1 // Copyright 2009 The Go Authors. All rights reserved.
2 // Use of this source code is governed by a BSD-style
3 // license that can be found in the LICENSE file.
5 // Multiprecision decimal numbers.
6 // For floating-point formatting only; not general purpose.
7 // Only operations are assign and (binary) left/right shift.
8 // Can do binary floating point in multiprecision decimal precisely
9 // because 2 divides 10; cannot do decimal floating point
10 // in multiprecision binary precisely.
16 nd int // number of digits used
17 dp int // decimal point
19 trunc bool // discarded nonzero digits beyond d[:nd]
22 func (a *decimal) String() string {
31 buf := make([]byte, n)
38 // zeros fill space between decimal point and digits
43 w += digitZero(buf[w : w+-a.dp])
44 w += copy(buf[w:], a.d[0:a.nd])
47 // decimal point in middle of digits
48 w += copy(buf[w:], a.d[0:a.dp])
51 w += copy(buf[w:], a.d[a.dp:a.nd])
54 // zeros fill space between digits and decimal point
55 w += copy(buf[w:], a.d[0:a.nd])
56 w += digitZero(buf[w : w+a.dp-a.nd])
58 return string(buf[0:w])
61 func digitZero(dst []byte) int {
68 // trim trailing zeros from number.
69 // (They are meaningless; the decimal point is tracked
70 // independent of the number of digits.)
71 func trim(a *decimal) {
72 for a.nd > 0 && a.d[a.nd-1] == '0' {
81 func (a *decimal) Assign(v uint64) {
84 // Write reversed decimal in buf.
89 buf[n] = byte(v + '0')
94 // Reverse again to produce forward decimal in a.d.
96 for n--; n >= 0; n-- {
104 // Maximum shift that we can do in one pass without overflow.
105 // Signed int has 31 bits, and we have to be able to accommodate 9<<k.
108 // Binary shift right (* 2) by k bits. k <= maxShift to avoid overflow.
109 func rightShift(a *decimal, k uint) {
110 r := 0 // read pointer
111 w := 0 // write pointer
113 // Pick up enough leading digits to cover first shift.
115 for ; n>>k == 0; r++ {
118 // a == 0; shouldn't get here, but handle anyway.
133 // Pick up a digit, put down a digit.
134 for ; r < a.nd; r++ {
138 a.d[w] = byte(dig + '0')
143 // Put down extra digits.
148 a.d[w] = byte(dig + '0')
160 // Cheat sheet for left shift: table indexed by shift count giving
161 // number of new digits that will be introduced by that shift.
163 // For example, leftcheats[4] = {2, "625"}. That means that
164 // if we are shifting by 4 (multiplying by 16), it will add 2 digits
165 // when the string prefix is "625" through "999", and one fewer digit
166 // if the string prefix is "000" through "624".
168 // Credit for this trick goes to Ken.
170 type leftCheat struct {
171 delta int // number of new digits
172 cutoff string // minus one digit if original < a.
175 var leftcheats = []leftCheat{
176 // Leading digits of 1/2^i = 5^i.
177 // 5^23 is not an exact 64-bit floating point number,
178 // so have to use bc for the math.
180 seq 27 | sed 's/^/5^/' | bc |
181 awk 'BEGIN{ print "\tleftCheat{ 0, \"\" }," }
183 log2 = log(2)/log(10)
184 printf("\tleftCheat{ %d, \"%s\" },\t// * %d\n",
185 int(log2*NR+1), $0, 2**NR)
194 {2, "15625"}, // * 64
195 {3, "78125"}, // * 128
196 {3, "390625"}, // * 256
197 {3, "1953125"}, // * 512
198 {4, "9765625"}, // * 1024
199 {4, "48828125"}, // * 2048
200 {4, "244140625"}, // * 4096
201 {4, "1220703125"}, // * 8192
202 {5, "6103515625"}, // * 16384
203 {5, "30517578125"}, // * 32768
204 {5, "152587890625"}, // * 65536
205 {6, "762939453125"}, // * 131072
206 {6, "3814697265625"}, // * 262144
207 {6, "19073486328125"}, // * 524288
208 {7, "95367431640625"}, // * 1048576
209 {7, "476837158203125"}, // * 2097152
210 {7, "2384185791015625"}, // * 4194304
211 {7, "11920928955078125"}, // * 8388608
212 {8, "59604644775390625"}, // * 16777216
213 {8, "298023223876953125"}, // * 33554432
214 {8, "1490116119384765625"}, // * 67108864
215 {9, "7450580596923828125"}, // * 134217728
218 // Is the leading prefix of b lexicographically less than s?
219 func prefixIsLessThan(b []byte, s string) bool {
220 for i := 0; i < len(s); i++ {
231 // Binary shift left (/ 2) by k bits. k <= maxShift to avoid overflow.
232 func leftShift(a *decimal, k uint) {
233 delta := leftcheats[k].delta
234 if prefixIsLessThan(a.d[0:a.nd], leftcheats[k].cutoff) {
238 r := a.nd // read index
239 w := a.nd + delta // write index
242 // Pick up a digit, put down a digit.
243 for r--; r >= 0; r-- {
244 n += (int(a.d[r]) - '0') << k
249 a.d[w] = byte(rem + '0')
256 // Put down extra digits.
262 a.d[w] = byte(rem + '0')
270 if a.nd >= len(a.d) {
277 // Binary shift left (k > 0) or right (k < 0).
278 func (a *decimal) Shift(k int) {
281 // nothing to do: a == 0
284 leftShift(a, maxShift)
287 leftShift(a, uint(k))
290 rightShift(a, maxShift)
293 rightShift(a, uint(-k))
297 // If we chop a at nd digits, should we round up?
298 func shouldRoundUp(a *decimal, nd int) bool {
299 if nd < 0 || nd >= a.nd {
302 if a.d[nd] == '5' && nd+1 == a.nd { // exactly halfway - round to even
303 // if we truncated, a little higher than what's recorded - always round up
307 return nd > 0 && (a.d[nd-1]-'0')%2 != 0
309 // not halfway - digit tells all
310 return a.d[nd] >= '5'
313 // Round a to nd digits (or fewer).
314 // If nd is zero, it means we're rounding
315 // just to the left of the digits, as in
317 func (a *decimal) Round(nd int) {
318 if nd < 0 || nd >= a.nd {
321 if shouldRoundUp(a, nd) {
328 // Round a down to nd digits (or fewer).
329 func (a *decimal) RoundDown(nd int) {
330 if nd < 0 || nd >= a.nd {
337 // Round a up to nd digits (or fewer).
338 func (a *decimal) RoundUp(nd int) {
339 if nd < 0 || nd >= a.nd {
344 for i := nd - 1; i >= 0; i-- {
346 if c < '9' { // can stop after this digit
354 // Change to single 1 with adjusted decimal point.
360 // Extract integer part, rounded appropriately.
361 // No guarantees about overflow.
362 func (a *decimal) RoundedInteger() uint64 {
364 return 0xFFFFFFFFFFFFFFFF
368 for i = 0; i < a.dp && i < a.nd; i++ {
369 n = n*10 + uint64(a.d[i]-'0')
371 for ; i < a.dp; i++ {
374 if shouldRoundUp(a, a.dp) {