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std.math.big.int: changed llshr and llshl implementation

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samy007 2025-04-14 20:46:06 +02:00
parent b635b37249
commit b08924e938
1 changed files with 100 additions and 55 deletions
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155
lib/std/math/big/int.zig
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@ -17,7 +17,6 @@ const Endian = std.builtin.Endian;
const Signedness = std.builtin.Signedness;
const native_endian = builtin.cpu.arch.endian();
/// Returns the number of limbs needed to store `scalar`, which must be a
/// primitive integer value.
/// Note: A comptime-known upper bound of this value that may be used
@ -210,7 +209,7 @@ pub const Mutable = struct {
for (self.limbs[0..self.len]) |limb| {
std.debug.print("{x} ", .{limb});
}
std.debug.print("capacity={} positive={}\n", .{ self.limbs.len, self.positive });
std.debug.print("len={} capacity={} positive={}\n", .{ self.len, self.limbs.len, self.positive });
}
/// Clones an Mutable and returns a new Mutable with the same value. The new Mutable is a deep copy and
@ -1104,8 +1103,8 @@ pub const Mutable = struct {
/// Asserts there is enough memory to fit the result. The upper bound Limb count is
/// `a.limbs.len + (shift / (@sizeOf(Limb) * 8))`.
pub fn shiftLeft(r: *Mutable, a: Const, shift: usize) void {
llshl(r.limbs, a.limbs, shift);
r.normalize(a.limbs.len + (shift / limb_bits) + 1);
const new_len = llshl(r.limbs, a.limbs, shift);
r.normalize(new_len);
r.positive = a.positive;
}
@ -1173,8 +1172,8 @@ pub const Mutable = struct {
// This shift should not be able to overflow, so invoke llshl and normalize manually
// to avoid the extra required limb.
llshl(r.limbs, a.limbs, shift);
r.normalize(a.limbs.len + (shift / limb_bits));
const new_len = llshl(r.limbs, a.limbs, shift);
r.normalize(new_len);
r.positive = a.positive;
}
@ -1182,7 +1181,7 @@ pub const Mutable = struct {
/// r and a may alias.
///
/// Asserts there is enough memory to fit the result. The upper bound Limb count is
/// `a.limbs.len - (shift / (@sizeOf(Limb) * 8))`.
/// `a.limbs.len - (shift / (@bitSizeOf(Limb)))`.
pub fn shiftRight(r: *Mutable, a: Const, shift: usize) void {
const full_limbs_shifted_out = shift / limb_bits;
const remaining_bits_shifted_out = shift % limb_bits;
@ -1210,9 +1209,9 @@ pub const Mutable = struct {
break :nonzero a.limbs[full_limbs_shifted_out] << not_covered != 0;
};
llshr(r.limbs, a.limbs, shift);
const new_len = llshr(r.limbs, a.limbs, shift);
r.len = a.limbs.len - full_limbs_shifted_out;
r.len = new_len;
r.positive = a.positive;
if (nonzero_negative_shiftout) r.addScalar(r.toConst(), -1);
r.normalize(r.len);
@ -1971,7 +1970,7 @@ pub const Const = struct {
for (self.limbs[0..self.limbs.len]) |limb| {
std.debug.print("{x} ", .{limb});
}
std.debug.print("positive={}\n", .{self.positive});
std.debug.print("len={} positive={}\n", .{ self.len, self.positive });
}
pub fn abs(self: Const) Const {
@ -2673,7 +2672,7 @@ pub const Managed = struct {
for (self.limbs[0..self.len()]) |limb| {
std.debug.print("{x} ", .{limb});
}
std.debug.print("capacity={} positive={}\n", .{ self.limbs.len, self.isPositive() });
std.debug.print("len={} capacity={} positive={}\n", .{ self.len(), self.limbs.len, self.isPositive() });
}
/// Negate the sign.
@ -3711,68 +3710,114 @@ fn lldiv0p5(quo: []Limb, rem: *Limb, a: []const Limb, b: HalfLimb) void {
}
}
fn llshl(r: []Limb, a: []const Limb, shift: usize) void {
@setRuntimeSafety(debug_safety);
assert(a.len >= 1);
/// Performs r = a << shift and returns the amount of limbs affected
///
/// if a and r overlaps, then r.ptr >= a.ptr is asserted
/// r must have the capacity to store a << shift
fn llshl(r: []Limb, a: []const Limb, shift: usize) usize {
std.debug.assert(a.len >= 1);
if (slicesOverlap(a, r))
std.debug.assert(@intFromPtr(r.ptr) >= @intFromPtr(a.ptr));
const interior_limb_shift = @as(Log2Limb, @truncate(shift));
if (shift == 0) {
if (a.ptr != r.ptr)
std.mem.copyBackwards(Limb, r[0..a.len], a);
return a.len;
}
if (shift >= limb_bits) {
const limb_shift = shift / limb_bits;
const affected = llshl(r[limb_shift..], a, shift % limb_bits);
@memset(r[0..limb_shift], 0);
return limb_shift + affected;
}
// shift is guaranteed to be < limb_bits
const bit_shift: Log2Limb = @truncate(shift);
const opposite_bit_shift: Log2Limb = @truncate(limb_bits - bit_shift);
// We only need the extra limb if the shift of the last element overflows.
// This is useful for the implementation of `shiftLeftSat`.
if (a[a.len - 1] << interior_limb_shift >> interior_limb_shift != a[a.len - 1]) {
assert(r.len >= a.len + (shift / limb_bits) + 1);
const overflows = a[a.len - 1] >> opposite_bit_shift != 0;
if (overflows) {
std.debug.assert(r.len >= a.len + 1);
} else {
assert(r.len >= a.len + (shift / limb_bits));
std.debug.assert(r.len >= a.len);
}
const limb_shift = shift / limb_bits + 1;
var carry: Limb = 0;
var i: usize = 0;
while (i < a.len) : (i += 1) {
const src_i = a.len - i - 1;
const dst_i = src_i + limb_shift;
const src_digit = a[src_i];
r[dst_i] = carry | @call(.always_inline, math.shr, .{
Limb,
src_digit,
limb_bits - @as(Limb, @intCast(interior_limb_shift)),
});
carry = (src_digit << interior_limb_shift);
var i: usize = a.len;
if (overflows) {
// r is asserted to be large enough above
r[a.len] = a[a.len - 1] >> opposite_bit_shift;
}
while (i > 1) {
i -= 1;
r[i] = (a[i - 1] >> opposite_bit_shift) | (a[i] << bit_shift);
}
r[0] = a[0] << bit_shift;
r[limb_shift - 1] = carry;
@memset(r[0 .. limb_shift - 1], 0);
return a.len + @intFromBool(overflows);
}
fn llshr(r: []Limb, a: []const Limb, shift: usize) void {
@setRuntimeSafety(debug_safety);
assert(a.len >= 1);
assert(r.len >= a.len - (shift / limb_bits));
/// Performs r = a >> shift and returns the amount of limbs affected
///
/// if a and r overlaps, then r.ptr <= a.ptr is asserted
/// r must have the capacity to store a >> shift
///
/// See tests below for examples of behaviour
fn llshr(r: []Limb, a: []const Limb, shift: usize) usize {
if (slicesOverlap(a, r))
std.debug.assert(@intFromPtr(r.ptr) <= @intFromPtr(a.ptr));
const limb_shift = shift / limb_bits;
const interior_limb_shift = @as(Log2Limb, @truncate(shift));
if (a.len == 0) return 0;
if (shift == 0) {
std.debug.assert(r.len >= a.len);
if (a.ptr != r.ptr)
std.mem.copyForwards(Limb, r[0..a.len], a);
return a.len;
}
if (shift >= limb_bits) {
if (shift / limb_bits >= a.len) {
r[0] = 0;
return 1;
}
return llshr(r, a[shift / limb_bits ..], shift % limb_bits);
}
// shift is guaranteed to be < limb_bits
const bit_shift: Log2Limb = @truncate(shift);
const opposite_bit_shift: Log2Limb = @truncate(limb_bits - bit_shift);
// special case, where there is a risk to set r to 0
if (a.len == 1) {
r[0] = a[0] >> bit_shift;
return 1;
}
if (a.len == 0) {
r[0] = 0;
return 1;
}
// if the most significant limb becomes 0 after the shift
const shrink = a[a.len - 1] >> bit_shift == 0;
std.debug.assert(r.len >= a.len - @intFromBool(!shrink));
var i: usize = 0;
while (i < a.len - limb_shift) : (i += 1) {
const dst_i = i;
const src_i = dst_i + limb_shift;
const src_digit = a[src_i];
const src_digit_next = if (src_i + 1 < a.len) a[src_i + 1] else 0;
const carry = @call(.always_inline, math.shl, .{
Limb,
src_digit_next,
limb_bits - @as(Limb, @intCast(interior_limb_shift)),
});
r[dst_i] = carry | (src_digit >> interior_limb_shift);
while (i < a.len - 1) : (i += 1) {
r[i] = (a[i] >> bit_shift) | (a[i + 1] << opposite_bit_shift);
}
if (!shrink)
r[i] = a[i] >> bit_shift;
return a.len - @intFromBool(shrink);
}
// r = ~r
fn llnot(r: []Limb) void {
for (r) |*elem| {
elem.* = ~elem.*;
}
@ -4107,7 +4152,7 @@ fn llsquareBasecase(r: []Limb, x: []const Limb) void {
}
// Each product appears twice, multiply by 2
llshl(r, r[0 .. 2 * x_norm.len], 1);
_ = llshl(r, r[0 .. 2 * x_norm.len], 1);
for (x_norm, 0..) |v, i| {
// Compute and add the squares