1 files changed, 74 insertions, 0 deletions
diff --git a/misc/cgo/gmp/pi.go b/misc/cgo/gmp/pi.go
new file mode 100644
index 0000000..5ea0349
--- /dev/null
+++ b/misc/cgo/gmp/pi.go
@@ -0,0 +1,74 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build ignore
+// +build ignore
+
+package main
+
+import (
+	big "."
+	"fmt"
+	"runtime"
+)
+
+var (
+	tmp1  = big.NewInt(0)
+	tmp2  = big.NewInt(0)
+	numer = big.NewInt(1)
+	accum = big.NewInt(0)
+	denom = big.NewInt(1)
+	ten   = big.NewInt(10)
+)
+
+func extractDigit() int64 {
+	if big.CmpInt(numer, accum) > 0 {
+		return -1
+	}
+	tmp1.Lsh(numer, 1).Add(tmp1, numer).Add(tmp1, accum)
+	big.DivModInt(tmp1, tmp2, tmp1, denom)
+	tmp2.Add(tmp2, numer)
+	if big.CmpInt(tmp2, denom) >= 0 {
+		return -1
+	}
+	return tmp1.Int64()
+}
+
+func nextTerm(k int64) {
+	y2 := k*2 + 1
+	accum.Add(accum, tmp1.Lsh(numer, 1))
+	accum.Mul(accum, tmp1.SetInt64(y2))
+	numer.Mul(numer, tmp1.SetInt64(k))
+	denom.Mul(denom, tmp1.SetInt64(y2))
+}
+
+func eliminateDigit(d int64) {
+	accum.Sub(accum, tmp1.Mul(denom, tmp1.SetInt64(d)))
+	accum.Mul(accum, ten)
+	numer.Mul(numer, ten)
+}
+
+func main() {
+	i := 0
+	k := int64(0)
+	for {
+		d := int64(-1)
+		for d < 0 {
+			k++
+			nextTerm(k)
+			d = extractDigit()
+		}
+		eliminateDigit(d)
+		fmt.Printf("%c", d+'0')
+
+		if i++; i%50 == 0 {
+			fmt.Printf("\n")
+			if i >= 1000 {
+				break
+			}
+		}
+	}
+
+	fmt.Printf("\n%d calls; bit sizes: %d %d %d\n", runtime.NumCgoCall(), numer.Len(), accum.Len(), denom.Len())
+}