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authorMark Dickinson <dickinsm@gmail.com>2010-01-13 18:21:53 (GMT)
committerMark Dickinson <dickinsm@gmail.com>2010-01-13 18:21:53 (GMT)
commitb26d56ac18df454e373e01a405a622d412d5bd53 (patch)
tree3419a83ee871f8718666e9ecc559b44af948cd79 /Python
parent5f76d132a56a92f1911e16afb59443d5b44f8d32 (diff)
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Add comments explaining the role of the bigcomp function in dtoa.c.
Diffstat (limited to 'Python')
-rw-r--r--Python/dtoa.c61
1 files changed, 60 insertions, 1 deletions
diff --git a/Python/dtoa.c b/Python/dtoa.c
index 1fe20f4..504ad1f 100644
--- a/Python/dtoa.c
+++ b/Python/dtoa.c
@@ -1151,7 +1151,66 @@ sulp(U *x, BCinfo *bc)
return ulp(x);
}
-/* return 0 on success, -1 on failure */
+/* The bigcomp function handles some hard cases for strtod, for inputs
+ with more than STRTOD_DIGLIM digits. It's called once an initial
+ estimate for the double corresponding to the input string has
+ already been obtained by the code in _Py_dg_strtod.
+
+ The bigcomp function is only called after _Py_dg_strtod has found a
+ double value rv such that either rv or rv + 1ulp represents the
+ correctly rounded value corresponding to the original string. It
+ determines which of these two values is the correct one by
+ computing the decimal digits of rv + 0.5ulp and comparing them with
+ the digits of s0.
+
+ In the following, write dv for the absolute value of the number represented
+ by the input string.
+
+ Inputs:
+
+ s0 points to the first significant digit of the input string.
+
+ rv is a (possibly scaled) estimate for the closest double value to the
+ value represented by the original input to _Py_dg_strtod. If
+ bc->scale is nonzero, then rv/2^(bc->scale) is the approximation to
+ the input value.
+
+ bc is a struct containing information gathered during the parsing and
+ estimation steps of _Py_dg_strtod. Description of fields follows:
+
+ bc->dp0 gives the position of the decimal point in the input string
+ (if any), relative to the start of s0. If there's no decimal
+ point, it points to one past the last significant digit.
+
+ bc->dp1 gives the position immediately following the decimal point in
+ the input string, relative to the start of s0. If there's no
+ decimal point, it points to one past the last significant digit.
+
+ bc->dplen gives the length of the decimal separator. In the current
+ implementation, which only allows '.' as a decimal separator, it's
+ 1 if a separator is present in the significant digits of s0, and 0
+ otherwise.
+
+ bc->dsign is 1 if rv < decimal value, 0 if rv >= decimal value. In
+ normal use, it should almost always be 1 when bigcomp is entered.
+
+ bc->e0 gives the exponent of the input value, such that dv = (integer
+ given by the bd->nd digits of s0) * 10**e0
+
+ bc->nd gives the total number of significant digits of s0.
+
+ bc->nd0 gives the number of significant digits of s0 before the
+ decimal separator. If there's no decimal separator, bc->nd0 ==
+ bc->nd.
+
+ bc->scale is the value used to scale rv to avoid doing arithmetic with
+ subnormal values. It's either 0 or 2*P (=106).
+
+ Outputs:
+
+ On successful exit, rv/2^(bc->scale) is the closest double to dv.
+
+ Returns 0 on success, -1 on failure (e.g., due to a failed malloc call). */
static int
bigcomp(U *rv, const char *s0, BCinfo *bc)