From 1a57267a17c2fc17fb6e104846fabc3e363c326c Mon Sep 17 00:00:00 2001
From: Emile <git@emile.space>
Date: Fri, 16 Aug 2024 19:50:26 +0200
Subject: initial commit

---
 vendor/github.com/remyoudompheng/bigfft/fermat.go | 216 ++++++++++++++++++++++
 1 file changed, 216 insertions(+)
 create mode 100644 vendor/github.com/remyoudompheng/bigfft/fermat.go

(limited to 'vendor/github.com/remyoudompheng/bigfft/fermat.go')

diff --git a/vendor/github.com/remyoudompheng/bigfft/fermat.go b/vendor/github.com/remyoudompheng/bigfft/fermat.go
new file mode 100644
index 0000000..200ee57
--- /dev/null
+++ b/vendor/github.com/remyoudompheng/bigfft/fermat.go
@@ -0,0 +1,216 @@
+package bigfft
+
+import (
+	"math/big"
+)
+
+// Arithmetic modulo 2^n+1.
+
+// A fermat of length w+1 represents a number modulo 2^(w*_W) + 1. The last
+// word is zero or one. A number has at most two representatives satisfying the
+// 0-1 last word constraint.
+type fermat nat
+
+func (n fermat) String() string { return nat(n).String() }
+
+func (z fermat) norm() {
+	n := len(z) - 1
+	c := z[n]
+	if c == 0 {
+		return
+	}
+	if z[0] >= c {
+		z[n] = 0
+		z[0] -= c
+		return
+	}
+	// z[0] < z[n].
+	subVW(z, z, c) // Substract c
+	if c > 1 {
+		z[n] -= c - 1
+		c = 1
+	}
+	// Add back c.
+	if z[n] == 1 {
+		z[n] = 0
+		return
+	} else {
+		addVW(z, z, 1)
+	}
+}
+
+// Shift computes (x << k) mod (2^n+1).
+func (z fermat) Shift(x fermat, k int) {
+	if len(z) != len(x) {
+		panic("len(z) != len(x) in Shift")
+	}
+	n := len(x) - 1
+	// Shift by n*_W is taking the opposite.
+	k %= 2 * n * _W
+	if k < 0 {
+		k += 2 * n * _W
+	}
+	neg := false
+	if k >= n*_W {
+		k -= n * _W
+		neg = true
+	}
+
+	kw, kb := k/_W, k%_W
+
+	z[n] = 1 // Add (-1)
+	if !neg {
+		for i := 0; i < kw; i++ {
+			z[i] = 0
+		}
+		// Shift left by kw words.
+		// x = a·2^(n-k) + b
+		// x<<k = (b<<k) - a
+		copy(z[kw:], x[:n-kw])
+		b := subVV(z[:kw+1], z[:kw+1], x[n-kw:])
+		if z[kw+1] > 0 {
+			z[kw+1] -= b
+		} else {
+			subVW(z[kw+1:], z[kw+1:], b)
+		}
+	} else {
+		for i := kw + 1; i < n; i++ {
+			z[i] = 0
+		}
+		// Shift left and negate, by kw words.
+		copy(z[:kw+1], x[n-kw:n+1])            // z_low = x_high
+		b := subVV(z[kw:n], z[kw:n], x[:n-kw]) // z_high -= x_low
+		z[n] -= b
+	}
+	// Add back 1.
+	if z[n] > 0 {
+		z[n]--
+	} else if z[0] < ^big.Word(0) {
+		z[0]++
+	} else {
+		addVW(z, z, 1)
+	}
+	// Shift left by kb bits
+	shlVU(z, z, uint(kb))
+	z.norm()
+}
+
+// ShiftHalf shifts x by k/2 bits the left. Shifting by 1/2 bit
+// is multiplication by sqrt(2) mod 2^n+1 which is 2^(3n/4) - 2^(n/4).
+// A temporary buffer must be provided in tmp.
+func (z fermat) ShiftHalf(x fermat, k int, tmp fermat) {
+	n := len(z) - 1
+	if k%2 == 0 {
+		z.Shift(x, k/2)
+		return
+	}
+	u := (k - 1) / 2
+	a := u + (3*_W/4)*n
+	b := u + (_W/4)*n
+	z.Shift(x, a)
+	tmp.Shift(x, b)
+	z.Sub(z, tmp)
+}
+
+// Add computes addition mod 2^n+1.
+func (z fermat) Add(x, y fermat) fermat {
+	if len(z) != len(x) {
+		panic("Add: len(z) != len(x)")
+	}
+	addVV(z, x, y) // there cannot be a carry here.
+	z.norm()
+	return z
+}
+
+// Sub computes substraction mod 2^n+1.
+func (z fermat) Sub(x, y fermat) fermat {
+	if len(z) != len(x) {
+		panic("Add: len(z) != len(x)")
+	}
+	n := len(y) - 1
+	b := subVV(z[:n], x[:n], y[:n])
+	b += y[n]
+	// If b > 0, we need to subtract b<<n, which is the same as adding b.
+	z[n] = x[n]
+	if z[0] <= ^big.Word(0)-b {
+		z[0] += b
+	} else {
+		addVW(z, z, b)
+	}
+	z.norm()
+	return z
+}
+
+func (z fermat) Mul(x, y fermat) fermat {
+	if len(x) != len(y) {
+		panic("Mul: len(x) != len(y)")
+	}
+	n := len(x) - 1
+	if n < 30 {
+		z = z[:2*n+2]
+		basicMul(z, x, y)
+		z = z[:2*n+1]
+	} else {
+		var xi, yi, zi big.Int
+		xi.SetBits(x)
+		yi.SetBits(y)
+		zi.SetBits(z)
+		zb := zi.Mul(&xi, &yi).Bits()
+		if len(zb) <= n {
+			// Short product.
+			copy(z, zb)
+			for i := len(zb); i < len(z); i++ {
+				z[i] = 0
+			}
+			return z
+		}
+		z = zb
+	}
+	// len(z) is at most 2n+1.
+	if len(z) > 2*n+1 {
+		panic("len(z) > 2n+1")
+	}
+	// We now have
+	// z = z[:n] + 1<<(n*W) * z[n:2n+1]
+	// which normalizes to:
+	// z = z[:n] - z[n:2n] + z[2n]
+	c1 := big.Word(0)
+	if len(z) > 2*n {
+		c1 = addVW(z[:n], z[:n], z[2*n])
+	}
+	c2 := big.Word(0)
+	if len(z) >= 2*n {
+		c2 = subVV(z[:n], z[:n], z[n:2*n])
+	} else {
+		m := len(z) - n
+		c2 = subVV(z[:m], z[:m], z[n:])
+		c2 = subVW(z[m:n], z[m:n], c2)
+	}
+	// Restore carries.
+	// Substracting z[n] -= c2 is the same
+	// as z[0] += c2
+	z = z[:n+1]
+	z[n] = c1
+	c := addVW(z, z, c2)
+	if c != 0 {
+		panic("impossible")
+	}
+	z.norm()
+	return z
+}
+
+// copied from math/big
+//
+// basicMul multiplies x and y and leaves the result in z.
+// The (non-normalized) result is placed in z[0 : len(x) + len(y)].
+func basicMul(z, x, y fermat) {
+	// initialize z
+	for i := 0; i < len(z); i++ {
+		z[i] = 0
+	}
+	for i, d := range y {
+		if d != 0 {
+			z[len(x)+i] = addMulVVW(z[i:i+len(x)], x, d)
+		}
+	}
+}
-- 
cgit 1.4.1