package main

import (
	"fmt"
	"math"
	"strconv"
	"strings"
)

// Pure-Go hexagonal binning for RX coverage display. We deliberately avoid the
// CGO-based uber/h3-go (this project builds with CGO_ENABLED=0). Points are
// projected to Web Mercator and snapped to a pointy-top hex grid whose size
// depends on the display resolution. Cell ids are "res:q:r" (axial coords).
// At city/region scale this looks like H3/mapme.sh coverage without any deps.

const hexEarthRadius = 6378137.0 // Web Mercator sphere radius (m)

// hexTargetPx is the desired on-screen hex size (point-to-point height) in CSS
// pixels. mercUPPZ0 is Web Mercator units per pixel at zoom 0 (world span / 256);
// Leaflet halves it each zoom level, independent of latitude. Sizing the hex in
// these units therefore renders it at a constant ~hexTargetPx at every zoom — the
// old fixed-meter buckets looked like specks when zoomed out (issue: hexes too small).
const hexTargetPx = 28.0
const mercUPPZ0 = 156543.03392

func hexMercator(lat, lon float64) (float64, float64) {
	x := hexEarthRadius * lon * math.Pi / 180
	y := hexEarthRadius * math.Log(math.Tan(math.Pi/4+lat*math.Pi/360))
	return x, y
}

func hexInvMercator(x, y float64) (lat, lon float64) {
	lon = x / hexEarthRadius * 180 / math.Pi
	lat = (2*math.Atan(math.Exp(y/hexEarthRadius)) - math.Pi/2) * 180 / math.Pi
	return lat, lon
}

// hexSizeForRes is the hex circumradius (center→corner) in Web Mercator units for a
// display resolution. Resolution equals the Leaflet zoom level (see zoomToHexRes), so
// the size scales as 2^-zoom and the hex keeps a constant ~hexTargetPx on-screen size
// regardless of zoom. hexCellAt (binning) and hexBoundary (drawing) both read this, so
// they stay consistent for a given cell id.
func hexSizeForRes(res int) float64 {
	return (hexTargetPx / 2) * mercUPPZ0 / math.Pow(2, float64(res))
}

// hexMaxLat is the Web Mercator latitude limit. The projection (hexMercator)
// diverges toward ±90° — tan(π/4 + lat·π/360) → ∞ — so points beyond this would
// produce NaN cell rings via hexInvMercator. Coverage is therefore only defined
// within ±hexMaxLat; polar submissions are clamped to the edge (#17).
const hexMaxLat = 85.05112878

// hexCellAt returns a stable cell id ("res:q:r") for the lat/lon at res. Latitude
// is clamped to ±hexMaxLat so near-polar points bin to the edge instead of
// producing NaN geometry.
func hexCellAt(lat, lon float64, res int) string {
	if lat > hexMaxLat {
		lat = hexMaxLat
	} else if lat < -hexMaxLat {
		lat = -hexMaxLat
	}
	size := hexSizeForRes(res)
	x, y := hexMercator(lat, lon)
	q := (math.Sqrt(3)/3*x - 1.0/3*y) / size
	r := (2.0 / 3 * y) / size
	qi, ri := hexRound(q, r)
	return fmt.Sprintf("%d:%d:%d", res, qi, ri)
}

// hexRound rounds fractional axial coords to the nearest hex via cube rounding.
func hexRound(q, r float64) (int, int) {
	x, z := q, r
	y := -x - z
	rx, ry, rz := math.Round(x), math.Round(y), math.Round(z)
	dx, dy, dz := math.Abs(rx-x), math.Abs(ry-y), math.Abs(rz-z)
	switch {
	case dx > dy && dx > dz:
		rx = -ry - rz
	case dy > dz:
		ry = -rx - rz
	default:
		rz = -rx - ry
	}
	return int(rx), int(rz)
}

// hexBoundary returns the cell's 6 corners as a closed [lon,lat] ring (GeoJSON
// order), or nil if the cell id is malformed.
func hexBoundary(cellID string) [][2]float64 {
	res, q, r, ok := parseHexCell(cellID)
	if !ok {
		return nil
	}
	size := hexSizeForRes(res)
	cx := size * (math.Sqrt(3)*float64(q) + math.Sqrt(3)/2*float64(r))
	cy := size * (1.5 * float64(r))
	ring := make([][2]float64, 0, 7)
	for i := 0; i < 6; i++ {
		ang := math.Pi / 180 * float64(60*i-30)
		lat, lon := hexInvMercator(cx+size*math.Cos(ang), cy+size*math.Sin(ang))
		ring = append(ring, [2]float64{lon, lat})
	}
	ring = append(ring, ring[0]) // close the ring
	return ring
}

func parseHexCell(id string) (res, q, r int, ok bool) {
	p := strings.Split(id, ":")
	if len(p) != 3 {
		return 0, 0, 0, false
	}
	a, e1 := strconv.Atoi(p[0])
	b, e2 := strconv.Atoi(p[1])
	c, e3 := strconv.Atoi(p[2])
	if e1 != nil || e2 != nil || e3 != nil {
		return 0, 0, 0, false
	}
	return a, b, c, true
}