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+module Day15
+
+open System.IO
+open FSharp.Collections.ParallelSeq
+open FParsec
+open Common
+
+let tuner = 4_000_000
+
+type Sensor =
+    { Pos: Vec2
+      Radius: int
+      NearestBeacon: Vec2 }
+
+let rangeCount (first, last) = max 0 (last - first + 1)
+
+type RangeSet =
+    | RangeSet of (int * int) list
+
+    static member empty = RangeSet []
+
+    static member private merge =
+        function
+        | ((_, l1) as r1) :: ((f2, _) as r2) :: t when l1 + 1 < f2 -> r1 :: RangeSet.merge (r2 :: t)
+        | (f1, l1) :: (_, l2) :: t -> RangeSet.merge <| (f1, max l1 l2) :: t
+        | other -> other
+
+    static member add set range =
+        if rangeCount range = 0 then
+            set
+        else
+            let (RangeSet list) = set
+            RangeSet(RangeSet.merge (Util.insertSorted range list))
+
+    static member count(RangeSet list) = List.map rangeCount list |> List.sum
+
+    static member gap =
+        function
+        | (RangeSet ([ (_, x); _ ])) -> Some(x + 1)
+        | _ -> None
+
+let sensorList =
+    let parseSensor line =
+        let ppos =
+            tuple2 (pstring "x=" >>. pint32) (pstring ", y=" >>. pint32)
+            |>> Vec2
+
+        let pline =
+            pstring "Sensor at " >>. ppos
+            .>> pstring ": closest beacon is at "
+            .>>. ppos
+
+        let (sensorPos, beaconPos) = Util.parse pline line
+
+        { Pos = sensorPos
+          Radius = Vec2.mahattanDist sensorPos beaconPos
+          NearestBeacon = beaconPos }
+
+    Seq.map parseSensor >> List.ofSeq
+
+let rowCoverages y sensors =
+    let coverage ({ Radius = radius; Pos = pos }) =
+        let offset = radius - abs (Vec2.y pos - y)
+        (Vec2.x pos - offset, Vec2.x pos + offset)
+
+    sensors
+    |> Seq.map coverage
+    |> Seq.fold RangeSet.add RangeSet.empty
+
+let solution1 y input =
+    let sensors = sensorList input
+
+    let beaconsInRow =
+        sensors
+        |> Seq.choose (fun ({ NearestBeacon = b }) ->
+            if Vec2.y b = y then
+                Some(Vec2.x b)
+            else
+                None)
+        |> Util.countDistinct
+
+    sensors
+    |> rowCoverages y
+    |> RangeSet.count
+    |> (fun count -> count - beaconsInRow)
+
+let solution2 input =
+    let sensors = sensorList input
+
+    seq { 0..tuner }
+    |> PSeq.pick (fun y ->
+        sensors
+        |> rowCoverages y
+        |> RangeSet.gap
+        |> Option.map (fun x -> int64 x * int64 tuner + int64 y))
+
+let test = File.ReadLines("test.txt")
+assert (solution1 10 test = 26)
+assert (solution2 test = 56000011)
+
+let input = File.ReadLines("input.txt")
+printfn "%A" <| solution1 2_000_000 input
+printfn "%A" <| solution2 input