ndarray — go-ndarray

A pure-Go (CGO=0) NumPy-style N-dimensional array library. Row-major (C-order) strided arrays with:

• Thirteen dtypes — bool, int8…int64, uint8…uint64, float32/float64, complex64/complex128, chosen at run time with NumPy 2 promotion (AsType, ZerosOf, FromSlice, typed access with Data[T]/Item[T], weak Go scalars with Scalar). Float64 is the default and the SIMD-accelerated one (docs/design-dtypes.md).
• Creation — New/Zeros/Ones/Full/FromData/Arange/Linspace/ Eye/Identity.
• Shape & views — Reshape (with -1 inference), Ravel/Flatten, Transpose, Squeeze/ExpandDims, and NumPy basic-indexing Slice returning strided views that share data.
• Elementwise — Add/Sub/Mul/Div (+ scalar) with full NumPy broadcasting, Map/Neg/Abs, math ufuncs (Sqrt/Exp/Log/Sin/ Cos/…), broadcasting comparisons (Greater/Equal/… returning Bool masks), Maximum/Minimum, Logical*, and the complex parts Real/Imag/Conj/Angle.
• Reductions — whole-array (Sum/Prod/Max/Min/Mean), per-axis (SumAxis/… with keepdims), index reductions (ArgMax/ArgMin flat and per-axis), cumulative scans (CumSum/CumProd), Clip, and Where.
• Indexing — basic-indexing Slice views plus boolean/fancy indexing: MaskSelect (a[mask]), Nonzero (flatnonzero), and Take.
• Manipulation — Concatenate/Stack/VStack/HStack.
• Linear algebra — MatMul/Dot/Inner/Outer; package linalg is numpy.linalg in pure Go: Solve, Inv, Det, Slogdet, Cholesky, QR (all four modes), SVD/SVDVals, Eigh/Eigvalsh, Eig/Eigvals, Lstsq, Pinv, MatrixRank, Norm, Cond, MatrixPower, MultiDot — stacked (..., M, N) inputs (run in parallel), every dtype, LAPACK's algorithms (divide and conquer for the SVD and Hermitian eigenproblems), checked against numpy 2.5.3 on 1,040 cases. See docs/linalg.md for accuracy and speed.
• Fourier transforms — package fft is numpy.fft on arrays (FFT/RFFT/FFT2/FFTN/… with n/s, axes and norm, single precision kept), on go-fft.
• Random numbers — package random is numpy.random with NumPy's numbers for the same seed: SeedSequence, PCG64 (default), PCG64DXSM, MT19937, Philox, SFC64; Generator (DefaultRNG: uniform, integers, ziggurat normal/exponential, gamma, beta, binomial, Poisson, choice, shuffle, … 40 samplers) and the legacy RandomState behind random.Seed(0) / random.Rand(…). Bit-exact against NumPy 2.5.3 on x86-64; see docs/random.md for the few ULP-level exceptions (C-library log/exp rounding) and NumPy's own aarch64 drift.
• Notebooks and storage — Repr prints what NumPy 2.5.3's repr prints with its default options, character for character: the same digits (8 at most, the shortest that read back), padding, scientific switch (at 1e8, 1e6 for float32), line breaks at 75 columns, summary above 1000 elements and shape=/dtype= suffixes. It is a port of arrayprint.py, checked against the expected strings of NumPy's own test_arrayprint.py. Display gives a notebook a text and an HTML table, and MarshalBinary/UnmarshalBinary encode an array the same way on every architecture (so gob and the go-notebook kernel keep arrays).
• Memory reuse — Workspace, an arena for loops: bind the inputs with ws.Use, compute as usual, ws.Reset() at the end of each pass. Results stop costing the garbage collector anything (see below).

Try it without installing anything: the playground runs go-ndarray compiled to WebAssembly in your browser.

Loops: Workspace

Every operation returns a new array, and in a loop those results are garbage the collector must chase. NumPy frees its temporaries the moment their reference count drops; Go cannot. On a Zen 3 that made x + y on 1 024 elements 3× slower than NumPy, and a chain like sqrt(x*y + x) 3.4× slower. A Workspace gives the results of one pass a single arena that Reset hands back:

ws := ndarray.NewWorkspace()
for step := 0; step < steps; step++ {
	x := ws.Use(state)          // results computed from x come from ws
	p, _ := x.Mul(w)
	q, _ := p.Add(bias)
	state = q.Sqrt().Detach()   // the one result that outlives the pass
	ws.Reset()                  // everything else is recycled
}

After the first pass nothing is allocated, nothing is zeroed that the kernel overwrites anyway, and the memory stays warm in cache: x + y on 1 024 elements goes from 2.6 µs to 0.38 µs (2.1× NumPy), sqrt(x*y + x) on 256 Ki from 1.18 ms to 0.39 ms (7.5× NumPy). ⚠ An array allocated from the workspace is invalid after Reset; Detach is how a result leaves it. The *Into methods (AddInto, SqrtInto, …) remain the way to reuse one named buffer.

The numeric inner loops are kept behind a narrow kernel API. Behind it, large elementwise ops, reductions and products run multicore (across GOMAXPROCS). On amd64, arm64, ppc64le, loong64, riscv64 and s390x the hot loops are go-asmgen-generated SIMD kernels: sum, add/sub/mul/div, sqrt and the dot product (SSE2 or AVX2/FMA on amd64, NEON on arm64, VSX on ppc64le, LASX on loong64, RVV on riscv64 and the vector facility on s390x, the last three when the CPU has them), max/min too on amd64, arm64 and s390x (z14 and later), and a panel-packed, cache-blocked GEMM with an SIMD-FMA micro-kernel (NEON 4×8; AVX2/FMA 6×8 chosen at run time, SSE2 fallback; VSX 8×8; LASX 8×8; RVV 6×8; s390x 4×8), the OpenBLAS/BLIS structure. The 32-bit targets run the same pure-Go code those kernels are tested against. Exp and Log are ports of Arm's optimized-routines (≤ 0.51 ULP measured, against up to 0.88 and 0.72 for Go's math.Exp/math.Log on arm64), and they are correct where amd64's math.Exp returns +Inf (golang/go#81995) and math.Log is wrong on subnormals (golang/go#56600).

Measured against NumPy 2.5.3 (OpenBLAS 0.3.34) on an AMD Zen 3, 16 cores (full tables in docs/perf.md):

• Faster: whole-array reductions (Sum/Mean/Max at 4 Mi: 2.3–4.5×), row reductions (SumAxis(1) 2.2×, MaxAxis(1) 1.7×), Exp from 256 Ki elements (2.4–5.6×), the *Into forms of Add/Mul/Sqrt (faster or at parity at every size), and MatMul against single-threaded OpenBLAS (3.7–5.9× from 512²); in a Workspace, x + y and sqrt(x*y + x) match or beat NumPy at every size measured.
• At parity: SumAxis(0), Inner against 16-thread OpenBLAS, Log on one core.
• Slower: MatMul against 16-thread OpenBLAS at 512² and up (0.42× at 512², 0.51× at 1024², measured in one run with OpenBLAS; at 256² ours is 1.44× faster); Dot and mat·vec against NumPy's 16-thread BLAS (0.69× and 0.52× since v0.6.0's helper pool, measured in one run; 0.16× before); Exp below 256 Ki elements (0.82×); the allocating forms of elementwise ops up to 256 Ki elements, Concatenate/Stack and slice copies outside a Workspace (0.2–0.5×: Go's allocator and garbage collector, which Workspace largely removes).

On Apple silicon the GEMM reached parity with tuned BLAS at 1024² (OpenBLAS in an arm64 VM, single-threaded vecLib on an M4 Max), and every product beats the pure-Go gonum 4–10× (BENCHMARKS.md). On the other targets, against go-ndarray's own scalar code on one core: on a RISC-V SpacemiT X60 the RVV kernels cut Sum by up to 5.8× and MatMul time by 31–45%; on an IBM z17 the s390x kernels cut MatMul 1024² from 340 to 70 ms and Max by 92–96% (docs/perf.md, SIMD coverage). It is a standalone, reusable module and the cgo-free ndarray backend behind go-embedded-ruby's NDArray class.

⚠️ Status: NumPy parity for the core surface, in 13 dtypes. Creation, slicing/views, broadcasting elementwise + ufuncs, reductions (incl. arg/cumulative/clip/where), manipulation, linear algebra and Workspace are complete, 100%-covered, and differentially checked against NumPy. CI runs the suite on amd64, arm64, 386, and riscv64/loong64/ppc64le/s390x/arm under qemu, on Linux, macOS and Windows, and compiles it for every GOOS/GOARCH pair Go supports; v0.1.0 and v0.2.0 were also run on real amd64, arm64, ppc64le, riscv64 and loong64 hardware, v0.3.0 on real amd64, arm64, ppc64le and loong64. See docs/plan-ndarray.md for the roadmap (SIMD for float32, linear algebra decompositions).

Why this module?

gonum is matrix-centric and its optimized assembly is amd64-only. More broadly, Ruby has no cgo-free ndarray (Numo::NArray, NMatrix are C extensions). A pure-Go core whose kernels are generated for every arch is therefore a durable foundation. The numeric loops live in internal/kernels, each with a pure-Go reference implementation; go-asmgen-generated SIMD kernels replace them on every 64-bit target Go supports (amd64, arm64, ppc64le, loong64, riscv64, s390x), behind the same API and held to the same tests.

Goroutines

Operations on large arrays are spread over GOMAXPROCS goroutines. After the first one, the package keeps up to GOMAXPROCS-1 helper goroutines alive. When an operation ends, they poll for the next one for 200 µs and then block, the way OpenBLAS's threads do, so operations issued back to back do not pay to wake threads. Waking threads per call made a 2^20-element dot slower on 8 POWER9 cores than on 4. The helpers are never stopped: a goroutine-leak check (goleak) that runs after ndarray operations will list them.

Untrusted input

Shapes, indices and data can come from someone else (a file, a request), so every operation answers a shape it cannot represent with an error wrapping ErrShapeMismatch instead of a panic or a wrong array. That covers results as well as inputs: (2^32, 0) @ (0, 2^32) holds no data, yet its result would have 2^64 elements, so MatMul refuses it, as NumPy does. The one exception is Outer, which has no error result: it panics when its result is too big, which takes operands of more than 2^30 elements each. An empty array whose other axis is huge, (2^58, 0), costs nothing to reduce, concatenate or scan.

A shape that is representable can still be far larger than memory, and Go cannot recover from running out of it: the process dies, where NumPy raises MemoryError. Several operations produce more than they are given: broadcasting ((n, 1) + (1, n) is n² elements), Outer, MatMul, and summing the empty axis of (0, n) (n zeros, as in NumPy). When shapes come from someone else, bound the result's size before computing it.

The assembly kernels take raw pointers. Their Go wrappers check every operand the kernel reads or writes against the length it is given, so a mistake is an index panic in Go, never an access outside a slice; a test places each operand flush against an inaccessible page to prove the kernels themselves stay inside (see SECURITY.md).

Example

import "github.com/go-ndarray/ndarray"

a, _ := ndarray.Arange(0, 6, 1)     // [0 1 2 3 4 5]
m, _ := a.Reshape(2, -1)            // -1 inferred -> [[0 1 2] [3 4 5]]

row, _ := ndarray.FromData([]float64{10, 20, 30}, 3)
sum, _ := m.Add(row)                // broadcast (2,3)+(3,) -> (2,3)

t := m.Transpose()                  // zero-copy (3,2) view
total := m.Sum()                    // 15
cols, _ := m.SumAxis(0, false)      // sum down each column -> [3 5 7]

// NumPy basic-indexing views (share data):
col0, _ := m.Slice(ndarray.All(), ndarray.A(0))     // m[:,0] -> [0 3]
sub, _ := m.Slice(ndarray.R(0, 2), ndarray.Step(2)) // m[0:2, ::2]
row1, _ := m.Slice(ndarray.A(1))                    // m[1]: unindexed axes are whole

// ufuncs and masks
roots := m.Sqrt()
mask, _ := m.Greater(ndarray.Scalar(2))             // Bool mask of m > 2
big, _ := m.MaskSelect(mask)                        // m[m > 2] -> [3 4 5]

// dtypes, with NumPy's promotion
i8, _ := ndarray.FromSlice([]int8{1, 2, 3}, 3)
x, _ := i8.Add(ndarray.Scalar(1))                   // int8: a Go int is weak
y, _ := i8.Div(i8)                                  // float64: true division
z := i8.AsType(ndarray.Complex128)                  // complex128

// numpy.fft
spec, _ := fft.RFFT(row)                            // complex128, 2 bins

// numpy.random, the same numbers as NumPy for the same seed
rng := random.DefaultRNG(42)                        // np.random.default_rng(42)
noise, _ := rng.StandardNormal(2, 3)                // rng.standard_normal((2, 3))
random.Seed(0)                                      // np.random.seed(0)
u, _ := random.Rand(3)                              // [0.5488135 0.71518937 0.60276338]

// linear algebra
b, _ := ndarray.Arange(0, 6, 1)
b, _ = b.Reshape(3, 2)
prod, _ := m.MatMul(b)              // (2,3)·(3,2) -> (2,2)
inv, _ := linalg.Inv(prod)          // np.linalg.inv
left, sv, vh, _ := linalg.SVD(m, false) // np.linalg.svd(m, full_matrices=False)

License

BSD-3-Clause. See LICENSE.