py/parsenum: fix rounding error when float ends in .0

dlech · dlech · commit c8e8da6e8daa · 2020-03-28T17:08:36.000-05:00
It is common code style to write floating point numbers with trailing
`.0` instead of just trailing `.`. When there is a trailing `.0`, the
current parser will multiply the number by 10 and then divide it by
10 again later. For certain numbers, this can cause rounding.

This adds a check for the special case of `.0` to avoid the extra work.
A further check is added to avoid calling pow() when exp_val is 0. Even
in cases where there is not a rounding problem, this should be more
efficient, especially for soft float implementations.
diff --git a/py/parsenum.c b/py/parsenum.c
@@ -265,6 +265,11 @@ mp_obj_t mp_parse_num_decimal(const char *str, size_t len, bool allow_imag, bool
                 }
             } else if (in == PARSE_DEC_IN_INTG && dig == '.') {
                 in = PARSE_DEC_IN_FRAC;
+                // check for case of single '0' after decimal to avoid rounding error
+                if (str + 1 == top && *str == '0') {
+                    str++;
+                    break;
+                }
             } else if (in != PARSE_DEC_IN_EXP && ((dig | 0x20) == 'e')) {
                 in = PARSE_DEC_IN_EXP;
                 if (str < top) {
@@ -309,7 +314,7 @@ mp_obj_t mp_parse_num_decimal(const char *str, size_t len, bool allow_imag, bool
         // of slightly erroneous values.
         if (exp_val < 0 && exp_val >= -EXACT_POWER_OF_10) {
             dec_val /= MICROPY_FLOAT_C_FUN(pow)(10, -exp_val);
-        } else {
+        } else if (exp_val > 0) {
             dec_val *= MICROPY_FLOAT_C_FUN(pow)(10, exp_val);
         }
     }
diff --git a/tests/float/float_parse.py b/tests/float/float_parse.py
@@ -34,3 +34,8 @@
 # check small decimals are as close to their true value as possible
 for n in range(1, 10):
     print(float('0.%u' % n) == n / 10)
+
+# Issue #5831 - make sure trailing 0 does not cause rounding with 32-bit float
+# 14187745 has exact 32-bit representation but 141877450 does not
+print(int(float("14187745.")))
+print(int(float("14187745.0")))
