#136: Start working on database persistence

vendor/github.com/remyoudompheng/bigfft/scan.go

@@ -0,0 +1,70 @@
+package bigfft
+
+import (
+	"math/big"
+)
+
+// FromDecimalString converts the base 10 string
+// representation of a natural (non-negative) number
+// into a *big.Int.
+// Its asymptotic complexity is less than quadratic.
+func FromDecimalString(s string) *big.Int {
+	var sc scanner
+	z := new(big.Int)
+	sc.scan(z, s)
+	return z
+}
+
+type scanner struct {
+	// powers[i] is 10^(2^i * quadraticScanThreshold).
+	powers []*big.Int
+}
+
+func (s *scanner) chunkSize(size int) (int, *big.Int) {
+	if size <= quadraticScanThreshold {
+		panic("size < quadraticScanThreshold")
+	}
+	pow := uint(0)
+	for n := size; n > quadraticScanThreshold; n /= 2 {
+		pow++
+	}
+	// threshold * 2^(pow-1) <= size < threshold * 2^pow
+	return quadraticScanThreshold << (pow - 1), s.power(pow - 1)
+}
+
+func (s *scanner) power(k uint) *big.Int {
+	for i := len(s.powers); i <= int(k); i++ {
+		z := new(big.Int)
+		if i == 0 {
+			if quadraticScanThreshold%14 != 0 {
+				panic("quadraticScanThreshold % 14 != 0")
+			}
+			z.Exp(big.NewInt(1e14), big.NewInt(quadraticScanThreshold/14), nil)
+		} else {
+			z.Mul(s.powers[i-1], s.powers[i-1])
+		}
+		s.powers = append(s.powers, z)
+	}
+	return s.powers[k]
+}
+
+func (s *scanner) scan(z *big.Int, str string) {
+	if len(str) <= quadraticScanThreshold {
+		z.SetString(str, 10)
+		return
+	}
+	sz, pow := s.chunkSize(len(str))
+	// Scan the left half.
+	s.scan(z, str[:len(str)-sz])
+	// FIXME: reuse temporaries.
+	left := Mul(z, pow)
+	// Scan the right half
+	s.scan(z, str[len(str)-sz:])
+	z.Add(z, left)
+}
+
+// quadraticScanThreshold is the number of digits
+// below which big.Int.SetString is more efficient
+// than subquadratic algorithms.
+// 1232 digits fit in 4096 bits.
+const quadraticScanThreshold = 1232