Implement UBJ and JSON models for latest XGBoost

internal/xgjson/detect.go

@@ -0,0 +1,121 @@
+package xgjson
+
+import (
+	"bufio"
+	"encoding/binary"
+	"bytes"
+)
+
+// LooksLikeJSON peeks at the buffered reader and returns true if the content
+// appears to be an XGBoost JSON model file. It does not advance the reader.
+//
+// Heuristic: the first non-whitespace byte must be '{', and the string
+// "\"learner\"" (the mandatory top-level key in every XGBoost model) must
+// appear within the first 512 peeked bytes.
+func LooksLikeJSON(r *bufio.Reader) (bool, error) {
+	buf, err := r.Peek(512)
+	if len(buf) == 0 {
+		return false, err
+	}
+
+	// Find first non-whitespace byte; it must be '{'
+	firstNonWS := -1
+	for i, b := range buf {
+		if b != ' ' && b != '\t' && b != '\r' && b != '\n' {
+			firstNonWS = i
+			break
+		}
+	}
+	if firstNonWS < 0 || buf[firstNonWS] != '{' {
+		return false, nil
+	}
+
+	// Confirm the mandatory top-level key is present in the peeked window.
+	// Every XGBoost JSON model has "learner" as its first object key.
+	return bytes.Contains(buf, []byte(`"learner"`)), nil
+}
+
+// LooksLikeUBJ peeks at the buffered reader and returns true if the content
+// appears to be an XGBoost UBJ (Universal Binary JSON) model file. It does
+// not advance the reader.
+//
+// Heuristic: UBJ objects start with '{' (0x7B), and the first key in an
+// XGBoost UBJ model is always "learner". We verify the first byte is '{' and
+// then parse the UBJ key-length encoding + key bytes from the peek buffer,
+// checking that the first key is exactly "learner".
+//
+// UBJ key lengths are encoded as: <type-marker> <integer-bytes>, where the
+// marker is one of: 'i' (int8), 'U' (uint8), 'I' (int16 BE), 'l' (int32 BE),
+// 'L' (int64 BE). XGBoost currently uses 'L', so we handle all widths for
+// robustness.
+func LooksLikeUBJ(r *bufio.Reader) (bool, error) {
+	// 32 bytes is more than enough: 1 ('{') + 1 (marker) + 8 (int64) + 7 ("learner") = 17
+	buf, err := r.Peek(32)
+	if len(buf) < 2 {
+		return false, err
+	}
+
+	if buf[0] != '{' {
+		return false, nil
+	}
+
+	// Parse the key length from buf[1:]
+	keyLen, headerLen, ok := ubjReadLength(buf[1:])
+	if !ok {
+		return false, nil
+	}
+
+	// "learner" is 7 bytes
+	if keyLen != 7 {
+		return false, nil
+	}
+
+	start := 1 + headerLen
+	end := start + 7
+	if end > len(buf) {
+		return false, nil
+	}
+
+	return bytes.Equal(buf[start:end], []byte("learner")), nil
+}
+
+// ubjReadLength parses a UBJ integer-length from b (starting at b[0] which is
+// the type marker). Returns (value, bytesConsumed, ok).
+func ubjReadLength(b []byte) (value int, consumed int, ok bool) {
+	if len(b) < 1 {
+		return 0, 0, false
+	}
+	marker := b[0]
+	switch marker {
+	case 'i': // int8 — 1 data byte
+		if len(b) < 2 {
+			return 0, 0, false
+		}
+		return int(int8(b[1])), 2, true
+	case 'U': // uint8 — 1 data byte
+		if len(b) < 2 {
+			return 0, 0, false
+		}
+		return int(b[1]), 2, true
+	case 'I': // int16 BE — 2 data bytes
+		if len(b) < 3 {
+			return 0, 0, false
+		}
+		v := int16(binary.BigEndian.Uint16(b[1:3]))
+		return int(v), 3, true
+	case 'l': // int32 BE — 4 data bytes
+		if len(b) < 5 {
+			return 0, 0, false
+		}
+		v := int32(binary.BigEndian.Uint32(b[1:5]))
+		return int(v), 5, true
+	case 'L': // int64 BE — 8 data bytes
+		if len(b) < 9 {
+			return 0, 0, false
+		}
+		v := binary.BigEndian.Uint64(b[1:9])
+		return int(v), 9, true
+	default:
+		return 0, 0, false
+	}
+}

internal/xgjson/detect_test.go

@@ -0,0 +1,110 @@
+package xgjson_test
+
+import (
+	"bufio"
+	"bytes"
+	"os"
+	"testing"
+
+	"github.com/dmitryikh/leaves/internal/xgjson"
+)
+
+const testdataDir = "testdata/"
+
+func TestLooksLikeJSON(t *testing.T) {
+	t.Run("real JSON model", func(t *testing.T) {
+		r := openBuf(t, testdataDir+"test_binary_logistic.json")
+		if ok, err := xgjson.LooksLikeJSON(r); err != nil || !ok {
+			t.Errorf("LooksLikeJSON(json) = (%v, %v), want (true, nil)", ok, err)
+		}
+	})
+
+	t.Run("UBJ file is not JSON", func(t *testing.T) {
+		r := openBuf(t, testdataDir+"test_binary_logistic.ubj")
+		if ok, _ := xgjson.LooksLikeJSON(r); ok {
+			t.Error("LooksLikeJSON(ubj) = true, want false")
+		}
+	})
+
+	t.Run("empty reader", func(t *testing.T) {
+		r := bufio.NewReader(bytes.NewReader(nil))
+		if ok, _ := xgjson.LooksLikeJSON(r); ok {
+			t.Error("LooksLikeJSON(empty) = true, want false")
+		}
+	})
+
+	t.Run("random bytes", func(t *testing.T) {
+		r := bufio.NewReader(bytes.NewReader([]byte{0x00, 0x01, 0x02, 0x03, 0xFF}))
+		if ok, _ := xgjson.LooksLikeJSON(r); ok {
+			t.Error("LooksLikeJSON(random bytes) = true, want false")
+		}
+	})
+
+	t.Run("reader is not consumed", func(t *testing.T) {
+		f, err := os.Open(testdataDir + "test_binary_logistic.json")
+		if err != nil {
+			t.Fatal(err)
+		}
+		defer f.Close()
+		r := bufio.NewReader(f)
+		xgjson.LooksLikeJSON(r) //nolint:errcheck
+		// Peek must not consume: a second call should return the same result.
+		if ok, _ := xgjson.LooksLikeJSON(r); !ok {
+			t.Error("second LooksLikeJSON call returned false — reader was consumed")
+		}
+	})
+}
+
+func TestLooksLikeUBJ(t *testing.T) {
+	t.Run("real UBJ model", func(t *testing.T) {
+		r := openBuf(t, testdataDir+"test_binary_logistic.ubj")
+		if ok, err := xgjson.LooksLikeUBJ(r); err != nil || !ok {
+			t.Errorf("LooksLikeUBJ(ubj) = (%v, %v), want (true, nil)", ok, err)
+		}
+	})
+
+	t.Run("JSON file is not UBJ", func(t *testing.T) {
+		r := openBuf(t, testdataDir+"test_binary_logistic.json")
+		if ok, _ := xgjson.LooksLikeUBJ(r); ok {
+			t.Error("LooksLikeUBJ(json) = true, want false")
+		}
+	})
+
+	t.Run("empty reader", func(t *testing.T) {
+		r := bufio.NewReader(bytes.NewReader(nil))
+		if ok, _ := xgjson.LooksLikeUBJ(r); ok {
+			t.Error("LooksLikeUBJ(empty) = true, want false")
+		}
+	})
+
+	t.Run("random bytes", func(t *testing.T) {
+		r := bufio.NewReader(bytes.NewReader([]byte{0x00, 0x01, 0x02, 0x03, 0xFF}))
+		if ok, _ := xgjson.LooksLikeUBJ(r); ok {
+			t.Error("LooksLikeUBJ(random bytes) = true, want false")
+		}
+	})
+
+	t.Run("reader is not consumed", func(t *testing.T) {
+		f, err := os.Open(testdataDir + "test_binary_logistic.ubj")
+		if err != nil {
+			t.Fatal(err)
+		}
+		defer f.Close()
+		r := bufio.NewReader(f)
+		xgjson.LooksLikeUBJ(r) //nolint:errcheck
+		// Peek must not consume: a second call should return the same result.
+		if ok, _ := xgjson.LooksLikeUBJ(r); !ok {
+			t.Error("second LooksLikeUBJ call returned false — reader was consumed")
+		}
+	})
+}
+
+func openBuf(t *testing.T, path string) *bufio.Reader {
+	t.Helper()
+	f, err := os.Open(path)
+	if err != nil {
+		t.Fatalf("open %s: %v", path, err)
+	}
+	t.Cleanup(func() { f.Close() })
+	return bufio.NewReader(f)
+}

internal/xgjson/testdata/README.md

@@ -0,0 +1,22 @@
+# XGBoost Test Model Data
+
+This directory contains generated XGBoost model files used by the `xgjson` package tests.
+
+## Files
+
+- `train.py` — Python script that generates the test models
+- `test1.json` — XGBoost model in JSON format (XGBoost 2.x/3.x)
+- `test1.ubj` — XGBoost model in Universal Binary JSON format (XGBoost 2.x/3.x)
+- `test1_expected.json` — Input feature vectors with expected raw scores and probabilities
+
+## Regenerating
+
+Requires [uv](https://docs.astral.sh/uv/).
+
+```sh
+cd internal/xgjson/testdata
+go generate
+```
+
+This trains a small XGBoost binary classifier (10 estimators, max_depth=3) on synthetic data
+with 3 features, then writes the three output files above. Commit all three output files.

internal/xgjson/testdata/generate.go

@@ -0,0 +1,3 @@
+//go:generate uv run train.py
+
+package testdata

internal/xgjson/testdata/test_binary_logistic.json

@@ -0,0 +1 @@
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