reset dec='\0' to detect numbers while dec is undecided

Author: MichaelChirico

NEWS.md

@@ -6,6 +6,10 @@
 
 1. In `DT[, variable := value]`, when value is class `POSIXlt`, we automatically coerce it to class `POSIXct` instead, [#1724](https://github.com/Rdatatable/data.table/issues/1724). Thanks to @linzhp for the report, and Benjamin Schwendinger for the fix.
 
+## BUG FIXES
+
+1. `fread()` automatically detects timestamps with microseconds again, [#6440](https://github.com/Rdatatable/data.table/issues/6440). This was a regression due to interference with new `dec='auto'` support. Thanks @kav2k for the concise report and @MichaelChirico for the fix.
+
 ## NOTES
 
 1. Tests run again when some Suggests packages are missing, [#6411](https://github.com/Rdatatable/data.table/issues/6411). Thanks @aadler for the note and @MichaelChirico for the fix.

src/fread.c

@@ -1038,7 +1038,7 @@ static void parse_iso8601_timestamp(FieldParseContext *ctx)
   if (second == NA_FLOAT64 || second < 0 || second >= 60)
     goto fail;
 
-  if (*ch == 'Z') {
+  if (*ch == 'Z') { 
     ch++; // "Zulu time"=UTC
   } else {
     if (*ch == ' ')
@@ -1075,7 +1075,6 @@ static void parse_iso8601_timestamp(FieldParseContext *ctx)
 
   date_only:
 
-  //Rprintf("date=%d\thour=%d\tz_hour=%d\tminute=%d\ttz_minute=%d\tsecond=%.1f\n", date, hour, tz_hour, minute, tz_minute, second);
   // cast upfront needed to prevent silent overflow
   *target = 86400*(double)date + 3600*(hour - tz_hour) + 60*(minute - tz_minute) + second;
 
@@ -1236,9 +1235,13 @@ static int detect_types( const char **pch, int8_t type[], int ncol, bool *bumped
         }
       }
       ch = fieldStart;
-      if (autoDec && IS_DEC_TYPE(tmpType[field]) && dec == '.') { // . didn't parse a double; try ,
-        dec = ',';
-        continue;
+      if (autoDec && IS_DEC_TYPE(tmpType[field])) {
+        if (dec == '.') { // '.' didn't parse a double; try ','
+          dec = ',';
+          continue; // i.e., restart since tmpType not incremented
+        } else if (dec == ',') { // ',' didn't parse a double either; reset
+          dec = '\0';
+        }
       }
       while (++tmpType[field]<CT_STRING && disabled_parsers[tmpType[field]]) {};
       *bumped = true;

src/fread.h

@@ -36,7 +36,7 @@ typedef enum {
   NUMTYPE          // placeholder for the number of types including drop; used for allocation and loop bounds
 } colType;
 
-#define IS_DEC_TYPE(x) ((x) == CT_FLOAT64 || (x) == CT_FLOAT64_EXT) // types where dec matters
+#define IS_DEC_TYPE(x) ((x) == CT_FLOAT64 || (x) == CT_FLOAT64_EXT || (x) == CT_ISO8601_TIME) // types where dec matters
 
 extern int8_t typeSize[NUMTYPE];
 extern const char typeName[NUMTYPE][10];