llvm-project
65 строк · 3.1 Кб
1from math import *2
3"""
4This script is used to generate a table used by
5libc/src/__support/high_precision_decimal.h.
6
7For the ith entry in the table there are two values (indexed starting at 0).
8The first value is the number of digits longer the second value would be if
9multiplied by 2^i.
10The second value is the smallest number that would create that number of
11additional digits (which in base ten is always 5^i). Anything less creates one
12fewer digit.
13
14As an example, the 3rd entry in the table is {1, "125"}. This means that if
15125 is multiplied by 2^3 = 8, it will have exactly one more digit.
16Multiplying it out we get 125 * 8 = 1000. 125 is the smallest number that gives
17that extra digit, for example 124 * 8 = 992, and all larger 3 digit numbers
18also give only one extra digit when multiplied by 8, for example 8 * 999 = 7992.
19This makes sense because 5^3 * 2^3 = 10^3, the smallest 4 digit number.
20
21For numbers with more digits we can ignore the digits past what's in the second
22value, since the most significant digits determine how many extra digits there
23will be. Looking at the previous example, if we have 1000, and we look at just
24the first 3 digits (since 125 has 3 digits), we see that 100 < 125, so we get
25one fewer than 1 extra digits, which is 0.
26Multiplying it out we get 1000 * 8 = 8000, which fits the expectation.
27Another few quick examples:
28For 1255, 125 !< 125, so 1 digit more: 1255 * 8 = 10040
29For 9999, 999 !< 125, so 1 digit more: 9999 * 8 = 79992
30
31Now let's try an example with the 10th entry: {4, "9765625"}. This one means
32that 9765625 * 2^10 will have 4 extra digits.
33Let's skip straight to the examples:
34For 1, 1 < 9765625, so 4-1=3 extra digits: 1 * 2^10 = 1024, 1 digit to 4 digits is a difference of 3.
35For 9765624, 9765624 < 9765625 so 3 extra digits: 9765624 * 1024 = 9999998976, 7 digits to 10 digits is a difference of 3.
36For 9765625, 9765625 !< 9765625 so 4 extra digits: 9765625 * 1024 = 10000000000, 7 digits to 11 digits is a difference of 4.
37For 9999999, 9999999 !< 9765625 so 4 extra digits: 9999999 * 1024 = 10239998976, 7 digits to 11 digits is a difference of 4.
38For 12345678, 1234567 < 9765625 so 3 extra digits: 12345678 * 1024 = 12641974272, 8 digits to 11 digits is a difference of 3.
39
40
41This is important when left shifting in the HPD class because it reads and
42writes the number backwards, and to shift in place it needs to know where the
43last digit should go. Since a binary left shift by i bits is the same as
44multiplying by 2^i we know that looking up the ith element in the table will
45tell us the number of additional digits. If the first digits of the number
46being shifted are greater than or equal to the digits of 5^i (the second value
47of each entry) then it is just the first value in the entry, else it is one
48fewer.
49"""
50
51
52# Generate Left Shift Table
53outStr = ""54for i in range(61):55tenToTheI = 10**i56fiveToTheI = 5**i57outStr += "{"58# The number of new digits that would be created by multiplying 5**i by 2**i59outStr += str(ceil(log10(tenToTheI) - log10(fiveToTheI)))60outStr += ', "'61if not i == 0:62outStr += str(fiveToTheI)63outStr += '"},\n'64
65print(outStr)66