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Task/P-Adic-square-roots/00-META.yaml Normal file
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---
from: http://rosettacode.org/wiki/P-Adic_square_roots
note: mathematics

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Task/P-Adic-square-roots/00-TASK.txt Normal file
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;Task.
Convert rational a/b to its approximate [[wp:Hensel%27s_lemma#Hensel's_lemma_for_p-adic_numbers|p-adic square root]]. To check the result,
square the root and construct rational m/n to compare with radicand a/b.
For rational reconstruction Lagrange's [[wp:Lattice_reduction|lattice basis reduction]] algorithm is used.
'''Recipe:''' find root {{math|''x<sub>1</sub>'' modulo p}} and build a sequence of solutions
{{math|''f''(''x<sub>k</sub>'') &#8801; 0 (mod p<sup>k</sup>)}},
<br/>using the lifting equation
{{math|''x<sub>k+1</sub>'' &#61; ''x<sub>k</sub>'' + ''d<sub>k</sub>'' * ''p<sup>k</sup>''&nbsp;}}
with {{math|''d<sub>k</sub>'' &#61; &#8211;(''f''(''x<sub>k</sub>'') &#47; ''p<sup>k</sup>'') &#47;
''f''&nbsp;&#8242;(''x<sub>1</sub>'') (mod p)}}.
<br/>The multipliers {{math|''d<sub>k</sub>''}} are the successive p-adic digits to find.
If evaluation of {{math|''f''(''x'') &#61; ''bx<sup>2</sup>'' &#8211; ''a''}} overflows,
the expansion is cut off and might be too short to retrieve the radicand.
Setting a higher precision won't help, using a programming language with built-in
large integer support will.
;Related task.
[[p-Adic numbers, basic]]
;Reference.
[https://www.uvm.edu/~cvincen1/files/teaching/spring2017-math255/quadraticequation.pdf]
Solving {{math|''x<sup>2</sup>'' &#8801; ''a'' (mod n)}}
__TOC__

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Task/P-Adic-square-roots/FreeBASIC/p-adic-square-roots.basic Normal file
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' ***********************************************
'subject: p-adic square roots, Hensel lifting.
'tested : FreeBasic 1.07.0
'The root is squared, approximated by a
'rational, and compared with radicand a/b.
const emx = 48
'exponent maximum
const amx = 700000
'tentative argument maximum
'------------------------------------------------
const Mxd = cdbl(2)^53 - 1
'max. float64 integer
const Pmax = 32749
'max. prime < 2^15
type ratio
as longint a, b
end type
type padic
declare function sqrt (byref q as ratio, byval sw as integer) as integer
'p-adic square root of q = a/b, set sw to print
declare sub printf (byval sw as integer)
'print expansion, set sw to print rational
declare function crat (byval sw as integer) as ratio
'rational reconstruction
declare sub cmpt (byref a as padic)
'let self:= complement_a
declare sub sqr (byref a as padic)
'let self:= a ^ 2
as long d(-emx to emx - 1)
as integer v
end type
'global variables
dim shared as long p1, p = 7
'default prime
dim shared as integer k = 11
'precision
#define min(a, b) iif((a) > (b), b, a)
'------------------------------------------------
'p-adic square root of g = a/b
function padic.sqrt (byref g as ratio, byval sw as integer) as integer
dim as longint a = g.a, b = g.b
dim as longint q, x, pk, pm
dim as long f1, r, s, t
dim i as integer, f as double
sqrt = 0
if b = 0 then return 1
if b < 0 then b = -b: a = -a
if p < 2 or k < 1 then return 1
'max. short prime
p = min(p, Pmax)
if sw then
'echo numerator, denominator,
print a;"/";str(b);" + ";
'prime and precision
print "O(";str(p);"^";str(k);")"
end if
'initialize
v = 0
p1 = p - 1
for i = -emx to emx - 1
d(i) = 0: next
if a = 0 then return 0
'valuation
do until b mod p
b \= p: v -= 1
loop
do until a mod p
a \= p: v += 1
loop
if (v and 1) = 1 then
'odd valuation
print "non-residue mod"; p
return -1
end if
'max. array length
k = min(k + v, emx - 1)
k -= v: v shr= 1
if abs(a) > amx or b > amx then return -1
if p = 2 then
'1 / b = b (mod 8)
'a / b = 1 (mod 8)
t = a * b
if (t and 7) - 1 then
print "non-residue mod 8"
return -1
end if
else
'find root for small p
for r = 1 to p1
q = b * r * r - a
if q mod p = 0 then exit for
next r
if r = p then
print "non-residue mod"; p
return -1
end if
'f'(r) = 2br
t = b * r shl 1
s = 0
t mod= p
'modular inverse for small p
for f1 = 1 to p1
s += t
if s > p1 then s -= p
if s = 1 then exit for
next f1
if f1 = p then
print "impossible inverse mod"
return -1
end if
end if
'evaluate f(x)
#macro evalf(x)
f = b * x * cdbl(x / pk)
f -= cdbl(a / pk)
'overflow
if f > Mxd then exit for
q = clngint(f)
#endmacro
if p = 2 then
'initialize
x = 1
d(v) = 1
d(v + 1) = 0
pk = 4
for i = v + 2 to k - 1 + v
pk shl= 1
'2-power overflow
if pk < 1 then exit for
evalf(x)
'next digit
d(i) = iif(q and 1, 1, 0)
'lift x
x += d(i) * (pk shr 1)
next i
else
'-1 / f'(x) mod p
f1 = p - f1
x = r
d(v) = x
pk = 1
for i = v + 1 to k - 1 + v
pm = pk: pk *= p
if pk \ pm - p then exit for
evalf(x)
d(i) = q * f1 mod p
if d(i) < 0 then d(i) += p
x += d(i) * pk
next i
end if
k = i - v
if sw then print "lift:";x;" mod";p;"^";str(k)
end function
'------------------------------------------------
'rational reconstruction
function padic.crat (byval sw as integer) as ratio
dim as integer i, j, t = min(v, 0)
dim as longint s, pk, pm
dim as long q, x, y
dim as double f, h
dim r as ratio
'weighted digit sum
s = 0: pk = 1
for i = t to k - 1 + v
pm = pk: pk *= p
if pk \ pm - p then
'overflow
pk = pm: exit for
end if
s += d(i) * pm '(mod pk)
next i
'lattice basis reduction
dim as longint m(1) = {pk, s}
dim as longint n(1) = {0, 1}
'norm(v)^2
h = cdbl(s) * s + 1
i = 0: j = 1
'Lagrange's algorithm
do
f = m(i) * (m(j) / h)
f += n(i) * (n(j) / h)
'Euclidean step
q = int(f +.5)
m(i) -= q * m(j)
n(i) -= q * n(j)
f = h
h = cdbl(m(i)) * m(i)
h += cdbl(n(i)) * n(i)
'compare norms
if h < f then
'interchange vectors
swap i, j
else
exit do
end if
loop
x = m(j): y = n(j)
if y < 0 then y = -y: x = -x
'check determinant
t = abs(m(i) * y - x * n(i)) = pk
if t = 0 then
print "crat: fail"
x = 0: y = 1
else
'negative powers
for i = v to -1
y *= p: next
if sw then
print x;
if y > 1 then print "/";str(y);
print
end if
end if
r.a = x: r.b = y
return r
end function
'print expansion
sub padic.printf (byval sw as integer)
dim as integer i, t = min(v, 0)
for i = k - 1 + t to t step -1
print d(i);
if i = 0 andalso v < 0 then print ".";
next i
print
'rational approximation
if sw then crat(sw)
end sub
'------------------------------------------------
'let self:= complement_a
sub padic.cmpt (byref a as padic)
dim i as integer, r as padic
dim as long c = 1
with r
.v = a.v
for i = .v to k +.v
c += p1 - a.d(i)
'carry
if c > p1 then
.d(i) = c - p: c = 1
else
.d(i) = c: c = 0
end if
next i
end with
this = r
end sub
'let self:= a ^ 2
sub padic.sqr (byref a as padic)
dim as long ptr rp, ap = @a.d(a.v)
dim as longint q, c = 0
dim as integer i, j
dim r as padic
with r
.v = a.v shl 1
rp = @.d(.v)
for i = 0 to k
for j = 0 to i
c += ap[j] * ap[i - j]
next j
'Euclidean step
q = c \ p
rp[i] = c - q * p
c = q
next i
end with
this = r
end sub
'main
'------------------------------------------------
dim as integer sw
dim as padic a, c
dim as ratio q, r
width 64, 30
cls
' -7 + O(2^7)
data -7,1, 2,7
data 9,1, 2,8
data 17,1, 2,9
data 497,10496, 2,18
data 10496,497, 2,19
data -577215,664901, 3,23
data 15403,26685, 3,18
data -1,1, 5,8
data 86,25, 5,8
data 2150,1, 5,8
data 2,1, 7,8
data 11696,621467, 7,11
data -27764,11521, 7,11
data -27584,12953, 7,11
data -166420,135131, 11,11
data 14142,135623, 5,15
data -255,256, 257,3
data 0,0, 0,0
print
do
read q.a,q.b, p,k
sw = a.sqrt(q, 1)
if sw = 1 then exit do
if sw then ? : continue do
print "sqrt +/-"
print "...";
a.printf(0)
a.cmpt(a)
print "...";
a.printf(0)
c.sqr(a)
print "sqrt^2"
print " ";
c.printf(0)
r = c.crat(1)
'{r = q}
if q.a * r.b - r.a * q.b then
print "fail: sqrt^2"
end if
print : ?
loop
end

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{-# LANGUAGE KindSignatures, DataKinds #-}
import Data.Ratio
import Data.List (find)
import GHC.TypeLits
import Padic
pSqrt :: KnownNat p => Rational -> Padic p
pSqrt r = res
where
res = maybe Null mkUnit series
(a, b) = (numerator r, denominator r)
series = case modulo res of
2 | eqMod 4 a 3 -> Nothing
| not (eqMod 8 a 1) -> Nothing
| otherwise -> Just $ 1 : 0 : go 8 1
where
go pk x =
let q = ((b*x*x - a) `div` pk) `mod` 2
in q : go (2*pk) (x + q * (pk `div` 2))
p -> do
y <- find (\x -> eqMod p (b*x*x) a) [1..p-1]
df <- recipMod p (2*b*y)
let go pk x =
let f = (b*x*x - a) `div` pk
d = (f * (p - df)) `mod` p
in x `div` (pk `div` p) : go (p*pk) (x + d*pk)
Just $ go p y
eqMod :: Integral a => a -> a -> a -> Bool
eqMod p a b = a `mod` p == b `mod` p

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using Nemo, LinearAlgebra
set_printing_mode(FlintPadicField, :terse)
""" convert to Rational (rational reconstruction) """
function toRational(pa::padic)
rat = lift(QQ, pa)
r, den = BigInt(numerator(rat)), Int(denominator(rat))
p, k = Int(prime(parent(pa))), Int(precision(pa))
N = BigInt(p^k)
a1, a2 = [N, 0], [r, 1]
while dot(a1, a1) > dot(a2, a2)
q = dot(a1, a2) // dot(a2, a2)
a1, a2 = a2, a1 - round(q) * a2
end
if dot(a1, a1) < N
return (Rational{Int}(a1[1]) // Rational{Int}(a1[2])) // Int(den)
else
return Int(r) // den
end
end
function dstring(pa::padic)
u, v, n, p, k = pa.u, pa.v, pa.N, pa.parent.p, pa.parent.prec_max
d = digits(v > 0 ? u * p^v : u, base=p, pad=k)
return prod([i == k + v && v != 0 ? "$x . " : "$x " for (i, x) in enumerate(reverse(d))])
end
const DATA = [
[-7, 1, 2, 7],
[9, 1, 2, 8],
[17, 1, 2, 9],
[-1, 1, 5, 8],
[86, 25, 5, 8],
[2150, 1, 5, 8],
[2, 1, 7, 8],
[3029, 4821, 7, 9],
[379, 449, 7, 8],
[717, 8, 11, 7],
[1414, 213, 41, 5],
[-255, 256, 257, 3]
]
for (num1, den1, P, K) in DATA
Qp = PadicField(P, K)
a = Qp(QQ(num1 // den1))
c = sqrt(a)
r = toRational(c * c)
println(a, "\nsqrt +/-\n", dstring(c), "\n", dstring(-c), "\nCheck sqrt^2:\n", dstring(c * c), "\n", r, "\n")
end

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import strformat
const
Emx = 64 # Exponent maximum.
Amx = 6000 # Argument maximum.
PMax = 32749 # Prime maximum.
type
Ratio = tuple[a, b: int]
Padic = object
p: int # Prime.
k: int # Precision.
v: int
d: array[-Emx..(Emx-1), int]
PadicError = object of ValueError
proc sqrt(pa: var Padic; q: Ratio; sw: bool) =
## Return the p-adic square root of q = a/b. Set sw to print.
var (a, b) = q
var i, x: int
if b == 0:
raise newException(PadicError, &"Wrong rational: {a}/{b}" )
if b < 0:
b = -b
a = -a
if pa.p < 2:
raise newException(PadicError, &"Wrong value for p: {pa.p}")
if pa.k < 1:
raise newException(PadicError, &"Wrong value for k: {pa.k}")
pa.p = min(pa.p, PMax) # Maximum short prime.
if sw: echo &"{a}/{b} + 0({pa.p}^{pa.k})"
# Initialize.
pa.v = 0
pa.d.reset()
if a == 0: return
# Valuation.
while b mod pa.p == 0:
b = b div pa.p
dec pa.v
while a mod pa.p == 0:
a = a div pa.p
inc pa.v
if (pa.v and 1) != 0:
# Odd valuation.
raise newException(PadicError, &"Non-residue mod {pa.p}.")
# Maximum array length.
pa.k = min(pa.k + pa.v, Emx - 1) - pa.v
pa.v = pa.v shr 1
if abs(a) > Amx or b > Amx:
raise newException(PadicError, &"Rational exceeding limits: {a}/{b}.")
if pa.p == 2:
# 1 / b = b (mod 8); a / b = 1 (mod 8).
if (a * b and 7) - 1 != 0:
raise newException(PadicError, "Non-residue mod 8.")
# Initialize.
x = 1
pa.d[pa.v] = 1
pa.d[pa.v + 1] = 0
var pk = 4
i = pa.v + 2
while i < pa.k + pa.v:
pk *= 2
let f = b * x * x - a
let q = f div pk
if f != q * pk: break # Overflow.
# Next digit.
pa.d[i] = if (q and 1) != 0: 1 else: 0
# Lift "x".
x += pa.d[i] * (pk shr 1)
inc i
else:
# Find root for small "p".
var r = 1
while r < pa.p:
if (b * r * r - a) mod pa.p == 0: break
inc r
if r == pa.p:
raise newException(PadicError, &"Non-residue mod {pa.p}.")
let t = (b * r shl 1) mod pa.p
var s = 0
# Modular inverse for small "p".
var f1 = 1
while f1 < pa.p:
inc s, t
if s >= pa.p: dec s, pa.p
if s == 1: break
inc f1
if f1 == pa.p:
raise newException(PadicError, "Impossible to compute inverse modulo")
f1 = pa.p - f1
x = r
pa.d[pa.v] = x
var pk = 1
i = pa.v + 1
while i < pa.k + pa.v:
pk *= pa.p
let f = b * x * x - a
let q = f div pk
if f != q * pk: break # Overflow.
pa.d[i] = q * f1 mod pa.p
if pa.d[i] < 0: pa.d[i] += pa.p
x += pa.d[i] * pk
inc i
pa.k = i - pa.v
if sw: echo &"lift: {x} mod {pa.p}^{pa.k}"
proc crat(pa: Padic; sw: bool): Ratio =
## Rational reconstruction.
# Weighted digit sum.
var
s = 0
pk = 1
for i in min(pa.v, 0)..<(pa.k + pa.v):
let pm = pk
pk *= pa.p
if pk div pm - pa.p != 0:
# Overflow.
pk = pm
break
s += pa.d[i] * pm
# Lattice basis reduction.
var
m = [pk, s]
n = [0, 1]
i = 0
j = 1
s = s * s + 1
# Lagrange's algorithm.
while true:
# Euclidean step.
var q = ((m[i] * m[j] + n[i] * n[j]) / s).toInt
m[i] -= q * m[j]
n[i] -= q * n[j]
q = s
s = m[i] * m[i] + n[i] * n[i]
# Compare norms.
if s < q: swap i, j # Interchange vectors.
else: break
var x = m[j]
var y = n[j]
if y < 0:
y = -y
x = -x
# Check determinant.
if abs(m[i] * y - x * n[i]) != pk:
raise newException(PadicError, "Rational reconstruction failed.")
# Negative powers.
for i in pa.v..(-1): y *= pa.p
if sw: echo x, if y > 1: '/' & $y else: ""
result = (x, y)
func cmpt(pa: Padic): Padic =
## Return the complement.
result = Padic(p: pa.p, k: pa.k, v: pa.v)
var c = 1
for i in pa.v..(pa.k + pa.v):
inc c, pa.p - 1 - pa.d[i]
if c >= pa.p:
result.d[i] = c - pa.p
c = 1
else:
result.d[i] = c
c = 0
func sqr(pa: Padic): Padic =
## Return the square of a P-adic number.
result = Padic(p: pa.p, k: pa.k, v: pa.v * 2)
var c = 0
for i in 0..pa.k:
for j in 0..i:
c += pa.d[pa.v + j] * pa.d[pa.v + i - j]
# Euclidean step.
let q = c div pa.p
result.d[result.v + i] = c - q * pa.p
c = q
func `$`(pa: Padic): string =
## String representation.
let t = min(pa.v, 0)
for i in countdown(pa.k - 1 + t, t):
result.add $pa.d[i]
if i == 0 and pa.v < 0: result.add "."
result.add " "
when isMainModule:
const Data = [[-7, 1, 2, 7],
[9, 1, 2, 8],
[17, 1, 2, 9],
[497, 10496, 2, 18],
[10496, 497, 2, 19],
[3141, 5926, 3, 15],
[2718, 281, 3, 13],
[-1, 1, 5, 8],
[86, 25, 5, 8],
[2150, 1, 5, 8],
[2,1, 7, 8],
[-2645, 28518, 7, 9],
[3029, 4821, 7, 9],
[379, 449, 7, 8],
[717, 8, 11, 7],
[1414, 213, 41, 5],
[-255, 256, 257, 3]]
for d in Data:
try:
let q: Ratio = (d[0], d[1])
var a = Padic(p: d[2], k: d[3])
a.sqrt(q, true)
echo "sqrt +/-"
echo "...", a
a = a.cmpt()
echo "...", a
let c = sqr(a)
echo "sqrt^2"
echo " ", c
let r = c.crat(true)
if q.a * r.b - r.a * q.b != 0:
echo "fail: sqrt^2"
echo ""
except PadicError:
echo getCurrentExceptionMsg()

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constant EMX = 48 // exponent maximum (if indexing starts at -EMX)
constant DMX = 1e5 // approximation loop maximum
constant AMX = 700000 // argument maximum
constant PMAX = 32749 // prime maximum
// global variables
integer p1 = 0
integer p = 7 // default prime
integer k = 11 // precision
type Ratio(sequence r)
return length(r)=2 and integer(r[1]) and integer(r[2])
end type
class Padic
integer v = 0
sequence d = repeat(0,EMX*2)
function square_root(Ratio g, integer sw)
-- p-adic square root of g = a/b
integer {a,b} = g
atom f, q, pk, x
integer f1, r, s, t, i, res = 0
if b = 0 then return 1 end if
if b < 0 then
b = -b
a = -a
end if
if p < 2 or k < 1 then return 1 end if
-- max. short prime
p = min(p, PMAX)
if sw then
-- numerator, denominator, prime, precision
printf(1,"%d/%d + O(%d^%d)\n",{a,b,p,k})
end if
-- initialize
v = 0
p1 = p - 1
sequence ntd = repeat(0,2*EMX) -- (new this.d)
if a = 0 then return 0 end if
-- valuation
while remainder(b,p)=0 do
b /= p
v -= 1
end while
while remainder(a,p)=0 do
a /= p
v += 1
end while
if remainder(v,2) then
-- odd valuation
printf(1,"(1)non-residue mod %d\n",p)
return -1
end if
-- max. array length
k = min(k + v, EMX - 1) - v
v = floor(v/2)
if abs(a) > AMX or b > AMX then return -1 end if
if p = 2 then
--1 / b = b (mod 8)
--a / b = 1 (mod 8)
t = a * b
if mod(t,8)-1 then
printf(1,"(2)non-residue mod 8\n")
return -1
end if
else
-- find root for small p
for r = 1 to p1 do
f = b * r * r - a
if mod(f,p) = 0 then exit end if
end for
if r = p then
printf(1,"(3)non-residue mod %d\n", p)
return -1
end if
-- f'(r) = 2br
t = b * r * 2
s = 0
t = mod(t,p)
-- modular inverse for small p
for f1 = 1 to p1 do
s += t
if s > p1 then s -= p end if
if s = 1 then exit end if
end for
if f1 = p then
printf(1,"impossible inverse mod\n")
return -1
end if
end if
if p = 2 then
-- initialize
x = 1
ntd[v+EMX+1] = 1
ntd[v+EMX+2] = 0
pk = 4
for i = v+2 to k-1+v do
pk *= 2
f = b * x * x - a
q = floor(f/pk)
-- overflow
if f != q * pk then exit end if
-- next digit
ntd[i+EMX+1] = and_bits(q,1)
-- lift x
x += ntd[i+EMX+1] * floor(pk/2)
end for
else
-- -1 / f'(x) mod p
f1 = p - f1
x = r
ntd[v+EMX+1] = x
pk = 1
for i = v+1 to k-1 do
pk *= p
f = b * x * x - a
q = floor(f/pk)
-- overflow
if f - q * pk then exit end if
r = mod(q*f1,p)
if r < 0 then r += p end if
ntd[i+EMX+1] = r
x += r * pk
end for
end if
this.d = ntd
k = i-v
if sw then
printf(1,"lift: %d mod %d^%d\n",{x,p,k})
end if
return 0
end function
function square()
integer c = 0
Padic r = new()
r.v = this.v * 2
sequence td = this.d,
rd = r.d
for i=0 to k do
for j=0 to i do
c += td[v+j+EMX+1] * td[v+i-j+EMX+1]
end for
// Euclidean step
integer q = floor(c/p)
rd[r.v+i+EMX+1] = c - q*p
c = q
end for
r.d = rd
return r
end function
function complement()
integer c = 1
Padic r = new({v})
sequence rd = r.d
for i=v to k+v do
integer dx = i+EMX+1
c += p1 - this.d[dx]
if c>p1 then
rd[dx] = c - p
c = 1
else
rd[dx] = c
c = 0
end if
end for
r.d = rd
return r
end function
function crat(integer sw)
-- rational reconstruction
integer i, j, t = min(v, 0)
Ratio r
atom f
integer x, y
atom p1,pk, q, s
-- weighted digit sum
s = 0
pk = 1
for i = t to k-1+v do
p1 = pk
pk *= p
if floor(pk/p1) - p then
-- overflow
pk = p1
exit
end if
s += d[i+EMX+1] * p1 --(mod pk)
end for
-- lattice basis reduction
sequence m = {pk, s},
n = {0, 1}
i = 1
j = 2
s = s * s + 1 -- norm(v)^2
-- Lagrange's algorithm
while true do
f = (m[i] * m[j] + n[i] * n[j]) / s
-- Euclidean step
q = floor(f +.5)
m[i] -= q * m[j]
n[i] -= q * n[j]
q = s
s = m[i] * m[i] + n[i] * n[i]
-- compare norms
if s < q then
-- interchange vectors
{i,j} = {j,i}
else
exit
end if
end while
x = m[j]
y = n[j]
if y < 0 then
y = -y
x = -x
end if
-- check determinant
t = abs(m[i] * y - x * n[i]) == pk
if not t then
printf(1,"crat: fail\n")
x = 0
y = 1
else
-- negative powers
for i = v to -1 do
y *= p
end for
if sw then
-- printf(1,iff(y=1?"%d":"%d/%d"),{x*sgn,y})
printf(1,iff(y=1?"%d\n":"%d/%d\n"),{x,y})
end if
end if
r = {x,y}
return r
end function
procedure prntf(bool sw)
-- print expansion
integer t = min(v, 0)
for i=k-1+t to t by -1 do
printf(1,"%d",d[i+EMX+1])
printf(1,iff(i=0 and v<0?". ":" "))
end for
printf(1,"\n")
// rational approximation
if sw then crat(sw) end if
end procedure
end class
constant tests = {
{{-7,1},2,7},
--/*
{{9,1},2,8},
{{17,1},2,9},
{{497,10496},2,18},
{{10496,497},2,19},
{{3141,5926},3,17},
{{2718,281},3,15},
{{-1,1},5,8},
{{86,25},5,8},
{{2150,1},5,10},
{{2,1},7,8},
{{-2645,28518},7,9},
{{3029,4821},7,9},
{{379,449},7,8},
{{717,8},11,7},
{{1414,213},41,5},
--*/
{{-255,256},257,3}
}
Padic a = new(), c
Ratio q, r
for i=1 to length(tests) do
{q,p,k} = tests[i]
integer sw = a.square_root(q, 1)
if sw=1 then exit end if
if sw=0 then
printf(1,"square_root +/-\n")
printf(1,"... ")
a.prntf(0)
a = a.complement()
printf(1,"... ")
a.prntf(0)
c = a.square()
printf(1,"square_root^2\n")
printf(1," ")
c.prntf(0)
r = c.crat(1)
if q[1] * r[2] - r[1] * q[2] then
printf(1,"fail: square_root^2\n")
end if
end if
printf(1,"\n")
end for

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import "/dynamic" for Struct
import "/big" for BigInt
// constants
var EMX = 64 // exponent maximum (if indexing starts at -EMX)
var AMX = 6000 // argument maximum
var PMAX = 32749 // prime maximum
// global variables
var P1 = 0
var P = 7 // default prime
var K = 11 // precision
var Ratio = Struct.create("Ratio", ["a", "b"])
class Padic {
// uninitialized
construct new() {
_v = 0
_d = List.filled(2 * EMX, 0) // add EMX to index to be consistent wih FB
}
// properties
v { _v }
v=(o) { _v = o }
d { _d }
// (re)initialize 'this' to the square root of a Ratio, set 'sw' to print
sqrt(g, sw) {
var a = g.a
var b = g.b
if (b == 0) return 1
if (b < 0) {
b = -b
a = -a
}
if (P < 2 || K < 1) return 1
P = P.min(PMAX) // maximum short prime
if (sw != 0) {
System.write("%(a)/%(b) + ") // numerator, denominator
System.print("0(%(P)^%(K))") // prime, precision
}
// (re)initialize
_v = 0
P1 = P - 1
_d = List.filled(2 * EMX, 0)
if (a == 0) return 0
//valuation
while (b%P== 0) {
b = (b/P).truncate
_v = _v - 1
}
while (a%P == 0) {
a = (a/P).truncate
_v = _v + 1
}
if ((_v & 1) == 1) {
// odd valuation
System.print("non-residue mod %(P)")
return -1
}
K = (K + _v).min(EMX - 1) - _v // maximum array length
_v = (_v/2).truncate
if (a.abs > AMX || b > AMX) return -1
var bb = BigInt.new(b) // to avoid overflowing 'f(x) = b * x * x – a'
var r
var s
var t
var f
var f1
if (P == 2) {
t = a * b
if ((t & 7) - 1 != 0) {
System.print("non-residue mod 8")
return -1
}
} else {
// find root for small P
r = 1
while (r <= P1) {
f = bb * r * r - a
if ((f % P) == 0) break
r = r + 1
}
if (r == P) {
System.print("non-residue mod %(P)")
return -1
}
t = 2 * b * r
s = 0
t = t % P
// modular inverse for small P
f1 = 1
while (f1 <= P1) {
s = s + t
if (s > P1) s = s - P
if (s == 1) break
f1 = f1 + 1
}
if (f1 == P) {
System.print("impossible inverse mod")
return -1
}
}
var x
var pk
var q
var i
if (P == 2) {
// initialize
x = 1
_d[_v+EMX] = 1
_d[_v+1+EMX] = 0
pk = 4
i = _v + 2
while (i <= K - 1 + _v) {
pk = pk * 2
f = bb * x * x - a
q = f / pk
// overflow
if (f != q * pk) break
// next digit
_d[i+EMX] = ((q & 1) != 0) ? 1 : 0
// lift x
x = x + _d[i+EMX]*(pk >> 1)
i = i + 1
}
} else {
f1 = P - f1
x = r
_d[_v+EMX] = x
pk = 1
i = _v + 1
while (i <= K - 1 + _v) {
pk = pk * P
f = bb * x * x - a
q = f / pk
// overflow
if (f != q * pk) break
_d[i+EMX] = q.toSmall * f1 % P
if (_d[i+EMX] < 0) _d[i+EMX] = _d[i+EMX] + P
x = x + _d[i+EMX]*pk
i = i + 1
}
}
K = i - _v
if (sw != 0) System.print("lift: %(x) mod %(P)^%(K)")
return 0
}
// rational reconstruction
crat(sw) {
var t = _v.min(0)
// weighted digit sum
var s = 0
var pk = 1
for (i in t..K-1+_v) {
P1 = pk
pk = pk * P
if (((pk/P1).truncate - P) != 0) {
// overflow
pk = p1
break
}
s = s + _d[i+EMX]*P1
}
// lattice basis reduction
var m = [pk, s]
var n = [0, 1]
var i = 0
var j = 1
s = s * s + 1
// Lagrange's algorithm
while (true) {
var f = (m[i] * m[j] + n[i] * n[j]) / s
// Euclidean step
var q = (f + 0.5).floor
m[i] = m[i] - q*m[j]
n[i] = n[i] - q*n[j]
q = s
s = m[i] * m[i] + n[i] * n[i]
// compare norms
if (s < q) {
// interchange vectors
var z = i
i = j
j = z
} else {
break
}
}
var x = m[j]
var y = n[j]
if (y < 0) {
y = -y
x = -x
}
// check determinant
t = (m[i]*y - x*n[i]).abs == pk
if (!t) {
System.print("crat: fail")
x = 0
y = 1
} else {
// negative powers
var i = _v
while (i <= -1) {
y = y * P
i = i + 1
}
if (sw != 0) {
System.write(x)
if (y > 1) System.write("/%(y)")
System.print()
}
}
return Ratio.new(x, y)
}
// print expansion
printf(sw) {
var t = _v.min(0)
for (i in K - 1 + t..t) {
System.write(_d[i + EMX])
if (i == 0 && _v < 0) System.write(".")
System.write(" ")
}
System.print()
// rational approximation
if (sw != 0) crat(sw)
}
// complement
cmpt {
var c = 1
var r = Padic.new()
r.v = _v
for (i in r.v..K + r.v) {
c = c + P1 - _d[i+EMX]
if (c > P1) {
r.d[i+EMX] = c - P
c = 1
} else {
r.d[i+EMX] = c
c = 0
}
}
return r
}
// square
sqr {
var c = 0
var r = Padic.new()
r.v = _v * 2
for (i in 0..K) {
for (j in 0..i) c = c + _d[_v+j+EMX] * _d[_v+i-j+EMX]
// Euclidean step
var q = (c/P).truncate
r.d[r.v+i+EMX] = c - q*P
c = q
}
return r
}
}
var data = [
[-7, 1, 2, 7],
[9, 1, 2, 8],
[17, 1, 2, 9],
[497, 10496, 2, 18],
[10496, 497, 2, 19],
[3141, 5926, 3, 15],
[2718, 281, 3, 13],
[-1, 1, 5, 8],
[86, 25, 5, 8],
[2150, 1, 5, 8],
[2,1, 7, 8],
[-2645, 28518, 7, 9],
[3029, 4821, 7, 9],
[379, 449, 7, 8],
[717, 8, 11, 7],
[1414, 213, 41, 5],
[-255, 256, 257, 3]
]
var sw = 0
var a = Padic.new()
var c = Padic.new()
for (d in data) {
var q = Ratio.new(d[0], d[1])
P = d[2]
K = d[3]
sw = a.sqrt(q, 1)
if (sw == 1) break
if (sw == 0) {
System.print("sqrt +/-")
System.write("...")
a.printf(0)
a = a.cmpt
System.write("...")
a.printf(0)
c = a.sqr
System.print("sqrt^2")
System.write(" ")
c.printf(0)
var r = c.crat(1)
if (q.a * r.b - r.a * q.b != 0) {
System.print("fail: sqrt^2")
}
System.print()
}
}