py/parsenum: Refactor float parsing code.

This commit extracts from the current float parsing code two functions
which could be reused elsewhere in MicroPython.

The code used to multiply a float x by a power of 10 is also simplified by
applying the binary exponent separately from the power of 5.  This avoids
the risk of overflow in the intermediate stage, before multiplying by x.

Signed-off-by: Yoctopuce dev <[email protected]>

Author: yoctopuce

py/parsenum.c

@@ -195,6 +195,8 @@ mp_obj_t mp_parse_num_integer(const char *restrict str_, size_t len, int base, m
     }
 }
 
+#if MICROPY_PY_BUILTINS_FLOAT
+
 enum {
     REAL_IMAG_STATE_START = 0,
     REAL_IMAG_STATE_HAVE_REAL = 1,
@@ -207,25 +209,39 @@ typedef enum {
     PARSE_DEC_IN_EXP,
 } parse_dec_in_t;
 
-#if MICROPY_PY_BUILTINS_FLOAT
 // MANTISSA_MAX is used to retain precision while not overflowing mantissa
-// SMALL_NORMAL_VAL is the smallest power of 10 that is still a normal float
-// EXACT_POWER_OF_10 is the largest value of x so that 10^x can be stored exactly in a float
-//   Note: EXACT_POWER_OF_10 is at least floor(log_5(2^mantissa_length)). Indeed, 10^n = 2^n * 5^n
-//   so we only have to store the 5^n part in the mantissa (the 2^n part will go into the float's
-//   exponent).
+#define MANTISSA_MAX (sizeof(mp_float_uint_t) == 8 ? 0x1999999999999998ULL : 0x19999998U)
+
+// MAX_EXACT_POWER_OF_5 is the largest value of x so that 5^x can be stored exactly in a float
 #if MICROPY_FLOAT_IMPL == MICROPY_FLOAT_IMPL_FLOAT
-#define MANTISSA_MAX 0x19999998U
-#define SMALL_NORMAL_VAL (1e-37F)
-#define SMALL_NORMAL_EXP (-37)
-#define EXACT_POWER_OF_10 (9)
+#define MAX_EXACT_POWER_OF_5 (10)
 #elif MICROPY_FLOAT_IMPL == MICROPY_FLOAT_IMPL_DOUBLE
-#define MANTISSA_MAX 0x1999999999999998ULL
-#define SMALL_NORMAL_VAL (1e-307)
-#define SMALL_NORMAL_EXP (-307)
-#define EXACT_POWER_OF_10 (22)
+#define MAX_EXACT_POWER_OF_5 (22)
 #endif
 
+// Helper to compute `num * (10.0 ** dec_exp)`
+mp_float_t mp_decimal_exp(mp_float_t num, int dec_exp) {
+
+    if (dec_exp == 0 || num == MICROPY_FLOAT_CONST(0.0)) {
+        return num;
+    }
+    mp_float_union_t res = {num};
+    // Multiply first by (2.0 ** dec_exp) via the exponent
+    // - this will ensure that the result of `pow()` is always in mp_float_t range
+    //   when the result is expected to be in mp_float_t range (e.g. during format)
+    // - we don't need to care about p.exp overflow, as (5.0 ** dec_exp) will anyway
+    //   force the final result toward the proper edge if needed (0.0 or inf)
+    res.p.exp += dec_exp;
+    // Use positive exponents when they are more precise then negative
+    if (dec_exp < 0 && dec_exp >= -MAX_EXACT_POWER_OF_5) {
+        res.f /= MICROPY_FLOAT_C_FUN(pow)(5, -dec_exp);
+    } else {
+        res.f *= MICROPY_FLOAT_C_FUN(pow)(5, dec_exp);
+    }
+    return (mp_float_t)res.f;
+}
+
+
 // Break out inner digit accumulation routine to ease trailing zero deferral.
 static mp_float_uint_t accept_digit(mp_float_uint_t p_mantissa, unsigned int dig, int *p_exp_extra, int in) {
     // Core routine to ingest an additional digit.
@@ -244,6 +260,85 @@ static mp_float_uint_t accept_digit(mp_float_uint_t p_mantissa, unsigned int dig
         return p_mantissa;
     }
 }
+
+// Helper to parse an unsigned decimal number into a mp_float_t
+const char *mp_parse_float_internal(const char *str, size_t len, mp_float_t *res) {
+    const char *top = str + len;
+
+    parse_dec_in_t in = PARSE_DEC_IN_INTG;
+    bool exp_neg = false;
+    mp_float_uint_t mantissa = 0;
+    int exp_val = 0;
+    int exp_extra = 0;
+    int trailing_zeros_intg = 0, trailing_zeros_frac = 0;
+    while (str < top) {
+        unsigned int dig = *str++;
+        if ('0' <= dig && dig <= '9') {
+            dig -= '0';
+            if (in == PARSE_DEC_IN_EXP) {
+                // don't overflow exp_val when adding next digit, instead just truncate
+                // it and the resulting float will still be correct, either inf or 0.0
+                // (use INT_MAX/2 to allow adding exp_extra at the end without overflow)
+                if (exp_val < (INT_MAX / 2 - 9) / 10) {
+                    exp_val = 10 * exp_val + dig;
+                }
+            } else {
+                if (dig == 0 || mantissa >= MANTISSA_MAX) {
+                    // Defer treatment of zeros in fractional part.  If nothing comes afterwards, ignore them.
+                    // Also, once we reach MANTISSA_MAX, treat every additional digit as a trailing zero.
+                    if (in == PARSE_DEC_IN_INTG) {
+                        ++trailing_zeros_intg;
+                    } else {
+                        ++trailing_zeros_frac;
+                    }
+                } else {
+                    // Time to un-defer any trailing zeros.  Intg zeros first.
+                    while (trailing_zeros_intg) {
+                        mantissa = accept_digit(mantissa, 0, &exp_extra, PARSE_DEC_IN_INTG);
+                        --trailing_zeros_intg;
+                    }
+                    while (trailing_zeros_frac) {
+                        mantissa = accept_digit(mantissa, 0, &exp_extra, PARSE_DEC_IN_FRAC);
+                        --trailing_zeros_frac;
+                    }
+                    mantissa = accept_digit(mantissa, dig, &exp_extra, in);
+                }
+            }
+        } else if (in == PARSE_DEC_IN_INTG && dig == '.') {
+            in = PARSE_DEC_IN_FRAC;
+        } else if (in != PARSE_DEC_IN_EXP && ((dig | 0x20) == 'e')) {
+            in = PARSE_DEC_IN_EXP;
+            if (str < top) {
+                if (str[0] == '+') {
+                    str++;
+                } else if (str[0] == '-') {
+                    str++;
+                    exp_neg = true;
+                }
+            }
+            if (str == top) {
+                return NULL;
+            }
+        } else if (dig == '_') {
+            continue;
+        } else {
+            // unknown character
+            str--;
+            break;
+        }
+    }
+
+    // work out the exponent
+    if (exp_neg) {
+        exp_val = -exp_val;
+    }
+    exp_val += exp_extra + trailing_zeros_intg;
+
+    // At this point, we just need to multiply the mantissa by its base 10 exponent.
+    *res = (mp_float_t)mp_decimal_exp(mantissa, exp_val);
+
+    return str;
+}
 #endif // MICROPY_PY_BUILTINS_FLOAT
 
 #if MICROPY_PY_BUILTINS_COMPLEX
@@ -295,91 +390,9 @@ parse_start:;
         dec_val = MICROPY_FLOAT_C_FUN(nan)("");
     } else {
         // string should be a decimal number
-        parse_dec_in_t in = PARSE_DEC_IN_INTG;
-        bool exp_neg = false;
-        mp_float_uint_t mantissa = 0;
-        int exp_val = 0;
-        int exp_extra = 0;
-        int trailing_zeros_intg = 0, trailing_zeros_frac = 0;
-        while (str < top) {
-            unsigned int dig = *str++;
-            if ('0' <= dig && dig <= '9') {
-                dig -= '0';
-                if (in == PARSE_DEC_IN_EXP) {
-                    // don't overflow exp_val when adding next digit, instead just truncate
-                    // it and the resulting float will still be correct, either inf or 0.0
-                    // (use INT_MAX/2 to allow adding exp_extra at the end without overflow)
-                    if (exp_val < (INT_MAX / 2 - 9) / 10) {
-                        exp_val = 10 * exp_val + dig;
-                    }
-                } else {
-                    if (dig == 0 || mantissa >= MANTISSA_MAX) {
-                        // Defer treatment of zeros in fractional part.  If nothing comes afterwards, ignore them.
-                        // Also, once we reach MANTISSA_MAX, treat every additional digit as a trailing zero.
-                        if (in == PARSE_DEC_IN_INTG) {
-                            ++trailing_zeros_intg;
-                        } else {
-                            ++trailing_zeros_frac;
-                        }
-                    } else {
-                        // Time to un-defer any trailing zeros.  Intg zeros first.
-                        while (trailing_zeros_intg) {
-                            mantissa = accept_digit(mantissa, 0, &exp_extra, PARSE_DEC_IN_INTG);
-                            --trailing_zeros_intg;
-                        }
-                        while (trailing_zeros_frac) {
-                            mantissa = accept_digit(mantissa, 0, &exp_extra, PARSE_DEC_IN_FRAC);
-                            --trailing_zeros_frac;
-                        }
-                        mantissa = accept_digit(mantissa, dig, &exp_extra, in);
-                    }
-                }
-            } else if (in == PARSE_DEC_IN_INTG && dig == '.') {
-                in = PARSE_DEC_IN_FRAC;
-            } else if (in != PARSE_DEC_IN_EXP && ((dig | 0x20) == 'e')) {
-                in = PARSE_DEC_IN_EXP;
-                if (str < top) {
-                    if (str[0] == '+') {
-                        str++;
-                    } else if (str[0] == '-') {
-                        str++;
-                        exp_neg = true;
-                    }
-                }
-                if (str == top) {
-                    goto value_error;
-                }
-            } else if (dig == '_') {
-                continue;
-            } else {
-                // unknown character
-                str--;
-                break;
-            }
-        }
-
-        // work out the exponent
-        if (exp_neg) {
-            exp_val = -exp_val;
-        }
-
-        // apply the exponent, making sure it's not a subnormal value
-        exp_val += exp_extra + trailing_zeros_intg;
-        dec_val = (mp_float_t)mantissa;
-        if (exp_val < SMALL_NORMAL_EXP) {
-            exp_val -= SMALL_NORMAL_EXP;
-            dec_val *= SMALL_NORMAL_VAL;
-        }
-
-        // At this point, we need to multiply the mantissa by its base 10 exponent. If possible,
-        // we would rather manipulate numbers that have an exact representation in IEEE754. It
-        // turns out small positive powers of 10 do, whereas small negative powers of 10 don't.
-        // So in that case, we'll yield a division of exact values rather than a multiplication
-        // 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 {
-            dec_val *= MICROPY_FLOAT_C_FUN(pow)(10, exp_val);
+        str = mp_parse_float_internal(str, top - str, &dec_val);
+        if (!str) {
+            goto value_error;
         }
     }
 

py/parsenum.h

@@ -34,6 +34,11 @@
 
 mp_obj_t mp_parse_num_integer(const char *restrict str, size_t len, int base, mp_lexer_t *lex);
 
+#if MICROPY_PY_BUILTINS_FLOAT
+mp_float_t mp_decimal_exp(mp_float_t num, int dec_exp);
+const char *mp_parse_float_internal(const char *str, size_t len, mp_float_t *res);
+#endif
+
 #if MICROPY_PY_BUILTINS_COMPLEX
 mp_obj_t mp_parse_num_decimal(const char *str, size_t len, bool allow_imag, bool force_complex, mp_lexer_t *lex);