Source file src/pkg/strconv/decimal.go

     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.
     4	
     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.
    11	
    12	package strconv
    13	
    14	type decimal struct {
    15		d     [800]byte // digits, big-endian representation
    16		nd    int       // number of digits used
    17		dp    int       // decimal point
    18		neg   bool      // negative flag
    19		trunc bool      // discarded nonzero digits beyond d[:nd]
    20	}
    21	
    22	func (a *decimal) String() string {
    23		n := 10 + a.nd
    24		if a.dp > 0 {
    25			n += a.dp
    26		}
    27		if a.dp < 0 {
    28			n += -a.dp
    29		}
    30	
    31		buf := make([]byte, n)
    32		w := 0
    33		switch {
    34		case a.nd == 0:
    35			return "0"
    36	
    37		case a.dp <= 0:
    38			// zeros fill space between decimal point and digits
    39			buf[w] = '0'
    40			w++
    41			buf[w] = '.'
    42			w++
    43			w += digitZero(buf[w : w+-a.dp])
    44			w += copy(buf[w:], a.d[0:a.nd])
    45	
    46		case a.dp < a.nd:
    47			// decimal point in middle of digits
    48			w += copy(buf[w:], a.d[0:a.dp])
    49			buf[w] = '.'
    50			w++
    51			w += copy(buf[w:], a.d[a.dp:a.nd])
    52	
    53		default:
    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])
    57		}
    58		return string(buf[0:w])
    59	}
    60	
    61	func digitZero(dst []byte) int {
    62		for i := range dst {
    63			dst[i] = '0'
    64		}
    65		return len(dst)
    66	}
    67	
    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' {
    73			a.nd--
    74		}
    75		if a.nd == 0 {
    76			a.dp = 0
    77		}
    78	}
    79	
    80	// Assign v to a.
    81	func (a *decimal) Assign(v uint64) {
    82		var buf [24]byte
    83	
    84		// Write reversed decimal in buf.
    85		n := 0
    86		for v > 0 {
    87			v1 := v / 10
    88			v -= 10 * v1
    89			buf[n] = byte(v + '0')
    90			n++
    91			v = v1
    92		}
    93	
    94		// Reverse again to produce forward decimal in a.d.
    95		a.nd = 0
    96		for n--; n >= 0; n-- {
    97			a.d[a.nd] = buf[n]
    98			a.nd++
    99		}
   100		a.dp = a.nd
   101		trim(a)
   102	}
   103	
   104	// Maximum shift that we can do in one pass without overflow.
   105	// A uint has 32 or 64 bits, and we have to be able to accommodate 9<<k.
   106	const uintSize = 32 << (^uint(0) >> 63)
   107	const maxShift = uintSize - 4
   108	
   109	// Binary shift right (/ 2) by k bits.  k <= maxShift to avoid overflow.
   110	func rightShift(a *decimal, k uint) {
   111		r := 0 // read pointer
   112		w := 0 // write pointer
   113	
   114		// Pick up enough leading digits to cover first shift.
   115		var n uint
   116		for ; n>>k == 0; r++ {
   117			if r >= a.nd {
   118				if n == 0 {
   119					// a == 0; shouldn't get here, but handle anyway.
   120					a.nd = 0
   121					return
   122				}
   123				for n>>k == 0 {
   124					n = n * 10
   125					r++
   126				}
   127				break
   128			}
   129			c := uint(a.d[r])
   130			n = n*10 + c - '0'
   131		}
   132		a.dp -= r - 1
   133	
   134		var mask uint = (1 << k) - 1
   135	
   136		// Pick up a digit, put down a digit.
   137		for ; r < a.nd; r++ {
   138			c := uint(a.d[r])
   139			dig := n >> k
   140			n &= mask
   141			a.d[w] = byte(dig + '0')
   142			w++
   143			n = n*10 + c - '0'
   144		}
   145	
   146		// Put down extra digits.
   147		for n > 0 {
   148			dig := n >> k
   149			n &= mask
   150			if w < len(a.d) {
   151				a.d[w] = byte(dig + '0')
   152				w++
   153			} else if dig > 0 {
   154				a.trunc = true
   155			}
   156			n = n * 10
   157		}
   158	
   159		a.nd = w
   160		trim(a)
   161	}
   162	
   163	// Cheat sheet for left shift: table indexed by shift count giving
   164	// number of new digits that will be introduced by that shift.
   165	//
   166	// For example, leftcheats[4] = {2, "625"}.  That means that
   167	// if we are shifting by 4 (multiplying by 16), it will add 2 digits
   168	// when the string prefix is "625" through "999", and one fewer digit
   169	// if the string prefix is "000" through "624".
   170	//
   171	// Credit for this trick goes to Ken.
   172	
   173	type leftCheat struct {
   174		delta  int    // number of new digits
   175		cutoff string // minus one digit if original < a.
   176	}
   177	
   178	var leftcheats = []leftCheat{
   179		// Leading digits of 1/2^i = 5^i.
   180		// 5^23 is not an exact 64-bit floating point number,
   181		// so have to use bc for the math.
   182		// Go up to 60 to be large enough for 32bit and 64bit platforms.
   183		/*
   184			seq 60 | sed 's/^/5^/' | bc |
   185			awk 'BEGIN{ print "\t{ 0, \"\" }," }
   186			{
   187				log2 = log(2)/log(10)
   188				printf("\t{ %d, \"%s\" },\t// * %d\n",
   189					int(log2*NR+1), $0, 2**NR)
   190			}'
   191		*/
   192		{0, ""},
   193		{1, "5"},                                           // * 2
   194		{1, "25"},                                          // * 4
   195		{1, "125"},                                         // * 8
   196		{2, "625"},                                         // * 16
   197		{2, "3125"},                                        // * 32
   198		{2, "15625"},                                       // * 64
   199		{3, "78125"},                                       // * 128
   200		{3, "390625"},                                      // * 256
   201		{3, "1953125"},                                     // * 512
   202		{4, "9765625"},                                     // * 1024
   203		{4, "48828125"},                                    // * 2048
   204		{4, "244140625"},                                   // * 4096
   205		{4, "1220703125"},                                  // * 8192
   206		{5, "6103515625"},                                  // * 16384
   207		{5, "30517578125"},                                 // * 32768
   208		{5, "152587890625"},                                // * 65536
   209		{6, "762939453125"},                                // * 131072
   210		{6, "3814697265625"},                               // * 262144
   211		{6, "19073486328125"},                              // * 524288
   212		{7, "95367431640625"},                              // * 1048576
   213		{7, "476837158203125"},                             // * 2097152
   214		{7, "2384185791015625"},                            // * 4194304
   215		{7, "11920928955078125"},                           // * 8388608
   216		{8, "59604644775390625"},                           // * 16777216
   217		{8, "298023223876953125"},                          // * 33554432
   218		{8, "1490116119384765625"},                         // * 67108864
   219		{9, "7450580596923828125"},                         // * 134217728
   220		{9, "37252902984619140625"},                        // * 268435456
   221		{9, "186264514923095703125"},                       // * 536870912
   222		{10, "931322574615478515625"},                      // * 1073741824
   223		{10, "4656612873077392578125"},                     // * 2147483648
   224		{10, "23283064365386962890625"},                    // * 4294967296
   225		{10, "116415321826934814453125"},                   // * 8589934592
   226		{11, "582076609134674072265625"},                   // * 17179869184
   227		{11, "2910383045673370361328125"},                  // * 34359738368
   228		{11, "14551915228366851806640625"},                 // * 68719476736
   229		{12, "72759576141834259033203125"},                 // * 137438953472
   230		{12, "363797880709171295166015625"},                // * 274877906944
   231		{12, "1818989403545856475830078125"},               // * 549755813888
   232		{13, "9094947017729282379150390625"},               // * 1099511627776
   233		{13, "45474735088646411895751953125"},              // * 2199023255552
   234		{13, "227373675443232059478759765625"},             // * 4398046511104
   235		{13, "1136868377216160297393798828125"},            // * 8796093022208
   236		{14, "5684341886080801486968994140625"},            // * 17592186044416
   237		{14, "28421709430404007434844970703125"},           // * 35184372088832
   238		{14, "142108547152020037174224853515625"},          // * 70368744177664
   239		{15, "710542735760100185871124267578125"},          // * 140737488355328
   240		{15, "3552713678800500929355621337890625"},         // * 281474976710656
   241		{15, "17763568394002504646778106689453125"},        // * 562949953421312
   242		{16, "88817841970012523233890533447265625"},        // * 1125899906842624
   243		{16, "444089209850062616169452667236328125"},       // * 2251799813685248
   244		{16, "2220446049250313080847263336181640625"},      // * 4503599627370496
   245		{16, "11102230246251565404236316680908203125"},     // * 9007199254740992
   246		{17, "55511151231257827021181583404541015625"},     // * 18014398509481984
   247		{17, "277555756156289135105907917022705078125"},    // * 36028797018963968
   248		{17, "1387778780781445675529539585113525390625"},   // * 72057594037927936
   249		{18, "6938893903907228377647697925567626953125"},   // * 144115188075855872
   250		{18, "34694469519536141888238489627838134765625"},  // * 288230376151711744
   251		{18, "173472347597680709441192448139190673828125"}, // * 576460752303423488
   252		{19, "867361737988403547205962240695953369140625"}, // * 1152921504606846976
   253	}
   254	
   255	// Is the leading prefix of b lexicographically less than s?
   256	func prefixIsLessThan(b []byte, s string) bool {
   257		for i := 0; i < len(s); i++ {
   258			if i >= len(b) {
   259				return true
   260			}
   261			if b[i] != s[i] {
   262				return b[i] < s[i]
   263			}
   264		}
   265		return false
   266	}
   267	
   268	// Binary shift left (* 2) by k bits.  k <= maxShift to avoid overflow.
   269	func leftShift(a *decimal, k uint) {
   270		delta := leftcheats[k].delta
   271		if prefixIsLessThan(a.d[0:a.nd], leftcheats[k].cutoff) {
   272			delta--
   273		}
   274	
   275		r := a.nd         // read index
   276		w := a.nd + delta // write index
   277	
   278		// Pick up a digit, put down a digit.
   279		var n uint
   280		for r--; r >= 0; r-- {
   281			n += (uint(a.d[r]) - '0') << k
   282			quo := n / 10
   283			rem := n - 10*quo
   284			w--
   285			if w < len(a.d) {
   286				a.d[w] = byte(rem + '0')
   287			} else if rem != 0 {
   288				a.trunc = true
   289			}
   290			n = quo
   291		}
   292	
   293		// Put down extra digits.
   294		for n > 0 {
   295			quo := n / 10
   296			rem := n - 10*quo
   297			w--
   298			if w < len(a.d) {
   299				a.d[w] = byte(rem + '0')
   300			} else if rem != 0 {
   301				a.trunc = true
   302			}
   303			n = quo
   304		}
   305	
   306		a.nd += delta
   307		if a.nd >= len(a.d) {
   308			a.nd = len(a.d)
   309		}
   310		a.dp += delta
   311		trim(a)
   312	}
   313	
   314	// Binary shift left (k > 0) or right (k < 0).
   315	func (a *decimal) Shift(k int) {
   316		switch {
   317		case a.nd == 0:
   318			// nothing to do: a == 0
   319		case k > 0:
   320			for k > maxShift {
   321				leftShift(a, maxShift)
   322				k -= maxShift
   323			}
   324			leftShift(a, uint(k))
   325		case k < 0:
   326			for k < -maxShift {
   327				rightShift(a, maxShift)
   328				k += maxShift
   329			}
   330			rightShift(a, uint(-k))
   331		}
   332	}
   333	
   334	// If we chop a at nd digits, should we round up?
   335	func shouldRoundUp(a *decimal, nd int) bool {
   336		if nd < 0 || nd >= a.nd {
   337			return false
   338		}
   339		if a.d[nd] == '5' && nd+1 == a.nd { // exactly halfway - round to even
   340			// if we truncated, a little higher than what's recorded - always round up
   341			if a.trunc {
   342				return true
   343			}
   344			return nd > 0 && (a.d[nd-1]-'0')%2 != 0
   345		}
   346		// not halfway - digit tells all
   347		return a.d[nd] >= '5'
   348	}
   349	
   350	// Round a to nd digits (or fewer).
   351	// If nd is zero, it means we're rounding
   352	// just to the left of the digits, as in
   353	// 0.09 -> 0.1.
   354	func (a *decimal) Round(nd int) {
   355		if nd < 0 || nd >= a.nd {
   356			return
   357		}
   358		if shouldRoundUp(a, nd) {
   359			a.RoundUp(nd)
   360		} else {
   361			a.RoundDown(nd)
   362		}
   363	}
   364	
   365	// Round a down to nd digits (or fewer).
   366	func (a *decimal) RoundDown(nd int) {
   367		if nd < 0 || nd >= a.nd {
   368			return
   369		}
   370		a.nd = nd
   371		trim(a)
   372	}
   373	
   374	// Round a up to nd digits (or fewer).
   375	func (a *decimal) RoundUp(nd int) {
   376		if nd < 0 || nd >= a.nd {
   377			return
   378		}
   379	
   380		// round up
   381		for i := nd - 1; i >= 0; i-- {
   382			c := a.d[i]
   383			if c < '9' { // can stop after this digit
   384				a.d[i]++
   385				a.nd = i + 1
   386				return
   387			}
   388		}
   389	
   390		// Number is all 9s.
   391		// Change to single 1 with adjusted decimal point.
   392		a.d[0] = '1'
   393		a.nd = 1
   394		a.dp++
   395	}
   396	
   397	// Extract integer part, rounded appropriately.
   398	// No guarantees about overflow.
   399	func (a *decimal) RoundedInteger() uint64 {
   400		if a.dp > 20 {
   401			return 0xFFFFFFFFFFFFFFFF
   402		}
   403		var i int
   404		n := uint64(0)
   405		for i = 0; i < a.dp && i < a.nd; i++ {
   406			n = n*10 + uint64(a.d[i]-'0')
   407		}
   408		for ; i < a.dp; i++ {
   409			n *= 10
   410		}
   411		if shouldRoundUp(a, a.dp) {
   412			n++
   413		}
   414		return n
   415	}
   416	

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