1 files changed, 392 insertions, 0 deletions
diff --git a/src/pkg/strconv/decimal.go b/src/pkg/strconv/decimal.go
new file mode 100644
index 000000000..38d9c47fb
--- /dev/null
+++ b/src/pkg/strconv/decimal.go
@@ -0,0 +1,392 @@
+// 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.
+
+// Multiprecision decimal numbers.
+// For floating-point formatting only; not general purpose.
+// Only operations are assign and (binary) left/right shift.
+// Can do binary floating point in multiprecision decimal precisely
+// because 2 divides 10; cannot do decimal floating point
+// in multiprecision binary precisely.
+
+package strconv
+
+import "bytes"
+
+type decimal struct {
+	// TODO(rsc): Can make d[] a bit smaller and add
+	// truncated bool;
+	d [2000] byte;	// digits
+	nd int;	// number of digits used
+	dp int;	// decimal point
+};
+func (a *decimal) String() string;
+func (a *decimal) Assign(v uint64);
+func (a *decimal) Shift(k int) *decimal;
+func (a *decimal) Round(nd int) *decimal;
+func (a *decimal) RoundUp(nd int) *decimal;
+func (a *decimal) RoundDown(nd int) *decimal;
+func (a *decimal) RoundedInteger() uint64;
+
+
+func digitZero(dst []byte) int;
+
+func (a *decimal) String() string {
+	n := 10 + a.nd;
+	if a.dp > 0 {
+		n += a.dp;
+	}
+	if a.dp < 0 {
+		n += -a.dp;
+	}
+
+	buf := make([]byte, n);
+	w := 0;
+	switch {
+	case a.nd == 0:
+		return "0";
+
+	case a.dp <= 0:
+		// zeros fill space between decimal point and digits
+		buf[w] = '0';
+		w++;
+		buf[w] = '.';
+		w++;
+		w += digitZero(buf[w:w+-a.dp]);
+		w += bytes.Copy(buf[w:w+a.nd], a.d[0:a.nd]);
+
+	case a.dp < a.nd:
+		// decimal point in middle of digits
+		w += bytes.Copy(buf[w:w+a.dp], a.d[0:a.dp]);
+		buf[w] = '.';
+		w++;
+		w += bytes.Copy(buf[w:w+a.nd-a.dp], a.d[a.dp:a.nd]);
+
+	default:
+		// zeros fill space between digits and decimal point
+		w += bytes.Copy(buf[w:w+a.nd], a.d[0:a.nd]);
+		w += digitZero(buf[w:w+a.dp-a.nd]);
+	}
+	return string(buf[0:w]);
+}
+
+func copy(dst []byte, src []byte) int {
+	for i := 0; i < len(dst); i++ {
+		dst[i] = src[i];
+	}
+	return len(dst);
+}
+
+func digitZero(dst []byte) int {
+	for i := 0; i < len(dst); i++ {
+		dst[i] = '0';
+	}
+	return len(dst);
+}
+
+// trim trailing zeros from number.
+// (They are meaningless; the decimal point is tracked
+// independent of the number of digits.)
+func trim(a *decimal) {
+	for a.nd > 0 && a.d[a.nd-1] == '0' {
+		a.nd--;
+	}
+	if a.nd == 0 {
+		a.dp = 0;
+	}
+}
+
+// Assign v to a.
+func (a *decimal) Assign(v uint64) {
+	var buf [50]byte;
+
+	// Write reversed decimal in buf.
+	n := 0;
+	for v > 0 {
+		v1 := v/10;
+		v -= 10*v1;
+		buf[n] = byte(v + '0');
+		n++;
+		v = v1;
+	}
+
+	// Reverse again to produce forward decimal in a.d.
+	a.nd = 0;
+	for n--; n>=0; n-- {
+		a.d[a.nd] = buf[n];
+		a.nd++;
+	}
+	a.dp = a.nd;
+	trim(a);
+}
+
+func newDecimal(i uint64) *decimal {
+	a := new(decimal);
+	a.Assign(i);
+	return a;
+}
+
+// Maximum shift that we can do in one pass without overflow.
+// Signed int has 31 bits, and we have to be able to accomodate 9<<k.
+const maxShift = 27
+
+// Binary shift right (* 2) by k bits.  k <= maxShift to avoid overflow.
+func rightShift(a *decimal, k uint) {
+	r := 0;	// read pointer
+	w := 0;	// write pointer
+
+	// Pick up enough leading digits to cover first shift.
+	n := 0;
+	for ; n>>k == 0; r++ {
+		if r >= a.nd {
+			if n == 0 {
+				// a == 0; shouldn't get here, but handle anyway.
+				a.nd = 0;
+				return;
+			}
+			for n>>k == 0 {
+				n = n*10;
+				r++;
+			}
+			break;
+		}
+		c := int(a.d[r]);
+		n = n*10 + c-'0';
+	}
+	a.dp -= r-1;
+
+	// Pick up a digit, put down a digit.
+	for ; r < a.nd; r++ {
+		c := int(a.d[r]);
+		dig := n>>k;
+		n -= dig<<k;
+		a.d[w] = byte(dig+'0');
+		w++;
+		n = n*10 + c-'0';
+	}
+
+	// Put down extra digits.
+	for n > 0 {
+		dig := n>>k;
+		n -= dig<<k;
+		a.d[w] = byte(dig+'0');
+		w++;
+		n = n*10;
+	}
+
+	a.nd = w;
+	trim(a);
+}
+
+// Cheat sheet for left shift: table indexed by shift count giving
+// number of new digits that will be introduced by that shift.
+//
+// For example, leftcheats[4] = {2, "625"}.  That means that
+// if we are shifting by 4 (multiplying by 16), it will add 2 digits
+// when the string prefix is "625" through "999", and one fewer digit
+// if the string prefix is "000" through "624".
+//
+// Credit for this trick goes to Ken.
+
+type leftCheat struct {
+	delta int;	// number of new digits
+	cutoff string;	//   minus one digit if original < a.
+}
+
+var leftcheats = []leftCheat {
+	// Leading digits of 1/2^i = 5^i.
+	// 5^23 is not an exact 64-bit floating point number,
+	// so have to use bc for the math.
+	/*
+	seq 27 | sed 's/^/5^/' | bc |
+	awk 'BEGIN{ print "\tleftCheat{ 0, \"\" }," }
+	{
+		log2 = log(2)/log(10)
+		printf("\tleftCheat{ %d, \"%s\" },\t// * %d\n",
+			int(log2*NR+1), $0, 2**NR)
+	}'
+	 */
+	leftCheat{ 0, "" },
+	leftCheat{ 1, "5" },	// * 2
+	leftCheat{ 1, "25" },	// * 4
+	leftCheat{ 1, "125" },	// * 8
+	leftCheat{ 2, "625" },	// * 16
+	leftCheat{ 2, "3125" },	// * 32
+	leftCheat{ 2, "15625" },	// * 64
+	leftCheat{ 3, "78125" },	// * 128
+	leftCheat{ 3, "390625" },	// * 256
+	leftCheat{ 3, "1953125" },	// * 512
+	leftCheat{ 4, "9765625" },	// * 1024
+	leftCheat{ 4, "48828125" },	// * 2048
+	leftCheat{ 4, "244140625" },	// * 4096
+	leftCheat{ 4, "1220703125" },	// * 8192
+	leftCheat{ 5, "6103515625" },	// * 16384
+	leftCheat{ 5, "30517578125" },	// * 32768
+	leftCheat{ 5, "152587890625" },	// * 65536
+	leftCheat{ 6, "762939453125" },	// * 131072
+	leftCheat{ 6, "3814697265625" },	// * 262144
+	leftCheat{ 6, "19073486328125" },	// * 524288
+	leftCheat{ 7, "95367431640625" },	// * 1048576
+	leftCheat{ 7, "476837158203125" },	// * 2097152
+	leftCheat{ 7, "2384185791015625" },	// * 4194304
+	leftCheat{ 7, "11920928955078125" },	// * 8388608
+	leftCheat{ 8, "59604644775390625" },	// * 16777216
+	leftCheat{ 8, "298023223876953125" },	// * 33554432
+	leftCheat{ 8, "1490116119384765625" },	// * 67108864
+	leftCheat{ 9, "7450580596923828125" },	// * 134217728
+}
+
+// Is the leading prefix of b lexicographically less than s?
+func prefixIsLessThan(b []byte, s string) bool {
+	for i := 0; i < len(s); i++ {
+		if i >= len(b) {
+			return true;
+		}
+		if b[i] != s[i] {
+			return b[i] < s[i];
+		}
+	}
+	return false;
+}
+
+// Binary shift left (/ 2) by k bits.  k <= maxShift to avoid overflow.
+func leftShift(a *decimal, k uint) {
+	delta := leftcheats[k].delta;
+	if prefixIsLessThan(a.d[0:a.nd], leftcheats[k].cutoff) {
+		delta--;
+	}
+
+	r := a.nd;	// read index
+	w := a.nd + delta;	// write index
+	n := 0;
+
+	// Pick up a digit, put down a digit.
+	for r--; r >= 0; r-- {
+		n += (int(a.d[r])-'0') << k;
+		quo := n/10;
+		rem := n - 10*quo;
+		w--;
+		a.d[w] = byte(rem+'0');
+		n = quo;
+	}
+
+	// Put down extra digits.
+	for n > 0 {
+		quo := n/10;
+		rem := n - 10*quo;
+		w--;
+		a.d[w] = byte(rem+'0');
+		n = quo;
+	}
+
+	if w != 0 {
+		// TODO: Remove - has no business panicking.
+		panicln("strconv: bad leftShift", w);
+	}
+	a.nd += delta;
+	a.dp += delta;
+	trim(a);
+}
+
+// Binary shift left (k > 0) or right (k < 0).
+// Returns receiver for convenience.
+func (a *decimal) Shift(k int) *decimal {
+	switch {
+	case a.nd == 0:
+		// nothing to do: a == 0
+	case k > 0:
+		for k > maxShift {
+			leftShift(a, maxShift);
+			k -= maxShift;
+		}
+		leftShift(a, uint(k));
+	case k < 0:
+		for k < -maxShift {
+			rightShift(a, maxShift);
+			k += maxShift;
+		}
+		rightShift(a, uint(-k));
+	}
+	return a;
+}
+
+// If we chop a at nd digits, should we round up?
+func shouldRoundUp(a *decimal, nd int) bool {
+	if nd <= 0 || nd >= a.nd {
+		return false;
+	}
+	if a.d[nd] == '5' && nd+1 == a.nd {	// exactly halfway - round to even
+		return (a.d[nd-1] - '0') % 2 != 0;
+	}
+	// not halfway - digit tells all
+	return a.d[nd] >= '5';
+}
+
+// Round a to nd digits (or fewer).
+// Returns receiver for convenience.
+func (a *decimal) Round(nd int) *decimal {
+	if nd <= 0 || nd >= a.nd {
+		return a;
+	}
+	if(shouldRoundUp(a, nd)) {
+		return a.RoundUp(nd);
+	}
+	return a.RoundDown(nd);
+}
+
+// Round a down to nd digits (or fewer).
+// Returns receiver for convenience.
+func (a *decimal) RoundDown(nd int) *decimal {
+	if nd <= 0 || nd >= a.nd {
+		return a;
+	}
+	a.nd = nd;
+	trim(a);
+	return a;
+}
+
+// Round a up to nd digits (or fewer).
+// Returns receiver for convenience.
+func (a *decimal) RoundUp(nd int) *decimal {
+	if nd <= 0 || nd >= a.nd {
+		return a;
+	}
+
+	// round up
+	for i := nd-1; i >= 0; i-- {
+		c := a.d[i];
+		if c < '9' {	 // can stop after this digit
+			a.d[i]++;
+			a.nd = i+1;
+			return a;
+		}
+	}
+
+	// Number is all 9s.
+	// Change to single 1 with adjusted decimal point.
+	a.d[0] = '1';
+	a.nd = 1;
+	a.dp++;
+	return a;
+}
+
+// Extract integer part, rounded appropriately.
+// No guarantees about overflow.
+func (a *decimal) RoundedInteger() uint64 {
+	if a.dp > 20 {
+		return 0xFFFFFFFFFFFFFFFF;
+	}
+	var i int;
+	n := uint64(0);
+	for i = 0; i < a.dp && i < a.nd; i++ {
+		n = n*10 + uint64(a.d[i] - '0');
+	}
+	for ; i < a.dp; i++ {
+		n *= 10;
+	}
+	if shouldRoundUp(a, a.dp) {
+		n++;
+	}
+	return n;
+}
+