| // Copyright 2021 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. |
| |
| package strconv |
| |
| import ( |
| "math/bits" |
| ) |
| |
| // binary to decimal conversion using the Ryū algorithm. |
| // |
| // See Ulf Adams, "Ryū: Fast Float-to-String Conversion" (doi:10.1145/3192366.3192369) |
| // |
| // Fixed precision formatting is a variant of the original paper's |
| // algorithm, where a single multiplication by 10^k is required, |
| // sharing the same rounding guarantees. |
| |
| // ryuFtoaFixed32 formats mant*(2^exp) with prec decimal digits. |
| func ryuFtoaFixed32(d *decimalSlice, mant uint32, exp int, prec int) { |
| if prec < 0 { |
| panic("ryuFtoaFixed32 called with negative prec") |
| } |
| if prec > 9 { |
| panic("ryuFtoaFixed32 called with prec > 9") |
| } |
| // Zero input. |
| if mant == 0 { |
| d.nd, d.dp = 0, 0 |
| return |
| } |
| // Renormalize to a 25-bit mantissa. |
| e2 := exp |
| if b := bits.Len32(mant); b < 25 { |
| mant <<= uint(25 - b) |
| e2 += int(b) - 25 |
| } |
| // Choose an exponent such that rounded mant*(2^e2)*(10^q) has |
| // at least prec decimal digits, i.e |
| // mant*(2^e2)*(10^q) >= 10^(prec-1) |
| // Because mant >= 2^24, it is enough to choose: |
| // 2^(e2+24) >= 10^(-q+prec-1) |
| // or q = -mulByLog2Log10(e2+24) + prec - 1 |
| q := -mulByLog2Log10(e2+24) + prec - 1 |
| |
| // Now compute mant*(2^e2)*(10^q). |
| // Is it an exact computation? |
| // Only small positive powers of 10 are exact (5^28 has 66 bits). |
| exact := q <= 27 && q >= 0 |
| |
| di, dexp2, d0 := mult64bitPow10(mant, e2, q) |
| if dexp2 >= 0 { |
| panic("not enough significant bits after mult64bitPow10") |
| } |
| // As a special case, computation might still be exact, if exponent |
| // was negative and if it amounts to computing an exact division. |
| // In that case, we ignore all lower bits. |
| // Note that division by 10^11 cannot be exact as 5^11 has 26 bits. |
| if q < 0 && q >= -10 && divisibleByPower5(uint64(mant), -q) { |
| exact = true |
| d0 = true |
| } |
| // Remove extra lower bits and keep rounding info. |
| extra := uint(-dexp2) |
| extraMask := uint32(1<<extra - 1) |
| |
| di, dfrac := di>>extra, di&extraMask |
| roundUp := false |
| if exact { |
| // If we computed an exact product, d + 1/2 |
| // should round to d+1 if 'd' is odd. |
| roundUp = dfrac > 1<<(extra-1) || |
| (dfrac == 1<<(extra-1) && !d0) || |
| (dfrac == 1<<(extra-1) && d0 && di&1 == 1) |
| } else { |
| // otherwise, d+1/2 always rounds up because |
| // we truncated below. |
| roundUp = dfrac>>(extra-1) == 1 |
| } |
| if dfrac != 0 { |
| d0 = false |
| } |
| // Proceed to the requested number of digits |
| formatDecimal(d, uint64(di), !d0, roundUp, prec) |
| // Adjust exponent |
| d.dp -= q |
| } |
| |
| // ryuFtoaFixed64 formats mant*(2^exp) with prec decimal digits. |
| func ryuFtoaFixed64(d *decimalSlice, mant uint64, exp int, prec int) { |
| if prec > 18 { |
| panic("ryuFtoaFixed64 called with prec > 18") |
| } |
| // Zero input. |
| if mant == 0 { |
| d.nd, d.dp = 0, 0 |
| return |
| } |
| // Renormalize to a 55-bit mantissa. |
| e2 := exp |
| if b := bits.Len64(mant); b < 55 { |
| mant = mant << uint(55-b) |
| e2 += int(b) - 55 |
| } |
| // Choose an exponent such that rounded mant*(2^e2)*(10^q) has |
| // at least prec decimal digits, i.e |
| // mant*(2^e2)*(10^q) >= 10^(prec-1) |
| // Because mant >= 2^54, it is enough to choose: |
| // 2^(e2+54) >= 10^(-q+prec-1) |
| // or q = -mulByLog2Log10(e2+54) + prec - 1 |
| // |
| // The minimal required exponent is -mulByLog2Log10(1025)+18 = -291 |
| // The maximal required exponent is mulByLog2Log10(1074)+18 = 342 |
| q := -mulByLog2Log10(e2+54) + prec - 1 |
| |
| // Now compute mant*(2^e2)*(10^q). |
| // Is it an exact computation? |
| // Only small positive powers of 10 are exact (5^55 has 128 bits). |
| exact := q <= 55 && q >= 0 |
| |
| di, dexp2, d0 := mult128bitPow10(mant, e2, q) |
| if dexp2 >= 0 { |
| panic("not enough significant bits after mult128bitPow10") |
| } |
| // As a special case, computation might still be exact, if exponent |
| // was negative and if it amounts to computing an exact division. |
| // In that case, we ignore all lower bits. |
| // Note that division by 10^23 cannot be exact as 5^23 has 54 bits. |
| if q < 0 && q >= -22 && divisibleByPower5(mant, -q) { |
| exact = true |
| d0 = true |
| } |
| // Remove extra lower bits and keep rounding info. |
| extra := uint(-dexp2) |
| extraMask := uint64(1<<extra - 1) |
| |
| di, dfrac := di>>extra, di&extraMask |
| roundUp := false |
| if exact { |
| // If we computed an exact product, d + 1/2 |
| // should round to d+1 if 'd' is odd. |
| roundUp = dfrac > 1<<(extra-1) || |
| (dfrac == 1<<(extra-1) && !d0) || |
| (dfrac == 1<<(extra-1) && d0 && di&1 == 1) |
| } else { |
| // otherwise, d+1/2 always rounds up because |
| // we truncated below. |
| roundUp = dfrac>>(extra-1) == 1 |
| } |
| if dfrac != 0 { |
| d0 = false |
| } |
| // Proceed to the requested number of digits |
| formatDecimal(d, di, !d0, roundUp, prec) |
| // Adjust exponent |
| d.dp -= q |
| } |
| |
| var uint64pow10 = [...]uint64{ |
| 1, 1e1, 1e2, 1e3, 1e4, 1e5, 1e6, 1e7, 1e8, 1e9, |
| 1e10, 1e11, 1e12, 1e13, 1e14, 1e15, 1e16, 1e17, 1e18, 1e19, |
| } |
| |
| // formatDecimal fills d with at most prec decimal digits |
| // of mantissa m. The boolean trunc indicates whether m |
| // is truncated compared to the original number being formatted. |
| func formatDecimal(d *decimalSlice, m uint64, trunc bool, roundUp bool, prec int) { |
| max := uint64pow10[prec] |
| trimmed := 0 |
| for m >= max { |
| a, b := m/10, m%10 |
| m = a |
| trimmed++ |
| if b > 5 { |
| roundUp = true |
| } else if b < 5 { |
| roundUp = false |
| } else { // b == 5 |
| // round up if there are trailing digits, |
| // or if the new value of m is odd (round-to-even convention) |
| roundUp = trunc || m&1 == 1 |
| } |
| if b != 0 { |
| trunc = true |
| } |
| } |
| if roundUp { |
| m++ |
| } |
| if m >= max { |
| // Happens if di was originally 99999....xx |
| m /= 10 |
| trimmed++ |
| } |
| // render digits (similar to formatBits) |
| n := uint(prec) |
| d.nd = int(prec) |
| v := m |
| for v >= 100 { |
| var v1, v2 uint64 |
| if v>>32 == 0 { |
| v1, v2 = uint64(uint32(v)/100), uint64(uint32(v)%100) |
| } else { |
| v1, v2 = v/100, v%100 |
| } |
| n -= 2 |
| d.d[n+1] = smallsString[2*v2+1] |
| d.d[n+0] = smallsString[2*v2+0] |
| v = v1 |
| } |
| if v > 0 { |
| n-- |
| d.d[n] = smallsString[2*v+1] |
| } |
| if v >= 10 { |
| n-- |
| d.d[n] = smallsString[2*v] |
| } |
| for d.d[d.nd-1] == '0' { |
| d.nd-- |
| trimmed++ |
| } |
| d.dp = d.nd + trimmed |
| } |
| |
| // ryuFtoaShortest formats mant*2^exp with prec decimal digits. |
| func ryuFtoaShortest(d *decimalSlice, mant uint64, exp int, flt *floatInfo) { |
| if mant == 0 { |
| d.nd, d.dp = 0, 0 |
| return |
| } |
| // If input is an exact integer with fewer bits than the mantissa, |
| // the previous and next integer are not admissible representations. |
| if exp <= 0 && bits.TrailingZeros64(mant) >= -exp { |
| mant >>= uint(-exp) |
| ryuDigits(d, mant, mant, mant, true, false) |
| return |
| } |
| ml, mc, mu, e2 := computeBounds(mant, exp, flt) |
| if e2 == 0 { |
| ryuDigits(d, ml, mc, mu, true, false) |
| return |
| } |
| // Find 10^q *larger* than 2^-e2 |
| q := mulByLog2Log10(-e2) + 1 |
| |
| // We are going to multiply by 10^q using 128-bit arithmetic. |
| // The exponent is the same for all 3 numbers. |
| var dl, dc, du uint64 |
| var dl0, dc0, du0 bool |
| if flt == &float32info { |
| var dl32, dc32, du32 uint32 |
| dl32, _, dl0 = mult64bitPow10(uint32(ml), e2, q) |
| dc32, _, dc0 = mult64bitPow10(uint32(mc), e2, q) |
| du32, e2, du0 = mult64bitPow10(uint32(mu), e2, q) |
| dl, dc, du = uint64(dl32), uint64(dc32), uint64(du32) |
| } else { |
| dl, _, dl0 = mult128bitPow10(ml, e2, q) |
| dc, _, dc0 = mult128bitPow10(mc, e2, q) |
| du, e2, du0 = mult128bitPow10(mu, e2, q) |
| } |
| if e2 >= 0 { |
| panic("not enough significant bits after mult128bitPow10") |
| } |
| // Is it an exact computation? |
| if q > 55 { |
| // Large positive powers of ten are not exact |
| dl0, dc0, du0 = false, false, false |
| } |
| if q < 0 && q >= -24 { |
| // Division by a power of ten may be exact. |
| // (note that 5^25 is a 59-bit number so division by 5^25 is never exact). |
| if divisibleByPower5(ml, -q) { |
| dl0 = true |
| } |
| if divisibleByPower5(mc, -q) { |
| dc0 = true |
| } |
| if divisibleByPower5(mu, -q) { |
| du0 = true |
| } |
| } |
| // Express the results (dl, dc, du)*2^e2 as integers. |
| // Extra bits must be removed and rounding hints computed. |
| extra := uint(-e2) |
| extraMask := uint64(1<<extra - 1) |
| // Now compute the floored, integral base 10 mantissas. |
| dl, fracl := dl>>extra, dl&extraMask |
| dc, fracc := dc>>extra, dc&extraMask |
| du, fracu := du>>extra, du&extraMask |
| // Is it allowed to use 'du' as a result? |
| // It is always allowed when it is truncated, but also |
| // if it is exact and the original binary mantissa is even |
| // When disallowed, we can subtract 1. |
| uok := !du0 || fracu > 0 |
| if du0 && fracu == 0 { |
| uok = mant&1 == 0 |
| } |
| if !uok { |
| du-- |
| } |
| // Is 'dc' the correctly rounded base 10 mantissa? |
| // The correct rounding might be dc+1 |
| cup := false // don't round up. |
| if dc0 { |
| // If we computed an exact product, the half integer |
| // should round to next (even) integer if 'dc' is odd. |
| cup = fracc > 1<<(extra-1) || |
| (fracc == 1<<(extra-1) && dc&1 == 1) |
| } else { |
| // otherwise, the result is a lower truncation of the ideal |
| // result. |
| cup = fracc>>(extra-1) == 1 |
| } |
| // Is 'dl' an allowed representation? |
| // Only if it is an exact value, and if the original binary mantissa |
| // was even. |
| lok := dl0 && fracl == 0 && (mant&1 == 0) |
| if !lok { |
| dl++ |
| } |
| // We need to remember whether the trimmed digits of 'dc' are zero. |
| c0 := dc0 && fracc == 0 |
| // render digits |
| ryuDigits(d, dl, dc, du, c0, cup) |
| d.dp -= q |
| } |
| |
| // mulByLog2Log10 returns math.Floor(x * log(2)/log(10)) for an integer x in |
| // the range -1600 <= x && x <= +1600. |
| // |
| // The range restriction lets us work in faster integer arithmetic instead of |
| // slower floating point arithmetic. Correctness is verified by unit tests. |
| func mulByLog2Log10(x int) int { |
| // log(2)/log(10) ≈ 0.30102999566 ≈ 78913 / 2^18 |
| return (x * 78913) >> 18 |
| } |
| |
| // mulByLog10Log2 returns math.Floor(x * log(10)/log(2)) for an integer x in |
| // the range -500 <= x && x <= +500. |
| // |
| // The range restriction lets us work in faster integer arithmetic instead of |
| // slower floating point arithmetic. Correctness is verified by unit tests. |
| func mulByLog10Log2(x int) int { |
| // log(10)/log(2) ≈ 3.32192809489 ≈ 108853 / 2^15 |
| return (x * 108853) >> 15 |
| } |
| |
| // computeBounds returns a floating-point vector (l, c, u)×2^e2 |
| // where the mantissas are 55-bit (or 26-bit) integers, describing the interval |
| // represented by the input float64 or float32. |
| func computeBounds(mant uint64, exp int, flt *floatInfo) (lower, central, upper uint64, e2 int) { |
| if mant != 1<<flt.mantbits || exp == flt.bias+1-int(flt.mantbits) { |
| // regular case (or denormals) |
| lower, central, upper = 2*mant-1, 2*mant, 2*mant+1 |
| e2 = exp - 1 |
| return |
| } else { |
| // border of an exponent |
| lower, central, upper = 4*mant-1, 4*mant, 4*mant+2 |
| e2 = exp - 2 |
| return |
| } |
| } |
| |
| func ryuDigits(d *decimalSlice, lower, central, upper uint64, |
| c0, cup bool) { |
| lhi, llo := divmod1e9(lower) |
| chi, clo := divmod1e9(central) |
| uhi, ulo := divmod1e9(upper) |
| if uhi == 0 { |
| // only low digits (for denormals) |
| ryuDigits32(d, llo, clo, ulo, c0, cup, 8) |
| } else if lhi < uhi { |
| // truncate 9 digits at once. |
| if llo != 0 { |
| lhi++ |
| } |
| c0 = c0 && clo == 0 |
| cup = (clo > 5e8) || (clo == 5e8 && cup) |
| ryuDigits32(d, lhi, chi, uhi, c0, cup, 8) |
| d.dp += 9 |
| } else { |
| d.nd = 0 |
| // emit high part |
| n := uint(9) |
| for v := chi; v > 0; { |
| v1, v2 := v/10, v%10 |
| v = v1 |
| n-- |
| d.d[n] = byte(v2 + '0') |
| } |
| d.d = d.d[n:] |
| d.nd = int(9 - n) |
| // emit low part |
| ryuDigits32(d, llo, clo, ulo, |
| c0, cup, d.nd+8) |
| } |
| // trim trailing zeros |
| for d.nd > 0 && d.d[d.nd-1] == '0' { |
| d.nd-- |
| } |
| // trim initial zeros |
| for d.nd > 0 && d.d[0] == '0' { |
| d.nd-- |
| d.dp-- |
| d.d = d.d[1:] |
| } |
| } |
| |
| // ryuDigits32 emits decimal digits for a number less than 1e9. |
| func ryuDigits32(d *decimalSlice, lower, central, upper uint32, |
| c0, cup bool, endindex int) { |
| if upper == 0 { |
| d.dp = endindex + 1 |
| return |
| } |
| trimmed := 0 |
| // Remember last trimmed digit to check for round-up. |
| // c0 will be used to remember zeroness of following digits. |
| cNextDigit := 0 |
| for upper > 0 { |
| // Repeatedly compute: |
| // l = Ceil(lower / 10^k) |
| // c = Round(central / 10^k) |
| // u = Floor(upper / 10^k) |
| // and stop when c goes out of the (l, u) interval. |
| l := (lower + 9) / 10 |
| c, cdigit := central/10, central%10 |
| u := upper / 10 |
| if l > u { |
| // don't trim the last digit as it is forbidden to go below l |
| // other, trim and exit now. |
| break |
| } |
| // Check that we didn't cross the lower boundary. |
| // The case where l < u but c == l-1 is essentially impossible, |
| // but may happen if: |
| // lower = ..11 |
| // central = ..19 |
| // upper = ..31 |
| // and means that 'central' is very close but less than |
| // an integer ending with many zeros, and usually |
| // the "round-up" logic hides the problem. |
| if l == c+1 && c < u { |
| c++ |
| cdigit = 0 |
| cup = false |
| } |
| trimmed++ |
| // Remember trimmed digits of c |
| c0 = c0 && cNextDigit == 0 |
| cNextDigit = int(cdigit) |
| lower, central, upper = l, c, u |
| } |
| // should we round up? |
| if trimmed > 0 { |
| cup = cNextDigit > 5 || |
| (cNextDigit == 5 && !c0) || |
| (cNextDigit == 5 && c0 && central&1 == 1) |
| } |
| if central < upper && cup { |
| central++ |
| } |
| // We know where the number ends, fill directly |
| endindex -= trimmed |
| v := central |
| n := endindex |
| for n > d.nd { |
| v1, v2 := v/100, v%100 |
| d.d[n] = smallsString[2*v2+1] |
| d.d[n-1] = smallsString[2*v2+0] |
| n -= 2 |
| v = v1 |
| } |
| if n == d.nd { |
| d.d[n] = byte(v + '0') |
| } |
| d.nd = endindex + 1 |
| d.dp = d.nd + trimmed |
| } |
| |
| // mult64bitPow10 takes a floating-point input with a 25-bit |
| // mantissa and multiplies it with 10^q. The resulting mantissa |
| // is m*P >> 57 where P is a 64-bit element of the detailedPowersOfTen tables. |
| // It is typically 31 or 32-bit wide. |
| // The returned boolean is true if all trimmed bits were zero. |
| // |
| // That is: |
| // m*2^e2 * round(10^q) = resM * 2^resE + ε |
| // exact = ε == 0 |
| func mult64bitPow10(m uint32, e2, q int) (resM uint32, resE int, exact bool) { |
| if q == 0 { |
| // P == 1<<63 |
| return m << 6, e2 - 6, true |
| } |
| if q < detailedPowersOfTenMinExp10 || detailedPowersOfTenMaxExp10 < q { |
| // This never happens due to the range of float32/float64 exponent |
| panic("mult64bitPow10: power of 10 is out of range") |
| } |
| pow := detailedPowersOfTen[q-detailedPowersOfTenMinExp10][1] |
| if q < 0 { |
| // Inverse powers of ten must be rounded up. |
| pow += 1 |
| } |
| hi, lo := bits.Mul64(uint64(m), pow) |
| e2 += mulByLog10Log2(q) - 63 + 57 |
| return uint32(hi<<7 | lo>>57), e2, lo<<7 == 0 |
| } |
| |
| // mult128bitPow10 takes a floating-point input with a 55-bit |
| // mantissa and multiplies it with 10^q. The resulting mantissa |
| // is m*P >> 119 where P is a 128-bit element of the detailedPowersOfTen tables. |
| // It is typically 63 or 64-bit wide. |
| // The returned boolean is true is all trimmed bits were zero. |
| // |
| // That is: |
| // m*2^e2 * round(10^q) = resM * 2^resE + ε |
| // exact = ε == 0 |
| func mult128bitPow10(m uint64, e2, q int) (resM uint64, resE int, exact bool) { |
| if q == 0 { |
| // P == 1<<127 |
| return m << 8, e2 - 8, true |
| } |
| if q < detailedPowersOfTenMinExp10 || detailedPowersOfTenMaxExp10 < q { |
| // This never happens due to the range of float32/float64 exponent |
| panic("mult128bitPow10: power of 10 is out of range") |
| } |
| pow := detailedPowersOfTen[q-detailedPowersOfTenMinExp10] |
| if q < 0 { |
| // Inverse powers of ten must be rounded up. |
| pow[0] += 1 |
| } |
| e2 += mulByLog10Log2(q) - 127 + 119 |
| |
| // long multiplication |
| l1, l0 := bits.Mul64(m, pow[0]) |
| h1, h0 := bits.Mul64(m, pow[1]) |
| mid, carry := bits.Add64(l1, h0, 0) |
| h1 += carry |
| return h1<<9 | mid>>55, e2, mid<<9 == 0 && l0 == 0 |
| } |
| |
| func divisibleByPower5(m uint64, k int) bool { |
| if m == 0 { |
| return true |
| } |
| for i := 0; i < k; i++ { |
| if m%5 != 0 { |
| return false |
| } |
| m /= 5 |
| } |
| return true |
| } |
| |
| // divmod1e9 computes quotient and remainder of division by 1e9, |
| // avoiding runtime uint64 division on 32-bit platforms. |
| func divmod1e9(x uint64) (uint32, uint32) { |
| if !host32bit { |
| return uint32(x / 1e9), uint32(x % 1e9) |
| } |
| // Use the same sequence of operations as the amd64 compiler. |
| hi, _ := bits.Mul64(x>>1, 0x89705f4136b4a598) // binary digits of 1e-9 |
| q := hi >> 28 |
| return uint32(q), uint32(x - q*1e9) |
| } |
