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bigint: Improve add.

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This commit is contained in:
Jeroen van Rijn
2021-07-16 01:01:48 +02:00
parent 18dda6ff9d
commit d57e1be89f
2 changed files with 57 additions and 14 deletions
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core/math/bigint/basic.odin
+54 -11
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@@ -12,6 +12,12 @@ package bigint
import "core:mem"
import "core:intrinsics"
/*
===========================
User-level routines
===========================
*/
/*
High-level addition. Handles sign.
*/
@@ -40,6 +46,49 @@ add :: proc(dest, a, b: ^Int) -> (err: Error) {
return _sub(dest, x, y);
}
/*
High-level subtraction, dest = number - decrease. Handles signs.
*/
sub :: proc(dest, number, decrease: ^Int) -> (err: Error) {
dest := dest; x := number; y := decrease;
_panic_if_uninitialized(number); _panic_if_uninitialized(decrease); _panic_if_uninitialized(dest);
if x.sign != y.sign {
/*
Subtract a negative from a positive, OR subtract a positive from a negative.
In either case, ADD their magnitudes and use the sign of the first number.
*/
dest.sign = x.sign;
return _add(dest, x, y);
}
/*
Subtract a positive from a positive, OR negative from a negative.
First, take the difference between their magnitudes, then...
*/
if cmp_mag(x, y) == .Less_Than {
/*
The second has a larger magnitude.
The result has the *opposite* sign from the first number.
*/
dest.sign = .Negative if is_pos(x) else .Zero_or_Positive;
x, y = y, x;
} else {
/*
The first has a larger or equal magnitude.
Copy the sign from the first.
*/
dest.sign = x.sign;
}
return _sub(dest, x, y);
}
/*
==========================
Low-level routines
==========================
*/
/*
Low-level addition, unsigned.
Handbook of Applied Cryptography, algorithm 14.7.
@@ -121,23 +170,17 @@ _add :: proc(dest, a, b: ^Int) -> (err: Error) {
return .OK;
}
/*
Low-level subtraction. Assumes |a| > |b|.
Low-level subtraction, dest = number - decrease. Assumes |number| > |decrease|.
Handbook of Applied Cryptography, algorithm 14.9.
*/
_sub :: proc(dest, a, b: ^Int) -> (err: Error) {
dest := dest; x := a; y := b;
_panic_if_uninitialized(a); _panic_if_uninitialized(b); _panic_if_uninitialized(dest);
for n in 0..=12 {
dest.digit[n] = DIGIT(n);
dest.used = n+1;
}
_sub :: proc(dest, number, decrease: ^Int) -> (err: Error) {
dest := dest; x := number; y := decrease;
_panic_if_uninitialized(number); _panic_if_uninitialized(decrease); _panic_if_uninitialized(dest);
old_used := dest.used;
min_used := y.used;
max_used := a.used;
max_used := x.used;
i: int;
err = grow(dest, max(max_used, _DEFAULT_DIGIT_COUNT));