symmetry operation triplets: allow decimal fractions

Some COD CIFs have decimal fractions such as "-x+0.25", ".5+Y", "1.25000-y".

include/gemmi/symmetry.hpp

@@ -262,10 +262,23 @@ inline std::array<int, 4> parse_triplet_part(const std::string& s) {
       fail("wrong or unsupported triplet format: " + s);
     int r_idx;
     int den = 1;
-    if (*c >= '0' && *c <= '9') {
+    if ((*c >= '0' && *c <= '9') || *c == '.') {
       // syntax examples in this branch: "1", "-1/2", "+2*x", "1/2 * b"
       char* endptr;
-      num *= std::strtol(c, &endptr, 10);
+      int n = std::strtol(c, &endptr, 10);
+      // some COD CIFs have decimal fractions ("-x+0.25", ".5+Y", "1.25000-y")
+      if (*endptr == '.') {
+        // avoiding strtod() etc which is locale-dependent
+        double fract = n;
+        for (double denom = 0.1; *++endptr >= '0' && *endptr <= '9'; denom *= 0.1)
+          fract += int(*endptr - '0') * denom;
+        double rounded = std::round(fract * num);
+        if (std::fabs(rounded - fract * num) > 0.05)
+          fail("unexpected number in a symmetry triplet part: " + s);
+        num = int(rounded);
+      } else {
+        num *= n;
+      }
       if (*endptr == '/')
         den = std::strtol(endptr + 1, &endptr, 10);
       if (*endptr == '*') {

tests/test_symmetry.py

@@ -46,6 +46,12 @@
     "1+Y"   : [ 0, D, 0,  D],
     "-2+Y"  : [ 0, D, 0, -D*2],
     "-z-5/6": [ 0, 0,-D, -D*5//6],
+    # some CIF files from COD have decimal fractions
+    "0.5-x":  [-D, 0, 0,  D//2],
+    ".5+Y":   [ 0, D, 0, D//2],
+    "-Z+0.25": [ 0, 0, -D, D//4],
+    "x+0.6666666667": [ D, 0, 0, D*2//3],
+    "1.25000-y": [ 0, -D, 0, D*5//4],
 }
 
 # From PLATON manual (WinGX v1.80, Chapter 9.3 PLATON),