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v/vlib/math/big/special_array_ops.v

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module big
import math
import math.bits
import strings
// suppose operand_a bigger than operand_b and both not null.
// Both quotient and remaider are already allocated but of length 0
fn newton_divide_array_by_array(operand_a []u32, operand_b []u32, mut quotient []u32, mut remainder []u32) {
// tranform back to Integers (on the stack without allocation)
a := Integer{
signum: 1
digits: operand_a
}
b := Integer{
signum: 1
digits: operand_b
}
k := bit_length(a) + bit_length(b) // a*b < 2**k
mut x := integer_from_int(2) // 0 < x < 2**(k+1)/b // initial guess for convergence
// https://en.wikipedia.org/wiki/Division_algorithm#Newton%E2%80%93Raphson_division
// use 48/17 - 32/17.D (divisor)
initial_guess := (((integer_from_int(48) - (integer_from_int(32) * b)) * integer_from_i64(0x0f0f0f0f0f0f0f0f)).rshift(64)).neg() // / 17 == 0x11
if initial_guess > zero_int {
x = initial_guess
}
mut lastx := integer_from_int(0)
pow2_k_plus_1 := pow2(k + 1) // outside of the loop to optimize allocatio
for lastx != x { // main loop
lastx = x
x = (x * (pow2_k_plus_1 - (x * b))).rshift(u32(k))
}
if x * b < pow2(k) {
x.inc()
}
mut q := (a * x).rshift(u32(k))
// possible adjustments. see literature
if q * b > a {
q.dec()
}
mut r := a - (q * b)
if r >= b {
q.inc()
r -= b
}
quotient = q.digits
remainder = r.digits
for remainder.len > 0 && remainder.last() == 0 {
remainder.delete_last()
}
}
[inline]
fn bit_length(a Integer) int {
return a.digits.len * 32 - bits.leading_zeros_32(a.digits.last())
}
[inline]
fn debug_u32_str(a []u32) string {
mut sb := strings.new_builder(30)
sb.write_string('[')
mut first := true
for i in 0 .. a.len {
if !first {
sb.write_string(', ')
}
sb.write_string('0x${a[i].hex()}')
first = false
}
sb.write_string(']')
return sb.str()
}
// karatsuba algorithm for multiplication
// possible optimisations:
// - transform one or all the recurrences in loops
fn karatsuba_multiply_digit_array(operand_a []u32, operand_b []u32, mut storage []u32) {
// base case necessary to end recursion
if operand_a.len == 0 || operand_b.len == 0 {
for storage.len > 0 {
storage.delete_last()
}
return
}
if operand_a.len < operand_b.len {
multiply_digit_array(operand_b, operand_a, mut storage)
return
}
if operand_b.len == 1 {
multiply_array_by_digit(operand_a, operand_b[0], mut storage)
return
}
// karatsuba
// thanks to the base cases we can pass zero-length arrays to the mult func
half := math.max(operand_a.len, operand_b.len) / 2
if half <= 0 {
panic('Unreachable. Both array have 1 length and multiply_array_by_digit should have been called')
}
a_l := operand_a[0..half]
a_h := operand_a[half..]
mut b_l := []u32{}
mut b_h := []u32{}
if half <= operand_b.len {
b_l = operand_b[0..half]
b_h = operand_b[half..]
} else {
b_l = unsafe { operand_b }
// b_h = []u32{}
}
// use storage for p_1 to avoid allocation and copy later
multiply_digit_array(a_h, b_h, mut storage)
mut p_3 := []u32{len: a_l.len + b_l.len + 1, init: 0}
multiply_digit_array(a_l, b_l, mut p_3)
mut tmp_1 := []u32{len: math.max(a_h.len, a_l.len) + 1, init: 0}
mut tmp_2 := []u32{len: math.max(b_h.len, b_l.len) + 1, init: 0}
add_digit_array(a_h, a_l, mut tmp_1)
add_digit_array(b_h, b_l, mut tmp_2)
mut p_2 := []u32{len: operand_a.len + operand_b.len + 1, init: 0}
multiply_digit_array(tmp_1, tmp_2, mut p_2)
subtract_in_place(mut p_2, storage) // p_1
subtract_in_place(mut p_2, p_3)
// return p_1.lshift(2 * u32(half * 32)) + p_2.lshift(u32(half * 32)) + p_3
lshift_byte_in_place(mut storage, 2 * half)
lshift_byte_in_place(mut p_2, half)
add_in_place(mut storage, p_2)
add_in_place(mut storage, p_3)
for storage.len > 0 && storage.last() == 0 {
storage.delete_last()
}
}
[inline]
fn pow2(k int) Integer {
mut ret := []u32{len: (k >> 5) + 1, init: 0}
bit_set(mut ret, k)
return Integer{
signum: 1
digits: ret
}
}
// optimized left shift of full byte(s) in place. byte_nb must be positive
fn lshift_byte_in_place(mut a []u32, byte_nb int) {
a_len := a.len
// control or allocate capacity
for _ in a_len .. a_len + byte_nb {
a << u32(0)
}
for index := a_len - 1; index >= 0; index-- {
a[index + byte_nb] = a[index]
}
for index in 0 .. byte_nb {
a[index] = u32(0)
}
}
// operand b can be greater than operand a
// the capacity of both array is supposed to be sufficient
[inline]
fn add_in_place(mut a []u32, b []u32) {
len_a := a.len
len_b := b.len
max := math.max(len_a, len_b)
min := math.min(len_a, len_b)
mut carry := u64(0)
for index in 0 .. min {
partial := carry + a[index] + b[index]
a[index] = u32(partial)
carry = u32(partial >> 32)
}
if len_a >= len_b {
for index in min .. max {
partial := carry + a[index]
a[index] = u32(partial)
carry = u32(partial >> 32)
}
} else {
for index in min .. max {
partial := carry + b[index]
a << u32(partial)
carry = u32(partial >> 32)
}
}
}
// a := a - b supposed a >= b
fn subtract_in_place(mut a []u32, b []u32) {
len_a := a.len
len_b := b.len
max := math.max(len_a, len_b)
min := math.min(len_a, len_b)
mut carry := u32(0)
mut new_carry := u32(0)
for index in 0 .. min {
if a[index] < (b[index] + carry) {
new_carry = 1
} else {
new_carry = 0
}
a[index] -= (b[index] + carry)
carry = new_carry
}
if len_a >= len_b {
for index in min .. max {
if a[index] < carry {
new_carry = 1
} else {
new_carry = 0
}
a[index] -= carry
carry = new_carry
}
} else { // if len.b > len.a return zero
for a.len > 0 {
a.delete_last()
}
}
}