/// What can be written at a position: the top-level names of every module
/// the file sees, nearest first, the compiler's builtins, and the special
/// forms. The client filters them as the word is typed; the doc of one is
/// sent when the client asks for it, through `completionItem/resolve`.
module BjolangLsp.Completion

open System
open Bjolang
open BjolangLsp.Symbols

/// The heads the parsers recognise themselves, which no module defines. A
/// copy of the REPL's list in `Repl.fs`, which is private to it.
let private specialForms =
    [ "->"; "and"; "begin"; "bjo"; "bjoroutine"; "by-colour"; "case"; "cast"; "def"; "def*"; "def/hash-extend"
      "def/macro"; "def/mutable"; "def/pattern"; "def/trait"; "defbjo"; "defbjouble"; "defun"; "dyn"
      "export"; "fun"; "if"; "impl"; "impl/extern"; "import"; "import/class"; "import/extern"; "include"
      "let"; "let/mono"; "letrec"; "loop"; "match"; "module"; "not"; "or"; "parameterize"; "re-export"
      "record"; "record-ref"; "record-set"; "record-set!"; "seq"; "seql"; "set!"; "spawn"; "spawn-evt"
      "spawn/daemon"; "spawn/detached"; "struct"; "struct-ref"; "struct-set"; "struct-set!"; "task->event"
      "try"; "type"; "type-rec"; "unless"; "when"; "with-open"; "with-return"; "yield"; "yield-from" ]

/// LSP's `CompletionItemKind`, the members of it used here.
type ItemKind =
    | Method = 2
    | Function = 3
    | Field = 5
    | Variable = 6
    | Class = 7
    | Interface = 8
    | Keyword = 14
    | EnumMember = 20
    | Struct = 22

type Item =
    { Label: string
      Kind: ItemKind
      /// Where it comes from: a module's file name, `builtin`, `special form`.
      From: string }

let private itemKind (k: Kind) =
    match k with
    | Kind.Function -> ItemKind.Function
    | Kind.Variable -> ItemKind.Variable
    | Kind.Class -> ItemKind.Class
    | Kind.Interface -> ItemKind.Interface
    | Kind.Method -> ItemKind.Method
    | Kind.Field -> ItemKind.Field
    | Kind.EnumMember -> ItemKind.EnumMember
    | Kind.Struct -> ItemKind.Struct
    | _ -> ItemKind.Variable

/// The word the cursor ends, as the start column of it on its line. A word
/// is what the lexer reads as one symbol, so `list-` and `string->` are
/// words and the item replaces all of it.
let wordStart (line: string) (column: int) : int =
    let mutable i = min column line.Length

    while i > 0 && Lexer.isSymbolChar line[i - 1] do
        i <- i - 1

    i

/// Whether `column` of `line` is in a comment or a string, where nothing is
/// offered. Read off the line alone, so a string that began on an earlier
/// line is not seen.
let inText (line: string) (column: int) : bool =
    let mutable inString = false
    let mutable inComment = false
    let mutable i = 0

    while i < min column line.Length && not inComment do
        match line[i] with
        | '\\' when inString -> i <- i + 1
        | '#' when not inString && i + 1 < line.Length && line[i + 1] = '\\' -> i <- i + 2
        | '"' -> inString <- not inString
        | ';' when not inString -> inComment <- true
        | _ -> ()

        i <- i + 1

    inString || inComment

/// Everything that can be written in the file of `text`, each name once, the
/// nearest definition of it winning.
let items (path: string) (text: string) : Item list =
    let seen = Collections.Generic.HashSet<string>()

    let fromModules =
        [ for m in visible path text do
              let from = IO.Path.GetFileName m.File

              for s in m.Symbols do
                  // An `impl` is not a name, and a field is read with
                  // `record-ref` rather than written on its own.
                  if s.Kind <> Kind.Module && s.Kind <> Kind.Field && seen.Add s.Name then
                      yield { Label = s.Name; Kind = itemKind s.Kind; From = from } ]

    let builtins =
        [ for name in Prelude.builtinNames do
              if seen.Add name then
                  // `Cons`, `None`, `Err`: the builtin constructors.
                  let kind = if Char.IsUpper name[0] then ItemKind.EnumMember else ItemKind.Function
                  yield { Label = name; Kind = kind; From = "builtin" } ]

    let special =
        [ for name in specialForms do
              if seen.Add name then
                  yield { Label = name; Kind = ItemKind.Keyword; From = "special form" } ]

    fromModules @ builtins @ special