The Eisel-Lemire ParseNumberF64 Algorithm

Summary: ParseNumberF64, StringToDouble and similarly named functions take a string like "12.5" (one two dot five) and return a 64-bit double-precision floating point number like 12.5 (twelve point five). Some numbers (like 12.3) aren’t exactly representable as an f64 but ParseNumberF64 still has to return the best approximation. In March 2020, Daniel Lemire published some source code for a new, fast algorithm to do this, based on an original idea by Michael Eisel. Here’s how it works.


Fallback Implementation

First, a caveat. The Eisel-Lemire algorithm is very fast (Lemire’s blog post contains impressive benchmark numbers, e.g. 9 times faster than the C standard library’s strtod) but it isn’t comprehensive. There are a small proportion of strings that are valid numbers but it cannot parse, where Eisel-Lemire will fail over to a fallback ParseNumberF64 implementation.

The primary goal is speed, for 99+% of the cases, not 100% coverage. As long as Eisel-Lemire doesn’t claim false positives, the combined approach is both fast and correct. Update on 2020-10-08: To be clear, combining with the fallback means that Eisel-Lemire-with-the-fallback is (much) faster than just-the-fallback, for ‘only’ 99+% of the cases, but still correct for 100% of the cases, including subnormal numbers, infinities and all the rest.

If falling back to strtod, know that it can be sensitive to locale-related environment variables (i.e. whether twelve point five is "12.5" or "12,5"). Discussing fallback algorithms any further is out of scope for this blog post. Update on 2020-11-02: the Simple Decimal Conversion fallback algorithm is discussed in the next blog post.


Let [I .. J] denote the half-open range of numbers simultaneously greater than or equal to I and less than J. The lower bound is inclusive but the upper bound is exclusive.

Let [I ..= J] denote a closed range, where the upper bound is now inclusive and its constraint is now “less than or equal to”.

Let (X ** Y) denote X raised to the Yth power. For example, here are some different ways to write “one thousand”:

Similarly, here are some different ways to write “sixty four”:

Exponents can be negative. (10 ** -1) is one tenth and (2 ** -3) is one eighth.

Let A ~MOD+ B denote modular addition, where the modulus is usually clear from the context. For example, working with u8 values would use a modulus of 256. (100 + 200) would normally be 300, which overflows a u8, but (100 ~MOD+ 200) would be 44 without overflow. In C/C++, for unsigned integer types, the "~MOD+" operator is simply spelled "+".

Double-Precision Floating Point

In C/C++, this type is called double. Go calls it float64. Rust calls it f64. We’ll use f64 in this blog post, as well as u64 for 64-bit unsigned integers and i32 for 32-bit signed integers.

Wikipedia’s double-precision floating point article has a lot of detail. More briefly, a 64-bit value (e.g. 0x40840000_00000000) is split into:

Let AsF64(0x40840000_00000000) denote reinterpreting those 64 bits as an f64 bit pattern. Its value is therefore (1.25 * (2 ** 9)), which is 640 in decimal. An equivalent derivation starts with 0x00140000_00000000 = 5629499534213120 and then (5629499534213120 >> (52-9)) = 640.

Similarly, AsF64(0x43400000_00000000) and AsF64(0x43400000_00000001) are 9007199254740992 and 9007199254740994 in decimal, also known as ((1<<53) + 0) and ((1<<53) + 2). The integer in between, 9007199254740993 = ((1<<53) + 1), is not exactly representable as an f64. Relatedly, the slightly smaller 9007199254740991 = ((1<<53) - 1) is also known in JavaScript as Number.MAX_SAFE_INTEGER.

The sign bit (corresponding to a leading "-" minus sign in the string form) is trivial to parse and we won’t spend any more time discussing it.

Non-normal numbers include subnormal numbers (with a biased exponent of 0x000) and non-finite numbers (with a biased exponent of 0x7FF and whose value is either infinite or Not-a-Number). We similarly won’t spend much time on these.

Round To Even

Typically, when rounding a decimal fraction to an integer, 7.3 rounds down to 7 and 7.6 rounds up to 8. Rounding numbers like 7.5, half-way between two integers, is subject to more debate. One option is rounding to even, alternating between rounding down and up:

Properly parsing f64 values similarly rounds to even: the evenness of the least significant bit of the 53-bit mantissa. This isn’t necessarily the same as rounding the overall value to an integer:

Static Single Assignment

For clarity, this blog post presents the Eisel-Lemire algorithm in Static Single Assignment form. For example, a separate AdjE2_1 variable is defined below, based on AdjE2_0, instead of destructively modifying a single AdjE2 variable over time. Implementations are obviously free to use a more traditional imperative programming style.

Multiplying Two u64 Values

Some compilers (and some instruction sets) provide a built-in u128 representation for multiplying two u64 values without overflow. When they don’t, it’s relatively straightfoward to implement with u64 operations:

Pre-computed Powers-of-10

The smallest and largest positive, finite f64 values, DBL_TRUE_MIN and DBL_MAX, are approximately 4.94e-324 and 1.80e+308. We’ll pre-compute two approximations, called the narrow (low resolution) and wide (high resolution) approximations, to each power-of-10 in a range, such as from 1e-325 to 1e+308 inclusive. Implementations can choose a smaller range, discussed in the Exp10 Range section below.

For each base-10 exponent E10, the narrow approximation to (10 ** E10) is the unique pair of a u64-typed mantissa M64 and an i32-typed base-2 exponent E2 such that:

The >= in the first condition is == when the approximation is exact. When inexact, the approximation rounds down (truncates). Whether exact or not, the residual R64, defined as:

implies that R64 is in the range [0 .. 1].

Here’s an exact example, for 1e3, also known as (250 * 4) or (0xFA << 2):

Here’s an inexact example, for 1e43:

Specifically, these two numbers bracket 1e43:

The (0xE596B7B0_C643C719, 79) pair represents an inclusive-lower exclusive-upper bound range for 1e43.

Look-Up Table Columns

The narrow powers-of-10 look-up table has two columns for each E10 row: M64 and NarrowBiasedE2. The NarrowBiasedE2 value is E2 plus a NarrowBias constant (the magical number 1150) which is discussed later.

The wide approximation is like the narrow one except its mantissa M128 is 128 bits instead of 64.

The wide powers-of-10 look-up table splits M128’s bits in half to have three columns: M128Lo, M128Hi and WideBiasedE2. For the 1e43 example, these are 0x6D9CCD05_D0000000, 0xE596B7B0_C643C719 and (WideBias + 15). The last two columns are shared with the narrow look-up table (M64 = M128Hi and WideBias = NarrowBias + 64 = 1214) so that a single table holds both the narrow and wide approximations.

The powers-of-10 look-up table is generated by this Go program. The two u64 columns are explicitly printed, and the one i32 column is implied by a linear expression with slope log(10)/log(2).

Eisel-Lemire Algorithm

Man:Exp10 Form

Parsing starts by converting the string to an integer mantissa and base-10 exponent. For example:

Small-Value Fast Path

If the mantissa is zero, then the parsed f64 is trivially zero.

If the mantissa is non-zero but less than (1 << 53) then it is still exactly representable as an f64. Likewise, the first 23 powers of 10, from 1e0 to 1e22, are also exactly representable.

Thus, parsing "67800.0" can be done by simply converting (in the C++ static_cast<double> sense) the u64 678 to an f64 678 and then multiplying by 1e2. Converting "3.14159" is similar, except dividing instead of multiplying by 1e5. Such cases don’t need to run Eisel-Lemire or the fallback algorithm.

This Go snippet agrees:

smallPowersOf10 := [23]float64{
    1e0, 1e1, 1e2, 1e3, 1e4, 1e5, 1e6, 1e7,
    1e8, 1e9, 1e10, 1e11, 1e12, 1e13, 1e14, 1e15,
    1e16, 1e17, 1e18, 1e19, 1e20, 1e21, 1e22,
u := uint64(314159)
f := float64(u)
e := smallPowersOf10[5]
fmt.Printf("0x%016X\n", math.Float64bits(f/e))
fmt.Printf("0x%016X\n", math.Float64bits(3.14159))
// Output:
// 0x400921F9F01B866E
// 0x400921F9F01B866E

Man Range

As mentioned earlier, the Eisel-Lemire algorithm is not comprehensive. For example, the fallback applies if the mantissa part of the Man:Exp10 form overflows a u64. In practice, it’s easier to check the looser condition that Man has at most 19 decimal digits (and is non-zero):

Exp10 Range

Similarly, the fallback applies when Exp10 is outside a certain range. In Lemire’s original code, the range is [-325 ..= 308]. The Wuffs library implementation uses a smaller range, [-307 ..= 288] because it leads to a smaller look-up table. More importantly, combining the smaller range with the Man range of [1 ..= UINT64_MAX], approximately [1 ..= 1.85e+19], means that (Man * (10 ** Exp10)) is in the range [1e-307 ..= 1.85e+307]. This is entirely within the range of normal (neither subnormal nor non-finite) f64 values: DBL_MIN and DBL_MAX are approximately 2.23e–308 and 1.80e+308. Note that the awkwardly named (but C++ standard) DBL_MIN constant is larger than DBL_TRUE_MIN.


Continuing with the parsing "1.23e45" example, let TV denote the true numerical value 1.23e45 (not just the closest f64 value).

With the equivalent Man:Exp10 form: 123e43, the Exp10 part indexes the look-up table. For 1e43, recall that M64 is 0xE596B7B0_C643C719 and NarrowBiasedE2 is (NarrowBias + 79):

The next step is to normalize the u64 123 value so that its high bit is set. In hexadecimal, 0x00000000_0000007B has 57 leading zero bits (CLZ or Count Leading Zeroes is a common bit-manipulation function that typically has hardware and compiler support). The zero mantissa case was handled above, so the non-zero mantissa here has a well-defined CLZ. Shifting Man left by CLZ(Man) gives a normalized mantissa, NorMan, whose high bit is set. We’ll also track AdjE2_0, and adjusted base-2 exponent, based on the look-up table’s NarrowBiasedE2 and this shift:

We won’t need it just yet, but as we’re defining the CLZ(arg) function to return the count of leading zeroes, let’s also define the LSB(arg) and MSB(arg) functions to return the Least and Most Significant Bits. For a u64 arg, LSB(arg) = (arg & 1) and MSB(arg) = (arg >> 63).

Rounding Ranges

The essential idea is that, after converting the input string to the normalized Man:Exp10 form, we combine 64 bits of input mantissa with 64 bits of Exp10 mantissa to produce more than enough for the 53 bits of f64 mantissa. The f64 base-2 exponent is basically AdjE2_0 with one or two more tweaks, described below.

The 64+64 intermediate mantissa bits will need to be properly rounded to produce the right 53 f64 mantissa bits. Furthermore, the look-up table doesn’t always give the exact value of (10 ** Exp10), only a range. Still, we are often able to produce a conclusive rounding when every number in that range would round to the same 53 bits.

An analogy is rounding a number to the nearest integer when only knowing the first three decimal digits (a lower bound). If you know that a number is in the range [10.234 .. 10.235] then you know that the nearest integer is 10, even if you don’t know exactly what the number is. Similarly, anything in the range [3.999 .. 4.000] certainly rounds to 4. Subtly, rounding anything in the range [8.499 .. 8.500] also certainly rounds to 8 because the upper bound of a .. range is exclusive. However, rounding a number in the range [8.500 .. 8.501] is ambiguous. “Eight and a half exactly” rounds down (per round to even) but “eight and a half and a little bit more” rounds up. When the Eisel-Lemire algorithm encounters an ambiguous case, it simply fails over to the fallback algorithm.

“Knowing the first three decimal digits” means that the size of a range like [10.234 .. 10.235] is 0.001. Let’s call that an example of a “1-unit range”, for an appropriate definition of “unit”. A “2-unit range”, like [10.234 .. 10.236] still round unambiguously, as does [3.999 .. 4.001] and [8.498 .. 8.500]. The patterns to look out for are [8.499 .. 8.501] and [8.500 .. 8.502].

More on this later, but to recap, for decimal digits:


NorMan and M64 are both u64 values whose high bits are set, so multiplying them together produces a u128 value W that has only 0 or 1 leading zero bits. Split W into high and low 64-bit halves, WHi and WLo, and WHi likewise has only 0 or 1 leading zero bits.

A small-scale analogy is that multiplying (without overflow) two u8 values both in the range [0x80 ..= 0xFF] produces a u16 value in the range [0x4000 ..= 0xFE01], so its high 8 bits are a u8 in the range [0x40 ..= 0xFE]. If that u8’s high bit (the 0x80 bit) is 0 then its second-highest bit (the 0x40 bit) must be 1.

Anyway, we already knew:


Wider Approximation

When scaled by an appropriate power-of-2 (i.e. for an appropriately defined “unit”), [WHi .. (WHi + 1)] is therefore a 1-unit range that contains the scaled (NorMan * M64).

Recall that while Man is exact and NorMan = (Man * (2 ** CLZ(Man))) is exact, M64 and E2 form an approximation. The difference between the power-of-10 approximation (M64 * (2 ** E2)) and the true power-of-10 (10 ** M10) is (R64 * (2 ** E2)). Therefore the difference between:

Focusing just on the (NorMan * R64) part of ET, NorMan is a u64 and therefore less than (2 ** 64) at W scale, so it must be less than 1 at WHi scale. R64 is less than 1. Therefore, ET is less than (1 * 1) at WHi scale. Combining that 1-unit for ET with the range at the top of this section gives that [WHi .. (WHi + 2)] is therefore a 2-unit range that contains the scaled true value TV.

As discussed in the “Rounding Ranges” section above, a 2-unit range is good enough to work with unless we’re in the base-2 equivalent of the 499 case. As we’ll see in the following sections, we’re about to shift right by 9 or 10 bits and then again by 1 more bit, so the base-2 equivalent of 499 is that the low 10 bits are 0x1FF or the low 11 bits are 0x3FF. Recall that the primary goal is speed, not perfect coverage. A fast and simple check for both is that ((WHi & 0x1FF) == 0x1FF).

Even if that condition holds, we can still proceed if we can narrow the 2-unit range to a 1-unit range. First, the error term ET is (NorMan * R64) at W scale, which is less than NorMan at the same W scale. Equivalently, (W + NorMan) is an upper bound for TV. If (WLo + NorMan) does not overflow a u64 then the high 64 bits of that upper bound are the same as WHi and we have a 1-unit range, starting at WHi, at WHi scale. The test for overflow is that ((WLo ~MOD+ NorMan) < NorMan).

Thus, if either ((WHi & 0x1FF) != 0x1FF) or ((WLo ~MOD+ NorMan) >= NorMan) are true, and that’s the case for the "1.23e45" example, then we can simply rename W to X and skip the rest of this section.

Otherwise, we might have a 499 case but all is not yet lost. We can refine our approximation to TV. Before, we used (NorMan * M64), a 128-bit value, based on the narrow approximation to (10 ** E10). This time, we would use (NorMan * M128), a 192-bit value, based on the wide approximation.

The 192-bit computation is largely straightfoward but uninteresting and we won’t dwell on it for this blog post. At the end of it, we set X to its high 128 bits and there’s another “fail over to the fallback” check, similar to the two-part check a few paragraphs above that started with ((WHi & 0x1FF) != 0x1FF), but it has three parts instead of two, because 192 is three times 64.

Shifting to 54 Bits

Let XHi and XLo be X’s high and low 64 bits. Recall that CLZ(X) is either 0 or 1, so that CLZ(XHi) must also be either 0 or 1 and that, either way, (CLZ(XHi) + MSB(XHi) == 1).

Now, we know that XHi has either 0 or 1 leading zero bits. Shifting X right by (9 + MSB(XHi)) therefore results in a u64 that has exactly 10 leading 0 bits and then a 1 bit: a 54-bit number. We also tweak AdjE2_0 (and for this example, tweak it by zero) to produce AdjE2_1:

Half-way Ambiguity

We now detect the equivalent of the 500 ambiguity, discussed in the “Rounding Ranges” section above. Like the 499 case, a necessary condition is that the low 10 bits of XHi are 0x200 or the low 11 bits are 0x400. Again, a slightly faster-looser check for both is that ((XHi & 0x1FF) == 0x000) and that (LSB(X54) == 1). Another necessary condition is that XLo is all zeroes, otherwise we’d have “a half and a little bit more”. Finally, with round to even, ambiguity requires that the equivalent of the ‘integer part’ be even, which is that (LSB(X54 >> 1) == 0).

That multiple-part condition can be re-arranged to be (XLo == 0) and ((XHi & 0x1FF) == 0) and ((X54 & 3) == 1). If all three are true, Eisel-Lemire fails over to the fallback.

Otherwise, rounding to 53 bits (exactly what we need for an f64’s 52-bit mantissa with an explicit 53rd bit that’s 1) just depends on LSB(X54): 0 means to round down and 1 means to round up.

From 54 to 53 Bits

This simply involves adding X54’s low bit to itself and then right shifting by 1:

Note that (X54 + 1) can overflow 54 bits. It does not, in this case, but if it did (i.e. if (X53 >> 53) was 1 instead of 0), shift and add by 1 more:

The RetExp value started with the magical NarrowBiasE2 constant. That magical number 1150 (which is 1023 + 127) was chosen so that RetExp here is exactly the 11-bit f64 base-2 exponent, including its 1023 bias.

In Lemire’s original code, there is one final fail-over check that RetExp is in the range [0x001 .. 0x7FF]. Too small and we’re encroaching on subnormal f64 space. Too large and we’re encroaching on nonfinite f64 space. The Wuffs library implementation is tighter, as discussed in the Exp10 Range section above, so it can skip the check here. This trades off speeding up the common cases for slowing down the rare cases.

Packing the 52 bits of RetMan with 11 bits of RetExp (left shifted by 52) produces our final f64 return value:

This Go snippet agrees that AsF64(0x494B93DA_907BD0A4) is the closest approximation to 1.23e45 (and the Go compiler, as of version 1.15 released in August 2020, does not use the Eisel-Lemire algorithm):

fmt.Printf("0x%016X\n", math.Float64bits(1.23e45))
const m = 0x1B93DA_907BD0A4
const e = 0x494
fmt.Printf("%46v\n", big.NewInt(0).Lsh(big.NewInt(1), e-1023-52))
fmt.Printf("%v\n", big.NewInt(0).Lsh(big.NewInt(m-1), e-1023-52))
fmt.Printf("%v\n", big.NewInt(0).Lsh(big.NewInt(m+0), e-1023-52))
fmt.Printf("%v\n", big.NewInt(0).Lsh(big.NewInt(m+1), e-1023-52))
// Output:
// 0x494B93DA907BD0A4
//                 158456325028528675187087900672
// 1229999999999999815358543982490949384520335360
// 1229999999999999973814869011019624571608236032
// 1230000000000000132271194039548299758696136704


Update on 2020-11-02: link to a richer test suite.

The nigeltao/parse-number-fxx-test-data repository contains many test cases, one per line, that look like:

3C00 3F800000 3FF0000000000000 1
3D00 3FA00000 3FF4000000000000 1.25
3D9A 3FB33333 3FF6666666666666 1.4
57B7 42F6E979 405EDD2F1A9FBE77 123.456
622A 44454000 4088A80000000000 789
7C00 7F800000 7FF0000000000000 123.456e789

Parsing the fourth column (the string form) should produce the third column (the 64 bits of the f64 form). In this snippet, the final line’s f64 representation is infinity. As before, DBL_MAX is approximately 1.80e+308.

The test cases (in string form) were found by running the equivalent of /usr/bin/strings and /bin/grep over various source code repositories like google/double-conversion and ulfjack/ryu. The f64 form was then calculated using Go’s strconv.ParseFloat function.

Hooking up a test program to that data set verifies that, on my computer:

all agree on the f64 form for over several million unique test cases. Everything but C’s strtod should also be locale-independent.

Source Code

This blog post is much, much longer than the actual source code. The core function is about 80 lines of C/C++ code, excluding comments and the powers-of-10 table. lemire/fast_double_parser is the original C++ implementation. google/wuffs has a C re-implementation. There are also Julia and Rust re-implementations.

Update on 2021-02-21: if you just want to see the code, Go 1.16 (released February 2021) has a 70 line implementation (plus another 70 lines for float32 vs float64, plus 700 lines for the powers-of-10 table).

Published: 2020-10-07