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constantine
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Fix carry and modulus offset in bigint

This commit is contained in:
Mamy André-Ratsimbazafy 2020-02-10 02:58:37 +01:00
parent 3033ae30f8
commit ade919b003
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2 changed files with 10 additions and 13 deletions
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7
constantine/bigints.nim
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@ -256,12 +256,9 @@ func shlAddMod[bits](a: var BigInt[bits], c: Word, M: BigInt[bits]) =
block: # q*p
var qp_hi: Word
unsafeExtendedPrecMul(qp_hi, qp_lo, q, M.limbs[i]) # q * p
# assert qp_lo.isMsbSet.not.bool
# assert carry.isMsbSet.not.bool
qp_lo += carry # Add carry from previous limb
let qp_carry = qp_lo.isMsbSet
carry = mux(qp_carry, qp_hi + One, qp_hi) # New carry
carry = qp_hi shl 1 + qp_lo.isMsbSet.Word # New carry
qp_lo = qp_lo and MaxWord # Normalize to u63
block: # a*2^63 - q*p
@ -312,5 +309,5 @@ func reduce*[aBits, mBits](r: var BigInt[mBits], a: BigInt[aBits], M: BigInt[mBi
const aOffset = a.limbs.len - M.limbs.len
copyLimbs(r, 0, a, aOffset, M.limbs.len - 1)
r.limbs[^1] = Zero
for i in countdown(aOffset, 0):
for i in countdown(aOffset-1, 0):
r.shlAddMod(a.limbs[i], M)

16
tests/test_bigints.nim
View File

@ -160,29 +160,29 @@ suite "Modular operations - small modulus - Stint specific failures highlighted
let v = 174261910798982'u64
let a = BigInt[64].fromUint(u)
let m = BigInt[48].fromUint(v)
let m = BigInt[49].fromUint(v)
var r: BigInt[48]
var r: BigInt[49]
r.reduce(a, m)
# Copy the result in a conveniently sized buffer
var rr: BigInt[48]
var rr: BigInt[49]
copyLimbs(rr, 0, r, 0, r.limbs.len)
check:
bool(rr == BigInt[48].fromUint(u mod v))
bool(rr == BigInt[49].fromUint(u mod v))
test "Modulo: 15080397990160655 mod 600432699691":
let u = 15080397990160655'u64
let v = 600432699691'u64
let a = BigInt[64].fromUint(u)
let m = BigInt[40].fromUint(v)
let m = BigInt[41].fromUint(v)
var r: BigInt[40]
var r: BigInt[41]
r.reduce(a, m)
# Copy the result in a conveniently sized buffer
var rr: BigInt[40]
var rr: BigInt[41]
copyLimbs(rr, 0, r, 0, r.limbs.len)
check:
bool(rr == BigInt[40].fromUint(u mod v))
bool(rr == BigInt[41].fromUint(u mod v))