refactor: remove internal/h3core and reorganize x/h3go

Remove the internal/h3core package and inline the shared H3 index-format
constants and pentagon base-cell table into both consumers, so neither
depends on the shared package:
- h3: bit-layout constants become literals (or C macros where the block
  already uses them); the pentagon table becomes a package-level var.
- x/h3go: same constants and pentagon table inlined, keeping the package
  fully cgo-free and self-contained.

Reorganize x/h3go alongside the inlining:
- Group const/var/type declaration blocks at the top of each source file.
- Split error vars into errors.go; rename h3go.go -> constants.go and add
  h3.go for the package doc.
- Move type declarations out of constants.go into the files where their
  methods live (projection primitives -> faceijk.go, etc.).
- Convert bboxFromGeoLoop/bboxesFromGeoPolygon to methods
  (GeoLoop.toBbox / GeoPolygon.toBboxes).
- Replace the shared Cell bit-manipulation methods with generic functions
  over the Index constraint (modeOf, reservedBits, resolution, indexDigit,
  indexDigitChecked, ownerCell) so DirectedEdge and Vertex no longer cast to
  Cell to reuse them; no DirectedEdge/Vertex -> Cell casts remain.
- Add a typed `mode` (iota: cellMode=1, directedEdgeMode=2, edgeMode=3 unused,
  vertexMode=4); modeOf returns it.

No behavior change; parity tests, 100% coverage, vet, and golangci-lint all pass.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: justinhwang

h3.go

@@ -39,8 +39,6 @@ import (
 	"strconv"
 	"strings"
 	"unsafe"
-
-	"github.com/uber/h3-go/v4/internal/h3core"
 )
 
 const (
@@ -49,7 +47,7 @@ const (
 	MaxCellBndryVerts = C.MAX_CELL_BNDRY_VERTS
 
 	// MaxResolution is the maximum H3 resolution a LatLng can be indexed to.
-	MaxResolution = h3core.MaxResolution
+	MaxResolution = C.MAX_H3_RES
 
 	// NumIcosaFaces is the number of faces on an icosahedron.
 	NumIcosaFaces = C.NUM_ICOSA_FACES
@@ -85,18 +83,18 @@ const (
 	// to avoid re-allocation.
 	latLngStringSize = 32
 
-	cellMode         = h3core.CellMode
-	directedEdgeMode = h3core.DirectedEdgeMode
-	vertexMode       = h3core.VertexMode
-	modeOffset       = h3core.ModeOffset
+	cellMode         = 1  // H3_CELL_MODE
+	directedEdgeMode = 2  // H3_DIRECTEDEDGE_MODE
+	vertexMode       = 4  // H3_VERTEX_MODE
+	modeOffset       = 59 // H3_MODE_OFFSET
 	reservedOffset   = 56 // H3_RESERVED_OFFSET
-	resolutionOffset = h3core.ResolutionOffset
-	baseCellOffset   = h3core.BaseCellOffset
-	perDigitOffset   = h3core.PerDigitOffset
-	digitMask        = h3core.DigitMask
+	resolutionOffset = 52 // H3_RES_OFFSET
+	baseCellOffset   = 45 // H3_BC_OFFSET
+	perDigitOffset   = 3  // H3_PER_DIGIT_OFFSET
+	digitMask        = 7  // H3_DIGIT_MASK
 	earthRadiusKm    = C.EARTH_RADIUS_KM
 
-	resolutionMask = h3core.ResolutionMask
+	resolutionMask = 0xF  // low 4 bits of the resolution field
 	baseCellMask   = 0x7F // 7 bits
 	modeMask       = 0xF  // 4 bits
 
@@ -131,6 +129,14 @@ var (
 	}
 	// compile-time check: pow7 must have exactly MaxResolution+1 entries.
 	_ = pow7[MaxResolution]
+
+	// isBaseCellPentagon maps base cell number to whether it is a pentagon.
+	// There are exactly 12 pentagons at every resolution, one for each vertex
+	// of the icosahedron.
+	isBaseCellPentagon = [128]bool{
+		4: true, 14: true, 24: true, 38: true, 49: true, 58: true,
+		63: true, 72: true, 83: true, 97: true, 107: true, 117: true,
+	}
 )
 
 // PolygonToCells containment modes
@@ -1590,7 +1596,7 @@ func hasAll7AfterRes(h uint64, res int) bool {
 }
 
 func hasDeletedSubsequence(h uint64, baseCell int) bool {
-	if !h3core.IsBaseCellPentagon[baseCell] {
+	if !isBaseCellPentagon[baseCell] {
 		return false
 	}
 	h <<= digitRegionOffset
@@ -1606,7 +1612,7 @@ func firstOneIndex(h uint64) int {
 }
 
 func isPentagonCell(c Cell) bool {
-	if !h3core.IsBaseCellPentagon[baseCellNumber(c)] {
+	if !isBaseCellPentagon[baseCellNumber(c)] {
 		return false
 	}
 	for r := 1; r <= resolution(c); r++ {

internal/h3core/h3core.go

@@ -18,26 +18,19 @@ package h3go
 
 import "math"
 
-// bbox is a geographic bounding box with degree coordinates, matching the units
-// of the public GeoPolygon. east < west indicates a box crossing the
-// antimeridian.
-type bbox struct {
-	north, south, east, west float64
-}
-
 // Longitude span constants in degrees, the units of the public API. They stand
 // in for the H3 C library's radian M_PI / M_2PI when detecting and normalizing
 // antimeridian-crossing geometry.
 const (
 	piDeg     = 180.0
 	twoPiDeg  = 360.0
 	halfPiDeg = 90.0
-)
 
-// twoPiRad is a full longitude turn in radians, used to normalize
-// antimeridian-crossing geometry where the math must run at the radian scale of
-// the H3 C library.
-const twoPiRad = 2 * math.Pi
+	// twoPiRad is a full longitude turn in radians, used to normalize
+	// antimeridian-crossing geometry where the math must run at the radian scale
+	// of the H3 C library.
+	twoPiRad = 2 * math.Pi
+)
 
 // longitudeNormalization selects how a longitude is shifted so two bounding
 // boxes, either of which may cross the antimeridian, can be compared in one
@@ -50,6 +43,13 @@ const (
 	normalizeWest
 )
 
+// bbox is a geographic bounding box with degree coordinates, matching the units
+// of the public GeoPolygon. east < west indicates a box crossing the
+// antimeridian.
+type bbox struct {
+	north, south, east, west float64
+}
+
 // isTransmeridian reports whether the bounding box crosses the antimeridian.
 func (b bbox) isTransmeridian() bool {
 	return b.east < b.west
@@ -68,10 +68,10 @@ func (b bbox) contains(point LatLng) bool {
 	return point.Lng >= b.west && point.Lng <= b.east
 }
 
-// bboxFromGeoLoop computes the bounding box of a loop of coordinates. It does not
+// toBbox computes the bounding box of a loop of coordinates. It does not
 // support loops with adjacent points more than 180° of longitude apart (treated
 // as antimeridian crossings) or loops containing a pole.
-func bboxFromGeoLoop(loop GeoLoop) bbox {
+func (loop GeoLoop) toBbox() bbox {
 	if len(loop) == 0 {
 		return bbox{}
 	}
@@ -111,14 +111,14 @@ func bboxFromGeoLoop(loop GeoLoop) bbox {
 	return out
 }
 
-// bboxesFromGeoPolygon returns the bounding box for the outer loop followed by
+// toBboxes returns the bounding box for the outer loop followed by
 // one for each hole, in order.
-func bboxesFromGeoPolygon(polygon GeoPolygon) []bbox {
+func (polygon GeoPolygon) toBboxes() []bbox {
 	bboxes := make([]bbox, len(polygon.Holes)+1)
-	bboxes[0] = bboxFromGeoLoop(polygon.GeoLoop)
+	bboxes[0] = polygon.GeoLoop.toBbox()
 
 	for i := range polygon.Holes {
-		bboxes[i+1] = bboxFromGeoLoop(polygon.Holes[i])
+		bboxes[i+1] = polygon.Holes[i].toBbox()
 	}
 
 	return bboxes

x/h3go/bbox.go

@@ -16,6 +16,13 @@
 
 package h3go
 
+type (
+	// CellBoundary is the ordered set of geographic vertices that outline a cell.
+	// It never has more vertices than a cell has topological vertices plus its
+	// distortion vertices.
+	CellBoundary []LatLng
+)
+
 // CellToBoundary returns the geographic boundary of a cell as an ordered list of
 // vertices in degrees.
 func CellToBoundary(c Cell) (CellBoundary, error) {