Split encoding into floating and integer functions

c/src/olc.c

@@ -82,8 +82,8 @@ int OLC_IsFull(const char* code, size_t size) {
   return is_full(&info);
 }
 
-int OLC_Encode(const OLC_LatLon* location, size_t length, char* code,
-               int maxlen) {
+int OLC_EncodeIntegers(long long int lat, long long int lng, size_t length,
+                       char* code, int maxlen) {
   // Limit the maximum number of digits in the code.
   if (length > kMaximumDigitCount) {
     length = kMaximumDigitCount;
@@ -94,51 +94,53 @@ int OLC_Encode(const OLC_LatLon* location, size_t length, char* code,
   if (length < kPairCodeLength && length % 2 == 1) {
     length = length + 1;
   }
-  // Adjust latitude and longitude so they fall into positive ranges.
-  double latitude = adjust_latitude(location->lat, length);
-  double longitude = normalize_longitude(location->lon);
+  // Convert latitude to positive range and clip.
+  lat += OLC_kLatMaxDegrees * kGridLatPrecisionInverse;
+  if (lat < 0) {
+    lat = 0;
+  } else if (lat >= 2 * OLC_kLatMaxDegrees * kGridLatPrecisionInverse) {
+    // Subtract one to bring it just inside 90 degrees lat.
+    lat = 2 * OLC_kLatMaxDegrees * kGridLatPrecisionInverse - 1;
+  }
+  // Convert longitude to positive range and normalise.
+  lng += OLC_kLonMaxDegrees * kGridLonPrecisionInverse;
+  while (lng < 0) {
+    lng += 2 * OLC_kLonMaxDegrees * kGridLonPrecisionInverse;
+  }
+  while (lng >= 2 * OLC_kLonMaxDegrees * kGridLonPrecisionInverse) {
+    lng -= 2 * OLC_kLonMaxDegrees * kGridLonPrecisionInverse;
+  }
 
   // Build up the code here, then copy it to the passed pointer.
   char fullcode[] = "12345678901234567";
 
   // Compute the code.
-  // This approach converts each value to an integer after multiplying it by
-  // the final precision. This allows us to use only integer operations, so
-  // avoiding any accumulation of floating point representation errors.
-
-  // Multiply values by their precision and convert to positive without any
-  // floating point operations.
-  long long int lat_val = kLatMaxDegrees * 2.5e7;
-  long long int lng_val = kLonMaxDegrees * 8.192e6;
-  lat_val += latitude * 2.5e7;
-  lng_val += longitude * 8.192e6;
-
   size_t pos = kMaximumDigitCount;
   // Compute the grid part of the code if necessary.
   if (length > kPairCodeLength) {
     for (size_t i = 0; i < kGridCodeLength; i++) {
-      int lat_digit = lat_val % kGridRows;
-      int lng_digit = lng_val % kGridCols;
+      int lat_digit = lat % kGridRows;
+      int lng_digit = lng % kGridCols;
       int ndx = lat_digit * kGridCols + lng_digit;
       fullcode[pos--] = kAlphabet[ndx];
       // Note! Integer division.
-      lat_val /= kGridRows;
-      lng_val /= kGridCols;
+      lat /= kGridRows;
+      lng /= kGridCols;
     }
   } else {
-    lat_val /= pow(kGridRows, kGridCodeLength);
-    lng_val /= pow(kGridCols, kGridCodeLength);
+    lat /= pow(kGridRows, kGridCodeLength);
+    lng /= pow(kGridCols, kGridCodeLength);
   }
   pos = kPairCodeLength;
   // Compute the pair section of the code.
   for (size_t i = 0; i < kPairCodeLength / 2; i++) {
-    int lat_ndx = lat_val % kEncodingBase;
-    int lng_ndx = lng_val % kEncodingBase;
+    int lat_ndx = lat % kEncodingBase;
+    int lng_ndx = lng % kEncodingBase;
     fullcode[pos--] = kAlphabet[lng_ndx];
     fullcode[pos--] = kAlphabet[lat_ndx];
     // Note! Integer division.
-    lat_val /= kEncodingBase;
-    lng_val /= kEncodingBase;
+    lat /= kEncodingBase;
+    lng /= kEncodingBase;
     if (i == 0) {
       fullcode[pos--] = kSeparator;
     }
@@ -165,6 +167,16 @@ int OLC_Encode(const OLC_LatLon* location, size_t length, char* code,
   return char_count;
 }
 
+int OLC_Encode(const OLC_LatLon* location, size_t length, char* code,
+               int maxlen) {
+  // Multiply degrees by precision. Use lround to explicitly round rather than
+  // truncate, which causes issues when using values like 0.1 that do not have
+  // precise floating point representations.
+  long long int lat = lround(location->lat * kGridLatPrecisionInverse);
+  long long int lng = lround(location->lon * kGridLonPrecisionInverse);
+  return OLC_EncodeIntegers(lat, lng, length, code, maxlen);
+}
+
 int OLC_EncodeDefault(const OLC_LatLon* location, char* code, int maxlen) {
   return OLC_Encode(location, kPairCodeLength, code, maxlen);
 }

c/src/olc.h

@@ -72,4 +72,4 @@ int OLC_Shorten(const char* code, size_t size, const OLC_LatLon* reference,
 int OLC_RecoverNearest(const char* short_code, size_t size,
                        const OLC_LatLon* reference, char* code, int maxlen);
 
-#endif
+#endif  // OLC_OPENLOCATIONCODE_H

c/src/olc_private.h

@@ -40,9 +40,9 @@ static const size_t kSeparatorPosition = 8;
 static const size_t kPairPrecisionInverse = 8000;
 // Inverse (1/) of the precision of the final grid digits in degrees.
 // Latitude is kEncodingBase^3 * kGridRows^kGridCodeLength
-static const size_t kGridLatPrecisionInverse = 2.5e7;
+static const long long int kGridLatPrecisionInverse = 2.5e7;
 // Longitude is kEncodingBase^3 * kGridColumns^kGridCodeLength
-static const size_t kGridLonPrecisionInverse = 8.192e6;
+static const long long int kGridLonPrecisionInverse = 8.192e6;
 // Latitude bounds are -kLatMaxDegrees degrees and +kLatMaxDegrees degrees
 // which we transpose to 0 and 180 degrees.
 static const double kLatMaxDegrees = OLC_kLatMaxDegrees;

test_data/encoding.csv

@@ -53,9 +53,9 @@
 # return a 15-digit code
 #
 ################################################################################
-37.539669125,-122.375069724,15,849VGJQF+VX7QR3J
-37.539669125,-122.375069724,16,849VGJQF+VX7QR3J
-37.539669125,-122.375069724,100,849VGJQF+VX7QR3J
+37.539669125,-122.375069724,15,849VGJQF+VX7QR3M
+37.539669125,-122.375069724,16,849VGJQF+VX7QR3M
+37.539669125,-122.375069724,100,849VGJQF+VX7QR3M
 ################################################################################
 #
 # Test floating point representation/rounding errors.
@@ -134,18 +134,18 @@
 #
 ################################################################################
 80.0100000001,58.57,15,CHGW2H6C+2222222
-80.0099999999,58.57,15,CHGW2H5C+X2RRRRR
+80.00999996,58.57,15,CHGW2H5C+X2RRRRR
 -80.0099999999,58.57,15,2HFWXHRC+2222222
--80.0100000001,58.57,15,2HFWXHQC+X2RRRRR
+-80.0100000399,58.57,15,2HFWXHQC+X2RRRRR
 ################################################################################
 #
 # Add a few other examples.
 #
 ################################################################################
-47.000000080000000,8.00022229,15,8FVC2222+235235C
-68.3500147997595,113.625636875353,15,9PWM9J2G+272FWJV
-38.1176000887231,165.441989844555,15,8VC74C9R+2QX445C
--28.1217794010122,-154.066811473758,15,5337VWHM+77PR2GR
+47.000000080000000,8.00022229,15,8FVC2222+235235F
+68.3500147997595,113.625636875353,15,9PWM9J2G+272FWR3
+38.1176000887231,165.441989844555,15,8VC74C9R+2QX445F
+-28.1217794010122,-154.066811473758,15,5337VWHM+77PR2P2
 ################################################################################
 #
 # Test short length.