mirror of https://github.com/ArduPilot/ardupilot
AP_OLC: Open Location Code support
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giacomo892
Peter Barker
parent
0a4ea250ad
commit
481d72255f
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/*
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* This file is free software: you can redistribute it and/or modify it
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* under the terms of the GNU General Public License as published by the
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* Free Software Foundation, either version 3 of the License, or
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* (at your option) any later version.
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*
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* This file is distributed in the hope that it will be useful, but
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* WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
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* See the GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License along
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* with this program. If not, see <http://www.gnu.org/licenses/>.
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*
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* AP_OLC is based on INAV olc.c implemention, thanks @fiam and other contributors.
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*/
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#include <math.h>
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#include <algorithm>
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#include "AP_OLC.h"
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#include <AP_Common/AP_Common.h>
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#include <AP_Math/AP_Math.h>
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// This is a port of https://github.com/google/open-location-code/blob/master/c/olc.c
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// to avoid double floating point math and use integer math as much as possible.
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static constexpr char SEPARATOR_CHAR = '+';
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static constexpr int SEPARATOR_POS = 8;
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static constexpr char PADDING_CHAR = '0';
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static constexpr int ENCODING_BASE = 20;
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static constexpr unsigned int PAIR_CODE_LEN = 10;
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static constexpr unsigned int CODE_LEN_MAX = 15;
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static constexpr int GRID_COLS = 4;
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static constexpr int GRID_ROWS = ENCODING_BASE / GRID_COLS;
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static constexpr olc_coord_t LAT_MAX = 90 * OLC_DEG_MULTIPLIER;
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static constexpr olc_coord_t LON_MAX = 180 * OLC_DEG_MULTIPLIER;
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olc_coord_t AP_OLC::grid_size;
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olc_coord_t AP_OLC::initial_resolution;
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const char AP_OLC::alphabet[] = "23456789CFGHJMPQRVWX";
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bool AP_OLC::inited = false;
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void AP_OLC::init_constants(void)
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{
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if (inited) {
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return;
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}
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inited = true;
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// Work out the encoding base exponent necessary to represent 360 degrees.
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olc_coord_t initial_exponent = floorf(logf(2 * (LON_MAX / OLC_DEG_MULTIPLIER)) / logf(ENCODING_BASE));
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// Work out the enclosing resolution (in degrees) for the grid algorithm.
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AP_OLC::grid_size = (1 / powf(ENCODING_BASE, PAIR_CODE_LEN / 2 - (initial_exponent + 1))) * OLC_DEG_MULTIPLIER;
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// Work out the initial resolution
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AP_OLC::initial_resolution = powf(ENCODING_BASE, initial_exponent) * OLC_DEG_MULTIPLIER;
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}
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// Compute the latitude precision value for a given code length. Lengths <= 10
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// have the same precision for latitude and longitude, but lengths > 10 have
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// different precisions due to the grid method having fewer columns than rows.
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float AP_OLC::compute_precision_for_length(int length)
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{
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// Magic numbers!
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if (length <= (int)PAIR_CODE_LEN) {
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return powf(ENCODING_BASE, floorf((length / -2) + 2));
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}
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return powf(ENCODING_BASE, -3) / powf(5, length - (int)PAIR_CODE_LEN);
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}
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olc_coord_t AP_OLC::adjust_latitude(olc_coord_t lat, size_t code_len)
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{
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if (lat < -LAT_MAX) {
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lat = -LAT_MAX;
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}
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if (lat > LAT_MAX) {
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lat = LAT_MAX;
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}
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if (lat >= LAT_MAX) {
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// Subtract half the code precision to get the latitude into the code area.
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olc_coord_t precision = compute_precision_for_length(code_len) * OLC_DEG_MULTIPLIER;
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lat -= precision / 2;
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}
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return lat;
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}
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olc_coord_t AP_OLC::normalize_longitude(olc_coord_t lon)
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{
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while (lon < -LON_MAX) {
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lon += LON_MAX;
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lon += LON_MAX;
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}
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while (lon >= LON_MAX) {
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lon -= LON_MAX;
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lon -= LON_MAX;
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}
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return lon;
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}
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// Encodes positive range lat,lon into a sequence of OLC lat/lon pairs. This
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// uses pairs of characters (latitude and longitude in that order) to represent
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// each step in a 20x20 grid. Each code, therefore, has 1/400th the area of
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// the previous code.
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unsigned AP_OLC::encode_pairs(uolc_coord_t lat, uolc_coord_t lon, size_t length, char *buf, size_t bufsize)
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{
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if ((length + 1) >= bufsize) {
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buf[0] = '\0';
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return 0;
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}
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unsigned pos = 0;
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olc_coord_t resolution = AP_OLC::initial_resolution;
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// Add two digits on each pass.
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for (size_t digit_count = 0;
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digit_count < length;
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digit_count += 2, resolution /= ENCODING_BASE) {
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size_t digit_value;
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// Do the latitude - gets the digit for this place and subtracts that
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// for the next digit.
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digit_value = lat / resolution;
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lat -= digit_value * resolution;
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buf[pos++] = alphabet[digit_value];
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// Do the longitude - gets the digit for this place and subtracts that
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// for the next digit.
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digit_value = lon / resolution;
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lon -= digit_value * resolution;
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buf[pos++] = alphabet[digit_value];
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// Should we add a separator here?
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if (pos == SEPARATOR_POS && pos < length) {
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buf[pos++] = SEPARATOR_CHAR;
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}
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}
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while (pos < SEPARATOR_POS) {
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buf[pos++] = PADDING_CHAR;
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}
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if (pos == SEPARATOR_POS) {
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buf[pos++] = SEPARATOR_CHAR;
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}
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buf[pos] = '\0';
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return pos;
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}
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// Encodes a location using the grid refinement method into an OLC string. The
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// grid refinement method divides the area into a grid of 4x5, and uses a
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// single character to refine the area. The grid squares use the OLC
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// characters in order to number the squares as follows:
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//
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// R V W X
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// J M P Q
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// C F G H
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// 6 7 8 9
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// 2 3 4 5
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//
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// This allows default accuracy OLC codes to be refined with just a single
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// character.
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int AP_OLC::encode_grid(uolc_coord_t lat, uolc_coord_t lon, size_t length,
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char *buf, size_t bufsize)
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{
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if ((length + 1) >= bufsize) {
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buf[0] = '\0';
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return 0;
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}
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int pos = 0;
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olc_coord_t lat_grid_size = AP_OLC::grid_size;
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olc_coord_t lon_grid_size = AP_OLC::grid_size;
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lat %= lat_grid_size;
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lon %= lon_grid_size;
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for (size_t i = 0; i < length; i++) {
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olc_coord_t lat_div = lat_grid_size / GRID_ROWS;
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olc_coord_t lon_div = lon_grid_size / GRID_COLS;
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if (lat_div == 0 || lon_div == 0) {
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// This case happens when OLC_DEG_MULTIPLIER doesn't have enough
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// precision for the requested length.
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break;
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}
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// Work out the row and column.
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size_t row = lat / lat_div;
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size_t col = lon / lon_div;
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lat_grid_size /= GRID_ROWS;
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lon_grid_size /= GRID_COLS;
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lat -= row * lat_grid_size;
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lon -= col * lon_grid_size;
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buf[pos++] = alphabet[row * GRID_COLS + col];
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}
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buf[pos] = '\0';
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return pos;
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}
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int AP_OLC::olc_encode(olc_coord_t lat, olc_coord_t lon, size_t length, char *buf, size_t bufsize)
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{
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int pos = 0;
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length = MIN(length, CODE_LEN_MAX);
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// Adjust latitude and longitude so they fall into positive ranges.
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uolc_coord_t alat = adjust_latitude(lat, length) + LAT_MAX;
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uolc_coord_t alon = normalize_longitude(lon) + LON_MAX;
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init_constants();
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pos += encode_pairs(alat, alon, MIN(length, PAIR_CODE_LEN), buf + pos, bufsize - pos);
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// If the requested length indicates we want grid refined codes.
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if (length > PAIR_CODE_LEN) {
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pos += encode_grid(alat, alon, length - PAIR_CODE_LEN, buf + pos, bufsize - pos);
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}
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buf[pos] = '\0';
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return pos;
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}
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@ -0,0 +1,50 @@
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/*
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* This file is free software: you can redistribute it and/or modify it
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* under the terms of the GNU General Public License as published by the
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* Free Software Foundation, either version 3 of the License, or
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* (at your option) any later version.
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*
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* This file is distributed in the hope that it will be useful, but
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* WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
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* See the GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License along
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* with this program. If not, see <http://www.gnu.org/licenses/>.
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*
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* AP_OLC is based on INAV olc.c implemention, thanks @fiam and other contributors.
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*/
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#pragma once
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#include <AP_Common/AP_Common.h>
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typedef int32_t olc_coord_t;
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typedef uint32_t uolc_coord_t;
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static constexpr olc_coord_t OLC_DEG_MULTIPLIER = 10000000; // 1e7
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class AP_OLC
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{
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public:
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// olc_encodes the given coordinates in lat and lon (deg * OLC_DEG_MULTIPLIER)
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// as an OLC code of the given length. It returns the number of characters
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// written to buf.
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static int olc_encode(olc_coord_t lat, olc_coord_t lon, size_t length, char *buf, size_t bufsize);
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private:
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static bool inited;
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// Initialized via init_constants()
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static olc_coord_t grid_size;
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static olc_coord_t initial_resolution;
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static const char alphabet[];
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static void init_constants(void);
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static float compute_precision_for_length(int length);
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static olc_coord_t adjust_latitude(olc_coord_t lat, size_t code_len);
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static olc_coord_t normalize_longitude(olc_coord_t lon);
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static unsigned encode_pairs(uolc_coord_t lat, uolc_coord_t lon, size_t length, char *buf, size_t bufsize);
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static int encode_grid(uolc_coord_t lat, uolc_coord_t lon, size_t length,char *buf, size_t bufsize);
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};
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