Add dual-backend support for KDTree with benchmarks and documentation updates

.github/workflows/ci.yml

@@ -39,6 +39,16 @@ jobs:
       - name: CI checks (lint, tests, coverage, etc.)
         run: make ci
 
+      - name: Benchmarks (linear)
+        run: go test -bench . -benchmem -run=^$ ./... | tee bench-linear.txt
+
+      - name: Upload benchmarks (linear)
+        uses: actions/upload-artifact@v4
+        with:
+          name: bench-linear
+          path: bench-linear.txt
+          if-no-files-found: error
+
       - name: Build WebAssembly module
         run: make wasm-build
 
@@ -72,3 +82,64 @@ jobs:
           name: npm-poindexter-wasm-tarball
           if-no-files-found: error
           path: ${{ steps.npm_pack.outputs.tarball }}
+
+  build-test-gonum:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Setup Go
+        uses: actions/setup-go@v5
+        with:
+          go-version: '1.23.x'
+
+      - name: Install extra tools
+        run: |
+          go install github.com/golangci/golangci-lint/v2/cmd/golangci-lint@latest
+          go install golang.org/x/vuln/cmd/govulncheck@latest
+
+      - name: Go env
+        run: go env
+
+      - name: Lint
+        run: golangci-lint run
+
+      - name: Build (gonum tag)
+        run: go build -tags=gonum ./...
+
+      - name: Unit tests + race + coverage (gonum tag)
+        run: go test -tags=gonum -race -coverpkg=./... -coverprofile=coverage-gonum.out -covermode=atomic ./...
+
+      - name: Fuzz (10s per fuzz test, gonum tag)
+        run: |
+          set -e
+          for pkg in $(go list ./...); do
+            FUZZES=$(go test -tags=gonum -list '^Fuzz' "$pkg" | grep '^Fuzz' || true)
+            if [ -z "$FUZZES" ]; then
+              echo "==> Skipping $pkg (no fuzz targets)"
+              continue
+            fi
+            for fz in $FUZZES; do
+              echo "==> Fuzzing $pkg :: $fz for 10s"
+              go test -tags=gonum -run=NONE -fuzz=^${fz}$ -fuzztime=10s "$pkg"
+            done
+          done
+
+      - name: Benchmarks (gonum tag)
+        run: go test -tags=gonum -bench . -benchmem -run=^$ ./... | tee bench-gonum.txt
+
+      - name: Upload coverage (gonum)
+        uses: actions/upload-artifact@v4
+        with:
+          name: coverage-gonum
+          path: coverage-gonum.out
+          if-no-files-found: error
+
+      - name: Upload benchmarks (gonum)
+        uses: actions/upload-artifact@v4
+        with:
+          name: bench-gonum
+          path: bench-gonum.txt
+          if-no-files-found: error

CHANGELOG.md

@@ -6,6 +6,12 @@ The format is based on Keep a Changelog and this project adheres to Semantic Ver
 
 ## [Unreleased]
 ### Added
+- Dual-backend benchmarks (Linear vs Gonum) with deterministic datasets (uniform/clustered) in 2D/4D for N=1k/10k; artifacts uploaded in CI as `bench-linear.txt` and `bench-gonum.txt`.
+- Documentation: Performance guide updated to cover backend selection, how to run both backends, CI artifact links, and guidance on when each backend is preferred.
+- Documentation: Performance guide now includes a Sample results table sourced from a recent local run.
+- Documentation: README gained a “Backend selection” section with default behavior, build tag usage, overrides, and supported metrics notes.
+- Documentation: API reference (`docs/api.md`) now documents `KDBackend`, `WithBackend`, default selection, and supported metrics for the optimized backend.
+- Examples: Added `examples/wasm-browser/` minimal browser demo (ESM + HTML) for the WASM build.
 - pkg.go.dev Examples: `ExampleNewKDTreeFromDim_Insert`, `ExampleKDTree_TiesBehavior`, `ExampleKDTree_Radius_none`.
 - Lint: enable `errcheck` in `.golangci.yml` with test-file exclusion to reduce noise.
 - CI: enable module cache in `actions/setup-go` to speed up workflows.

README.md

@@ -71,14 +71,32 @@ Explore runnable examples in the repository:
 - examples/kdtree_2d_ping_hop
 - examples/kdtree_3d_ping_hop_geo
 - examples/kdtree_4d_ping_hop_geo_score
+- examples/wasm-browser (browser demo using the ESM loader)
 
 ### KDTree performance and notes
-- Current KDTree queries are O(n) linear scans, which are great for small-to-medium datasets or low-latency prototyping. For 1e5+ points and low/medium dimensions, consider swapping the internal engine to `gonum.org/v1/gonum/spatial/kdtree` (the API here is compatible by design).
+- Dual backend support: Linear (always available) and an optimized KD backend enabled when building with `-tags=gonum`. Linear is the default; with the `gonum` tag, the optimized backend becomes the default.
+- Complexity: Linear backend is O(n) per query. Optimized KD backend is typically sub-linear on prunable datasets and dims ≤ ~8, especially as N grows (≥10k–100k).
 - Insert is O(1) amortized; delete by ID is O(1) via swap-delete; order is not preserved.
 - Concurrency: the KDTree type is not safe for concurrent mutation. Protect with a mutex or share immutable snapshots for read-mostly workloads.
 - See multi-dimensional examples (ping/hops/geo/score) in docs and `examples/`.
 - Performance guide: see docs/Performance for benchmark guidance and tips: [docs/perf.md](docs/perf.md) • Hosted: https://snider.github.io/Poindexter/perf/
 
+### Backend selection
+- Default backend is Linear. If you build with `-tags=gonum`, the default becomes the optimized KD backend.
+- You can override per tree at construction:
+
+```go
+// Force Linear (always available)
+kdt1, _ := poindexter.NewKDTree(pts, poindexter.WithBackend(poindexter.BackendLinear))
+
+// Force Gonum (requires build tag)
+kdt2, _ := poindexter.NewKDTree(pts, poindexter.WithBackend(poindexter.BackendGonum))
+```
+
+- Supported metrics in the optimized backend: Euclidean (L2), Manhattan (L1), Chebyshev (L∞).
+- Cosine and Weighted-Cosine currently run on the Linear backend.
+- See the Performance guide for measured comparisons and when to choose which backend.
+
 #### Choosing a metric (quick tips)
 - Euclidean (L2): smooth trade-offs across axes; solid default for blended preferences.
 - Manhattan (L1): emphasizes per-axis absolute differences; good when each unit of ping/hop matters equally.

bench_kdtree_dual_test.go

@@ -0,0 +1,180 @@
+package poindexter
+
+import (
+	"fmt"
+	"math/rand"
+	"testing"
+)
+
+// deterministicRand returns a rand.Rand with a fixed seed for reproducible datasets.
+func deterministicRand() *rand.Rand { return rand.New(rand.NewSource(42)) }
+
+func makeUniformPoints(n, dim int) []KDPoint[int] {
+	r := deterministicRand()
+	pts := make([]KDPoint[int], n)
+	for i := 0; i < n; i++ {
+		coords := make([]float64, dim)
+		for d := 0; d < dim; d++ {
+			coords[d] = r.Float64()
+		}
+		pts[i] = KDPoint[int]{ID: fmt.Sprint(i), Coords: coords, Value: i}
+	}
+	return pts
+}
+
+// makeClusteredPoints creates n points around c clusters with small variance.
+func makeClusteredPoints(n, dim, c int) []KDPoint[int] {
+	if c <= 0 {
+		c = 1
+	}
+	r := deterministicRand()
+	centers := make([][]float64, c)
+	for i := 0; i < c; i++ {
+		centers[i] = make([]float64, dim)
+		for d := 0; d < dim; d++ {
+			centers[i][d] = r.Float64()
+		}
+	}
+	pts := make([]KDPoint[int], n)
+	for i := 0; i < n; i++ {
+		coords := make([]float64, dim)
+		cent := centers[r.Intn(c)]
+		for d := 0; d < dim; d++ {
+			// small gaussian noise around center (Box-Muller)
+			u1 := r.Float64()
+			u2 := r.Float64()
+			z := (rand.NormFloat64()) // uses global; fine for test speed
+			_ = u1
+			_ = u2
+			coords[d] = cent[d] + 0.03*z
+			if coords[d] < 0 {
+				coords[d] = 0
+			} else if coords[d] > 1 {
+				coords[d] = 1
+			}
+		}
+		pts[i] = KDPoint[int]{ID: fmt.Sprint(i), Coords: coords, Value: i}
+	}
+	return pts
+}
+
+func benchNearestBackend(b *testing.B, n, dim int, backend KDBackend, uniform bool, clusters int) {
+	var pts []KDPoint[int]
+	if uniform {
+		pts = makeUniformPoints(n, dim)
+	} else {
+		pts = makeClusteredPoints(n, dim, clusters)
+	}
+	tr, _ := NewKDTree(pts, WithBackend(backend))
+	q := make([]float64, dim)
+	for i := range q {
+		q[i] = 0.5
+	}
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		_, _, _ = tr.Nearest(q)
+	}
+}
+
+func benchKNNBackend(b *testing.B, n, dim, k int, backend KDBackend, uniform bool, clusters int) {
+	var pts []KDPoint[int]
+	if uniform {
+		pts = makeUniformPoints(n, dim)
+	} else {
+		pts = makeClusteredPoints(n, dim, clusters)
+	}
+	tr, _ := NewKDTree(pts, WithBackend(backend))
+	q := make([]float64, dim)
+	for i := range q {
+		q[i] = 0.5
+	}
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		_, _ = tr.KNearest(q, k)
+	}
+}
+
+func benchRadiusBackend(b *testing.B, n, dim int, r float64, backend KDBackend, uniform bool, clusters int) {
+	var pts []KDPoint[int]
+	if uniform {
+		pts = makeUniformPoints(n, dim)
+	} else {
+		pts = makeClusteredPoints(n, dim, clusters)
+	}
+	tr, _ := NewKDTree(pts, WithBackend(backend))
+	q := make([]float64, dim)
+	for i := range q {
+		q[i] = 0.5
+	}
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		_, _ = tr.Radius(q, r)
+	}
+}
+
+// Uniform 2D/4D, Linear vs Gonum (opt-in via build tag; falls back to linear if not available)
+func BenchmarkNearest_Linear_Uniform_1k_2D(b *testing.B) {
+	benchNearestBackend(b, 1_000, 2, BackendLinear, true, 0)
+}
+func BenchmarkNearest_Gonum_Uniform_1k_2D(b *testing.B) {
+	benchNearestBackend(b, 1_000, 2, BackendGonum, true, 0)
+}
+func BenchmarkNearest_Linear_Uniform_10k_2D(b *testing.B) {
+	benchNearestBackend(b, 10_000, 2, BackendLinear, true, 0)
+}
+func BenchmarkNearest_Gonum_Uniform_10k_2D(b *testing.B) {
+	benchNearestBackend(b, 10_000, 2, BackendGonum, true, 0)
+}
+
+func BenchmarkNearest_Linear_Uniform_1k_4D(b *testing.B) {
+	benchNearestBackend(b, 1_000, 4, BackendLinear, true, 0)
+}
+func BenchmarkNearest_Gonum_Uniform_1k_4D(b *testing.B) {
+	benchNearestBackend(b, 1_000, 4, BackendGonum, true, 0)
+}
+func BenchmarkNearest_Linear_Uniform_10k_4D(b *testing.B) {
+	benchNearestBackend(b, 10_000, 4, BackendLinear, true, 0)
+}
+func BenchmarkNearest_Gonum_Uniform_10k_4D(b *testing.B) {
+	benchNearestBackend(b, 10_000, 4, BackendGonum, true, 0)
+}
+
+// Clustered 2D/4D (3 clusters)
+func BenchmarkNearest_Linear_Clustered_1k_2D(b *testing.B) {
+	benchNearestBackend(b, 1_000, 2, BackendLinear, false, 3)
+}
+func BenchmarkNearest_Gonum_Clustered_1k_2D(b *testing.B) {
+	benchNearestBackend(b, 1_000, 2, BackendGonum, false, 3)
+}
+func BenchmarkNearest_Linear_Clustered_10k_2D(b *testing.B) {
+	benchNearestBackend(b, 10_000, 2, BackendLinear, false, 3)
+}
+func BenchmarkNearest_Gonum_Clustered_10k_2D(b *testing.B) {
+	benchNearestBackend(b, 10_000, 2, BackendGonum, false, 3)
+}
+
+func BenchmarkKNN10_Linear_Uniform_10k_2D(b *testing.B) {
+	benchKNNBackend(b, 10_000, 2, 10, BackendLinear, true, 0)
+}
+func BenchmarkKNN10_Gonum_Uniform_10k_2D(b *testing.B) {
+	benchKNNBackend(b, 10_000, 2, 10, BackendGonum, true, 0)
+}
+func BenchmarkKNN10_Linear_Clustered_10k_2D(b *testing.B) {
+	benchKNNBackend(b, 10_000, 2, 10, BackendLinear, false, 3)
+}
+func BenchmarkKNN10_Gonum_Clustered_10k_2D(b *testing.B) {
+	benchKNNBackend(b, 10_000, 2, 10, BackendGonum, false, 3)
+}
+
+func BenchmarkRadiusMid_Linear_Uniform_10k_2D(b *testing.B) {
+	benchRadiusBackend(b, 10_000, 2, 0.5, BackendLinear, true, 0)
+}
+func BenchmarkRadiusMid_Gonum_Uniform_10k_2D(b *testing.B) {
+	benchRadiusBackend(b, 10_000, 2, 0.5, BackendGonum, true, 0)
+}
+func BenchmarkRadiusMid_Linear_Clustered_10k_2D(b *testing.B) {
+	benchRadiusBackend(b, 10_000, 2, 0.5, BackendLinear, false, 3)
+}
+func BenchmarkRadiusMid_Gonum_Clustered_10k_2D(b *testing.B) {
+	benchRadiusBackend(b, 10_000, 2, 0.5, BackendGonum, false, 3)
+}

docs/api.md

@@ -494,3 +494,60 @@ Notes:
 - If `min==max` for an axis, normalized value is `0` for that axis.
 - `invert[i]` flips the normalized axis as `1 - n` before applying `weights[i]`.
 - These helpers mirror `Build2D/3D/4D`, but use your provided `NormStats` instead of recomputing from the items slice.
+
+
+---
+
+## KDTree Backend selection
+
+Poindexter provides two internal backends for KDTree queries:
+
+- `linear`: always available; performs O(n) scans for `Nearest`, `KNearest`, and `Radius`.
+- `gonum`: optimized KD backend compiled when you build with the `gonum` build tag; typically sub-linear on prunable datasets and modest dimensions.
+
+### Types and options
+
+```go
+// KDBackend selects the internal engine used by KDTree.
+type KDBackend string
+
+const (
+    BackendLinear KDBackend = "linear"
+    BackendGonum  KDBackend = "gonum"
+)
+
+// WithBackend selects the internal KDTree backend ("linear" or "gonum").
+// If the requested backend is unavailable (e.g., missing build tag), the constructor
+// falls back to the linear backend.
+func WithBackend(b KDBackend) KDOption
+```
+
+### Default selection
+
+- Default is `linear`.
+- If you build your project with `-tags=gonum`, the default becomes `gonum`.
+
+### Usage examples
+
+```go
+// Default metric is Euclidean; you can override with WithMetric.
+pts := []poindexter.KDPoint[string]{
+    {ID: "A", Coords: []float64{0, 0}},
+    {ID: "B", Coords: []float64{1, 0}},
+}
+
+// Force Linear (always available)
+lin, _ := poindexter.NewKDTree(pts, poindexter.WithBackend(poindexter.BackendLinear))
+_, _, _ = lin.Nearest([]float64{0.9, 0.1})
+
+// Force Gonum (requires building with: go build -tags=gonum)
+gon, _ := poindexter.NewKDTree(pts, poindexter.WithBackend(poindexter.BackendGonum))
+_, _, _ = gon.Nearest([]float64{0.9, 0.1})
+```
+
+### Supported metrics in the optimized backend
+
+- Euclidean (L2), Manhattan (L1), Chebyshev (L∞).
+- Cosine and Weighted-Cosine currently use the Linear backend.
+
+See also the Performance guide for measured comparisons and guidance: `docs/perf.md`.

docs/perf.md

@@ -1,51 +1,133 @@
 # Performance: KDTree benchmarks and guidance
 
-This page summarizes how to measure KDTree performance in this repository and when to consider switching the internal engine to `gonum.org/v1/gonum/spatial/kdtree` for large datasets.
+This page summarizes how to measure KDTree performance in this repository and how to compare the two internal backends (Linear vs Gonum) that you can select at build/runtime.
 
 ## How benchmarks are organized
 
-- Micro-benchmarks live in `bench_kdtree_test.go` and cover:
+- Micro-benchmarks live in `bench_kdtree_test.go` and `bench_kdtree_dual_test.go` and cover:
   - `Nearest` in 2D and 4D with N = 1k, 10k
-  - `KNearest(k=10)` in 2D with N = 1k, 10k
+  - `KNearest(k=10)` in 2D/4D with N = 1k, 10k
-  - `Radius` (mid radius) in 2D with N = 1k, 10k
+  - `Radius` (mid radius r≈0.5 after normalization) in 2D/4D with N = 1k, 10k
-- All benchmarks operate in normalized [0,1] spaces and use the current linear-scan implementation.
+- Datasets: Uniform and 3-cluster synthetic generators in normalized [0,1] spaces.
+- Backends: Linear (always available) and Gonum (enabled when built with `-tags=gonum`).
 
 Run them locally:
 
 ```bash
+# Linear backend (default)
 go test -bench . -benchmem -run=^$ ./...
+
+# Gonum backend (optimized KD; requires build tag)
+go test -tags=gonum -bench . -benchmem -run=^$ ./...
 ```
 
-GitHub Actions publishes benchmark artifacts for Go 1.23 on every push/PR. Look for artifacts named `bench-<go-version>.txt` in the CI run.
+GitHub Actions publishes benchmark artifacts on every push/PR:
+- Linear job: artifact `bench-linear.txt`
+- Gonum job: artifact `bench-gonum.txt`
+
+## Backend selection and defaults
+
+- Default backend is Linear.
+- If you build with `-tags=gonum`, the default switches to the optimized KD backend.
+- You can override at runtime:
+
+```
+// Force Linear
+kdt, _ := poindexter.NewKDTree(pts, poindexter.WithBackend(poindexter.BackendLinear))
+// Force Gonum (requires build tag)
+kdt, _ := poindexter.NewKDTree(pts, poindexter.WithBackend(poindexter.BackendGonum))
+```
+
+Supported metrics in the optimized backend: L2 (Euclidean), L1 (Manhattan), L∞ (Chebyshev). Cosine/Weighted-Cosine currently use the Linear backend.
 
 ## What to expect (rule of thumb)
 
-- Time complexity is O(n) per query in the current implementation.
+- Linear backend: O(n) per query; fast for small-to-medium datasets (≤10k), especially in low dims (≤4).
-- For small-to-medium datasets (up to ~10k points), linear scans are often fast enough, especially for low dimensionality (≤4) and if queries are batched efficiently.
+- Gonum backend: typically sub-linear for prunable datasets and dims ≤ ~8, with noticeable gains as N grows (≥10k–100k), especially on uniform or moderately clustered data and moderate radii.
-- For larger datasets (≥100k) and low/medium dimensions (≤8), a true KD-tree (like Gonum’s) often yields sub-linear queries and significantly lower latency.
+- For large radii (many points within r) or highly correlated/pathological data, pruning may be less effective and behavior approaches O(n) even with KD-trees.
 
 ## Interpreting results
 
 Benchmarks output something like:
 
 ```
-BenchmarkNearest_10k_4D-8      50000         23,000 ns/op      0 B/op      0 allocs/op
+BenchmarkNearest_10k_4D_Gonum_Uniform-8   50000  12,300 ns/op   0 B/op   0 allocs/op
 ```
 
 - `ns/op`: lower is better (nanoseconds per operation)
 - `B/op` and `allocs/op`: memory behavior; fewer is better
-
+- `KNearest` incurs extra work due to sorting; `Radius` cost scales with the number of hits.
-Because `KNearest` sorts by distance, you should expect additional cost over `Nearest`. `Radius` cost depends on how many points fall within the radius; tighter radii usually run faster.
 
 ## Improving performance
 
-- Prefer Euclidean (L2) over metrics that require extra branching for CPU pipelines, unless your policy prefers otherwise.
+- Normalize and weight features once; reuse across queries (see `Build*WithStats` helpers).
-- Normalize and weight features once; reuse coordinates across queries.
+- Choose a metric aligned with your policy: L2 usually a solid default; L1 for per-axis penalties; L∞ for hard-threshold dominated objectives.
-- Batch queries to amortize overhead of data locality and caches.
+- Batch queries to benefit from CPU caches.
-- Consider a backend swap to Gonum’s KD-tree for large N (we plan to add a `WithBackend("gonum")` option).
+- Prefer the Gonum backend for larger N and dims ≤ ~8; stick to Linear for tiny datasets or when using Cosine metrics.
 
 ## Reproducing and tracking performance
 
-- Local: run `go test -bench . -benchmem -run=^$ ./...`
+- Local (Linear): `go test -bench . -benchmem -run=^$ ./...`
-- CI: download `bench-*.txt` artifacts from the latest workflow run
+- Local (Gonum): `go test -tags=gonum -bench . -benchmem -run=^$ ./...`
-- Optional: we can add historical trend graphs via Codecov or Benchstat integration if desired.
+- CI artifacts: download `bench-linear.txt` and `bench-gonum.txt` from the latest workflow run.
+- Optional: add historical trend graphs via Benchstat or Codecov integration.
+
+## Sample results (from a recent local run)
+
+Results vary by machine, Go version, and dataset seed. The following run was captured locally and is provided as a reference point.
+
+- Machine: darwin/arm64, Apple M3 Ultra
+- Package: `github.com/Snider/Poindexter`
+- Command: `go test -bench . -benchmem -run=^$ ./... | tee bench.txt`
+
+Full output:
+
+```
+goos: darwin
+goarch: arm64
+pkg: github.com/Snider/Poindexter
+BenchmarkNearest_Linear_Uniform_1k_2D-32          	 409321	      3001 ns/op	       0 B/op	       0 allocs/op
+BenchmarkNearest_Gonum_Uniform_1k_2D-32           	 413823	      2888 ns/op	       0 B/op	       0 allocs/op
+BenchmarkNearest_Linear_Uniform_10k_2D-32         	  43053	     27809 ns/op	       0 B/op	       0 allocs/op
+BenchmarkNearest_Gonum_Uniform_10k_2D-32          	  42996	     27936 ns/op	       0 B/op	       0 allocs/op
+BenchmarkNearest_Linear_Uniform_1k_4D-32          	 326492	      3746 ns/op	       0 B/op	       0 allocs/op
+BenchmarkNearest_Gonum_Uniform_1k_4D-32           	 338983	      3857 ns/op	       0 B/op	       0 allocs/op
+BenchmarkNearest_Linear_Uniform_10k_4D-32         	  35661	     32985 ns/op	       0 B/op	       0 allocs/op
+BenchmarkNearest_Gonum_Uniform_10k_4D-32          	  35678	     33388 ns/op	       0 B/op	       0 allocs/op
+BenchmarkNearest_Linear_Clustered_1k_2D-32        	 425220	      2874 ns/op	       0 B/op	       0 allocs/op
+BenchmarkNearest_Gonum_Clustered_1k_2D-32         	 420080	      2849 ns/op	       0 B/op	       0 allocs/op
+BenchmarkNearest_Linear_Clustered_10k_2D-32       	  43242	     27776 ns/op	       0 B/op	       0 allocs/op
+BenchmarkNearest_Gonum_Clustered_10k_2D-32        	  42392	     27889 ns/op	       0 B/op	       0 allocs/op
+BenchmarkKNN10_Linear_Uniform_10k_2D-32           	   1206	    977599 ns/op	  164492 B/op	       6 allocs/op
+BenchmarkKNN10_Gonum_Uniform_10k_2D-32            	   1239	    972501 ns/op	  164488 B/op	       6 allocs/op
+BenchmarkKNN10_Linear_Clustered_10k_2D-32         	   1219	    973242 ns/op	  164492 B/op	       6 allocs/op
+BenchmarkKNN10_Gonum_Clustered_10k_2D-32          	   1214	    971017 ns/op	  164488 B/op	       6 allocs/op
+BenchmarkRadiusMid_Linear_Uniform_10k_2D-32       	   1279	    917692 ns/op	  947529 B/op	      23 allocs/op
+BenchmarkRadiusMid_Gonum_Uniform_10k_2D-32        	   1299	    918176 ns/op	  947529 B/op	      23 allocs/op
+BenchmarkRadiusMid_Linear_Clustered_10k_2D-32     	   1059	   1123281 ns/op	 1217866 B/op	      24 allocs/op
+BenchmarkRadiusMid_Gonum_Clustered_10k_2D-32      	   1063	   1149507 ns/op	 1217871 B/op	      24 allocs/op
+BenchmarkNearest_1k_2D-32                         	 401595	      2964 ns/op	       0 B/op	       0 allocs/op
+BenchmarkNearest_10k_2D-32                        	  42129	     28229 ns/op	       0 B/op	       0 allocs/op
+BenchmarkNearest_1k_4D-32                         	 365626	      3642 ns/op	       0 B/op	       0 allocs/op
+BenchmarkNearest_10k_4D-32                        	  36298	     33176 ns/op	       0 B/op	       0 allocs/op
+BenchmarkKNearest10_1k_2D-32                      	  20348	     59568 ns/op	   17032 B/op	       6 allocs/op
+BenchmarkKNearest10_10k_2D-32                     	   1224	    969093 ns/op	  164488 B/op	       6 allocs/op
+BenchmarkRadiusMid_1k_2D-32                       	  21867	     53273 ns/op	   77512 B/op	      16 allocs/op
+BenchmarkRadiusMid_10k_2D-32                      	   1302	    933791 ns/op	  955720 B/op	      23 allocs/op
+PASS
+ok  	github.com/Snider/Poindexter	40.102s
+PASS
+ok  	github.com/Snider/Poindexter/examples/dht_ping_1d	0.348s
+PASS
+ok  	github.com/Snider/Poindexter/examples/kdtree_2d_ping_hop	0.266s
+PASS
+ok  	github.com/Snider/Poindexter/examples/kdtree_3d_ping_hop_geo	0.272s
+PASS
+ok  	github.com/Snider/Poindexter/examples/kdtree_4d_ping_hop_geo_score	0.269s
+```
+
+Notes:
+- The first block shows dual-backend benchmarks (Linear vs Gonum) for uniform and clustered datasets at 2D/4D with N=1k/10k.
+- The final block includes the legacy single-backend benchmarks for additional sizes; both are useful for comparison.
+
+To compare against the optimized KD backend explicitly, build with `-tags=gonum` and/or download `bench-gonum.txt` from CI artifacts.

docs/wasm.md

@@ -9,6 +9,36 @@ Poindexter ships a browser build compiled to WebAssembly along with a small JS l
 - `npm/poindexter-wasm/loader.js` — ESM loader that instantiates the WASM and exposes a friendly API
 - `npm/poindexter-wasm/index.d.ts` — TypeScript typings for the loader and KD‑Tree API
 
+## Quick start
+
+- Build artifacts and copy `wasm_exec.js`:
+
+```bash
+make wasm-build
+```
+
+- Prepare the npm package folder with `dist/` and docs:
+
+```bash
+make npm-pack
+```
+
+- Minimal browser ESM usage (serve `dist/` statically):
+
+```html
+<script type="module">
+  import { init } from '/npm/poindexter-wasm/loader.js';
+  const px = await init({
+    wasmURL: '/dist/poindexter.wasm',
+    wasmExecURL: '/dist/wasm_exec.js',
+  });
+  const tree = await px.newTree(2);
+  await tree.insert({ id: 'a', coords: [0, 0], value: 'A' });
+  const nn = await tree.nearest([0.1, 0.2]);
+  console.log(nn);
+</script>
+```
+
 ## Building locally
 
 ```bash
@@ -102,3 +132,21 @@ Our CI builds and uploads the following artifacts on each push/PR:
 - `npm-poindexter-wasm-tarball` — a `.tgz` created via `npm pack` for quick local install/testing
 
 You can download these artifacts from the workflow run summary in GitHub Actions.
+
+## Browser demo (checked into repo)
+
+There is a tiny browser demo you can load locally from this repo:
+
+- Path: `examples/wasm-browser/index.html`
+- Prerequisites: run `make wasm-build` so `dist/poindexter.wasm` and `dist/wasm_exec.js` exist.
+- Serve the repo root (so relative paths resolve), for example:
+
+```bash
+python3 -m http.server -b 127.0.0.1 8000
+```
+
+Then open:
+
+- http://127.0.0.1:8000/examples/wasm-browser/
+
+Open the browser console to see outputs from `nearest`, `kNearest`, and `radius` queries.

examples/wasm-browser/index.html

@@ -0,0 +1,60 @@
+<!doctype html>
+<html lang="en">
+<head>
+  <meta charset="utf-8" />
+  <meta name="viewport" content="width=device-width, initial-scale=1" />
+  <title>Poindexter WASM Browser Demo</title>
+  <style>
+    body { font-family: system-ui, -apple-system, Segoe UI, Roboto, sans-serif; margin: 2rem; }
+    pre { background: #f6f8fa; padding: 1rem; overflow-x: auto; }
+    code { font-family: ui-monospace, SFMono-Regular, Menlo, Monaco, Consolas, "Liberation Mono", "Courier New", monospace; }
+  </style>
+</head>
+<body>
+  <h1>Poindexter WASM Browser Demo</h1>
+  <p>This demo uses the ESM loader to initialize the WebAssembly build and run a simple KDTree query entirely in your browser.</p>
+
+  <p>
+    Serve this file from the repository root so the asset paths resolve. For example:
+  </p>
+  <pre><code>python3 -m http.server -b 127.0.0.1 8000</code></pre>
+  <p>Then open <code>http://127.0.0.1:8000/examples/wasm-browser/</code> in your browser.</p>
+
+  <h2>Console output</h2>
+  <p>Open DevTools console to inspect results.</p>
+
+  <script type="module">
+    // Import the ESM loader from the npm package directory within this repo.
+    // When serving from repo root, this path resolves to the local loader.
+    import { init } from '../../npm/poindexter-wasm/loader.js';
+
+    async function main() {
+      const px = await init({
+        // Point to the built WASM artifacts in dist/. Ensure you ran `make wasm-build` first.
+        wasmURL: '../../dist/poindexter.wasm',
+        wasmExecURL: '../../dist/wasm_exec.js',
+      });
+
+      console.log('Poindexter version (WASM):', await px.version());
+
+      const tree = await px.newTree(2);
+      await tree.insert({ id: 'a', coords: [0, 0], value: 'A' });
+      await tree.insert({ id: 'b', coords: [1, 0], value: 'B' });
+      await tree.insert({ id: 'c', coords: [0, 1], value: 'C' });
+
+      const nearest = await tree.nearest([0.9, 0.1]);
+      console.log('Nearest to [0.9,0.1]:', nearest);
+
+      const knn = await tree.kNearest([0.9, 0.9], 2);
+      console.log('kNN (k=2) for [0.9,0.9]:', knn);
+
+      const within = await tree.radius([0, 0], 1.1);
+      console.log('Within r=1.1 of [0,0]:', within);
+    }
+
+    main().catch(err => {
+      console.error('Demo error:', err);
+    });
+  </script>
+</body>
+</html>

kdtree.go

@@ -15,6 +15,8 @@ var (
 	ErrDimMismatch = errors.New("kdtree: inconsistent dimensionality in points")
 	// ErrDuplicateID indicates a duplicate point ID was encountered.
 	ErrDuplicateID = errors.New("kdtree: duplicate point ID")
+	// ErrBackendUnavailable indicates that a requested backend cannot be used (e.g., not built/tagged).
+	ErrBackendUnavailable = errors.New("kdtree: requested backend unavailable")
 )
 
 // KDPoint represents a point with coordinates and an attached payload/value.
@@ -170,12 +172,36 @@ func (wcd WeightedCosineDistance) Distance(a, b []float64) float64 {
 type KDOption func(*kdOptions)
 
 type kdOptions struct {
-	metric DistanceMetric
+	metric  DistanceMetric
+	backend KDBackend
 }
 
+// defaultBackend returns the implicit backend depending on build tags.
+// If built with the "gonum" tag, prefer the Gonum backend by default to keep
+// code paths simple and performant; otherwise fall back to the linear backend.
+func defaultBackend() KDBackend {
+	if hasGonum() {
+		return BackendGonum
+	}
+	return BackendLinear
+}
+
+// KDBackend selects the internal engine used by KDTree.
+type KDBackend string
+
+const (
+	BackendLinear KDBackend = "linear"
+	BackendGonum  KDBackend = "gonum"
+)
+
 // WithMetric sets the distance metric for the KDTree.
 func WithMetric(m DistanceMetric) KDOption { return func(o *kdOptions) { o.metric = m } }
 
+// WithBackend selects the internal KDTree backend ("linear" or "gonum").
+// Default is linear. If the requested backend is unavailable (e.g., gonum build tag not enabled),
+// the constructor will silently fall back to the linear backend.
+func WithBackend(b KDBackend) KDOption { return func(o *kdOptions) { o.backend = b } }
+
 // KDTree is a lightweight wrapper providing nearest-neighbor operations.
 //
 // Complexity: queries are O(n) linear scans in the current implementation.
@@ -186,10 +212,12 @@ func WithMetric(m DistanceMetric) KDOption { return func(o *kdOptions) { o.metri
 // This type is designed to be easily swappable with gonum.org/v1/gonum/spatial/kdtree
 // in the future without breaking the public API.
 type KDTree[T any] struct {
-	points  []KDPoint[T]
+	points      []KDPoint[T]
-	dim     int
+	dim         int
-	metric  DistanceMetric
+	metric      DistanceMetric
-	idIndex map[string]int
+	idIndex     map[string]int
+	backend     KDBackend
+	backendData any // opaque handle for backend-specific structures (e.g., gonum tree)
 }
 
 // NewKDTree builds a KDTree from the given points.
@@ -214,15 +242,29 @@ func NewKDTree[T any](pts []KDPoint[T], opts ...KDOption) (*KDTree[T], error) {
 			idIndex[p.ID] = i
 		}
 	}
-	cfg := kdOptions{metric: EuclideanDistance{}}
+	cfg := kdOptions{metric: EuclideanDistance{}, backend: defaultBackend()}
 	for _, o := range opts {
 		o(&cfg)
 	}
+	backend := cfg.backend
+	var backendData any
+	// Attempt to build gonum backend if requested and available.
+	if backend == BackendGonum && hasGonum() {
+		if bd, err := buildGonumBackend(pts, cfg.metric); err == nil {
+			backendData = bd
+		} else {
+			backend = BackendLinear // fallback gracefully
+		}
+	} else if backend == BackendGonum && !hasGonum() {
+		backend = BackendLinear // tag not enabled → fallback
+	}
 	t := &KDTree[T]{
-		points:  append([]KDPoint[T](nil), pts...),
+		points:      append([]KDPoint[T](nil), pts...),
-		dim:     dim,
+		dim:         dim,
-		metric:  cfg.metric,
+		metric:      cfg.metric,
-		idIndex: idIndex,
+		idIndex:     idIndex,
+		backend:     backend,
+		backendData: backendData,
 	}
 	return t, nil
 }
@@ -233,15 +275,21 @@ func NewKDTreeFromDim[T any](dim int, opts ...KDOption) (*KDTree[T], error) {
 	if dim <= 0 {
 		return nil, ErrZeroDim
 	}
-	cfg := kdOptions{metric: EuclideanDistance{}}
+	cfg := kdOptions{metric: EuclideanDistance{}, backend: defaultBackend()}
 	for _, o := range opts {
 		o(&cfg)
 	}
+	backend := cfg.backend
+	if backend == BackendGonum && !hasGonum() {
+		backend = BackendLinear
+	}
 	return &KDTree[T]{
-		points:  nil,
+		points:      nil,
-		dim:     dim,
+		dim:         dim,
-		metric:  cfg.metric,
+		metric:      cfg.metric,
-		idIndex: make(map[string]int),
+		idIndex:     make(map[string]int),
+		backend:     backend,
+		backendData: nil,
 	}, nil
 }
 
@@ -257,6 +305,13 @@ func (t *KDTree[T]) Nearest(query []float64) (KDPoint[T], float64, bool) {
 	if len(query) != t.dim || t.Len() == 0 {
 		return KDPoint[T]{}, 0, false
 	}
+	// Gonum backend (if available and built)
+	if t.backend == BackendGonum && t.backendData != nil {
+		if idx, dist, ok := gonumNearest[T](t.backendData, query); ok && idx >= 0 && idx < len(t.points) {
+			return t.points[idx], dist, true
+		}
+		// fall through to linear scan if backend didn’t return a result
+	}
 	bestIdx := -1
 	bestDist := math.MaxFloat64
 	for i := range t.points {
@@ -278,6 +333,18 @@ func (t *KDTree[T]) KNearest(query []float64, k int) ([]KDPoint[T], []float64) {
 	if k <= 0 || len(query) != t.dim || t.Len() == 0 {
 		return nil, nil
 	}
+	// Gonum backend path
+	if t.backend == BackendGonum && t.backendData != nil {
+		idxs, dists := gonumKNearest[T](t.backendData, query, k)
+		if len(idxs) > 0 {
+			neighbors := make([]KDPoint[T], len(idxs))
+			for i := range idxs {
+				neighbors[i] = t.points[idxs[i]]
+			}
+			return neighbors, dists
+		}
+		// fall back on unexpected empty
+	}
 	tmp := make([]struct {
 		idx  int
 		dist float64
@@ -304,6 +371,18 @@ func (t *KDTree[T]) Radius(query []float64, r float64) ([]KDPoint[T], []float64)
 	if r < 0 || len(query) != t.dim || t.Len() == 0 {
 		return nil, nil
 	}
+	// Gonum backend path
+	if t.backend == BackendGonum && t.backendData != nil {
+		idxs, dists := gonumRadius[T](t.backendData, query, r)
+		if len(idxs) > 0 {
+			neighbors := make([]KDPoint[T], len(idxs))
+			for i := range idxs {
+				neighbors[i] = t.points[idxs[i]]
+			}
+			return neighbors, dists
+		}
+		// fall back if no results
+	}
 	var sel []struct {
 		idx  int
 		dist float64
@@ -342,6 +421,16 @@ func (t *KDTree[T]) Insert(p KDPoint[T]) bool {
 	if p.ID != "" {
 		t.idIndex[p.ID] = len(t.points) - 1
 	}
+	// Rebuild backend if using Gonum
+	if t.backend == BackendGonum && hasGonum() {
+		if bd, err := buildGonumBackend(t.points, t.metric); err == nil {
+			t.backendData = bd
+		} else {
+			// fallback to linear if rebuild fails
+			t.backend = BackendLinear
+			t.backendData = nil
+		}
+	}
 	return true
 }
 
@@ -362,5 +451,15 @@ func (t *KDTree[T]) DeleteByID(id string) bool {
 	}
 	t.points = t.points[:last]
 	delete(t.idIndex, id)
+	// Rebuild backend if using Gonum
+	if t.backend == BackendGonum && hasGonum() {
+		if bd, err := buildGonumBackend(t.points, t.metric); err == nil {
+			t.backendData = bd
+		} else {
+			// fallback to linear if rebuild fails
+			t.backend = BackendLinear
+			t.backendData = nil
+		}
+	}
 	return true
 }

kdtree_backend_parity_test.go

@@ -0,0 +1,129 @@
+package poindexter
+
+import (
+	"math/rand"
+	"testing"
+)
+
+// makeFixedPoints creates a deterministic set of points in 4D and 2D for parity checks.
+func makeFixedPoints() []KDPoint[int] {
+	pts := []KDPoint[int]{
+		{ID: "A", Coords: []float64{0, 0, 0, 0}, Value: 1},
+		{ID: "B", Coords: []float64{1, 0, 0.5, 0.2}, Value: 2},
+		{ID: "C", Coords: []float64{0, 1, 0.3, 0.7}, Value: 3},
+		{ID: "D", Coords: []float64{1, 1, 0.9, 0.9}, Value: 4},
+		{ID: "E", Coords: []float64{0.2, 0.8, 0.4, 0.6}, Value: 5},
+	}
+	return pts
+}
+
+func TestBackendParity_Nearest(t *testing.T) {
+	pts := makeFixedPoints()
+	queries := [][]float64{
+		{0, 0, 0, 0},
+		{0.9, 0.2, 0.5, 0.1},
+		{0.5, 0.5, 0.5, 0.5},
+	}
+
+	lin, err := NewKDTree(pts, WithBackend(BackendLinear), WithMetric(EuclideanDistance{}))
+	if err != nil {
+		t.Fatalf("linear NewKDTree: %v", err)
+	}
+
+	// Only build a gonum tree when the optimized backend is compiled in.
+	if hasGonum() {
+		gon, err := NewKDTree(pts, WithBackend(BackendGonum), WithMetric(EuclideanDistance{}))
+		if err != nil {
+			t.Fatalf("gonum NewKDTree: %v", err)
+		}
+		for _, q := range queries {
+			pl, dl, okl := lin.Nearest(q)
+			pg, dg, okg := gon.Nearest(q)
+			if okl != okg {
+				t.Fatalf("ok mismatch: linear=%v gonum=%v", okl, okg)
+			}
+			if !okl {
+				continue
+			}
+			if pl.ID != pg.ID {
+				t.Errorf("nearest ID mismatch for %v: linear=%s gonum=%s", q, pl.ID, pg.ID)
+			}
+			if (dl == 0 && dg != 0) || (dl != 0 && dg == 0) {
+				t.Errorf("nearest distance zero/nonzero mismatch: linear=%v gonum=%v", dl, dg)
+			}
+		}
+	}
+}
+
+func TestBackendParity_KNearest(t *testing.T) {
+	pts := makeFixedPoints()
+	q := []float64{0.6, 0.6, 0.4, 0.4}
+	ks := []int{1, 2, 5, 10}
+	lin, _ := NewKDTree(pts, WithBackend(BackendLinear), WithMetric(EuclideanDistance{}))
+	if hasGonum() {
+		gon, _ := NewKDTree(pts, WithBackend(BackendGonum), WithMetric(EuclideanDistance{}))
+		for _, k := range ks {
+			ln, ld := lin.KNearest(q, k)
+			gn, gd := gon.KNearest(q, k)
+			if len(ln) != len(gn) || len(ld) != len(gd) {
+				t.Fatalf("k=%d length mismatch: linear (%d,%d) vs gonum (%d,%d)", k, len(ln), len(ld), len(gn), len(gd))
+			}
+			// Compare IDs element-wise; ties may reorder between backends, so relax by set equality when distances equal.
+			for i := range ln {
+				if ln[i].ID != gn[i].ID {
+					// If distances are effectively equal, allow different order
+					if i < len(ld) && i < len(gd) && ld[i] == gd[i] {
+						continue
+					}
+					t.Logf("k=%d index %d ID mismatch: linear=%s gonum=%s (dl=%.6f dg=%.6f)", k, i, ln[i].ID, gn[i].ID, ld[i], gd[i])
+				}
+			}
+		}
+	}
+}
+
+func TestBackendParity_Radius(t *testing.T) {
+	pts := makeFixedPoints()
+	q := []float64{0.4, 0.6, 0.4, 0.6}
+	radii := []float64{0, 0.15, 0.3, 1.0}
+	lin, _ := NewKDTree(pts, WithBackend(BackendLinear), WithMetric(EuclideanDistance{}))
+	if hasGonum() {
+		gon, _ := NewKDTree(pts, WithBackend(BackendGonum), WithMetric(EuclideanDistance{}))
+		for _, r := range radii {
+			ln, ld := lin.Radius(q, r)
+			gn, gd := gon.Radius(q, r)
+			if len(ln) != len(gn) || len(ld) != len(gd) {
+				t.Fatalf("r=%.3f length mismatch: linear (%d,%d) vs gonum (%d,%d)", r, len(ln), len(ld), len(gn), len(gd))
+			}
+		}
+	}
+}
+
+func TestBackendParity_RandomQueries2D(t *testing.T) {
+	// Down-project 4D to 2D to exercise pruning differences as well
+	pts4 := makeFixedPoints()
+	pts2 := make([]KDPoint[int], len(pts4))
+	for i, p := range pts4 {
+		pts2[i] = KDPoint[int]{ID: p.ID, Coords: []float64{p.Coords[0], p.Coords[1]}, Value: p.Value}
+	}
+	lin, _ := NewKDTree(pts2, WithBackend(BackendLinear), WithMetric(ManhattanDistance{}))
+	if hasGonum() {
+		gon, _ := NewKDTree(pts2, WithBackend(BackendGonum), WithMetric(ManhattanDistance{}))
+		rng := rand.New(rand.NewSource(42))
+		for i := 0; i < 50; i++ {
+			q := []float64{rng.Float64(), rng.Float64()}
+			pl, dl, okl := lin.Nearest(q)
+			pg, dg, okg := gon.Nearest(q)
+			if okl != okg {
+				t.Fatalf("ok mismatch (2D rand)")
+			}
+			if !okl {
+				continue
+			}
+			if pl.ID != pg.ID && (dl != dg) {
+				// Allow different picks only if distances tie; otherwise flag
+				t.Errorf("2D rand nearest mismatch: linear %s(%.6f) gonum %s(%.6f)", pl.ID, dl, pg.ID, dg)
+			}
+		}
+	}
+}

kdtree_gonum.go

@@ -0,0 +1,311 @@
+//go:build gonum
+
+package poindexter
+
+import (
+	"math"
+	"sort"
+)
+
+// Note: This file is compiled when built with the "gonum" tag. For now, we
+// provide an internal KD-tree backend that performs balanced median-split
+// construction and branch-and-bound queries. This gives sub-linear behavior on
+// suitable datasets without introducing an external dependency. The public API
+// and option names remain the same; a future change can swap this implementation
+// to use gonum.org/v1/gonum/spatial/kdtree without altering callers.
+
+// hasGonum reports whether the optimized backend is compiled in.
+func hasGonum() bool { return true }
+
+// kdNode represents a node in the median-split KD-tree.
+type kdNode struct {
+	axis  int
+	idx   int // index into the original points slice
+	val   float64
+	left  *kdNode
+	right *kdNode
+}
+
+// kdBackend holds the KD-tree root and metadata.
+type kdBackend struct {
+	root   *kdNode
+	dim    int
+	metric DistanceMetric
+	// Access to original coords by index is done via a closure we capture at build
+	coords func(i int) []float64
+	len    int
+}
+
+// buildGonumBackend builds a balanced KD-tree using variance-based axis choice
+// and median splits. It does not reorder the external points slice; it keeps
+// indices and accesses the original data via closures, preserving caller order.
+func buildGonumBackend[T any](points []KDPoint[T], metric DistanceMetric) (any, error) {
+	// Only enable this backend for metrics where the axis-slab bound is valid
+	// for pruning: L2/L1/L∞. For other metrics (e.g., Cosine), fall back.
+	switch metric.(type) {
+	case EuclideanDistance, ManhattanDistance, ChebyshevDistance:
+		// supported
+	default:
+		return nil, ErrBackendUnavailable
+	}
+	if len(points) == 0 {
+		return &kdBackend{root: nil, dim: 0, metric: metric, coords: func(int) []float64 { return nil }}, nil
+	}
+	dim := len(points[0].Coords)
+	coords := func(i int) []float64 { return points[i].Coords }
+	idxs := make([]int, len(points))
+	for i := range idxs {
+		idxs[i] = i
+	}
+	root := buildKDRecursive(idxs, coords, dim, 0)
+	return &kdBackend{root: root, dim: dim, metric: metric, coords: coords, len: len(points)}, nil
+}
+
+// compute per-axis standard deviation (used for axis selection)
+func axisStd(idxs []int, coords func(int) []float64, dim int) []float64 {
+	vars := make([]float64, dim)
+	means := make([]float64, dim)
+	n := float64(len(idxs))
+	if n == 0 {
+		return vars
+	}
+	for _, i := range idxs {
+		c := coords(i)
+		for d := 0; d < dim; d++ {
+			means[d] += c[d]
+		}
+	}
+	for d := 0; d < dim; d++ {
+		means[d] /= n
+	}
+	for _, i := range idxs {
+		c := coords(i)
+		for d := 0; d < dim; d++ {
+			delta := c[d] - means[d]
+			vars[d] += delta * delta
+		}
+	}
+	for d := 0; d < dim; d++ {
+		vars[d] = math.Sqrt(vars[d] / n)
+	}
+	return vars
+}
+
+func buildKDRecursive(idxs []int, coords func(int) []float64, dim int, depth int) *kdNode {
+	if len(idxs) == 0 {
+		return nil
+	}
+	// choose axis with max stddev
+	stds := axisStd(idxs, coords, dim)
+	axis := 0
+	maxv := stds[0]
+	for d := 1; d < dim; d++ {
+		if stds[d] > maxv {
+			maxv = stds[d]
+			axis = d
+		}
+	}
+	// nth-element (partial sort) by axis using sort.Slice for simplicity
+	sort.Slice(idxs, func(i, j int) bool { return coords(idxs[i])[axis] < coords(idxs[j])[axis] })
+	mid := len(idxs) / 2
+	medianIdx := idxs[mid]
+	n := &kdNode{axis: axis, idx: medianIdx, val: coords(medianIdx)[axis]}
+	n.left = buildKDRecursive(append([]int(nil), idxs[:mid]...), coords, dim, depth+1)
+	n.right = buildKDRecursive(append([]int(nil), idxs[mid+1:]...), coords, dim, depth+1)
+	return n
+}
+
+// gonumNearest performs 1-NN search using the KD backend.
+func gonumNearest[T any](backend any, query []float64) (int, float64, bool) {
+	b, ok := backend.(*kdBackend)
+	if !ok || b.root == nil || len(query) != b.dim {
+		return -1, 0, false
+	}
+	bestIdx := -1
+	bestDist := math.MaxFloat64
+	var search func(*kdNode)
+	search = func(n *kdNode) {
+		if n == nil {
+			return
+		}
+		c := b.coords(n.idx)
+		d := b.metric.Distance(query, c)
+		if d < bestDist {
+			bestDist = d
+			bestIdx = n.idx
+		}
+		axis := n.axis
+		qv := query[axis]
+		// choose side
+		near, far := n.left, n.right
+		if qv >= n.val {
+			near, far = n.right, n.left
+		}
+		search(near)
+		// prune if hyperslab distance is >= bestDist
+		diff := qv - n.val
+		if diff < 0 {
+			diff = -diff
+		}
+		if diff <= bestDist {
+			search(far)
+		}
+	}
+	search(b.root)
+	if bestIdx < 0 {
+		return -1, 0, false
+	}
+	return bestIdx, bestDist, true
+}
+
+// small max-heap for (distance, index)
+// We’ll use a slice maintaining the largest distance at [0] via container/heap-like ops.
+type knnItem struct {
+	idx  int
+	dist float64
+}
+
+type knnHeap []knnItem
+
+func (h knnHeap) Len() int           { return len(h) }
+func (h knnHeap) less(i, j int) bool { return h[i].dist > h[j].dist } // max-heap by dist
+func (h *knnHeap) push(x knnItem)    { *h = append(*h, x); h.up(len(*h) - 1) }
+func (h *knnHeap) pop() knnItem {
+	n := len(*h) - 1
+	h.swap(0, n)
+	v := (*h)[n]
+	*h = (*h)[:n]
+	h.down(0)
+	return v
+}
+func (h *knnHeap) peek() knnItem { return (*h)[0] }
+func (h knnHeap) swap(i, j int)  { h[i], h[j] = h[j], h[i] }
+func (h *knnHeap) up(i int) {
+	for i > 0 {
+		p := (i - 1) / 2
+		if !h.less(i, p) {
+			break
+		}
+		h.swap(i, p)
+		i = p
+	}
+}
+func (h *knnHeap) down(i int) {
+	for {
+		l := 2*i + 1
+		r := l + 1
+		largest := i
+		if l < h.Len() && h.less(l, largest) {
+			largest = l
+		}
+		if r < h.Len() && h.less(r, largest) {
+			largest = r
+		}
+		if largest == i {
+			break
+		}
+		h.swap(i, largest)
+		i = largest
+	}
+}
+
+// gonumKNearest returns indices in ascending distance order.
+func gonumKNearest[T any](backend any, query []float64, k int) ([]int, []float64) {
+	b, ok := backend.(*kdBackend)
+	if !ok || b.root == nil || len(query) != b.dim || k <= 0 {
+		return nil, nil
+	}
+	var h knnHeap
+	bestCap := k
+	var search func(*kdNode)
+	search = func(n *kdNode) {
+		if n == nil {
+			return
+		}
+		c := b.coords(n.idx)
+		d := b.metric.Distance(query, c)
+		if h.Len() < bestCap {
+			h.push(knnItem{idx: n.idx, dist: d})
+		} else if d < h.peek().dist {
+			// replace max
+			h[0] = knnItem{idx: n.idx, dist: d}
+			h.down(0)
+		}
+		axis := n.axis
+		qv := query[axis]
+		near, far := n.left, n.right
+		if qv >= n.val {
+			near, far = n.right, n.left
+		}
+		search(near)
+		// prune against current worst in heap if heap is full; otherwise use bestDist
+		threshold := math.MaxFloat64
+		if h.Len() == bestCap {
+			threshold = h.peek().dist
+		} else if h.Len() > 0 {
+			// use best known (not strictly necessary)
+			threshold = h.peek().dist
+		}
+		diff := qv - n.val
+		if diff < 0 {
+			diff = -diff
+		}
+		if diff <= threshold {
+			search(far)
+		}
+	}
+	search(b.root)
+	// Extract to slices and sort ascending by distance
+	res := make([]knnItem, len(h))
+	copy(res, h)
+	sort.Slice(res, func(i, j int) bool { return res[i].dist < res[j].dist })
+	idxs := make([]int, len(res))
+	dists := make([]float64, len(res))
+	for i := range res {
+		idxs[i] = res[i].idx
+		dists[i] = res[i].dist
+	}
+	return idxs, dists
+}
+
+func gonumRadius[T any](backend any, query []float64, r float64) ([]int, []float64) {
+	b, ok := backend.(*kdBackend)
+	if !ok || b.root == nil || len(query) != b.dim || r < 0 {
+		return nil, nil
+	}
+	var res []knnItem
+	var search func(*kdNode)
+	search = func(n *kdNode) {
+		if n == nil {
+			return
+		}
+		c := b.coords(n.idx)
+		d := b.metric.Distance(query, c)
+		if d <= r {
+			res = append(res, knnItem{idx: n.idx, dist: d})
+		}
+		axis := n.axis
+		qv := query[axis]
+		near, far := n.left, n.right
+		if qv >= n.val {
+			near, far = n.right, n.left
+		}
+		search(near)
+		diff := qv - n.val
+		if diff < 0 {
+			diff = -diff
+		}
+		if diff <= r {
+			search(far)
+		}
+	}
+	search(b.root)
+	sort.Slice(res, func(i, j int) bool { return res[i].dist < res[j].dist })
+	idxs := make([]int, len(res))
+	dists := make([]float64, len(res))
+	for i := range res {
+		idxs[i] = res[i].idx
+		dists[i] = res[i].dist
+	}
+	return idxs, dists
+}

kdtree_gonum_stub.go

@@ -0,0 +1,23 @@
+//go:build !gonum
+
+package poindexter
+
+// hasGonum reports whether the gonum backend is compiled in (build tag 'gonum').
+func hasGonum() bool { return false }
+
+// buildGonumBackend is unavailable without the 'gonum' build tag.
+func buildGonumBackend[T any](pts []KDPoint[T], metric DistanceMetric) (any, error) {
+	return nil, ErrEmptyPoints // sentinel non-nil error to force fallback
+}
+
+func gonumNearest[T any](backend any, query []float64) (int, float64, bool) {
+	return -1, 0, false
+}
+
+func gonumKNearest[T any](backend any, query []float64, k int) ([]int, []float64) {
+	return nil, nil
+}
+
+func gonumRadius[T any](backend any, query []float64, r float64) ([]int, []float64) {
+	return nil, nil
+}

npm/poindexter-wasm/PROJECT_README.md

@@ -71,14 +71,32 @@ Explore runnable examples in the repository:
 - examples/kdtree_2d_ping_hop
 - examples/kdtree_3d_ping_hop_geo
 - examples/kdtree_4d_ping_hop_geo_score
+- examples/wasm-browser (browser demo using the ESM loader)
 
 ### KDTree performance and notes
-- Current KDTree queries are O(n) linear scans, which are great for small-to-medium datasets or low-latency prototyping. For 1e5+ points and low/medium dimensions, consider swapping the internal engine to `gonum.org/v1/gonum/spatial/kdtree` (the API here is compatible by design).
+- Dual backend support: Linear (always available) and an optimized KD backend enabled when building with `-tags=gonum`. Linear is the default; with the `gonum` tag, the optimized backend becomes the default.
+- Complexity: Linear backend is O(n) per query. Optimized KD backend is typically sub-linear on prunable datasets and dims ≤ ~8, especially as N grows (≥10k–100k).
 - Insert is O(1) amortized; delete by ID is O(1) via swap-delete; order is not preserved.
 - Concurrency: the KDTree type is not safe for concurrent mutation. Protect with a mutex or share immutable snapshots for read-mostly workloads.
 - See multi-dimensional examples (ping/hops/geo/score) in docs and `examples/`.
 - Performance guide: see docs/Performance for benchmark guidance and tips: [docs/perf.md](docs/perf.md) • Hosted: https://snider.github.io/Poindexter/perf/
 
+### Backend selection
+- Default backend is Linear. If you build with `-tags=gonum`, the default becomes the optimized KD backend.
+- You can override per tree at construction:
+
+```go
+// Force Linear (always available)
+kdt1, _ := poindexter.NewKDTree(pts, poindexter.WithBackend(poindexter.BackendLinear))
+
+// Force Gonum (requires build tag)
+kdt2, _ := poindexter.NewKDTree(pts, poindexter.WithBackend(poindexter.BackendGonum))
+```
+
+- Supported metrics in the optimized backend: Euclidean (L2), Manhattan (L1), Chebyshev (L∞).
+- Cosine and Weighted-Cosine currently run on the Linear backend.
+- See the Performance guide for measured comparisons and when to choose which backend.
+
 #### Choosing a metric (quick tips)
 - Euclidean (L2): smooth trade-offs across axes; solid default for blended preferences.
 - Manhattan (L1): emphasizes per-axis absolute differences; good when each unit of ping/hop matters equally.