diff options
| -rw-r--r-- | Include/cpython/longintrepr.h | 31 |
1 files changed, 21 insertions, 10 deletions
diff --git a/Include/cpython/longintrepr.h b/Include/cpython/longintrepr.h index f5ccbb7..f037c7b 100644 --- a/Include/cpython/longintrepr.h +++ b/Include/cpython/longintrepr.h @@ -62,21 +62,32 @@ typedef long stwodigits; /* signed variant of twodigits */ #define PyLong_MASK ((digit)(PyLong_BASE - 1)) /* Long integer representation. + + Long integers are made up of a number of 30- or 15-bit digits, depending on + the platform. The number of digits (ndigits) is stored in the high bits of + the lv_tag field (lvtag >> _PyLong_NON_SIZE_BITS). + The absolute value of a number is equal to - SUM(for i=0 through abs(ob_size)-1) ob_digit[i] * 2**(SHIFT*i) - Negative numbers are represented with ob_size < 0; - zero is represented by ob_size == 0. - In a normalized number, ob_digit[abs(ob_size)-1] (the most significant + SUM(for i=0 through ndigits-1) ob_digit[i] * 2**(PyLong_SHIFT*i) + + The sign of the value is stored in the lower 2 bits of lv_tag. + + - 0: Positive + - 1: Zero + - 2: Negative + + The third lowest bit of lv_tag is reserved for an immortality flag, but is + not currently used. + + In a normalized number, ob_digit[ndigits-1] (the most significant digit) is never zero. Also, in all cases, for all valid i, - 0 <= ob_digit[i] <= MASK. + 0 <= ob_digit[i] <= PyLong_MASK. + The allocation function takes care of allocating extra memory - so that ob_digit[0] ... ob_digit[abs(ob_size)-1] are actually available. + so that ob_digit[0] ... ob_digit[ndigits-1] are actually available. We always allocate memory for at least one digit, so accessing ob_digit[0] - is always safe. However, in the case ob_size == 0, the contents of + is always safe. However, in the case ndigits == 0, the contents of ob_digit[0] may be undefined. - - CAUTION: Generic code manipulating subtypes of PyVarObject has to - aware that ints abuse ob_size's sign bit. */ typedef struct _PyLongValue { |