+  version : '0.0.0'
+  license : 'Apache-2.0',
+  meson_version : '>=0.50')
+
+install_headers('y16t.h')
+
+liby16t = shared_library('y16t',
+  files('y16t.c'),
+  version : '0.0.0',
+  c_args : ['-fno-plt', '-fno-unwind-tables', '-fno-exceptions', '-Werror=return-type'],
+  include_directories : include_directories('.')
+  install : true)
+
+pkg_mod = import('pkgconfig')
+pkg_mod.generate(
+  libraries : [liby16t],
+  version   : meson.project_version(),
+  name      : 'liby16t',
+  filebase  : 'y16t',
+  description : 'y16t 16-field quad tree library')

y16t.c

@@ -0,0 +1,38 @@
+// SPDX-License-Identifier: ISC
+#include "y16t.h"
+#include <arpa/inet.h>
+
+// conversion to ις-points interleaved coordinates
+static __attribute__((pure))
+uint8_t	y16t_loc2blk_coords(
+	const y16t_location_t loc, const uint32_t offset
+) {
+	const uint32_t byteoffset = offset / 4;
+	const uint8_t shiftval = 2 * (3 - offset % 4);
+	const uint8_t xres = (loc.x[byteoffset] >> shiftval) & 3;
+	const uint8_t yres = (loc.y[byteoffset] >> shiftval) & 3;
+	return (xres << 2) | yres;
+}
+
+void	y16t_lookup(
+	y16t_lookup_t * ret,
+	const uint32_t * data,
+	const size_t datalen,
+	const y16t_location_t * loc
+) {
+	// data_endblk is set to the first byte which doesn't constitute a full block
+	const size_t datalen_blks = datalen / 16;
+	const y16t_location_t loc_ = *loc;
+	size_t datoffset = ret->datoffset;
+
+	for ( ; /* bounds checks */ ret->inpoffset < loc_.xylen && datoffset < datalen_blks; ) {
+		datoffset = ntohl(data[16 * datoffset + y16t_loc2blk_coords(loc_, ret->inpoffset)]);
+		ret->inpoffset += 1;
+		if(0 == datoffset) {
+			// NULL pointer
+			break;
+		}
+	}
+
+	ret->datoffset = datoffset;
+}

y16t.h

@@ -0,0 +1,34 @@
+// SPDX-License-Identifier: ISC
+#pragma once
+#include <inttypes.h>
+#include <stddef.h>
+
+typedef struct {
+	uint8_t *x;
+	uint8_t *y;
+	// x and y should have the same length, and xylen = bytescount * 4
+	uint32_t xylen;
+} y16t_location_t;
+
+typedef struct {
+	// the offset indicates how far the lookup could traverse the tree
+	uint32_t inpoffset;
+
+	// the final data pointer, it doesn't have to be aligned to block boundaries
+	// if it ends up at the starting point, but the offset isn't 0,
+	// then a NULL pointer was hit
+	size_t datoffset;
+} y16t_lookup_t;
+
+// @param ret     ... the starting point; gets updated to the latest via the given path
+//                    reachable point
+// @param data    ... a pointer to a mmap-ed file. we convert pointers inside of the file
+//                    from network byte order to host byte order before following them.
+// @param datalen ... the length of the block that data points to, in 4 byte units (= file size in bytes / 4)
+__attribute__((pure, nonnull))
+void	y16t_lookup(
+	y16t_lookup_t * ret,
+	const uint32_t * data,
+	const size_t datalen,
+	const y16t_location_t * loc
+);