src/main/common/olc.c

   1 #include <math.h>
   2
   3 #include "common/maths.h"
   4 #include "common/olc.h"
   5
   6 // This is a port of https://github.com/google/open-location-code/blob/master/c/olc.c
   7 // to avoid double floating point math and use integer math as much as possible.
   8
   9 #define SEPARATOR_CHAR '+'
  10 #define SEPARATOR_POS 8
  11 #define PADDING_CHAR '0'
  12
  13 #define ENCODING_BASE 20
  14 #define PAIR_CODE_LEN 10u
  15 #define CODE_LEN_MAX 15u
  16
  17 #define GRID_COLS 4
  18 #define GRID_ROWS (ENCODING_BASE / GRID_COLS)
  19
  20
  21 #define LAT_MAX (90 * OLC_DEG_MULTIPLIER)
  22 #define LON_MAX (180 * OLC_DEG_MULTIPLIER)
  23
  24 static const char alphabet[] = "23456789CFGHJMPQRVWX";
  25
  26 // Initialized via init_constants()
  27 static olc_coord_t grid_size;
  28 static olc_coord_t initial_resolution;
  29
  30 static void init_constants(void)
  31 {
  32     static bool inited = 0;
  33     if (inited) {
  34         return;
  35     }
  36     inited = 1;
  37
  38     // Work out the encoding base exponent necessary to represent 360 degrees.
  39     olc_coord_t initial_exponent = floorf(logf(2 * (LON_MAX / OLC_DEG_MULTIPLIER)) / logf(ENCODING_BASE));
  40
  41     // Work out the enclosing resolution (in degrees) for the grid algorithm.
  42     grid_size = (1 / powf(ENCODING_BASE, PAIR_CODE_LEN / 2 - (initial_exponent + 1))) * OLC_DEG_MULTIPLIER;
  43
  44     // Work out the initial resolution
  45     initial_resolution = powf(ENCODING_BASE, initial_exponent) * OLC_DEG_MULTIPLIER;
  46 }
  47
  48 // Compute the latitude precision value for a given code length.  Lengths <= 10
  49 // have the same precision for latitude and longitude, but lengths > 10 have
  50 // different precisions due to the grid method having fewer columns than rows.
  51 static float compute_precision_for_length(int length)
  52 {
  53     // Magic numbers!
  54     if (length <= (int)PAIR_CODE_LEN) {
  55         return powf(ENCODING_BASE, floorf((length / -2) + 2));
  56     }
  57
  58     return powf(ENCODING_BASE, -3) / powf(5, length - (int)PAIR_CODE_LEN);
  59 }
  60
  61 static olc_coord_t adjust_latitude(olc_coord_t lat, size_t code_len)
  62 {
  63     if (lat < -LAT_MAX) {
  64         lat = -LAT_MAX;
  65     }
  66     if (lat > LAT_MAX) {
  67         lat = LAT_MAX;
  68     }
  69     if (lat >= LAT_MAX) {
  70         // Subtract half the code precision to get the latitude into the code area.
  71         olc_coord_t precision = compute_precision_for_length(code_len) * OLC_DEG_MULTIPLIER;
  72         lat -= precision / 2;
  73     }
  74     return lat;
  75 }
  76
  77 static olc_coord_t normalize_longitude(olc_coord_t lon)
  78 {
  79     while (lon < -LON_MAX) {
  80         lon += LON_MAX;
  81         lon += LON_MAX;
  82     }
  83     while (lon >= LON_MAX) {
  84         lon -= LON_MAX;
  85         lon -= LON_MAX;
  86     }
  87     return lon;
  88 }
  89
  90 // Encodes positive range lat,lon into a sequence of OLC lat/lon pairs.  This
  91 // uses pairs of characters (latitude and longitude in that order) to represent
  92 // each step in a 20x20 grid.  Each code, therefore, has 1/400th the area of
  93 // the previous code.
  94 static unsigned encode_pairs(uolc_coord_t lat, uolc_coord_t lon, size_t length, char *buf, size_t bufsize)
  95 {
  96     if ((length + 1) >= bufsize) {
  97         buf[0] = '\0';
  98         return 0;
  99     }
 100
 101     unsigned pos = 0;
 102     olc_coord_t resolution = initial_resolution;
 103     // Add two digits on each pass.
 104     for (size_t digit_count = 0;
 105          digit_count < length;
 106          digit_count += 2, resolution /= ENCODING_BASE) {
 107         size_t digit_value;
 108
 109         // Do the latitude - gets the digit for this place and subtracts that
 110         // for the next digit.
 111         digit_value = lat / resolution;
 112         lat -= digit_value * resolution;
 113         buf[pos++] = alphabet[digit_value];
 114
 115         // Do the longitude - gets the digit for this place and subtracts that
 116         // for the next digit.
 117         digit_value = lon / resolution;
 118         lon -= digit_value * resolution;
 119         buf[pos++] = alphabet[digit_value];
 120
 121         // Should we add a separator here?
 122         if (pos == SEPARATOR_POS && pos < length) {
 123             buf[pos++] = SEPARATOR_CHAR;
 124         }
 125     }
 126     while (pos < SEPARATOR_POS) {
 127         buf[pos++] = PADDING_CHAR;
 128     }
 129     if (pos == SEPARATOR_POS) {
 130         buf[pos++] = SEPARATOR_CHAR;
 131     }
 132     buf[pos] = '\0';
 133     return pos;
 134 }
 135
 136 // Encodes a location using the grid refinement method into an OLC string.  The
 137 // grid refinement method divides the area into a grid of 4x5, and uses a
 138 // single character to refine the area.  The grid squares use the OLC
 139 // characters in order to number the squares as follows:
 140 //
 141 //   R V W X
 142 //   J M P Q
 143 //   C F G H
 144 //   6 7 8 9
 145 //   2 3 4 5
 146 //
 147 // This allows default accuracy OLC codes to be refined with just a single
 148 // character.
 149 static int encode_grid(uolc_coord_t lat, uolc_coord_t lon, size_t length,
 150                        char *buf, size_t bufsize)
 151 {
 152     if ((length + 1) >= bufsize) {
 153         buf[0] = '\0';
 154         return 0;
 155     }
 156
 157     int pos = 0;
 158
 159     olc_coord_t lat_grid_size = grid_size;
 160     olc_coord_t lon_grid_size = grid_size;
 161
 162     lat %= lat_grid_size;
 163     lon %= lon_grid_size;
 164
 165     for (size_t i = 0; i < length; i++) {
 166         olc_coord_t lat_div = lat_grid_size / GRID_ROWS;
 167         olc_coord_t lon_div = lon_grid_size / GRID_COLS;
 168
 169         if (lat_div == 0 || lon_div == 0) {
 170             // This case happens when OLC_DEG_MULTIPLIER doesn't have enough
 171             // precision for the requested length.
 172             break;
 173         }
 174
 175         // Work out the row and column.
 176         size_t row = lat / lat_div;
 177         size_t col = lon / lon_div;
 178         lat_grid_size /= GRID_ROWS;
 179         lon_grid_size /= GRID_COLS;
 180         lat -= row * lat_grid_size;
 181         lon -= col * lon_grid_size;
 182         buf[pos++] = alphabet[row * GRID_COLS + col];
 183     }
 184     buf[pos] = '\0';
 185     return pos;
 186 }
 187
 188 int olc_encode(olc_coord_t lat, olc_coord_t lon, size_t length, char *buf, size_t bufsize)
 189 {
 190     int pos = 0;
 191
 192     length = MIN(length, CODE_LEN_MAX);
 193
 194     // Adjust latitude and longitude so they fall into positive ranges.
 195     uolc_coord_t alat = adjust_latitude(lat, length) + LAT_MAX;
 196     uolc_coord_t alon = normalize_longitude(lon) + LON_MAX;
 197
 198     init_constants();
 199
 200     pos += encode_pairs(alat, alon, MIN(length, PAIR_CODE_LEN), buf + pos, bufsize - pos);
 201     // If the requested length indicates we want grid refined codes.
 202     if (length > PAIR_CODE_LEN) {
 203         pos += encode_grid(alat, alon, length - PAIR_CODE_LEN, buf + pos, bufsize - pos);
 204     }
 205     buf[pos] = '\0';
 206     return pos;
 207 }