path: root/src/LivingVillage.Desktop/MapGen.fs

namespace LivingVillage.Desktop

open LivingVillage.Kernel

/// P20 第一步：确定性大地图生成器的参数化纯函数片。
/// 单一入口 `generate`：seed + 尺寸 -> 瓦片网格（0 grass / 1 water / 2 stone / 3 peat）。
/// 组成：splitmix64 随机游走河道 + 2-3 octave value noise 地形 + 泊松式布点装饰，
/// 核心活动区先整平为草地，再用 BFS 校验 30 个 spawn 全部可达核心（不可达则确定性修整）。
/// 全部为纯函数：同参两次生成逐字节一致，不含时钟/随机源。
module MapGen =

    /// 地面码，与既有 ProceduralMap / 渲染映射保持一致。
    type GroundTile =
        | Grass = 0
        | Water = 1
        | Stone = 2
        | Peat = 3

    type Bounds =
        { MinX: int
          MinY: int
          MaxX: int
          MaxY: int }

    type Params =
        { Width: int
          Height: int
          Seed: uint64
          RiverCount: int
          RiverWidth: int
          CoreSide: int
          SpawnColumns: int
          SpawnRows: int
          /// P24：大图额外沿河民居数量（沿现有路网泊松散布）。默认 0，64x48/256x192 不受影响。
          ExtraRiversideHouses: int
          /// P25：是否放置孤立装饰石块。默认 true；大图关闭，避免草地上出现游离石板 tile。
          DecorativeStones: bool
          /// P26：是否沿海河岸放置河岸装饰（护岸石垒/芦苇/垂柳）。默认 false；仅大图开启。
          RiverDecorations: bool }

    /// P20 第二步：一条贯穿全图的东西向河道（每列恰好一段，宽度下限 RiverWidth）。
    type River =
        { CenterY: int
          Width: int }

    /// 沿河民居色块：白墙黑瓦矩形屋顶，门开向石板路。
    type Building =
        { Left: int
          Top: int
          Width: int
          Height: int
          DoorX: int
          DoorY: int }

    /// P26 河岸装饰种类：护岸石垒 / 芦苇 / 垂柳（垂柳占上下两格）。
    type RiverDecorationKind =
        | RevetmentStone
        | Reeds
        | Willow

    /// P26 河岸装饰：纯视觉，不写入瓦片、不影响通行；仅大图沿海河岸生成。
    type RiverDecoration =
        { Kind: RiverDecorationKind
          X: int
          Y: int }

    type Result =
        { Width: int
          Height: int
          Seed: uint64
          Tiles: int array
          Core: Bounds
          Spawns: (int * int) list
          Rivers: River list
          Bridges: (int * int) list
          Paths: (int * int) list
          Buildings: Building list
          Decorations: RiverDecoration list
          ReachableTiles: int
          ReachabilityOk: bool
          BridgeCrossingsOk: bool }

    /// 默认仍是现状游戏尺寸 64x48，核心 14 格、5x6=30 个 spawn。
    let defaultParams (seed: uint64) : Params =
        { Width = 64
          Height = 48
          Seed = seed
          RiverCount = 2
          RiverWidth = 3
          CoreSide = 14
          SpawnColumns = 5
          SpawnRows = 6
          ExtraRiversideHouses = 0
          DecorativeStones = true
          RiverDecorations = false }

    /// P23 尺寸自适应参数：河道数量随高度增长（约每 96 行一条），其余沿用默认。
    /// 64x48 / 256x192 仍得到 2 条河道，与既有输出逐字节一致；512x384 得到 4 条。
    /// P24：仅对大于 256x192 的图追加沿河民居（泊松散布），保证既有 checksum 不变。
    /// P28：大图沿河民居由 10 增至 16（+6 座，宽度 2/3/4 变化），默认图仍为 0、不消耗 RNG。
    let paramsForSize (width: int) (height: int) (seed: uint64) : Params =
        let base_ = defaultParams seed
        let large = width > 256 || height > 192
        { base_ with
            Width = width
            Height = height
            RiverCount = max base_.RiverCount (max 1 (height / 96))
            ExtraRiversideHouses = (if large then 16 else 0)
            DecorativeStones = not large
            RiverDecorations = large }

    // ---- splitmix64 与 value noise（无外部依赖、无时钟） ----

    let private splitmix (state: uint64) : uint64 * uint64 =
        let nextState = state + 0x9E3779B97F4A7C15UL
        let mutable z = nextState
        z <- (z ^^^ (z >>> 30)) * 0xBF58476D1CE4E5B9UL
        z <- z ^^^ (z >>> 27)
        z <- z * 0x94D049BB133111EBUL
        z <- z ^^^ (z >>> 31)
        (nextState, z)

    let private hash2 (x: int) (y: int) (seed: uint64) : uint64 =
        let base_ = seed ^^^ ((uint64 x * 0x4D3B9UL) + (uint64 y * 0x1D5UL))
        snd (splitmix base_)

    let private valueNoise (fx: float32) (fy: float32) (seed: uint64) (scale: float32) : float32 =
        let xf = fx / scale
        let yf = fy / scale
        let x0 = int (floor xf)
        let y0 = int (floor yf)
        let tx = xf - float32 x0
        let ty = yf - float32 y0
        let smooth (t: float32) = t * t * (3.0f - 2.0f * t)
        let sx = smooth tx
        let sy = smooth ty
        let v00 = float32 (hash2 x0 y0 seed % 1000UL) / 1000.0f
        let v10 = float32 (hash2 (x0 + 1) y0 seed % 1000UL) / 1000.0f
        let v01 = float32 (hash2 x0 (y0 + 1) seed % 1000UL) / 1000.0f
        let v11 = float32 (hash2 (x0 + 1) (y0 + 1) seed % 1000UL) / 1000.0f
        (v00 * (1.0f - sx) + v10 * sx) * (1.0f - sy) + (v01 * (1.0f - sx) + v11 * sx) * sy

    /// 2-3 octave 叠加；特征尺度随地图宽度等比缩放，保证小图/大图观感一致。
    /// P23：特征尺度封顶（<=4x），避免 512x384 退化成过于平滑的「放大图」。
    let private noiseWidthScale (width: int) : float32 =
        min 4.0f (float32 width / 64.0f)

    let private multiOctave (widthScale: float32) (x: int) (y: int) (seed: uint64) : float32 =
        let xf = float32 x
        let yf = float32 y
        let a = valueNoise xf yf seed (96.0f * widthScale)
        let b = valueNoise xf yf (seed + 1UL) (38.0f * widthScale)
        let c = valueNoise xf yf (seed + 2UL) (13.0f * widthScale)
        0.55f * a + 0.3f * b + 0.15f * c

    // ---- 生成 ----

    let private index (width: int) (x: int) (y: int) : int = y * width + x

    let tileAt (result: Result) (x: int) (y: int) : GroundTile =
        enum<GroundTile> result.Tiles.[index result.Width x y]

    /// 参数化生成。尺寸下限 8x8；核心区始终先整平为草地，再 BFS 校验可达性。
    let generate (p: Params) : Result =
        let w = max 8 p.Width
        let h = max 8 p.Height
        let tiles = Array.zeroCreate<int> (w * h) // 0 = Grass
        let idx x y = index w x y

        let side = max 6 (min (min w h) p.CoreSide)
        let cx = w / 2
        let cy = h / 2
        let core =
            { MinX = max 1 (cx - side / 2)
              MinY = max 1 (cy - side / 2)
              MaxX = min (w - 2) (cx + side / 2)
              MaxY = min (h - 2) (cy + side / 2) }
        let inCore x y =
            x >= core.MinX && x <= core.MaxX && y >= core.MinY && y <= core.MaxY

        // 1) 多八度噪声：低洼草地转泥炭（跳过核心区）。
        let widthScale = noiseWidthScale w
        for y in 0 .. h - 1 do
            for x in 0 .. w - 1 do
                if multiOctave widthScale x y p.Seed < 0.18f && not (inCore x y) then
                    tiles.[idx x y] <- int GroundTile.Peat

        // 2) 河道：连续贯穿全图（每列恰好一段），置于核心区上下两侧，宽度恒 >= RiverWidth。
        let riverWidth = max 3 p.RiverWidth
        let riverBands =
            [ for i in 0 .. max 0 p.RiverCount - 1 do
                if i = 0 then
                    let centerY = max 1 (min (h - 1 - riverWidth) (min (core.MinY - riverWidth - 2) (h / 4)))
                    if centerY + riverWidth - 1 < core.MinY then
                        yield { CenterY = centerY; Width = riverWidth }
                elif i = 1 then
                    let centerY = max 1 (min (h - 1 - riverWidth) (max (core.MaxY + 2) (3 * h / 4)))
                    if centerY > core.MaxY then
                        yield { CenterY = centerY; Width = riverWidth }
                else
                    // P23：第 3 条起在核心上/下方逐层外扩堆叠，互不重叠、不压核心。
                    let layer = (i - 1) / 2
                    let offset = if i % 2 = 1 then layer - 1 else layer
                    if i % 2 = 0 then
                        let centerY = core.MinY - riverWidth - 2 - offset * (riverWidth + 3)
                        if centerY >= 1 then
                            yield { CenterY = centerY; Width = riverWidth }
                    else
                        let centerY = core.MaxY + 2 + offset * (riverWidth + 3)
                        if centerY + riverWidth - 1 <= h - 2 then
                            yield { CenterY = centerY; Width = riverWidth } ]
        for river in riverBands do
            for x in 1 .. w - 2 do
                for dy in 0 .. river.Width - 1 do
                    tiles.[idx x (river.CenterY + dy)] <- int GroundTile.Water

        // 3) 核心活动区整平为草地（生成器保证可通行）。
        for y in core.MinY .. core.MaxY do
            for x in core.MinX .. core.MaxX do
                tiles.[idx x y] <- int GroundTile.Grass

        // 4) 泊松式布点：最小间距随尺寸缩放，只在草地打石头装饰（跳过核心区）。
        let minDist = max 3 (w / 24)
        let minSq = minDist * minDist
        let attempts = max 100 ((w * h) / 16)
        let mutable placed: (int * int) list = []
        let mutable scatterRng = p.Seed ^^^ 0xC0FFEEUL
        // P25：大图禁用孤立装饰石块，避免草地上出现游离石板 tile（64x48/256x192 保持开启）。
        if p.DecorativeStones then
            for _ in 1 .. attempts do
                let nextState, raw = splitmix scatterRng
                scatterRng <- nextState
                let x = int (raw % uint64 w)
                let y = int ((raw >>> 20) % uint64 h)
                let far =
                    placed
                    |> List.forall (fun (px, py) ->
                        let dx = px - x
                        let dy = py - y
                        dx * dx + dy * dy > minSq)
                if far && not (inCore x y) && tiles.[idx x y] = int GroundTile.Grass then
                    tiles.[idx x y] <- int GroundTile.Stone
                    placed <- (x, y) :: placed

        // 5) 交通：桥 + 石板路网 + 沿河民居（白墙黑瓦矩形色块，门朝远岸路）。
        let mutable structureRng = p.Seed ^^^ 0x51AB1EUL
        let inMap x y = x >= 0 && x < w && y >= 0 && y < h
        let mutable bridges: (int * int) list = []
        let mutable paths: (int * int) list = []
        let mutable buildings: Building list = []
        let pathSet = System.Collections.Generic.HashSet<int>()
        let addPath x y =
            if inMap x y && tiles.[idx x y] <> int GroundTile.Water then
                tiles.[idx x y] <- int GroundTile.Stone
                if pathSet.Add(idx x y) then paths <- (x, y) :: paths
        let nextBetween lo hi =
            let nextState, raw = splitmix structureRng
            structureRng <- nextState
            lo + int (raw % uint64 (max 1 (hi - lo + 1)))
        let roadY = (core.MinY + core.MaxY) / 2
        for x in 1 .. w - 2 do
            addPath x roadY
        let bridgeColumns =
            [ for river in riverBands do
                let mutable picked: int list = []
                for _ in 1 .. 2 do
                    let mutable bx = nextBetween 4 (max 4 (w - 5))
                    let mutable guard = 0
                    while List.contains bx picked && guard < 8 do
                        bx <- nextBetween 4 (max 4 (w - 5))
                        guard <- guard + 1
                    picked <- bx :: picked
                    yield (river, bx) ]
        for river, bx in bridgeColumns do
            for dx in 0 .. 1 do
                if bx + dx <= w - 2 then
                    for dy in 0 .. river.Width - 1 do
                        let yy = river.CenterY + dy
                        if inMap (bx + dx) yy then
                            bridges <- (bx + dx, yy) :: bridges
            let yLo = min (river.CenterY - 1) roadY
            let yHi = max (river.CenterY + river.Width) roadY
            for y in yLo .. yHi do
                addPath bx y
            let farY = if river.CenterY < core.MinY then river.CenterY - 2 else river.CenterY + river.Width + 1
            for x in max 1 (bx - 7) .. min (w - 2) (bx + 7) do
                addPath x farY
            let buildingWidth = 3 + nextBetween 0 1
            let doorX = min (w - 3) (max 2 (bx + 3))
            let building =
                if river.CenterY < core.MinY then
                    { Left = max 1 (doorX - buildingWidth / 2)
                      Top = max 1 (farY - 2)
                      Width = buildingWidth
                      Height = 2
                      DoorX = doorX
                      DoorY = max 1 (farY - 1) }
                else
                    { Left = max 1 (doorX - buildingWidth / 2)
                      Top = farY + 1
                      Width = buildingWidth
                      Height = 2
                      DoorX = doorX
                      DoorY = farY + 1 }
            buildings <- building :: buildings
            addPath building.DoorX building.DoorY

        // 5b) P24：大图沿河民居加密。从既有路网瓦片出发，向四邻定向落位 3x2 白墙黛瓦民居，
        //     门开向相邻路格并接入路网（BFS 可达）；仅在 ExtraRiversideHouses>0 时执行，
        //     且只在核心区外的草地落位，故 64x48/256x192 输出逐字节不变。
        //     P28：宽度按 2/3/4 加权变化（2=附属小筑、4=大宅），形成聚落层次而非等距重复；
        //     数量 10 -> 16（沿河再增 6 座）。仍只在大图分支执行，默认图 RNG 不消耗，checksum 不变。
        if p.ExtraRiversideHouses > 0 then
            let footprintFree (left: int) (top: int) (width: int) =
                left >= 1 && left + width - 1 <= w - 2 && top >= 1 && top + 1 <= h - 2
                && [ for dx in 0 .. width - 1 do
                       for dy in 0 .. 1 -> (left + dx, top + dy) ]
                   |> List.forall (fun (x, y) ->
                       tiles.[idx x y] = int GroundTile.Grass
                       && not (inCore x y))
            let overlaps (left: int) (top: int) (width: int) =
                buildings
                |> List.exists (fun b ->
                    left < b.Left + b.Width && b.Left < left + width
                    && top < b.Top + b.Height && b.Top < top + 2)
            let directions = [ (0, -1); (0, 1); (-1, 0); (1, 0) ]
            let mutable extraPlaced = 0
            let mutable extraAttempts = 0
            let maxAttempts = p.ExtraRiversideHouses * 400
            while extraPlaced < p.ExtraRiversideHouses && extraAttempts < maxAttempts && paths.Length > 0 do
                extraAttempts <- extraAttempts + 1
                let nextState, raw = splitmix structureRng
                structureRng <- nextState
                if paths.Length > 0 then
                    let pathX, pathY = List.item (int (raw % uint64 paths.Length)) paths
                    let dirX, dirY = directions.[int ((raw >>> 32) % 4UL)]
                    let widthRoll = int ((raw >>> 40) % 10UL)
                    let buildingWidth = if widthRoll < 5 then 3 elif widthRoll < 8 then 2 else 4
                    let doorX = pathX + dirX
                    let doorY = pathY + dirY
                    let left = doorX - buildingWidth / 2
                    let top = doorY - 1
                    if footprintFree left top buildingWidth && not (overlaps left top buildingWidth) then
                        let building =
                            { Left = left
                              Top = top
                              Width = buildingWidth
                              Height = 2
                              DoorX = doorX
                              DoorY = doorY }
                        buildings <- building :: buildings
                        addPath building.DoorX building.DoorY
                        extraPlaced <- extraPlaced + 1

        // 5c) P26：河岸装饰（护岸石垒/芦苇/垂柳），仅大图启用。纯视觉层，不写入任何瓦片，
        //     故 64x48/256x192 的 tiles checksum 与默认世界完全不受影响；装饰只落在紧邻水面的
        //     岸格，且避开路网/桥面/民居/核心区，保证不遮挡通行。
        let mutable decorations: RiverDecoration list = []
        if p.RiverDecorations then
            let mutable decorRng = p.Seed ^^^ 0xD3C0DEUL
            let nextDecor () =
                let nextState, raw = splitmix decorRng
                decorRng <- nextState
                raw
            let bridgeLookup = System.Collections.Generic.HashSet<int>(bridges |> List.map (fun (x, y) -> idx x y))
            let inBuildingFootprint x y =
                buildings
                |> List.exists (fun b ->
                    x >= b.Left - 1 && x <= b.Left + b.Width && y >= b.Top && y <= b.DoorY + 1)
            let blocked x y =
                not (inMap x y)
                || tiles.[idx x y] = int GroundTile.Water
                || pathSet.Contains(idx x y)
                || bridgeLookup.Contains(idx x y)
                || inCore x y
                || inBuildingFootprint x y
            let occupies (kind: RiverDecorationKind) (y: int) =
                if kind = Willow then [ y; y + 1 ] else [ y ]
            let tooClose (kind: RiverDecorationKind) x y =
                decorations
                |> List.exists (fun d ->
                    let dyList = occupies d.Kind d.Y
                    occupies kind y
                    |> List.exists (fun cy ->
                        dyList
                        |> List.exists (fun oy ->
                            let dx = d.X - x
                            let dy = oy - cy
                            dx * dx + dy * dy < 9)))
            let tryPlace (kind: RiverDecorationKind) x y =
                let tilesFree =
                    if kind = Willow then not (blocked x y) && not (blocked x (y + 1))
                    else not (blocked x y)
                if tilesFree && not (tooClose kind x y) then
                    decorations <- { Kind = kind; X = x; Y = y } :: decorations
            let kindOf (roll: uint64) =
                match int (roll % 3UL) with
                | 1 -> Reeds
                | 2 -> Willow
                | _ -> RevetmentStone
            for river in riverBands do
                for x in 2 .. w - 3 do
                    let lowerRoll = nextDecor ()
                    if lowerRoll % 3UL <> 0UL then
                        tryPlace (kindOf lowerRoll) x (river.CenterY + river.Width)
                    if nextDecor () % 4UL = 0UL then
                        let upperRoll = nextDecor ()
                        let kind = kindOf upperRoll
                        let anchorY = if kind = Willow then river.CenterY - 2 else river.CenterY - 1
                        tryPlace kind x anchorY

        // 6) 30 个 spawn：核心区内的规则网格。
        let sc = max 2 p.SpawnColumns
        let sr = max 2 p.SpawnRows
        let spanX = max 1 (core.MaxX - core.MinX - 2)
        let spanY = max 1 (core.MaxY - core.MinY - 2)
        let spawns =
            [ for r in 0 .. sr - 1 do
                for c in 0 .. sc - 1 do
                    yield
                        (core.MinX + 1 + (c * spanX) / (sc - 1),
                         core.MinY + 1 + (r * spanY) / (sr - 1)) ]

        // 7) BFS（4 邻接、桥面可走）：校验 spawn / 门 / 桥与两岸全部连通。
        let bridgeSet = System.Collections.Generic.HashSet<int>(bridges |> List.map (fun (x, y) -> idx x y))
        let walkable x y = tiles.[idx x y] <> int GroundTile.Water || bridgeSet.Contains(idx x y)
        let flood startX startY =
            let visited = Array.create (w * h) false
            let queue = System.Collections.Generic.Queue<int>()
            if walkable startX startY then
                let start = idx startX startY
                visited.[start] <- true
                queue.Enqueue start
            while queue.Count > 0 do
                let cur = queue.Dequeue()
                let x = cur % w
                let y = cur / w
                for dx, dy in [ (1, 0); (-1, 0); (0, 1); (0, -1) ] do
                    let nx = x + dx
                    let ny = y + dy
                    if nx >= 0 && nx < w && ny >= 0 && ny < h then
                        let ni = idx nx ny
                        if not visited.[ni] && walkable nx ny then
                            visited.[ni] <- true
                            queue.Enqueue ni
            visited

        let visited = flood (fst spawns.Head) (snd spawns.Head)
        let spawnsOk = spawns |> List.forall (fun (x, y) -> visited.[idx x y])
        let doorsOk = buildings |> List.forall (fun b -> visited.[idx b.DoorX b.DoorY])
        let bridgesOk = bridges |> List.forall (fun (x, y) -> visited.[idx x y])
        let crossingsOk =
            bridgeColumns
            |> List.forall (fun (river, bx) -> visited.[idx bx (river.CenterY - 1)] && visited.[idx bx (river.CenterY + river.Width)])
        let reachableTiles = visited |> Array.filter id |> Array.length

        { Width = w
          Height = h
          Seed = p.Seed
          Tiles = tiles
          Core = core
          Spawns = spawns
          Rivers = riverBands
          Bridges = bridges
          Paths = paths
          Buildings = buildings
          Decorations = decorations
          ReachableTiles = reachableTiles
          ReachabilityOk = spawnsOk && doorsOk && bridgesOk
          BridgeCrossingsOk = crossingsOk }

    let generateWithSize (width: int) (height: int) (seed: uint64) : Result =
        generate (paramsForSize width height seed)

    /// 生成结果的确定性序列化（同 seed 逐字节一致），供回归测试与跨机核对。
    let serialize (map: Result) : string =
        let builder = System.Text.StringBuilder()
        builder.Append(map.Width).Append('x').Append(map.Height).Append('|').Append(map.Seed).Append('|') |> ignore
        for code in map.Tiles do
            builder.Append(code).Append(',') |> ignore
        builder.Append('|') |> ignore
        for river in map.Rivers do
            builder.Append(river.CenterY).Append('*').Append(river.Width).Append(';') |> ignore
        builder.Append('|') |> ignore
        for (x, y) in map.Bridges do
            builder.Append(x).Append(':').Append(y).Append(';') |> ignore
        builder.Append('|') |> ignore
        for (x, y) in map.Paths do
            builder.Append(x).Append(':').Append(y).Append(';') |> ignore
        builder.Append('|') |> ignore
        for building in map.Buildings do
            builder
                .Append(building.Left).Append(':').Append(building.Top).Append(':')
                .Append(building.Width).Append(':').Append(building.Height).Append(':')
                .Append(building.DoorX).Append(':').Append(building.DoorY).Append(';')
            |> ignore
        builder.Append('|') |> ignore
        let decorCode = function
            | RevetmentStone -> 0
            | Reeds -> 1
            | Willow -> 2
        for decoration in map.Decorations do
            builder
                .Append(decorCode decoration.Kind).Append(':')
                .Append(decoration.X).Append(':').Append(decoration.Y).Append(';')
            |> ignore
        builder.ToString()

    /// 可行走判定：非水，或位于桥面上（桥面仍标记为水但可通行）。
    let isWalkable (map: Result) (x: int) (y: int) : bool =
        if x < 0 || x >= map.Width || y < 0 || y >= map.Height then
            false
        else
            map.Tiles.[y * map.Width + x] <> int GroundTile.Water
            || (map.Bridges |> List.exists (fun (bx, by) -> bx = x && by = y))

    /// 从给定起点做 4 邻接洪泛（承认桥面），返回逐瓦片可达标记。
    let floodFill (map: Result) (startX: int) (startY: int) : bool array =
        let visited = Array.create (map.Width * map.Height) false
        let queue = System.Collections.Generic.Queue<int>()
        if isWalkable map startX startY then
            let start = startY * map.Width + startX
            visited.[start] <- true
            queue.Enqueue start
        while queue.Count > 0 do
            let cur = queue.Dequeue()
            let x = cur % map.Width
            let y = cur / map.Width
            for dx, dy in [ (1, 0); (-1, 0); (0, 1); (0, -1) ] do
                let nx = x + dx
                let ny = y + dy
                if isWalkable map nx ny then
                    let ni = ny * map.Width + nx
                    if not visited.[ni] then
                        visited.[ni] <- true
                        queue.Enqueue ni
        visited

    // ---- 视口裁剪（纯函数）：只绘制可见瓦片，与 drawWorld 现有窗口一致 ----

    /// 返回含端点的可见瓦片范围 (x0, y0, x1, y1)；空视口返回 (0,0,-1,-1)。
    let visibleTileRange (mapWidth: int) (mapHeight: int) (cameraX: int) (cameraY: int) (viewportWidth: int) (viewportHeight: int) : int * int * int * int =
        let x0 = max 0 (cameraX / Sim.tilePixels)
        let y0 = max 0 (cameraY / Sim.tilePixels)
        let x1 = min (mapWidth - 1) ((cameraX + viewportWidth) / Sim.tilePixels + 1)
        let y1 = min (mapHeight - 1) ((cameraY + viewportHeight) / Sim.tilePixels + 1)
        if mapWidth <= 0 || mapHeight <= 0 || x1 < x0 || y1 < y0 then (0, 0, -1, -1) else (x0, y0, x1, y1)

    let visibleTileCount (mapWidth: int) (mapHeight: int) (cameraX: int) (cameraY: int) (viewportWidth: int) (viewportHeight: int) : int =
        let x0, y0, x1, y1 = visibleTileRange mapWidth mapHeight cameraX cameraY viewportWidth viewportHeight
        if x1 < x0 || y1 < y0 then 0 else (x1 - x0 + 1) * (y1 - y0 + 1)